Monday, October 15, 2018

Valley Fill Passive Power Factor Correction method


Passive Power Factor Correction method

There are two types of Passive Power Factor Correction method; 
1) Capacitor Input Filter 
2) Valley Fill Power Factor Correction
Here we will discuss about Valley Fill Passive Power Factor Correction method.

Valley Fill Passive Power Factor Correction method is generally a circuit of two electrolytic capacitors, a resistor and two diodes.

The intend of the Valley Fill Passive Power Factor Correction method is to let the power converter to pull power straight off the AC line when the line voltage is larger than 50% of its peak voltage.

More details like working of Valley Fill Passive Power Factor Correction method you can find in my previous blog and the link for the same is;


Below is the circuit diagram of Valley Fill Passive Power Factor Correction method.

Power Factor Correction Method
Valley Fill Power Factor Correction Method

Selection of Component values

Now we will calculate the component values as per below specifications (consider it as an example).
VAC = 230V
VAC (min) = 90V
VAC (max) = 270V
POUT = 10W

The maximum i.e. highest bus voltage at the input of the power converter is,
VIN (max) = √2 x VAC (max)
= √2 x 270VAC
= 381.8V
= 382V
During this time, capacitors placed in  the valley fill circuit (C1 and C2) are in series and charged via the diode D2 and resistor R1.

If  the  capacitors  have  identical  capacitance  value,  the  peak  voltage  across  C1  and  C2 is;
VIN (max) / 2 = 191V
Often  a  20%  variance  in  capacitance  could  be  seen  between  like capacitors. Therefore a margin of 25% voltage rating should be considered.
Therefore the peak voltage across C1 and C2 becomes; 238.75V = 239V

Once  the  line  drops  below  50%  of  its  peak  voltage,  the  two capacitors  are  basically  placed  in parallel. The bus voltage VIN (min) is the lowermost voltage value at the power converter input.
VIN (min) at the minimum AC line voltage is,
VIN (min) = [√2 x VAC (min)] / 2
= [√2 x 90V] / 2 = 63.63V = 64V

At 60Hz, the total i.e. full time of a half AC line cycle is 8.33ms.

The power to the power converter is derived from the valley-fill capacitors when the AC line voltage is equal to or less than 50% of its peak voltage.

The  holdup  time  for  the  capacitors  equates  to;
tHOLD = (1/3) × 8.33ms  =  2.77ms

The valley-fill capacitor value can then be calculated as,
CTOTAL = {[POUT/ VIN (min)] x tHOLD} / VDROOP  
               = {[10/ 64] x 0.00277} / 20
               = {0.1562 x 0.00277} / 20
               = 0.000432 / 20
               = 0.0000216
               = 21µF
Therefore, C1 = C2 = 10µF

VDROOP is the voltage droop (drop) on the capacitors when they are delivering or supplying full power to the power converter.

Ideally VDROOP should be set to less than;
VDROOP = VIN (min) – VLED (max)
In order to ensure continuous load conduction at low line voltage.
Anyway, VDROOP is set to be 20V in the design example to avoid or prevent the need of very large valley-fill electrolytic capacitor.

A 20V VDROOP implies or indicates that the bus voltage VIN at the input of power converter will drop to 40V during part of the AC line cycle.

Let consider an example of the buck regulator, which needs VIN to be larger than the load voltage for regulation; the load will be off through part of the AC line cycle.

This has the consequence of reducing the real output load current at low AC input voltage. In this design example, the load current falls by roughly 20% from its nominal value at 90Vac.

Conclusion

Valley Fill Passive Power Factor Correction method is a low cost method for power factor improvement. This circuit can help user to achieve power factor in the range of 0.80 to 0.88.

Wednesday, October 10, 2018

Power Factor Correction and it's Modes of Operation


What is Power Factor?
We can define power factor as the ratio of the True (real) power (P) dissipated in the load to the apparent power (S) taken by the load, irrespective of the waveform.

Power Factor = True (Real) Power / Apparent Power (S)

What is Power Factor Correction?
Power Factor Correction is the process by which power factor correction circuit minimizes the input current distortion and makes the current in phase with the voltage.

More details on Power Factor and Power Factor Correction you can find on my previous blog and link for the same is;

Modes of Operation of Power Factor Correction?
We know that Power Factor Correction is implemented with either a passive or an active circuit to comply with regulations. Passive circuits contain a combination of large capacitors, inductors, and rectifiers that operate at the ac line frequency. Active circuits use a high frequency switching converter to regulate the input current harmonics. Active circuits operate at a higher frequency, which enables them to be physically smaller, weigh less, and operate more efficiently than a passive circuit. 

With proper control of an active PFC stage, nearly any difficult load emulates a linear resistance, which significantly reduces the harmonic current content. Active PFC circuits are the most popular way to meet harmonic content requirements because of the above mentioned benefits. Generally, active PFC circuits consist of inserting a PFC pre−converter between the rectifier bridge and the bulk capacitor. The boost (or step up) converter is the most popular topology for active power factor correction. With the proper control, it produces a constant voltage while consuming a sinusoidal current from the line.

Figure 1 shows the basic diagram of PFC circuit which contains; MOSFET which is acting as a switch, Capacitor and Inductor which is acting as energy storage element, a Diode (boost diode) which is working in reverse bias mode and a PFC controller which generates necessary Gating voltage.
PFC circuit
Basic Diagram of PFC circuit
As per this diagram we can say that their are two modes of operation of Power Factor Correction; Discontinuous Mode and Continuous Mode.

Discontinuous Mode: 

Discontinuous mode is when the boost converter’s MOSFET is turned ON when the inductor current reaches zero, and turned OFF when the inductor current meets the desired input reference voltage as shown in Figure 2. In this way, the input current waveform follows that of the input voltage, therefore attaining a power factor of close to 1.

Discontinuous mode can be used for SMPS that have power levels of 300W or less. In comparison with continuous mode devices, discontinuous ones use larger cores and have higher I²R and skin effect losses due to the larger inductor current swings. With the increased swing a larger input filter is also required. On the positive side, since discontinuous mode devices switch the boost MOSFET ON when the inductor current is at zero, there is no reverse recovery current (IRR) specification required on the boost diode. This means that less expensive diodes can be used. Here the average inductor current stays relatively low because the peak current is allowed to fall essentially to zero amperes. The Discontinues mode is easier to achieve. Here current is discontinuous, a very high peak current will be present, which requires a large EMI filter and some over sized components.

Continuous Mode: 

Continuous mode usually suits SMPS power levels greater than 300W. This is where the boost converter’s MOSFET does not switch ON when the boost inductor is at zero current, instead the current in the energy transfer inductor never reaches zero during the switching cycle as shown in Figure 3.

With this in mind, the voltage swing is less than in discontinuous mode which results in lower I²R losses and the lower ripple current results in lower inductor core losses. Less voltage swing also reduces EMI and allows for a smaller input filter to be used. Since the MOSFET is not being turned ON when the boost inductor’s current is at zero, a very fast reverse recovery diode is required to keep losses to a minimum. Here the average current runs higher. The Continuous mode results in power factor closer to unity.

CCM boost is a better choice for high power applications. For the CCM PFC, the full load inductor current ripple is typically designed to be 20-40% of the average input current by which Peak current become low and the RMS current factor with a trapezoidal waveform is reduced compared to a triangular waveform, reducing device conduction losses. Turn-off losses are lower due to switch off at much lower maximum current. Also the HF ripple current to be smoothed by the EMI filter is much lower in amplitude.

CCM encounters the turn-on losses in the MOSFET, which can be made worse by the boost rectifier reverse recovery loss due to reverse recovery charge, Qrr . For this reason, ultra-fast recovery diodes or silicon carbide Schottky Diodes with extreme low Qrr are needed for CCM mode.
Discontinuous Conduction Mode and Continuous Conduction Mode
PFC operation in Discontinuous mode and Continuous mode
Now we know that according to the current of inductor L, the operation modes can be specified as: CCM (Continuous Conduction Mode), DCM (Discontinuous Conduction Mode).  Also according to the current of inductor L, one more mode we can define and it is; CRM (Critical Conduction Mode). 

Critical Conduction Mode: 

The boost converter can operate in three modes: continuous conduction mode (CCM), discontinuous conduction mode (DCM), and critical conduction mode (CRM).

Critical Conduction mode is also called as Transitional mode or Borderline or Boundary Conduction Mode (BCM)

The critical conduction mode operates at the boundary of CCM and DCM. It is a control strategy in which the active switch turns on when the inductor current falls into zero point to remove the freewheeling diode reverse recovery. Please see Figure 4.
Critical Conduction Mode
PFC operation in Critical Conduction mode
CRM usually uses constant on-time control i.e. the line voltage is changing across the 60 Hz line cycle, the reset time for the boost inductor is varying, and the operating frequency will change as well in order to maintain the boundary mode operation. CRM say’s to the controller to sense the zero crossing inductor current in order to trigger the start of the next switching cycle.

The inductor peak current in CRM is twice of the average value, which greatly increases the MOSFET RMS current and turn-off current. But since every switching cycle starts at zero current, and typically with ZVS operation, turn-on loss of MOSFET is removed. Also, in CRM mode since the boost rectifier diode turns off at zero current, reverse recovery losses and noise in the boost diode are eliminated. The high input ripple current and its impact on the input EMI filter tends to eliminate CRM mode for high power designs. The input HF current ripple can be reduced by interleaved stages but this will increase the design cost. For low power applications, the CRM boost has the advantages in power saving and improving power density. The main design concerns for a CRM inductor are low HF core loss, low HF winding loss, and the stable value over the operating range (the inductor is essentially part of the timing circuit).

To better understand Critical Conduction Mode let’s look at the difference between discontinuous and continuous mode in a SMPS design such as a flyback converter. In discontinuous mode, the primary winding of the transformer has a dead time once the switch is turned off (including is a minimum winding reset time) and before it is energized again. Please see Figure 5.
Primary Current for Flyback Power Supply in Critical Conduction Mode
Primary Current for Flyback Power Supply in Critical Conduction Mode
In continuous mode, the primary winding has not fully exhausted all of its energy. We can see in Figure 6 that the primary winding does not start energizing at zero, somewhat left over current still resides in the winding.

In critical conduction mode there are no dead-time gaps between cycles and the inductor current is always at zero before the switch is turned on. Please see Figure 7.

Conclusion
We understood the Power Factor Correction and it’s working. According to inductor current, a mode of operation of Power Factor Correction is decided.

Friday, October 5, 2018

Resistor and Working of Resistor


Resistor

A resistor is a passive component which opposes the movement of current across it in other words; it limits or controls the flow of electrical current in an electronic circuit. Apart from current control in the circuit, resistors are used for; Adjustment of signal stages, voltage divider, to bias the device like transistors, for transmission line terminations, resistive loads, etc.
Resistor
Resistor

Resistor Symbol

Below image shows symbol of both fixed and variable type resistors.
Resistor Symbol
Resistor Symbol
Resistors are classified as; fixed and the variable type.

The fixed resistors have a fixed resistance value and this value is not adjustable. Fixed resistors have resistances which changes slightly with change in temperature, time or by operative voltage.

The variable resistors have a variable resistance value and this value is adjustable. These types of resistors can be used to amend circuit elements like amending volume or a varying the lamp intensity. Sometimes these resistors used as sensing component for heat, light, etc.

Working of Resistor

Below points presents details for working of resistor;

A resistor resists current because flowing electrons strike with atoms in the resistor and make them vibrate. By this, electrical energy is changed to heat energy. The quantity by which a resistor resists the flow of charge is measured as resistance “R”.

  • Resistor works according to Ohm's law

Basic principle of working of resistor or in other words behavior of resistor working depends on the Ohm’s law. As we know Ohm’s law states that; V = I / R.

Let’s explain this; as per Ohm's law the voltage (V) through a resistor is proportional to the current (I), where the constant of proportionality is the resistance (R) i.e. the electrical resistance of the conductor is constant.

So, in simple word’s we can say that, with the additional voltage applied to a resistor the extra current flows through it.

Let’s see an example; If a 100 ohm resistor is connected across the +ve and –ve terminals of a 24V power supply, then a current of 12 / 300 = 0.24A will flow through that resistor.

The resistor causes delay i.e. potential drop to flow of current, we can understand this by an example of a pipe through which the water flows. See the image at below;
Water flow in the pipe
Water flow in the pipe
Here the thickness of the pipe signifies the resistance. The narrow the pipe it’s tough for the water to get through the pipe and therefore larger the resistance. If you make the pipe size double, then the flow rate doubles i.e. if less resistance then more current will pass.

  • Series and parallel resistors

Resistor can be used to change a voltage to a current or a vice-versa. By connecting resistors in series or in parallel you can efficiently control the current. Let see in detail;

Resistors in series

Resistors in “Series” means, they are connected together in daisy form i.e. in a particular single line. Entire current passing across the first resistor must also pass through the second resistor and then third and so on, as there’s no other way for flow of current. Current flow has only one path i.e. resistors in series have a common current flow across them.

Below schematic shows that the resistors R1, R2, R3 and R4 are connected in series, in between points A and B and a common current (I) passing through them;
Resistors in Series
Resistors in Series
Here the total current that passing across these resistors in series will be the similar at any point.
Itotal = IR1 = IR2 = IR3 = IR4

Here the total resistance is the addition of all the individual resistance of the resistors in series.
Rtotal = R1 + R2 + R3 + R4 = 100 ohm + 200 ohm + 300 ohm + 400 ohm = 1k ohm

Here the total voltage across each resistor will be different and can be calculated by Ohm’s law. So, total voltage through the resistors is equivalent to the sum of the potential differences across each resistor.
Vtotal = VAB = VR1 + VR2 + VR3 + VR4 = 1V + 2V + 3V + 4V = 10V

In the circuit having resistors in series, if we apply the input voltage through the resistor pair and the output voltage is measured from the linking connection between them, then this circuit is called as Voltage Divider Circuit. This circuit produces an output voltage (Vout) that is a division of its input voltage (Vin) i.e. it turns large voltage to small voltage. Below schematic is for Voltage Divider;
Voltage Divider Circuit
Voltage Divider Circuit
Output voltage of voltage divider can be calculated by;
Vout = Vin [R2 (R1 + R2)]

Resistors in Parallel

Resistors in “Parallel” means, both the terminals of resistor or resistors are individually connected to each terminal of the other resistor or resistors.

Below schematic shows that the resistors R1, R2, R3 and R4 are connected in parallel, in between points A and B and uncommon current (I) passing through them;
Resistors in Parallel
Resistors in Parallel
Here in any branch the current passing across the resistors will be non-similar.
Itotal = I1 + I2 + I3 + I4. If Vin = 10V then;
Itotal = 0.1A + 0.05A + 0.03A + 0.025 = 0.205A

Here the total resistance is the addition of reciprocal (1/R) value of the individual resistances. In other words we can say that, the inverse of the equal resistance of two or more resistors attached in parallel is the addition of the inverses of the each single resistance.
1/Rtotal = (1/R1) + (1/R2) + (1/R3) + (1/R4) = (1/100 ohm) + (1/200 ohm) + (1/300 ohm) + (1/400 ohm) = 0.01 + 0.005 + 0.003 + 0.0025 = 0.0205 ohm

Rtotal = 1 / 0.0205 = 48.78 ohm

Here the voltage across each resistor will be same, i.e. the voltage drop through all of the resistors in a parallel resistive circuit is same.
Vin = VR1 = VR2 = VR3 = VR4 = 10V

The parallel resistor combination has the same voltage through it, the resistances might be different, and therefore the current passing across each resistor will be different as according to Ohms Law.

  • Resistor Power Rating

Power dissipation of resistor or resistor power rating also defines the selection and working of resistor. When a current passes across a resistor, there’s a loss of energy and the resistor heats. The larger the resistance the more hot it will become. The power supply forces the electrons across the resistor and this generates heat energy in the resistor. Resistor should withstand or dissipate this heat energy before its temperature rises extremely or it gets damage. The rate of change is the power of dissipation.

Resistor Power Rating, P = I²R = IV = V²R

Where; V = Voltage across the resistor, I = Current across resistor, R = Resistance of resistor.

All the resistor are available with maximum power dissipation rating like; 0.25W, 0.5W, 1W, etc. Normally up to 150W resistor power rating is available in online stores. Higher wattage than this can be made with special request from manufacturer. Normally higher wattage resistor consists of aluminium heat sink as shown in below image.
High Power Resistor
High Power Resistor
Power rating of a resistor should be more as compare to the actual power dissipated by the resistor in the application. Otherwise the resistor will be damage i.e. it will become more heated and its resistance value will be permanently change. With the rise in temperature there is change in the resistance value. Also, there will be chances of the resistor burning.

Resistors have maximum voltage rating which helps to reduce power dissipation of resistor having greater resistance value.

Inside the Resistor

Let’s see in general what is inside the resistor;
  • If you break a resistor in half and scratch the paint from the resistor, you can see a ceramic rod which is insulating in nature. 
  • Two discs are connected to the end of this ceramic rod. 
  • Middle part of this rod is covered by copper wire. This copper wire sets the resistance. 
  • The thin wire size and more copper turns sets high resistance and vice-versa. This type of resistor called as Wire Wound Resistor. 
  • In some low value resistors for lower power application, in place of copper, carbon film layers are used. 
  • Some of the resistor use carbon cylinder without any copper winding. 
  • Also, in some resistor in place of copper, Nichrome wire is used to get the required resistance.
  • According to applications different type of materials are used for setting the resistance.

Uses of Resistor

Below are some of the uses of resistor;

  • The important purpose of resistors in a circuit is to regulate the movement of current i.e. it controls the current before it is applied to components. We can see this in the application of LED’s, where resistor is used to limit the current. More current will destroy the LED.
  • Resistors with high wattage can dissipate more power through it. So, because of this property high wattage resistors can be used in in power distribution, controlling of motor and as a testing load for power converters or generators.
  • Voltage level can be reduce by voltage divider circuit by using resistor.
  • To adjust level of signals you can use resistors.
  • Transmission lines are ended with resistors.
  • Fixed resistors resistance varies very little with temperature or by applied voltage. So, these can be used for temperature critical applications. Resistors can be used as a heating element in appliances like heaters and toasters because resistors transform electrical energy into heat energy. One more example of this is Incandescent light bulbs in which filament turn hot because of the high temperature and by this light bulb produce light. This concept depends on power dissipation principal according to formula; P = I²R. Where I = Current in ampere, R = Resistance in ohms and finally P = Power in watts which decides the quantity of power generated by the resistor.
  • Variable resistors are used to regulate voltage in circuit.
  • For transistor biasing resistors are used to apply bias voltage to a transistor. Transistor mainly requires a small base voltage (approximately 0.6V) to make a big voltage flow by its collector or emitter pins. However the base of a transistor is somewhat weak to high currents, so a resistor is placed at this place to control the input current and to deliver a biasing voltage to transistor.
  • Timing circuit applications, resistors are used to determine charging time of the capacitor in a circuit. In the circuits for time setting the components used for timer and oscillator circuits are resistor and a capacitor. In this the time taken to charge or discharge a capacitor creates the initial time pulse or trigger-ON for the timing circuit. A resistor is used for this application to charge and discharge the capacitor and different value of resistance decides different time triggering interval.
  • Mostly resistors are used as a pull-up resistor to bias a microcontroller’s input pin to a recognized state.

Resistors
Resistors in a circuit along with other components

Conclusion

Resistors are most commonly used component of electronic circuits. Almost all types of circuits have its presence. Mostly saving the circuit by current control and never provide any power gain.

Monday, October 1, 2018

CE and CE Mark Process


What is CE?

CE is a certification mark that informs conformity of a product according to health, safety and environmental standards, so a manufacturer can easily sell products within the European internal market i.e. European Economic Area.
CE Mark
LED bulb with CE Mark
CE Full Form

CE full form is, “Conformité Européenne”, which means European Conformity.


CE Mark

The CE Mark on a product is the product manufacturer's announcement or declaration that the product fits the necessitate of the relevant EC directives.

The CE Mark consists of a logo, “CE”. Some times it consists of a Notified Body’s four digit identification number, who is engaged in the conformity assessment activity.
CE Mark
CE Logo

CE Marking Process

Below steps will clear CE Marking Process, subjecting to your product and the risk criteria associated with it;

  • First step is to search and find the directives related to your product. If there are more directives for the same product then please follow and comply all those related directives.
  • In the applicable directives, find the level up to which your product observes or follows the important necessities required for design and manufacturing.
  • There are numerous modules available for the Conformity Assessment Procedures as listed below, select the conformity assessment procedure from the below options i.e. modules according to the directive for your product.
Module A: Internal production control - It covers the design and production stages. This module does not need the intervention of any advised body.

Module Aa: Intervention of a Notified Body.

Module B: EC type-examination - It includes the design phase, and need be monitored or followed by a module needed for assessment in the production stage. An "EC type-examination" certificate is issued by a notified or advised body.

Module C: Conformity to type - It includes the production stage and follows module B. It offers the conformity by the type as defined in the EC type-examination certificate allotted conferring to module B. Module C does not need the intervention or involvement of a notified or reporting body.

Module D: Production quality assurance - It includes the production stage and follows module B. It originates from the quality assurance standard EN ISO 9002, by the involvement of a notified body, which is liable for approving and monitoring the quality system for; production, final product inspection and testing setup by the manufacturer.

Module E: Product quality assurance - It includes the production stage and follows module B. It originates from quality assurance standard EN ISO 9003, by the involvement of a notified body, which is liable for approving and monitoring the quality system for; production, final product inspection and testing setup by the manufacturer.

Module F: Product verification - It includes the production stage and follows module B. A notified body controls conformity to this type as defined in the EC type examination certificate allotted according to module B. It assigns a certificate of conformity to the product.

Module G: Unit verification – It includes the design and production stages. Every single product is inspected by a notified body, and this notified body issues a certificate of conformity.

Module H: Full quality assurance - It includes the design and production stages. It originates from quality assurance standard EN ISO 9001, with the involvement of a notified body which is responsible for approving and regulating the quality system for design, final product inspection and testing setup of the product.

The selected directives frequently use a sequence of queries about the scope or nature of your product to categorize the level of risk or hazard and mention to a chart called "Conformity Assessment Procedures", which contains all the possibilities accessible to a product manufacturer to certify their product and fix the CE Mark.
CE Mark
CE Mark on a Multimeter
There are two types of products at certification process;

Minimal Risk Products
A manufacturer can prepare a Declaration of Conformity and use CE mark as a self-certification on their product in the case the products falls in Minimal Risk criteria’s.

Greater Risk Products

Following points to be consider for products which have greater risks;

  • As per the respective directives these types of products to be certified by a “Notified Body” or as mentioned in the directive.
  • “Notified Bodies” are certified test labs and follows all the test procedures mentioned in the directives. They are approved by the government and follow the laws of European Commission. After approval by the government they work as autonomous test labs. These types of labs may be owned by government or by private sectors.  Whoever owns the lab, but they have to follow testing requirements as mentioned in the directives and should meet all the pre-defined criteria’s mentioned in the directives and laws.
  • Product manufacturers may choose a testing agency in any European Union state.
  • Details of these testing agencies or notified bodies can be find from European Commission’s official journal.
  • These notified bodies identify the standards needed for testing of a product.
  • These notified bodies perform tests on the product according to standards.
  • These notified bodies issues test certificate to the product.
  • These notified bodied evaluate or cross-check the submitted product testing files from product manufacturer.

Steps to get CE Certification from a Notified Body

Below steps will clear the CE marking process to get certification from Notified body;

  • First step is to identify the correct directive related to our product.
  • Next select a Notified Body, which have facility to test or examine our product.
  • For your product find an Official Representative in the European Union. This representative will help to prepare the Technical file or Documentation as per the directive.
  • Next step is to prepare a Declaration of Conformity. It contains details about the directives and standards which confirms your product, identification details of your product. It also contains name, address and signature of the manufacturer. This declaration will help to trace the product back to the manufacturer or the official representative in the European Union.
  • Next you have to register your product in European Union i.e. to get Certificate of Registration. Without this the products are not permitted to fix CE Mark and to place in the market. This is mostly with Class I Medical Devices.
  • At the last fix the CE Mark to your product. According to Council Directive 93/68/EEC fix the CE Mark on the product. This directive contains specific instruction for the fixing of CE Mark on the product i.e. instructions addressing the size and position of the Marking; fixing the CE Marking to products, packing and documents to be ship with the product and detailed restrictions on when and who is allowed to fix the CE Mark.
CE Mark
Correct CE Mark
Now let discuss about CE Technical File or Technical Documentation, which is very important part for product approval from Notified Body.


What is CE Technical File and What is CE technical file requirements?

CE Technical File


  • It contains all the information that confirms that the testing was performed correctly and that the product complies with appropriate standards.
  • It should be prepared by manufacturer or by Representative (as per directive).
  • It is necessary as, it delivers information on the design and manufacturing stages of the product.
  • The particulars comprised in the documentation depend on the type of the product and demonstrates the conformity of the product to the critical necessities of the applicable directive.
CE Technical File Requirements
The following points to be consider for the CE technical file requirements;

  • Overall description of the product.
  • General drawing of a product. Should also include design and manufacturing related drawings, components diagrams, diagram of sub-assemblies, and schematic of control circuits. Descriptions and clarifications should be there to understand these drawings, schematics and figures.
  • For hazards related to products, should include risk study or analysis along with detail method to eliminate hazard in that product.
  • Important necessities of the product related directives.
  • Should include the information of standards, which are used for the testing or analysis of the product.
  • Design related calculations.
  • Test reports and certificates of used components (can be arrange from the component manufacturers).
  • Instructions for product use.
Certificate of Compliance
Example of Certificate of Compliance

Conclusion

When a product manufacturer affix the CE Mark on a product it indicates that it complies with all the necessary Health and safety necessitate. Also, it confirms that the product is according to all relevant directives and ready to sell in European market. The process to get CE Mark for a product from a notified body is not very complicated, you have to properly follow the process and full fill all CE technical file requirements according to relevant directives.

Thursday, September 27, 2018

Types of Power Factor Correction


There are two types of Power Factor Correction techniques; Passive PFC and Active PFC.
Let’s understand one by one.

Passive Power Factor Correction

Passive components like inductors and capacitors are used in Passive PFC to correct reduced power factors. These capacitors and inductors are tuned to the line frequency in a low pass or band pass configuration. The Passive PFC normally uses a simple line-frequency LC filter to extend the current conduction angle and decrease the THD of the input current of the diode-capacitor rectifier. This type of power factor correction method corrects the power factor in between 0.7 to 0.85.
Physical size and weight of these filters at mains frequency makes them nasty, particularly when one considers that the circuitry size and form factor should be small.

Types of Passive Power Factor Correction;


Capacitor input filter

This type is also called as the π filter; it removes unwanted frequencies from a signal. This filter reduces the harmonic content of a current waveform by making sure that the cut-off frequency of the filter is just above the fundamental frequency. As a result, the best possible reduction of harmonics can be achieved. Below figure shows a π filter;

Passive PFC Circuit
pi Filter Circuit

Valley Fill PFC

One more way to achieve power factor improvement up to 0.85 using simple, low cost circuitry shown below;

Passive PFC Circuit
Valley Fill PFC Circuit
This power-factor corrector can be used in low-power applications, where a high effective ripple voltage on DC output can be tolerated. It is frequently used in electronic ballast applications. The circuit contains two capacitors and three diodes. The two capacitors are charged in series around the line peak to half of the peak line voltage. When the line voltage falls below the single capacitor voltage, the bridge rectifier diodes are reversely biased, which doesn’t allow current to flow. Valley-fill’s diodes then conduct and the capacitors are connected in parallel to feed the load.


Let’s discuss its working in detail (Refer Figure 2); the capacitors C1 and C2 are charged to ½ of the AC peak voltage in series via the diode D2 and resistor R1 on each half cycle of the rectified AC input. R1 is for reducing the peaks in the current waveform as the capacitors charge. They supply output current after the BUS voltage follows the sinusoidal waveform down to Vpeak/2. At this time the caps are essentially in parallel an supply load current until the rectified AC input again exceeds Vpeak/2 on the next half cycle. This valley fill passive PFC circuit presents good power factor (>0.85) and low THD (30%), the major drawback is the 50% DC BUS ripple witch result in a very high lamp current crest factor. The capacitor CX is for filtering the half-bridge inverter switching spikes which appears at DC BUS. Particularly at the light load condition or at the peak of AC input, a big spike occurs at every switching cycle when switching frequency decreases towards resonance causing load voltage and current to increase.

Valley Fill PFC
Valley Fill PFC incorporated in a 10W SMPS
Advantage of the Passive PFC is its easiness; the passive LC filter is a high-efficiency and low-cost PFC solution that could potentially meet the IEC 61000-3-2 Class D requirement in the low-power range. Other advantages counts simplicity, reliability and ruggedness, insensitivity to noise and surges, no generation of high-frequency electromagnetic interface (EMI) because of no high-frequency switching losses.

For higher power designs, the presence of heavier and bulkier filter inductors increases the size and weight of the passive components, which is a disadvantage of this Passive PFC technique. Also, Passive PFC does not use the full energy potential of the AC line. This technique is not able to effectively eliminate line current harmonics. PFC is not possible for universal input range.

Refer below figure for Passive PFC waveforms where AC input current is lagging with respect to AC input voltage hence both current and voltage are not purely in-phase.

Passive PFC
Current and Voltage in Passive PFC

Active Power Factor Correction

A typical switch mode power supply with no power factor correction may have a power factor of around 0.60, with passive power factor correction, it may be around 0.80, and with active power factor correction it would be 0.95 or better than this.

Active PFCs use active electronics circuits, which contain devices like MOSFETs, BJTs, and IGBTs. Active power factor correction can involve more circuitry than other methods, but can be very effective in its result. Active PFC offers improved THD and is considerably smaller and lighter than a passive PFC circuit. Active PFC operates at frequencies higher than the line frequency so that compensation of both distortion and displacement can occur within the timeframe of each line frequency cycle, resulting in corrected power factors of up to 0.99.

The aim of active power factor correction is to make the input to a power supply appear like a simple resistor. An active power factor corrector does this by programming the input current in response to the input voltage. As long as the ratio between the voltage and current is a constant the input will be resistive and the power factor will be near to 1.0.

Distortion in an active power factor corrector comes from several sources: the feedforward signals, the feedback loops, the output capacitor, the inductor and the input rectifiers.

Active PFC functions include:

  • Active wave shaping of the input current
  • Filtering of the high frequency switching
  • Feedback sensing of the source current for waveform control
  • Feedback control to regulate output voltage
Buck, boost, flyback and other converter topologies are used in active PFC circuits.

Let we discuss two basic types of active PFCs i.e. Types of Active Power Factor Correction;

Boost

The boost-circuit based PFC topology is the most popular. It is a good solution for complying with regulations. Here the output dc voltage is greater than its input dc voltage. Also, the input current is continuous and it generates the lowest level of conducted noise and the best input current waveform. The boost regulator input current must be programmed to be proportional to the input voltage waveform for power factor correction. Feedback is necessary to control the input current. It contains at least two semiconductor switches and at least one energy-storage element. All boost PFC circuit uses a controller IC for the purpose of ease of design, reduced circuit complexity, and to control the cost. Below figure shows a Boost Type PFC Circuit;

Active PFC Circuit
Boost based PFC Circuit
Let’s discuss its working in detail; When the MOSFET (S) is closed, the inductor (L) output is connected to ground and the voltage (Vi) is placed across it. The inductor current increases at a rate equal to Vi/L. When the switch is opened, however, the voltage across the inductor changes and is equal to VL-Vin. Current that was flowing in the inductor decays at a rate equal to (VL-Vi)/L.
The boost converter has the filter inductor on the input side, which provides a smooth continuous input current waveform. The continuous input current is much easier to filter, which is a major advantage of this design because any additional filtering needed on the converter input will increase the cost and reduces the power factor due to capacitive loading of the line.
Disadvantage to this technique is that the output voltage is always greater than the peak input voltage. 

The boost regulator input current must be forced or programmed to be proportional to the input voltage waveform for power factor correction. Feedback is necessary to control the input current and can be done by below mentioned methods;

Peak current mode control: This has a low gain, wide bandwidth current loop which generally makes it unsuitable for a high performance power factor corrector since there is a significant error between the programming signal and the current. This will produce distortion and a poor power factor.

Average current mode control: It is based on a simple concept. An amplifier is used in the feedback loop around the boost power stage so that input current tracks the programming signal with very little error. This is the advantage of average current mode control and it is what makes active power factor correction possible. Average current mode control is relatively easy to implement.


Buck

It is a converter in which the output voltage is less than the input voltage. It is like a voltage step-down converter. It is also a current step-up converter. In this technique energy is stored in the inductor. Below figure shows a Buck Type PFC Circuit;

Active PFC Circuit
Buck based PFC Circuit
Let’s discuss its working in detail; When the MOSFET is in the ON state the current flows to the load and energy is stored in both the inductor (L) and the capacitor (C) and no current flows through the diode as it is reverse-biased. When the MOSFET is in the OFF state, the energy stored in (L) is released reverse back into the circuit and the current flows via the load and diode. At some point when the load voltage begins to fall, the charge stored in C becomes the main source of current until the switch is ON again.

Advantage with Buck type PFC is that the inductor limits the rate of change of load current. But, the input current is discontinuous and a smoothing input filter is required. Buck converter provides one polarity of output voltage and unidirectional output current. It has high efficiency, more than 90%. Also as it generates the low voltage at output, we can apply this lower input voltage to the DC-DC output stage where we can use and achieve lower voltage rated semiconductors, optimized loss and size of isolation transformer and better performance.

Advantage of the Active PFC is its extensive suppression of line current harmonics. Also, it is able to operate in Universal Input range. Disadvantage is Complex and costly circuitry.

Refer below figure for Active PFC waveforms where AC input current is almost perfectly in-phase with AC input voltage.

Active PFC
Current and Voltage in Active PFC

Conclusion

Now you know that types of Power Factor Correction includes; Passive PFC and Active PFC. Both techniques are frequently used for power factor correction. According to application we can select which method is best for our application and the requirement. Cost increases in Active PFC techniques as it requires controlling IC, MOSFET and other electronic components but result in more accurate Power Factor Correction. On the other hand Passive PFC technique is cost effective solution as it requires low cost components like Capacitor or Inductor but resultant Power Factor Correction is much far than unity.

Monday, September 24, 2018

Automotive Electronics


Specially with the use of power electronics there are many Automotive Electronics applications. Some of the general and common applications are listed below;

Alternator

Best use of power electronics based automotive electronics is for automotive power generation i.e. use of Alternator. Alternators are used in automotive application to charge the battery and to power the electrical system when automotive engine is running. Let’s discuss in detail how alternator works in application related to automotive electronics;

The main modules of alternators are stator, rotor, diode and a voltage regulator

The rotor inside the alternator rotates fast, when the alternator belt or V-belt rotates the pulley on the alternator. The rotor is mainly a group of magnets that rotates with high speed, inside a stator which is a wounded form of copper wires.

This high speed rotation of rotor along with stator copper wires generates electricity by the principle of electromagnetism. This generated electricity lead through the copper wires to the diode by the means of wire harness. The diode changes the electricity from AC to DC current and used for charging of car battery.

Last module is voltage regulator which will off the flow of power to the battery if the voltage rises above a certain or pre-defined level, this will helps the battery from getting overcharged and heated. When the car battery is drained, current is allowed to flow again into it from the alternator and this process repeats again and again.

Figure 1 at below shows Alternator in detail.

automotive electronics
Alternator in detail

DC to DC Converters

Another best use of power electronics based automotive electronics is for power conversion in automobiles i.e. use of DC to DC Converters.

A DC - DC converter is a circuit which converts direct current (DC) from one voltage level to another voltage level.

DC-DC converter are used in automobiles for converting the high voltage of the battery pack used for the power train to a lower voltage for charging the secondary battery and operating secondary electric applications in electric vehicles.

A DC-DC converter uses switching method to change DC voltage level to another voltage level. The converted voltage may be high or low. This converted voltage is stored temporarily in transformer or capacitor for a short duration so that it is easily and continuously available for the external load at a different voltage. Apart from switching conversion method linear power conversion method are also used but it have disadvantage of power loss. Switching conversion provides high efficiency as fast switching components like MOSFET’s are used in the converters.

DC to DC converter topologies can be made bidirectional and able to flow power in both direction by changing all diodes with individually controlled active power rectification. A bidirectional converter is useful in automotive applications which need regenerative braking of vehicles, where power is supplied to the wheels while driving, but in return supplied by the wheels when braking.

Figure 2 at below shows a DC-DC Linear Regulated Power Supply.

automotive electronics
DC-DC Linear Regulated Power Supply

DC to AC Inverter

One more best use of power electronics based automotive electronics is for power inverters in automobiles i.e. use of DC to AC Inverters.

AC motors are used because of their efficiency for converting electrical energy into mechanical energy. Thus, for proper working of AC motors, DC to AC inverter is required which converts DC power stored in batteries to the AC power.

According to the quality of output inverters can be classified as;

     Square Wave Inverter
     Modified Sine Wave Inverter
     Pure Sine Wave Inverter

For the application H-bridge type of inverter can be used, the IGBT act as a switch (when a signal is applied to the gate, they turn on and then turn off when the signal is removed).

By closing first side upper and second side lower IGBT a positive DC supply is applied to the load. By closing second side upper and first side lower IGBT gives negative DC supply across the load. Switching control circuits are used to produce the required gate signals to produce the required PWM waveform.

Figure 3 at below shows a 12V DC to 240V AC Automotive Inverter.

automotive electronics
12V DC to 240V AC Automotive Inverter

Conclusion

Definitely there are many applications of power electronics based automotive electronics and here we discussed some of the common applications. Power electronics creates its special place in automobiles by providing the advantages of high efficiency, fast operation, high reliability, easiness, cost effectiveness, safety and by ease of placement.

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Author & Editor

Hi, welcome to my blog, “Power Electronics Talks”.

I am an Electronics Engineer and passionate about Power Electronics and latest Technology. By profession I am design and application engineer and play with circuits.

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