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Friday, December 7, 2018

Power Factor Correction (PFC) – Biasing Circuitry of L6562


We know very well what is, "Power Factor Correction" and why it is needed.
More details on "Power Factor and Power Factor Correction", you can find on my previous blog and link for the same is;
https://www.powerelectronicstalks.com/2018/09/what-is-power-factor-correction.html

More details on "Power Factor Correction (PFC) - Critical Conduction Mode Boost Converter Calculations using L6562", you can find on my previous blog and link for the same is;
https://www.powerelectronicstalks.com/2018/11/power-factor-correction-critical-conduction-mode-boost-converter-calculations-using-L6562.html

Below picture is of assembled boost circuit in a 120W power converter.

A boost circuit incorporated in a 120W power converter
Assembled boost circuit in a 120W power converter
While designing the Critical Conduction Mode boost converter we have to also focus on biasing circuitry i.e. the components which will control the working of L6562 we can say it in another words as;

Biasing Circuitry of L6562

Figure 1 shows the internal block diagram of L6562.

Internal schematic of L6562
Internal block diagram of L6562
Let’s discuss it more in detail pin by pin.


·        Pin 1 (INV): Internal schematic of L6562 shows that this pin is connected both to the inverting input of the error amplifier and to the DIS circuit block. Externally we have to connect a resistive divider between the boost regulated output voltage and this pin. The internal reference on the non-inverting input of the E/A is 2.5 V (typ), while the DIS intervention threshold is 27 µA (typ). RoutH and RoutL are then selected as follows:


RoutH = ΔVovp / 27µA = 55V / 27µA = 2.03MΩ = 2MΩ


RoutH / RoutL = Vout / 2.5V    ·    – 1 = 400V / 2.5V   ·    – 1   = 159


RoutL = RoutH / 159 = 2MΩ / 159 = 12.6kΩ

To get RoutL 12.6kΩ place 15kΩ parallel to 82kΩ.
For RoutH a resistor with a voltage rating >400 V is needed otherwise more resistors in series have to be used.

This pin can also be used as an ON/OFF control input if tied to GND by an open collector or open drain.

·        Pin 2 (COMP): This pin is the output of the error amplifier that is fed to one of the two inputs of the multiplier. Place a feedback compensation network in between this pin and INV (pin 1) having a narrow bandwidth in order to avoid the output voltage ripple (100 Hz) that would bring high distortion of the input current waveform.
We can find the capacitance value by setting the bandwidth (BW) from 20 to 30 Hz so, a capacitor can provide a low-frequency pole as well as a high DC gain.
Below equation can be use for calculating the value of single capacitor.
Ccompensation = 1 / [2π · (RoutH // RoutL) · BW]

A CRC network providing 2 poles and a zero is more suitable for constant power loads like a downstream converter.


                       The transfer functions of compensation networks are shown in Figure 2 and Figure 3;

Resistive load and Constant power load
Transfer functions of compensation networks

In our design we used combination of two capacitors and one resistor network.


CcompP = 150nF


CcompS = 2.2µF


RcompS = 22kΩ


·        Pin 3 (MULT): Internal schematic of L6562 shows that this pin is also a multiplier input. It is connected both to the output of the error amplifier and to the inverting input of PWM comparator. In application it is connected through a resistive divider, to the rectified mains to get a sinusoidal voltage reference.

The multiplier can be described by the relationship:
VCS = k · (VCOMP – 2.5V) · VMULT
Where,
VCS = It is the multiplier output. It is the reference for the current sense.
k = 0.38 (typ) is the multiplier gain.
VCOMP = It is the voltage available on Pin 2; i.e. Output of error amplifier.
VMULT = It is the voltage on pin 3.

The linear operation of the multiplier is guaranteed within the range 0 to 3 V of VMULT and the range 0 to 1.16 V (typ) of Vcs.

The procedure to properly set the operating point of the multiplier is;

First, the maximum peak value for VMULT, VMULTmax is selected. This value, which occurs at maximum mains voltage, should be 3V or nearly so in wide-range mains and less in case of single mains. The sense resistor selected is Rs = 0.55 and it is described in the detail about pin 4 of this section. The maximum peak value, occurring at maximum mains voltage is:
VMULTmax = [(ILpk · RS) / 1.1] · [VACmax / VACmin]
                     = [(1.812 · 0.55) / 1.1] · [265V/ 85V]
                     = 2.82V
Where, 1.1 V/V is the multiplier maximum slope.

The maximum required divider ratio is calculated as;
kp = (VMULTmax) / (2 · VACmax)
     = 2.82 / (1.414 · 265V)
     = 0.00752
     = 7.52 X 10¯³

Suppose a 200 μA current flowing into the multiplier divider, the lower resistor value can be calculated as;
RmultH = [(1-kp)/kp]RmultL
            = [(1 - 7.52 X 10¯³) / 7.52 X 10¯³] · 15kΩ
            = 1.97MΩ

In this application example RmultH = 2 M and RmultL = 15 k have been selected. Please note that for RmultH a resistor with a suitable voltage rating (>400 V) is needed, or more resistors in series must be used.

·        Pin 4 (CS): It is the inverting input of the current sense comparator. Instantaneous inductor current is sensed by L6562 by this pin, which is converted to a proportional voltage by an external sense resistor (Rs). As this signal crosses the threshold set by the multiplier output, the PWM latch is reset and the power MOSFET is turned off. The MOSFET stays in OFF-state until the PWM latch is reset by the ZCD signal. The pin is equipped with 200 ns leading-edge blanking to improve noise immunity.
                For 50W PFC the sense resistor value (Rs) can be calculated as follows;
RS < (VCSmin / ILpk)
RS < (1.0V / 1.812A) = 0.55
Where;
ILpk = Inductor’s maximum peak current. It is already calculated for 50W PFC solution. Please refer “Critical Conduction Mode Boost Converter Calculations using L6562”.
VCSmin = 1.0 V is the minimum voltage allowed on the L6562 current sense. It is given in the datasheet.

As the internal current sense clamping sets the maximum current that can flow in the inductor, the maximum peak of the inductor current is calculated considering the maximum voltage Vcsmax allowed on the L6562.

ILpkx = Vcsmax / RS = 1.16V / 0.55 = 2.10A
Where;
Vcsmax = 1.16V. It is given in the datasheet.
The calculated ILpkx is the limit at which the boost inductor saturates and it is used for calculating the inductor number of turns and air gap length.

The power dissipated in RS is given by;
PS = RS · (ISWRMS)² = 0.55 · (0.638A)² = 0.223W

As per the result two parallel resistors of 1.1with 0.25 W of power rating have been selected.

·        Pin 5 (ZCD): It is the input of the zero current detector circuit. In transition mode PFC, the ZCD pin is connected, through a limiting resistor, to the auxiliary winding of the boost inductor. The ZCD circuit is negative-going edge triggered. When the voltage on the pin falls below 0.7V, it sets the PWM latch and the MOSFET is turned on. Prior to falling below 0.7V, because of MOSFET’s turnoff the voltage on pin 5 must experience a positive-going edge exceeding 1.4V.

The maximum main-to-auxiliary winding turn ratio is given by;
nmax = nprimary / nauxiliary
           = [Vout – (2 · VACmax)] / 1.4V · 1.15
           = [400 – (2 · 265V)] / 1.4V · 1.15
           = 15.7
If the winding is also used for supplying the IC, the above criterion may not be well-suited with the Vcc voltage range; we have to design a self supply network.

The minimum value of the limiting resistor can be found considering the maximum voltage across the auxiliary winding with a selected turn ratio = 10 and assuming 0.8 mA current through the pin.

R1 = [(Vout/naux) – VZCDH] / 0.8mA
      = [(400V/10) – 5.7V] / 0.8mA  =  42.9k
R2 = [(2 · VACmax/naux) – VZCDL] / 0.8mA
      = [(2 · 265V/10) – 0V] / 0.8mA = 46.8k

VZCDH = 5.7 V and VZCDL = 0 V are the upper and lower ZCD clamp voltages of the L6562.
               Considering the higher value between the two calculated, RZCD = 47 k has been selected as the limiting resistor.

·        Pin 6 (GND): This pin acts as the current return both for the signal internal circuitry and for the gate drive current. When laying out the printed circuit board, these two paths should run separately.

·        Pin 7 (GD): It is the output of the L6562. The pin is able to drive an external MOSFET with 600 mA source and 800 mA sink capability. The high-level voltage of this pin is clamped at about 12 V to avoid excessive gate voltages in case the pin is supplied with a high Vcc. An internal pull-down circuit holds the pin low to avoid undesired switch-on of the external MOSFET because of some leakage current when the supply of the L6562 is below the UVLO threshold.

·        Pin 8 (VCC): At this pin supply is applied to make run L6562. This pin is externally connected to the startup circuit and to the self-supply circuit. To start the L6562, the voltage must exceed the startup threshold (typically 12.5 V). High value startup resistors (in the hundreds kΩ), should be use for reducing power consumption and optimizes system efficiency at low load. If the Vcc voltage exceeds 25V, an internal clamping circuitry, is activated in order to clamp the voltage.

Below figure shows location of biasing components in PFC Boost circuit using L6562.

Biasing components in PFC Boost circuit
Location of biasing components in PFC Boost circuit using L6562
Below pictures are of a boost circuit assembly on general board and it’s measured output voltage on Agilent DSO-X 2024A.

Assembled L6562A based boost circuit
L6562A based boost circuit assembled on general board for testing

Output of the Boost Circuit using L6562
Output of the Boost Circuit using L6562 measured on Agilent DSO-X 2024A

Reference : Referred STMicroelectronics "Solution for designing a transition mode PFC preregulator with the L6562A". Link is embed in title.

Friday, November 30, 2018

Hysteresis loop or B-H curve and Hysteresis loss


What is Hysteresis loop or B-H curve?

Hysteresis loop gives information about the magnetic properties of a material. By studying hysteresis loop all the magnetic properties related information of a material can be easily traced out.

In another word’s we can define Hysteresis loop as, “When a ferromagnetic material is magnetized in a one direction, it will not come back to zero magnetization when the applied magnetizing field is taken out. It must be driven back to zero by a magnetizing field in the opposite or reverse direction. If an alternating magnetic field is applied to the material, its magnetization will trace out a loop (in the form of curve graph) called a hysteresis loop.

The absence of re-traceability of the magnetization curve (H) is the property called as hysteresis and it is associated with the presence of magnetic domains in the material.

A hysteresis loop shows the relationship between the induced magnetic flux density (B) and the magnetizing force (H). This is the reason it is also called as the B-H curve. Below figure shows an example of hysteresis loop;

Hysteresis loop or B-H curve
Hysteresis loop or B-H curve
Below points explains the Hysteresis loop or B-H curve;

  • The loop is produced by measuring the magnetic flux (B) of a ferromagnetic material when the applied magnetizing force is changed (H).
  • A ferromagnetic material which has been never before magnetized or demagnetized ferromagnetic material will trail the dashed line (see the figure) as magnetizing force (H) is increased.
  • The dashed line shows that, the larger the quantity of current applied (H+), the stronger the magnetic field in the component (B+).
  • At "a" point nearly all of the magnetic domains are aligned and an extra increase in the magnetizing force will generate very little increase in magnetic flux.
  • The magnetic saturation point has been reached for the material.
  • When magnetizing force (H) is decreased to zero, the curve will move or change from "a" point to "b" point.
  • At this point, we can notice that some magnetic flux leftovers in the material even though the magnetizing force (H) is zero. This is called as the point of retentivity on the graph and shows the remanence or level of remaining magnetism in the material. Some of the magnetic domains stay aligned, but some magnetic domains lose their alignment.
  • With the application of magnetizing force in reverse direction, the curve moves to "c" point, where the flux has been decreased to zero. This point is called as coercivity point on the curve or loop. The reversed magnetizing force has reversed plenty of the domains, so that the remaining flux within the material is zero.
  • To remove the residual magnetism from the material a force has to be apply, this required force is called as the coercive force or coercivity of the material.
  • In the negative direction when magnetizing force is increased, the material will become again magnetically saturated or material under goes in saturation, but in the opposite or reverse direction i.e. towards "d" point.
  • Decreasing magnetizing force (H) to zero brings the curve to "e" point. The available level of remaining magnetism is equal to that achieved in the other direction.
  • Increasing magnetizing force (H) back in the positive direction will return or bring back the magnetic flux (B) to zero.
  • We can notice that, the curve did not return back to the beginning or origin of the graph because some magnetizing force is needed to remove the remaining or residual magnetism.
  • Now the curve in the graph will take a diverse or different path from “f” point back to the saturation point, here at this point it with complete the loop.
Below image shows B-H curve measurement on Oscilloscope;
B-H curve
B-H curve measurement on Oscilloscope

Advantages of Hysteresis loop or B-H curve

The outcome of magnetic hysteresis loop shows;

  • The magnetisation process of a ferromagnetic core.
  • The part of the curve the ferromagnetic core is magnetised decides flux density because this depends on the circuits previous history which gives the core a form of “memory”.
  • Ferromagnetic materials have memory because they stay magnetised after the external magnetic field has been taken out.
  • Relays, solenoids and transformers can be easily magnetised and demagnetised because, they are made up of Soft ferromagnetic materials such as silicon steel or iron, which have very narrow magnetic hysteresis loops resulting in very small amounts of residual magnetism.
  • Residual magnetism can be overcome by a coercive force; energy which is in use is dissipated as heat in the magnetic material. This heat is known as hysteresis loss, the material’s value of coercive force decides the amount of loss.
  • A very small coercive force can be made that have a very narrow hysteresis loop by adding additive’s to the iron metal such as silicon. Magnetisation and demagnetisation of soft magnetic materials with narrow hysteresis loops are easy.
B-H curve for Soft and Hard Material
Hysteresis loop for Soft and Hard Material

Applications of Hysteresis

There are varieties of applications of the hysteresis in ferromagnets. Many of the applications make use of their capability to hold a memory; like magnetic tape, computer hard disks and debit cards - credit cards. In these applications, hard magnets which have high coercivity like chromium and iron are required so the memory is not easily removed. Let understand it in detail;

Because of presence of magnetic domains in the material the magnetization curve is not re-traceable (which is termed as hysteresis). After re-orientation of magnetic domains, it will take some magnetizing field or energy to turn them back again. This characteristic of ferromagnetic materials is useful as a magnetic "memory". Some configurations of ferromagnetic materials will maintain a forced magnetization forever and are useful as "permanent magnets". The magnetic memory features of chromium and iron oxides are useful in audio tape recording and also for the magnetic storage of data on computer hard disks.

Soft magnets which have low coercivity for example iron oxide is used for the ferrite cores in electromagnets. The low coercivity decreases that energy loss related with hysteresis. This low energy loss at the time of hysteresis loop is the main reason of using soft iron for electric motors and transformer cores.

Hysteresis loss

As current flows in the forward and reverse directions the magnetization and demagnetization of the core happens which result in Hysteresis loss.

When we apply external magnetizing force to a material and as we increase the magnetizing force (current), the magnetic flux also increases, but when the magnetizing force (current) is decreased, the magnetic flux decreases gradually and not at the same rate. So, when the magnetizing force touches zero, the flux density didn’t come to zero and still has a positive value. Now the magnetizing force must be applied in the negative direction so the flux density reaches zero.

The link between the magnetizing force (H) and the magnetic flux density (B) is shown on a hysteresis loop or curve. The energy required for completing a full cycle of magnetizing and de-magnetizing is shown by the area of the hysteresis loop. Also this area of the loop characterizes the energy lost during this magnetisation process.

Below is the equation for hysteresis loss;
Where;

Pb = hysteresis loss (W)

η = Steinmetz hysteresis coefficient, depending on material (J/m³)

Bmax = maximum flux density (Wb/m²)

n = Steinmetz exponent, ranges from 1.5 to 2.5, depending on material

f = frequency of magnetic reversals per second (Hz)

V = volume of magnetic material (m³)

The hysteresis loss results in wasted energy which is proportional to the area of the magnetic hysteresis loop.

In AC transformers, hysteresis loss is always a problem where the current is continually changing the flow of direction and by this the magnetic poles continually flows in reverse direction and causes loss in the core.

Conclusion

Hysteresis loop provides information about the magnetic properties of a material. It is important that the B-H hysteresis loop is as small as possible so loss will be less because shape of B-H curve decides the loss. Bigger the area then more is the loss and vice-versa. The shape of hysteresis loop depends upon the nature of the material used i.e. iron or steel.

More details on Ferrite you can find in my previous blog;

Friday, November 23, 2018

Ferrite, Types of Ferrites and Ferrite Formula


Ferrite

Ferrite is usually ceramic, standardized material and it has ferrimagnetic properties. We can say in other words that, Ferrite is a ceramic material made by reacting or relating metal oxides into a magnetic material. Ferrites are insulators and having ferrimagnetic property i.e. they can be easily magnetized.

More details on Ferrite you can find in my previous blog;


Ferrite Formula

The ferrite formula is commonly stated as MeFe2O4.
Where “Me” signifies a divalent metal ion. e.g. Fe²⁺, Ni²⁺, Mn²⁺, Mg²⁺, Zn²⁺, Cu²⁺, Co²⁺, etc.


Types of Ferrites

On the basis of their resistance to being demagnetized, Ferrites can be divided into two types of ferrites; Soft and Hard Ferrites.

     1) Hard Ferrite Core

Hard Ferrite Cores have high coercivity i.e. it can withstand an external applied magnetic field without becoming demagnetized. The coercivity for ferromagnetic material is the strength of the externally applied magnetic field needed to decrease the magnetization of that material to zero, once the magnetization of that material has been bring to saturation.
Hard Ferrite Applications: Hard Ferrite Cores are used to make permanent magnets which can be used in speakers and motors.

     2) Soft Ferrite Core

Soft Ferrite Cores have low coercivity i.e. it can’t withstand an external applied magnetic field and easily demagnetized. They simply change their magnetization, and they act as conductors of magnetic fields.

Soft Ferrite Applications: Soft Ferrite Cores are used to make ferrite cores for high frequency applications like Inductors and Transformers.

Soft Ferrite Materials are characterised by Spinel or cubic crystal structure. Generally at high frequencies, soft ferrites have many advantages as compare to conventional metallic type materials. Different core geometries optimized for specific applications can be achieved by shaping ferrites.


Ferrites
Inductors and Transformers using Soft Ferrite Cores
Advantages of soft ferrites are listed below;

· Resistivity is high.
· Wide range of operating frequencies can be supported.
· With high permeability low loss can be achieved.
· Different types of material according to applications can be selected.
· Flexibility in the choice of core shapes.
· Low cost can be achieved.

Disadvantages of soft ferrites are listed below;

· Saturation flux density is low.
· Thermal conductivity is poor.
· Tensile strength is low.
· Fragile material.

There are two types of soft ferrite materials; Nickel-Zinc (NiZn) and Manganese-Zinc (MnZn).


Ferrites
Ferrite Powder
Let’s explain the difference between them;


Manganese-Zinc Ferrites

Ferrite Formula for Manganese-Zinc is  MnaZn(1-a)Fe2O4


Ferrites
MnZn Ferrite Cores
· This is most common type of soft ferrite and is used in many applications as compare to nickel-zinc ferrites.
· In the MnZn type a big range of materials is possible. The selection of material is mostly a purpose of the application that needs to be achieved.
· Mostly the application gives the required material characteristics, which helps to define the chemical composition and properties of the ferrite material.
· For frequencies less than 2 MHz Manganese-zinc ferrite is mainly used.
· The MnZn ferrites are categorized by the permeabilities (µ) > 1000 and saturation flux density (Bsat) to 5300 Gauss.
· Power, shielding and linear inductive components are some of the usual applications of MnZn Ferrites.
· The MnZn ferrite materials are usually used up to frequencies < 10 MHz.
· The MnZn ferrite materials can be used for pulse applications into the nanosecond range.
· MnZn materials are usually supports temperature range more than 200 °C.
· These cores are easily available in both regular and customized geometry shape i.e. client suggested or application specific.
· Manganese-zinc cores (MnZn) are inductive up to frequencies of 20MHz to 30MHz.
· At higher frequencies i.e. more than 80MHz the core material has more losses and is no longer operative.
· MnZn materials have a high permeability as compare to NiZn ferrites which have a low permeability.
· In the applications where the operating frequency is less than 5 MHz, Manganese-zinc ferrites are used.
· The common mode inductors are the exclusion, where the impedance of MnZn material makes it the finest choice up to frequency 70 MHz.

Nickel-Zinc Ferrites

Ferrite Formula for Nickel-Zinc is  NiaZn(1-a)Fe2O4

Ferrites
NiZn Ferrite Cores
· The NiZn based soft ferrites is considered by its high material resistivity. It’s very high as compare to MnZn ferrites.
· High resistivity NiZn ferrite makes it operational from 2 MHz to several hundred megahertz.
· An enormous number of nickel-zinc materials have been researched and developed to support such an extensive frequency range and different types of applications.
· The NiZn ferrite core material is considered by permeability’s (µ) up to 2500.
· Usual applications contains;
RF transformers (includes both: inductively coupled and transmission line broadband to support frequencies more than 1GHz).
Pulse power modules (here pulse shaping and fast switching up to < 10 nanoseconds is required).
For common inductive or interference suppression application in the range of MHz.
· These NiZn material cores are easily available in both regular and personalised geometry shape i.e. client suggested or application specific.
· Nickel-zinc material cores (NiZn) are inductive up to the frequency range of 60MHz. After which the core material act as a lossy for up to frequencies of 1GHz and above.
· Nickel-zinc ferrite cores have a higher resistivity as compare to Manganese-zinc ferrite cores and are used at frequencies from 2 MHz to more than hundred megahertz.
· NiZn ferrite cores are recommended for the application range from 70 MHz to several hundred GHz.

Basic Material Characteristics of MnZn and NiZn

Some of the basic material characteristics of both Manganese-zinc ferrites and Nickel-zinc ferrites are given below;

Material Characterisitics of Soft Ferrites
Sr. No.CharacteristicsMnZnNiZn
1Initial Permeability (µi)450 to 1500014 to 2000
2Saturation Flux Density (Bms)363mT to 530mT275mT to 470mT
3Curie Temperature (Tc)100°C < 300°C90°C < 300°C
4Resistivity (ρ)0.12Ωm to 10000Ωm>10⁶Ωm
5Density (d)4.70g/cm³ to 5.00g/cm³ 4.60g/cm³ to 5.10g/cm³ 


Conclusion

Both types of ferrites i.e. Soft Ferrite Cores and Hard Ferrite Cores have their own advantages and used according to the supporting applications. Nearly all applications include these ferrite cores. Hard ferrite cores which can be magnetized quite strongly are mostly used in motors. On the other hand Soft ferrite cores are found in nearly every electronics applications for the purpose of interference suppression, switching transformer for high frequency applications and radio frequency applications.

Author & Editor

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

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

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