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.

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