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Minimize The Cost Impact Of Your Power-Factor-Correction Circuit
Transition-mode PFC is the simpler and lower-cost approach, but if you need 300 W or more, consider continuous-current-mode PFC circuits.
Most applications that use power factor correction (PFC) are mandated by regulatory requirements like the IEC1000-3-2 harmonic reduction requirement in force since January 1, 2001. The regulation places certain limits on harmonic currents drawn by power supplies. This, in turn, requires an active PFC circuit. Some low-power, cost-sensitive applications still manage to use a passive PFC circuit. Comprised of only inductors and capacitors, a passive PFC circuit is simple and efficient for low-power applications. However, it requires a large bulk capacitor, can only be used in a narrow range input, can provide only a small hold-up time, and carries low-frequency ripple on the output. Hence, for applications over 200 W, active PFC is mandatory.
The easiest and most cost-effective way to implement an active PFC is via a boost circuit at the input end of the power supply. The two most popular types of boost topologies for implementing active PFC are transition mode (TM) and continuous-current mode (CCM) PFC.
TM and CCM refer to the way current flows in the boost inductor. In the CCM topology, the cycle-by-cycle inductor current doesn't fall to zero. The opposite of CCM mode is discontinuous-current mode. Here, cycle-by-cycle current falls to zero at the end of the cycle, and there's a dead time before the next cycle starts from a zero current. The TM topology is a discontinuous current-mode topology in which the next cycle starts just at the point where the current reaches zero. Thus, there's no dead time in each cycle.
TM- Vs. CCM-PFC TM-PFC is simpler and cheaper than CCM-PFC. It's widely used in lighting ballasts; adapters; and low-power, switching-mode power supplies (SMPS). This simpler control technique makes it possible to use low-cost, 8-pin PFC controllers, or an MCU where available. Control-loop implementation can take place by multiplying the instantaneous line voltage by the output of the error amplifier and generating a variable frequency-switching pattern. The other method is to set a fixed on time for the boost MOSFET. In both of these approaches, the zero current of the inductor is detected to start the next cycle. Hence, the peak inductor current is proportional to the instantaneous input line voltage, making the average input current similar to the input voltage for achieving a high power factor (Fig. 1a) .
CCM-PFC uses a fixed-frequency control. The sum of on and off times is constant. Relative to TM-PFC, inductor size is larger for a given output power-frequency combination. The slope of switching currents in CCM topology is smaller, and so is the peak inductor current, resulting in lower EMI and lower rms current through the boost inductor and MOSFET (Fig. 1b) . Lower rms current, in turn, reduces conduction losses. Hence, the CCM topology is the preferred option for higher-power applications (typically over 300 W).
The TM topology offers several advantages:
Because the control technique is simpler, TM controllers typically cost half the price of CCM topology controllers.
As the boost inductor needn't store energy at the end of each switching cycle, it can be smaller and cheaper.
Constant on-time implementation allows use of an MCU, saving the PFC controller cost in some applications.
Because the commutation of boost diode from on to off happens at zero current, slower diodes can be used, slashing cost.
Inner current feedback loop is much faster with feedback received every cycle by the inductor current's falling to zero.
But the CCM topology also has advantages:
Current slopes are much smaller compared to the TM topology for the same average input current, creating lower peak currents through the boost inductor. A lower slope and a lower peak current reduce the cost of EMI filtering circuits. Also, lower peak current means lower RMS current for a given input power and frequencies, resulting in lower conduction losses in the inductor and the boost MOSFET.
A constant switching frequency allows synchronization with downconverters. Also, for big enclosed systems like computers, the variable frequency of TM may not be acceptable.
Transition-Mode PFC Controllers The biggest variable that affects system cost in a TM-PFC implementation is the controller, even though it may cost less than the MOSFET or even the diode. Consider the features of some of the most-used controllers for TM-PFC.
Maximum Supply-Voltage Rating: While startup and nominal operating voltage for most controllers are the same, the maximum allowable VCC in some controllers is over 30 V. For a standard PFC application, this may not be beneficial. But it could be important for applications where PFC is implemented along with the dc-dc converter stage, such as in a single-stage converter. For example, in a constant-current-output power supply for a battery charger, output voltage may vary beyond the normal tolerance of 10 %, making the supply voltage on the IC change along with it. Hence, a larger VCC voltage range can be advantageous.
Error Amp: Some controllers contain transconductance amplifiers, as opposed to the more often seen simple voltage amplifiers. Transconductance amplifiers enable isolating the voltage feedback pin from the compensation pin, creating slightly more design flexibility. They also simplify using an optocoupler when necessary for feedback. On the other hand, they're more prone to pick up noise, particularly the one produced by the high di/dt of the drain current.
Overvoltage Protection (OVP)/Protection Against Feedback Loop Opening: Most TM-PFC controllers provide OVP. Some of them, however, have a differentiating element that protects against the feedback-loop opening. To implement this protection, the voltage feedback pin can't be the same as the OVP pin.
Startup And Quiescent Current: With many new standards requiring higher low-load efficiency, this parameter must be compared thoroughly.
Output Driver: Because the TM topology is intended for lower-power applications, a gate-drive current in the range of 0.5 to 1 A is usually available. The important thing to note, however, is switching off speed, which has a major impact on switching off losses. Fortunately, due to zero-current turn-on in a TM topology, switch-on losses don't depend on the driver.
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