Welcome back to the final part of our 3-part series on power supply design principles.  At Nuvation, we have been called upon to design power circuits and supplies to provide the sometimes large array of DC voltages required by a particular system.  Often times, our clients have a limited understanding of the complexity of these designs and as a result assume that the process is simple and consumes little time or talent.  In this series of articles I hope to provide a better understanding of the complexities of design and some of the thought processes involved in the design of power supplies. 

In part one, I discussed power supply design requirements, Power Factor, and an introduction to Power Factor Correction (PFC).  If you haven’t already read part one, please start with this link. In part two, I provided a better understanding of the complexities of PFC, specifically Passive PFC and an introduction to Active PFC.  To read part two, click here.

In this issue, I will complete my discussion on Active PFC, draw out a circuit using a boost converter and close with a list of pros and cons of active power factor correction.

Active PFC

For any design over 100W, the preferable type of PFC is Active Power Factor Correction (Active PFC) since it provides a lighter and more efficient power factor control.  Active PFC is comprised of a switching regulator operating at a high switching frequency, being able to generate a theoretical power factor of over 95%. Active Power Factor Correction automatically corrects for AC input voltage, and is capable of a wide range of input voltage. One disadvantage of Active PFC is the extra cost resulting from the additional complexity required in its implementation.

Figure 1; above depicts the basic elements of an active power factor correction circuit.  The control circuit measures both the input voltage (pin 2 on the controller) as well as the current (RS and pins 3 and 11 on the controller) and adjusts the switching time and duty cycle to present an in phase voltage and current load to the input.

The active PFC shown above is in the form of a boost regulator and as a result the voltage appearing across the load (R1) must be greater than the highest value of the peak voltage appearing at the input.  Normally, the DC voltage is set to 10 to 20V higher than the expected maximum peak input voltage.  Designing a universal input power supply (87-266Vrms at 47-63Hz) the DC output voltage from the PFC at the input to the DC-DC converter would be set at 386V to 396V.

Figure 2 shows the basic block diagram of a PFC boost regulator.  Unlike a standard power supply input there is no holdup capacitor directly across the bridge rectifier so that there is no large inrush current or transient currents as the input voltage rises above the voltage on the capacitor.  The PFC works by inducing a current in the inductor (L1, See Figure 1 above) and causing the current to track the input voltage.  The control circuit senses both the input voltage and the current flowing through the circuit.   By controlling the on time in the switch (Q1) that places L1 across the output of the rectifier, the current in the coil increases as the input voltage increases.   The switch is turned off periodically and the voltage at the drain end rises until the current in the inductor achieves the charge level.  Usually, this level is set to several volts higher than the bridge rectifier peak output voltage.  A boost regulator’s output voltage must be higher than the input voltage for the regulator to function correctly.

The DC output voltage of the boost regulator is also sensed and the charge discharge cycle on the inductor is adjusted to maintain a constant output voltage.  There is a requirement that the switching rate of the boost converter be much higher than the line frequency, typically these converters switch at rates of 20kHz to 100kHz.   The higher frequency allows for a small inductor to be used.  To compare, the inductor in the passive PFC described earlier needs to be in the range of 150mH – 300mH whereas the inductor required in the active PFC is in the order of 10µH - 30µH.  The difference is a full four orders of magnitude.  This allows the use of physically small and light low-loss parts.

Primary advantages of the Active PFC:

• Power factor ≥ 0.95
• Constant Intermediate voltage to drive the DC/DC converter, simplifies the requirements and the complexity of the DC/DC converter.
• Small, light inductive components.
• Wide range of input voltages, can work with 87Vrms – 266Vrms 47Hz-63Hz without switching
• Greater flexibility and control

The Primary disadvantages of the Active PFC are:

• Higher overall cost and complexity
• Requires better filtering to prevent high frequency hash from getting to the line
• Higher voltage components than would be required for a passive PFC

This concludes my three part series on power supply design requirements based on power factor requirements.  I discussed both passive and active PFC, distinguishing when to design with either method, and drew primary advantages and disadvantages for both.  For more information or to engage Nuvation on a Power Supply design or review, please email us at sales@nuvation.com.