Use power factor correction technology to minimize power consumption

As the use of consumer electronics and computers in homes and workplaces increases, power consumption costs are becoming more and more important. The need to reduce power consumption in user devices is driving greater energy efficiency in power supplies both inside and outside the devices.

For AC-DC power supplies ranging from hundreds of watts to kilowatts, the efficiency depends on power factor correction (PFC) and the efficiency of the subsequent DC-DC conversion. Although the trade-offs between cost and performance of DC-DC converters are well understood today, PFC technology has always lagged behind in terms of circuit and control technology. However, this situation has recently begun to change. This article will discuss some of the developments in this field of technology and how power supply design engineers can grasp various design perspectives and suggestions.

Losses in AC-DC Converters

The power losses in an AC-DC converter generally include:

Reverse recovery losses in the boost diode;

Input rectifier bridge losses;

Losses in EMI filters;

The loss of PFC power switch tube;

Inductor/choke losses.

Reverse recovery losses in boost diodes

PFC converters generally use two control techniques: continuous current mode (CCM) and boundary mode (BCM), the latter also known as modulation mode (TM) or critical mode (CRM). In CCM converters, the control IC uses a fixed frequency to adjust the duty cycle (PWM) to adjust the average current of the boost inductor. In BCM converters, the inductor current can return to zero before the switch is turned on, so it is a variable frequency control scheme.

When the MOSFET in the CCM converter is turned on, the boost rectifier diode will undergo a reverse recovery process (the process in which the reverse current in the diode disappears) because there is still inductor current flowing through the boost rectifier diode. This will cause power loss in the main MOSFET M1. In the BCM converter, the inductor current is basically zero when the MOSFET is turned on, which means that the soft switching function is realized. Therefore, the reverse recovery loss using the BCM control technology is minimized.

But the benefits of BCM are not without cost. The peak inductor current in BCM is twice as high as that in CCM; the higher peak inductor current causes larger conduction losses in both the MOSFET and the diode, and greater power losses in the inductor. Therefore, BCM mode converters are limited to applications with output power between 250W and 300W.

In addition, improvements in diode technology have improved the efficiency of PFC converters in CCM mode. Silicon carbide (SiC) rectifier diodes have significantly reduced reverse recovery effects, which helps solve the problem, but at a higher cost. Ultra-fast silicon diode products can also reduce reverse recovery losses, but at the expense of higher conduction losses.

Input rectifier bridge losses

The AC-DC converter has an input rectifier bridge composed of four slow recovery diodes. The power loss of these diodes is considerable. Therefore, there is a so-called "bridgeless PFC" technology, which replaces the lower two diodes of the rectifier bridge in Figure 1 with two controlled and driven MOSFETs as boost switches (note that the term "bridgeless" may be used incorrectly because the input rectifier diodes are still present). These bridge diodes act as boost diodes The AC-DC converter has an input rectifier bridge composed of four slow recovery diodes. The power loss of these diodes is considerable. Therefore, there is a so-called "bridgeless PFC" technology, which replaces the lower two diodes of the rectifier bridge in Figure 1 with two controlled and driven MOSFETs as boost switches (note that the term "bridgeless" may be used incorrectly because the input rectifier diodes are still present). These bridge diodes act as boost diodes, eliminating the boost diode component in traditional technology. In theory, this will improve efficiency because current only flows through two semiconductor devices at a time instead of three.

The problems faced by bridgeless PFC technology are current sensing, EMI and input voltage sensing. In addition, the active switching devices in the bridge rectifier must now be protected from transient changes in the input voltage. Moreover, since higher speed diodes must be used, inrush current protection is also a problem at higher powers. The latest PFC control technology, such as the FAN7528 using voltage mode control or controllers based on single-loop control technology, can at least avoid the problem of input voltage detection. Although conventional technology can be used to control the switches of the two bridges with a single drive signal from the control IC, new control technology is needed to achieve individual control of each power switch in order to achieve maximum power efficiency and low EMI.

Losses in EMI Filters

Reducing the size of the electromagnetic interference (EMI) filter also reduces the corresponding losses. PFC converters using multiple power stages are gaining acceptance in the industry due to the use of point-of-load processor power technology in DC-DC converters, namely the so-called "phase separation" or "interleaved channel" technology. Phase separation reduces the ripple current at the input, thereby reducing the size of the EMI filter. Phase separation also reduces the size of the entire boost inductor, and because the inductor is separated, it also helps improve heat dissipation.

PFC power switch loss

In order to reduce switching losses, it is necessary to consider the use of zero voltage switching (ZVS) or zero current switching (ZCS) technology. In BCM control (the technology used by Fairchild Semiconductor FAN7527B and FAN7528 controllers), the main MOSFET switch is turned on when the current is zero, reducing the conduction loss and thus reducing power consumption. This is a big advantage for low-power converters, but since the main loss comes from conduction loss when the power is higher, this advantage can only be reflected in applications below 300W.

Since the switching frequency of the PFC front end is relatively low, it is possible to use IGBTs (insulated gate bipolar transistors) to reduce conduction losses at high power. However, most applications still use MOSFETs due to their lower switching losses.

The main MOSFET switch can also be turned on with zero voltage. This requires adding some additional circuitry, including a small power MOSFET, rectifier, and inductor (Fairchild Semiconductor's FAN4822 uses these circuits). These components are equivalent to injecting a kind of "baby nutrition" into the switching circuit; through timing optimization and the use of resonance effects, the voltage across the main MOSFET switch is zero before turning on. Although this solution seems attractive, the circuit topology is very complicated.

Inductor/choke losses

The losses in the inductor can be reduced by minimizing the inductance, which is achieved by increasing the effective switching frequency, that is, using a control IC that can set the switching frequency externally. The trade-off of this approach is that the harmonic content increases and faster (and therefore more expensive) diodes may be required. Another consideration is the phase interleaving of the power stages; these power stages have the advantage of canceling ripple currents and can allow for higher peak currents. The higher allowed peak current means that smaller inductance is required, less copper is required, and therefore lower losses per choke coil.

Future development trends

The upcoming popular PFC technology is the boost follower PFC, which allows the output voltage to change with the input voltage. This technology boosts the voltage of the AC line to achieve the minimum voltage required by the subsequent DC-DC converter, thereby improving the overall efficiency of the PFC converter. However, this will lead to two cost-increasing factors: first, the design of the DC-DC converter is more complicated because it must work in a larger input voltage range (such as 200 to 400VDC); second, technologies with narrow input voltage ranges, such as the popular LLC resonant half-bridge, cannot be used.

Finally, for some new control techniques such as crossover and bridgeless PFC, the current lack of new viable analog control ICs means that digital control may be a desirable alternative. In fact, at least three digitally controlled AC-DC power supplies have been introduced to the market recently. Although the cost of many products seems prohibitive, it is still an exciting and worthwhile future development trend to watch, at least in the field of low-power applications.

Conclusion

The market for power supplies using active FPC technology is growing significantly faster than the general AC-DC market; the market demand for more efficient converters has increased. However, improving efficiency is not without cost, and a trade-off must be made between cost, number of components, reliability, and new technologies (see Table 1). Careful selection of components, combined with new control techniques and more optimized engineering methods, can significantly improve the efficiency of PFC converters.