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Digital control paves way for improved PFC design

Posted: 16 Jul 2014     Print Version  Bookmark and Share

Keywords:Digital control  power factor correction  THD  total harmonic injection  PFC 

The EMI filter X-Cap leads to the AC input current leading AC voltage (figure 4). Although the PFC current-control loop forces the inductor current IL to follow VAC, the reactive current of X-Cap IC leads VAC by 90 degrees. This results in the total current IAC leads VAC, which results in a poor power factor (PF). This situation gets worse at light-load and high-line. It will be very difficult for this PF to meet a rigorous specification.

The X-Cap reactive current can be compensated actively. Since the total capacitance of the X-Cap is known, the reactive current can be calculated, which then can be subtracted from the total input current IAC. The result is the ideal PFC inductor current. If we use this ideal inductor current as the current loop reference, then the X-Cap reactive current can be compensated and a high PF can be achieved.

Figure 4: X-Cap reactive current causes AC current-leading AC voltage.

This X-Cap reactive current compensation method was tested on a 360W single phase PFC. Table 1 shows the test results at VAC of 230V and 50Hz.

Table 1: Test results with the X-Cap compensation.

With this X-Cap compensation method, the PF is increased from 0.912 to 0.949, and the THD is also reduced from 9.79% to 9.39%.

A novel burst mode: AC cycle skipping
PFC efficiency gets lower and lower at light load, this is because switching loss, driving loss, and reverse recovery loss of semiconductor components becomes dominant at light load. Meanwhile, PFC may enter from continuous conduction mode (CCM) to discontinuous conduction mode (DCM). It causes the converter dynamics to change abruptly, and the current loop bandwidth reduces significantly. The small current feedback signal also makes control very difficult. As a result, the THD is getting worse.

A special burst-mode is developed once the PFC load is reduced to less than a pre-defined threshold. In this mode, depending on the load, one or more AC cycles are skipped by PFC. In other words, PFC turns off for one or more AC cycles, and turns back on for the next AC cycle. The turn on/turn off instant is at the AC zero-crossing, such that the whole AC cycle is skipped. Moreover, since the PFC turns on/turns off at the moment current equals zero, both stress and EMI noise are low.

The number of AC cycles to be skipped is reverse proportional to the load. The less the load, the more AC cycles will be skipped. A look up table can be generated between load and number of cycles to be skipped, such that AC cycles can be skipped as many as it can. Meanwhile, the output voltage ripple is maintained within a specified range.

Once PFC turns off, all the switching loss, driving loss, and reverse recovery loss are reduced to zero. The power loss is just the PFC stand-by power. Since the current is zero, THD is zero. When PFC turns on, since it needs to compensate the turn off period, it delivers a large amount of power which is more than average. This essentially operates PFC either at heavy load, or completely turns off. Since efficiency and THD are much better at heavy versus light load, light load efficiency is increased and THD is reduced.

Test results are shown in figures 5, 6 and 7.

Figure 5: AC cycle skipped. Channel 1 is AC voltage, channel 4 is AC current. Two AC cycles are skipped at this load.

Figure 6: Efficiency comparison with/without AC cycle skipping.

Figure 7: THD comparison with/without AC cycle skipping.

A low-cost input power and RMS current measurement solution
The real-time energy consumption measurement, including input real power and input RMS current measurement for off-line power supplies, nowadays is becoming ever more important. These measurements could be used to adjust power delivery and optimise energy usage. Traditionally input power and current are measured by a dedicated power metering chip and extra sensing circuits. While the power metering chip proved to be sufficient, it adds extra cost and design effort. Since the digital controller used in a digital PFC already has analogue-to-digital converters (ADCs) and a fast CPU, it could be used to do input power and current measurements as well.

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