Application of Power Factor Correction in Offline Power Supply

Offline switching power supplies usually use a rectifier bridge and input filter capacitors to absorb energy from the input. The large capacitor is charged at a point close to the peak of the AC input to provide energy to the unregulated BUS that provides energy for the inverter. The capacitor must be large enough so that when the line voltage is lower than the BUS voltage in the second half of the rectification, it will only provide energy for the subsequent period. Unfortunately, the input filter capacitor will cause the input current waveform to no longer be a sine wave, but a very narrow current waveform with a high peak value. The input power is only 0.5~0.65, and severe distortion causes grid pollution. The line current RMS value can reach twice the RMS value of the same sinusoidal current. A 120V, 15A line cannot even provide 1Kwde input power without causing the circuit breaker to operate. High power factor correction can provide almost twice the power with very low losses, so in many fields, high power factor correctors have become a demand. The high PFC described in this article is placed between the input rectifier and the BUS capacitor. The operating frequency is much greater than the line voltage frequency. The corrector absorbs a half-wave sinusoidal input current with the same phase as the line voltage. The current is controlled by comparing the BUS DC voltage with the reference voltage.

turn out:

1. Improve the power factor to 0.95~0.99.

2. Fewer harmonics (< 3% if necessary).

3. Uninterrupted operation in the line voltage range of 90~270V

4. Strictly control the BUS capacitance to keep the voltage fluctuation range very small, allowing low-cost and efficient design of the inverter.

5. Reduce the filter capacitance and reduce the cost.

6. Reduce the effective value of charging current and improve capacitor reliability.

Basic operating principle:

This article assumes that the PFC operating frequency is fs="100khz" and the grid frequency is 60hz. The corrector absorbs a current that varies proportionally with the half-sine wave voltage to obtain an input with a power factor close to 1. Therefore, the current and voltage are in phase at the input of the rectifier bridge. Of course, this is only with a pure resistance load. The correction circuit with this function is called a "resistance competitor".

Input current control uses a multiplier to multiply the sinusoidal half-wave representing the rectified input line voltage waveform by the control voltage to obtain VERR. VERR must be constant in each half-wave, so VERR can be controlled to control the RMS input current to control the energy absorbed from the grid in each half cycle. VERR represents the deviation between VDC and the reference voltage, which is amplified and converted into the output of the error amplifier. When VDC is low, VERR becomes larger, increasing the input power to compensate for the energy loss on the filter capacitor.

Power conversion: Although the corrector input current waveform is a sine wave, its output current ichg is a function of the square of the sine wave. The various operating parameters can be obtained by considering the corrector input/output power rather than the input/output voltage. Assuming a high input power factor correction, its frequency is much greater than the power frequency, and the energy stored and consumed in the corrector is negligible (the energy stored in the inductor is usually greater than the energy it transfers in each switching cycle, but it can be ignored in each half cycle of the power frequency). Therefore, the input and output powers are equal.

BOOST CIRCUITRY:

The most commonly used HPFC circuit, the output must always be greater than the input transient value. The input current does not need to be shut down, because the inductor is very small, the line pollution and EMI are reduced, and the line spike is absorbed by the inductor, which increases the system reliability.

In the continuous current mode, the input inductor makes the current control mode well applied to control the input current sinusoidally (current control actually controls the inductor current)

The location of the crystal makes it easy to drive because the S and E poles are referenced to the common terminal of the control circuit and the capacitor. The maximum voltage across the crystal is the capacitor voltage.

Its biggest disadvantage is that it cannot limit current because there is no series switch between input and output. It cannot control overload and overcurrent at startup, and only provides protection through the subsequent inverter part.

Also, it does not work when the input voltage is higher than the output voltage, which happens every time the power supply is turned on and the line voltage is disturbed for a long enough time. Soft start has no effect because the BOOST circuit does not operate in this case. The crystal is always turned off, but the input current will rise, and its peak value will be several times greater than the rated current value, causing the inductor to saturate, unless a current limiting circuit is added.

Slope compensation must be added to prevent system instability when D is greater than 0.5 (VIN < VDC/2). Because the inductor current varies with the input voltage, slope compensation is difficult to control. This problem can be avoided by reducing the bandwidth of the current inner loop so that the average inductor current is directly controlled instead of the peak current. Because the switching frequency is much higher than the grid frequency, there is a lot of room to control the bandwidth of the current loop.

Discontinuous inductor current mode cannot be used in HPFC circuits because at peak input voltage the inductor current drop is very narrow, so the ripple current is small. But in HPFC at input voltage peak, the line current is also at its peak. To have high peak current and low ripple, the inductor current must be continuous.

BUCK Circuit

Since the BUCK circuit requires that the input is always greater than the output, it is not used in HPFC. When the input current is a half-sine wave, when the voltage value is less than the BUS voltage, it stops working. However, the BUCK topology is very useful for current limiting (the bus has a switch tube ), which can be used as a supplement to BOOST.