Power factor correction in off-line power supply - Power Circuit - Circuit Diagram

Off-line switching power supplies typically use a rectifier bridge and an input filter capacitor to sink energy from the input, and a large capacitor charges near the peak of the AC input to power the unregulated BUS that powers the inverter. The capacity of the capacitor must be large enough to provide energy to the subsequent supply when the line voltage is lower than the BUS voltage during the second half of the rectification period. Unfortunately, the input filter capacitor will cause the input current waveform to be not sinusoidal, but a very narrow peak current waveform with an input power of only 0.5~0.65. Serious distortion causes grid pollution. The line current rms value can be up to twice the same sinusoidal current rms value. The 120V, 15A line does not even provide 1Kwde input power without causing circuit breaker action. While high power factor correction provides almost twice the power and low losses, high power factor correctors are a requirement in many areas.

The high PFC described in this paper is placed between the input rectification and the BUS capacitor. The operating frequency is much larger than the line voltage. The corrector absorbs the sinusoidal half-wave input current. The phase is the same as the line voltage. The current is controlled by comparing the BUS DC voltage with the reference voltage. .

The result is:

1. Improve the power factor to 0.95~0.99.

2. Less harmonics (< 3% if needed).

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

4. Strictly control the BUS capacitor to make the voltage fluctuation range small, allowing the inverter to be low cost and efficient design.

5. Reduce the filter capacitor and reduce the cost.

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

Basic operating principle:

This paper assumes that the PFC operating frequency is fs=100khz and the grid frequency is 60hz. The corrector absorbs the current that varies proportionally with the sinusoidal half-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 just a purely resistive load. A correction circuit with this function is called a "resistor competitor."

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

Power conversion: Although the calibrator inputs a sine wave with a current waveform, its output current ichg is a function of the square of the sine. Each operating parameter can be obtained by considering the input/output power of the corrector instead of the input/output voltage. Assuming high input power factor correction, the frequency is much greater than the power frequency, and the energy stored and consumed on the corrector is negligible (the energy stored by the inductor is usually greater than the energy delivered during each switching cycle, but at each power frequency half Can be ignored during the cycle). Therefore the input is equal to the output power.

BOOST circuit:

For the most commonly used HPFC circuits, the output must always be greater than the input transient value. The input current does not need to be turned off. Due to the small presence of the inductor, the line pollution and EMI are reduced. In addition, the SPIKE of the line is absorbed by the inductor, which increases the system reliability.

In continuous current mode, the input inductor allows the current control mode to be well applied to control the input current sinusoid (current control actual city control inductor current)

The position of the crystal makes it easy to drive because the S and E poles refer to the common end of the control circuit and capacitor. The maximum voltage of the crystal is the capacitor voltage.

The biggest drawback is that it cannot be limited because it has no series switch between input and output. It is not possible to control overload and start overcurrent, only protection is provided by the subsequent inverter section.

Also, when the input voltage is higher than the output voltage, it does not work, which occurs every time the power supply device is turned on and the line voltage is sufficiently long. The soft start has no effect because the BOOST circuit does not run in this case. The crystal is always off, but the input current will rise and its peak value will be greater than several times 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 input voltage, slope compensation is difficult to control. This problem can be avoided by reducing the current inner loop bandwidth. The average inductor current is directly controlled rather than intercepting the peak current. Because the switching frequency is much larger than the grid frequency, there is a lot of room to control the bandwidth of the current loop.

The discontinuous inductor current mode cannot be used in HPFC circuits because the inductor current drops very narrow at the peak input voltage and therefore the ripple current is small. But at the HPFC's peak input voltage, the line current is also at its peak. With 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 the HPFC. When the input current is a sine half wave, it stops working when its changed voltage value is less than the BUS voltage. Nonetheless, the BUCK topology is very useful when doing current limiting (the bus has a switch) that can be used as a complement to BOOST.