Design of off-line switching power supply
Time:2022-07-11
Views:2009
The off-line switching power supply usually uses rectifier bridge and input filter capacitor to absorb energy from the input, and the large capacitor is charged near the AC input peak to provide energy to the unadjusted bus that provides energy for the inverter. The capacity of the capacitor must be large enough. When the line voltage is lower than the bus voltage in the second half of the rectification period, it only provides energy to the subsequent.
Unfortunately, the input filter capacitor will cause the input current waveform to be not sinusoidal, but a very narrow current waveform with high peak value, The input power is only 0.5 "0.65, and serious distortion leads to grid pollution. The effective value of line current can reach twice the same effective value of sinusoidal current. 120V, 15A lines can not even provide 1kwde input power without causing the action of circuit breaker. However, high power factor correction can provide almost twice its power, and the loss is very low. Therefore, high power factor correctors have become a demand in many fields.
The high PFC described in this paper is placed between the input rectifier and the bus capacitor. The working frequency is much greater than the line voltage frequency. The corrector absorbs the sinusoidal half wave input current, and 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 necessary).
3. It operates without interruption in the 90 "270V line voltage range.
4. The bus capacitor is strictly controlled so that its voltage fluctuation range is very small, allowing the 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 the reliability of capacitance.
Basic operation principle:
This paper assumes that the PFC working frequency is fs=100khz and the power grid frequency is 60Hz. The corrector absorbs the current that changes in proportion to the sinusoidal half wave voltage to obtain an input with a power factor close to 1. Therefore, the current and voltage at the input of the rectifier bridge are in phase. Of course, this is only a pure resistance load. The correction circuit with this function is called "resistance competitor".
Input current control multiplies the sinusoidal half wave representing the voltage waveform of the rectifier input line by the control voltage through the multiplier 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 power grid every half cycle. Verr represents the deviation between VDC and reference voltage, which is amplified and transformed into the output of error amplifier. When the VDC is low, the Verr becomes larger, and the input power is increased to make up for the loss of energy on the filter capacitor.
Power conversion: Although the input current waveform of the corrector is a sine wave, its output current ICHG is a function of the square of the sine. Various operating parameters can be obtained by considering the input / output power of the corrector rather than the input / output voltage. Assuming high input power factor correction, its frequency is much greater than the power frequency, and the energy stored and consumed on the corrector is ignored (the energy stored by the inductance is usually greater than the energy transmitted in each switching cycle, but it can be ignored in each power frequency half cycle). Therefore, the input and output power are equal.
Boost circuit:
For 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 turned off. Because the inductance is very small, the line pollution and EMI are reduced. In addition, the spike of the line is absorbed by the inductance, which increases the reliability of the system.
In the current continuous mode, the input inductance makes the current control mode well applied to control the sinusoidal input current (current control actually controls the inductance 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 the capacitor. The maximum voltage of 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. Overload and startup overcurrent cannot be controlled, and protection is only provided through the subsequent inverter.
In addition, when the input voltage is higher than the output voltage, it does not work. This happens every time the power supply equipment is turned on and the line voltage is disordered for a long enough time. Soft start has no effect, because the boost circuit does not operate in this case. The crystal has been turned off, but the input current will rise, and its peak value will be greater than several times the rated current value, resulting in inductance saturation, unless another current limiting circuit is added.
Slope compensation must be added to prevent the system from being unstable when D is greater than 0.5 (VIN ＜ vdc/2). Because the inductive current changes with the input voltage, it is difficult to control the slope compensation. This problem can be avoided by reducing the current inner loop bandwidth, so that the average value of the inductive current is directly controlled rather than intercepting the peak current. Because the switching frequency is much greater than the grid frequency, there is a lot of room to control the current loop bandwidth.
The discontinuous inductive current mode cannot be used in HPFC circuits because the inductive current drop is very narrow at the peak input voltage, so the ripple current is very small. However, at the peak of input voltage of HPFC, the line current is also at its peak. With low ripple of peak current, the inductive 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 sinusoidal half wave, it stops working when the changed voltage value is less than the bus voltage. Nevertheless, the buck topology is very useful in current limiting (the bus has a switch tube), which can be used as a supplement to boost.
Unfortunately, the input filter capacitor will cause the input current waveform to be not sinusoidal, but a very narrow current waveform with high peak value, The input power is only 0.5 "0.65, and serious distortion leads to grid pollution. The effective value of line current can reach twice the same effective value of sinusoidal current. 120V, 15A lines can not even provide 1kwde input power without causing the action of circuit breaker. However, high power factor correction can provide almost twice its power, and the loss is very low. Therefore, high power factor correctors have become a demand in many fields.
The high PFC described in this paper is placed between the input rectifier and the bus capacitor. The working frequency is much greater than the line voltage frequency. The corrector absorbs the sinusoidal half wave input current, and 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 necessary).
3. It operates without interruption in the 90 "270V line voltage range.
4. The bus capacitor is strictly controlled so that its voltage fluctuation range is very small, allowing the 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 the reliability of capacitance.
Basic operation principle:
This paper assumes that the PFC working frequency is fs=100khz and the power grid frequency is 60Hz. The corrector absorbs the current that changes in proportion to the sinusoidal half wave voltage to obtain an input with a power factor close to 1. Therefore, the current and voltage at the input of the rectifier bridge are in phase. Of course, this is only a pure resistance load. The correction circuit with this function is called "resistance competitor".
Input current control multiplies the sinusoidal half wave representing the voltage waveform of the rectifier input line by the control voltage through the multiplier 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 power grid every half cycle. Verr represents the deviation between VDC and reference voltage, which is amplified and transformed into the output of error amplifier. When the VDC is low, the Verr becomes larger, and the input power is increased to make up for the loss of energy on the filter capacitor.
Power conversion: Although the input current waveform of the corrector is a sine wave, its output current ICHG is a function of the square of the sine. Various operating parameters can be obtained by considering the input / output power of the corrector rather than the input / output voltage. Assuming high input power factor correction, its frequency is much greater than the power frequency, and the energy stored and consumed on the corrector is ignored (the energy stored by the inductance is usually greater than the energy transmitted in each switching cycle, but it can be ignored in each power frequency half cycle). Therefore, the input and output power are equal.
Boost circuit:
For 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 turned off. Because the inductance is very small, the line pollution and EMI are reduced. In addition, the spike of the line is absorbed by the inductance, which increases the reliability of the system.
In the current continuous mode, the input inductance makes the current control mode well applied to control the sinusoidal input current (current control actually controls the inductance 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 the capacitor. The maximum voltage of 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. Overload and startup overcurrent cannot be controlled, and protection is only provided through the subsequent inverter.
In addition, when the input voltage is higher than the output voltage, it does not work. This happens every time the power supply equipment is turned on and the line voltage is disordered for a long enough time. Soft start has no effect, because the boost circuit does not operate in this case. The crystal has been turned off, but the input current will rise, and its peak value will be greater than several times the rated current value, resulting in inductance saturation, unless another current limiting circuit is added.
Slope compensation must be added to prevent the system from being unstable when D is greater than 0.5 (VIN ＜ vdc/2). Because the inductive current changes with the input voltage, it is difficult to control the slope compensation. This problem can be avoided by reducing the current inner loop bandwidth, so that the average value of the inductive current is directly controlled rather than intercepting the peak current. Because the switching frequency is much greater than the grid frequency, there is a lot of room to control the current loop bandwidth.
The discontinuous inductive current mode cannot be used in HPFC circuits because the inductive current drop is very narrow at the peak input voltage, so the ripple current is very small. However, at the peak of input voltage of HPFC, the line current is also at its peak. With low ripple of peak current, the inductive 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 sinusoidal half wave, it stops working when the changed voltage value is less than the bus voltage. Nevertheless, the buck topology is very useful in current limiting (the bus has a switch tube), which can be used as a supplement to boost.
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