“In order to improve the power factor of the power grid and reduce interference, most power supplies of flat-Panel TVs use active PFC circuits. , BCM critical type), but the basic structure is similar, both use BOOST boost topology.As shown in the figure below, this is a typical boost switching power supply. The basic idea is to divide the rectifier circuit and the large filter capacitor. By controlling the conduction of the PFC on-off tube, the input current can track the change of the input voltage and obtain the ideal power factor, reduce electromagnetic interference EMI and stabilize switching power supply
In order to improve the power factor of the power grid and reduce interference, most power supplies of flat-panel TVs use active PFC circuits. , BCM critical type), but the basic structure is similar, both use BOOST boost topology. As shown in the figure below, this is a typical boost switching power supply. The basic idea is to divide the rectifier circuit and the large filter capacitor. By controlling the conduction of the PFC on-off tube, the input current can track the change of the input voltage and obtain the ideal High power factor, reduce electromagnetic interference EMI and stabilize the working voltage of the switching tube in the switching power supply.
The figure below is a widely used boost switching power supply topology, I believe everyone is familiar with it. In this circuit, the PFC Inductor L stores energy when the MOS switch Q is turned on. When the switch is turned off, the inductor L induces a right positive and left negative voltage, and the stored energy is passed through the boost diode D1 pair. The large filter capacitor charges and outputs energy. Boost boost PFC inductor L is connected in parallel with a diode D2.
Statement 1: Reduce the impact of the surge voltage on the capacitor At the moment of power-on, the PFC inductor L will be limited to generate a huge self-inductance potential due to the surge current, resulting in circuit failure. Each time the power switch is turned on, the inductance can be any instantaneous value of the AC sine wave. If the moment when the power switch is turned on is near the maximum peak point of the sine wave, then a sudden change is added to the inductance. The voltage will cause a great self-inductance potential on the inductance L, which is more than twice the applied voltage, and form a large current to charge the capacitor behind, which will cause the fuse of the input circuit to blow. Cause filter capacitor and chopper switch Q breakdown. After the protection diode D2 is set, at the moment of turning on the power supply, D2 is turned on and charged to C, so that the current flowing through the PFC inductor L is greatly reduced, and the self-inductance potential generated is much smaller, which is very important for the filter capacitor and switch tube. The hazard and blowing of the fuse may be much less.
Statement 2: Reduce the impact of surge voltage on the boost diode. The diode shunts a part of the current of the PFC inductor and the boost diode branch, so it can protect the boost diode.
The above viewpoints all refer to the protective function of the diode D2, and they all have certain truths, but some of the above explanations are debatable.
We all know that the large energy storage filter capacitor C and the PFC inductor L behind the PFC circuit are connected in series, because the current on the inductor L cannot be abruptly changed. The PFC inductor itself limits the inrush current of the large filter capacitor C, and there will be no charging of the capacitor when a large self-inductance potential is generated on the inductance L1 at the moment when the power switch is turned on. Because the direction of the self-inductive potential is also left positive and right negative, this view is puzzling.
After connecting the protection shunt diode D2 in parallel, the impact on the filter capacitor will be larger and will not be reduced because there is no inductance limit in this path. Practice has also proved that after removing the diode D2, the surge impact on the capacitor C is reduced instead. The second point of view is to protect the booster tube D1. There is a certain truth, because D1 is a fast recovery diode, and its ability to withstand surge current is weak. Reducing the reverse recovery current and improving the surge voltage carrying capacity are mutually restrained, and D1 The common rectifier diode used has a strong ability to withstand surge current, such as the rated current of 1N5407 is 3A, and the surge current can reach 200A. However, since the boost diode D1 has the current limiting effect of the series-connected PFC inductor L, the author believes that the main function of the protection diode D2 is not only to protect the boost tube D1. Some data also indicate that the parallel diode D2 is to reduce the surge voltage during the boot process. This general statement is correct, but I think the surface of the protection diode D2 reduces the surge impact on the PFC inductor and boost diode, but in fact it is still There is an important role: to protect the PFC switch tube.
At the moment of power-on, the voltage of the filter capacitor has not yet been established. Since the large capacitor needs to be charged, the current through the PFC inductor is relatively large. It may be at the maximum value of the sine wave at the moment when the power switch is turned on. During the process, the PFC inductor L may have magnetic saturation. If the PFC circuit works at this time, it will be troublesome. The current flowing through the PFC switch tube will lose its limit and burn out the switch tube. In order to prevent the tragedy from happening, one method is to control the working sequence of the PFC circuit, that is, when the charging of the large capacitor is completed, then start the PFC circuit; another relatively simple method is to connect the PFC coil and the boost diode in parallel. The last bypass diode provides another branch for the charging of the large capacitor at the moment of startup, preventing a large current from flowing through the PFC coil and causing saturation, avoiding the overcurrent of the switch tube caused by the PFC circuit working momentarily, and protecting the switch tube. At the same time, the protection diode D2 also The current on the boost diode D1 is shunted, protecting the boost diode. In addition, the addition of D2 accelerates the charging process of the large capacitor, and the voltage on it is established in time, which also enables the voltage feedback loop of the PFC circuit to work in time, reducing the on-time of the PFC switch when it is turned on, and making the PFC circuit normal as soon as possible. Work.
To sum up, the function of diode D2 in the above circuit is to provide a charging path for the capacitor at the moment of startup or short-circuit of the load, and the PFC output voltage is lower than the input voltage in abnormal conditions, so as to prevent the danger caused by the magnetic saturation of the PFC inductance to the PFCMOS tube. At the same time, it also reduces the burden of the PFC inductor and boost diode, and plays a protective role. The function of the diode can still be said to reduce the impact of the surge voltage, but it is mainly to reduce the threat of the surge voltage to the switch tube, and it also has a shunt protection function for the boost diode, rather than protecting the filter capacitor. After the normal operation of the power-on, since the right side of D2 is the output voltage of B+PFC, the voltage is higher than the left side, and D2 is in a reverse bias cut-off state, which has no effect on the work of the circuit. rectifier diode.
In some power supplies, the capacitance behind the PFC is not large, and some are not connected to the protection diode D2, but if a large-capacity filter capacitor is used behind the PFC, this diode cannot be reduced, which is of great significance to the safety of the circuit .