Analysis and Error Analysis of Diodes on Boost PFC Inductors

The figure below is a widely used step-up switching power supply topology, I believe you are no stranger. In this circuit, the PFC inductor L stores energy when the MOS switch Q is turned on. When the switch is turned off, the right L-negative voltage is induced on the inductor L, and the energy stored during the turn-on is passed through the boost diode D1. Large filter capacitors charge and output energy. A diode D2 is connected to the Boost boost PFC inductor L.

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. Although the specific forms of the circuits are different, the working modes are different (CCM current continuous type, DCM discontinuous type). , BCM critical type), but the basic structure is similar, using BOOST boost topology. As shown in the figure below, this is a typical step-up 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 transistor, the input current can track the change of the input voltage. The power factor reduces electromagnetic interference EMI and stabilizes the operating voltage of the switching transistor in the switching power supply.

There are different opinions

There are some different opinions about the role of this diode in the power supply engineer. The excerpts are as follows:

One statement:

Reducing the surge voltage's impact on the capacitor limits the PFC inductor L to a large self-inductance potential due to the inrush current at the instant of startup, causing circuit failure. Each time the power switch is turned on, it can be applied to the inductor at any instantaneous value of the AC sine wave. If the power switch is turned on at the peak of the maximum value of the sine wave, then a change is added to the inductor. The voltage will cause a large self-inductance potential on the inductor L. The potential is more than twice the applied voltage, and a larger current is formed to charge the latter capacitor, which causes the fuse of the input circuit to be blown. Causes the filter capacitor and the chopper switch Q to breakdown. After setting the protection diode D2, at the moment of turning on the power, D2 is turned on and charges C, so that the current flowing through the PFC inductor L is greatly reduced, and the generated self-inductance potential is also much smaller. For the filter capacitor and the switch tube The hazard and the fuse blown may be much smaller.

Statement 2:

Reducing the surge voltage's impact on the boost diode This diode shunts a portion of the PFC inductor and the boost diode branch current, thus protecting the boost diode.

Misunderstanding

The above points all mention the protection of the diode D2, there are certain reasons, but some of the above explanations are debatable.

As we all know: the large energy storage filter capacitor C and 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 has a limiting effect on the surge current of the large filter capacitor C, and does not appear. The point of view is "the case where the capacitor is charged when a large self-inductance potential is generated on the inductor L1 at the moment when the power switch is turned on," because the direction of the self-inductance potential is also left-right and right-right, which is puzzling. After parallel protection of the shunt diode D2, this route has no limiting effect on the inductor, and the impact on the filter capacitor will be larger and will not decrease. Practice has also shown that after diode D2 is removed, the surge impact on capacitor C decreases. Viewpoint 2 protects the booster tube D1, there is a certain reason, because D1 is a fast recovery diode, the ability to withstand surge current is weak, reducing the reverse recovery current and increasing the surge voltage carrying capacity are mutually restrained, and D1 The common rectifier diode used has a strong ability to withstand surge current. For example, the rated current of the 1N5407 is 3A, and the inrush current can reach 200A. However, since the boost diode D1 has a current limiting effect of the PFC inductor L connected in series, the author believes that the most important function of the protection diode D2 is not only to protect the booster tube D1. Some data also indicate that the parallel diode D2 is to reduce the surge voltage during the boot process. This overall statement is correct, but I think that the surface of the protection diode D2 is reduced by the surge impact on the PFC inductor and the boost diode, but actually It has an important role: to protect the PFC switch tube.

At the moment of power-on, the voltage of the filter capacitor has not been established. Because the capacitor is charged, the current through the PFC inductor is relatively large. It is possible that the maximum value of the sine wave is at the moment when the power switch is turned on, and the capacitor is charged. During the process, the PFC inductor L may be magnetically saturated. If the PFC circuit is working at this time, it is troublesome, and the current flowing through the PFC switch tube will be lost and the switch tube will be burned out. In order to prevent the occurrence of tragedy, one method is to control the working timing of the PFC circuit, that is, when the charging of the large capacitor is completed, the PFC circuit is restarted; another 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 instant of starting, preventing large current from flowing through the PFC coil and causing saturation, avoiding the overcurrent of the switching tube caused by the PFC circuit working, protecting the switching tube, and the protection diode D2 The current on the boost diode D1 is shunted to protect the boost diode. In addition, the addition of D2 speeds up the charging process of the large capacitor, and the voltage on it is established in time, which can also make the voltage feedback loop of the PFC circuit work in time, reduce the conduction time of the PFC switch tube at startup, and make the PFC circuit as normal as possible. jobs.

In summary, the function of the diode D2 in the above circuit is to provide a charging path for the capacitor under the abnormal condition of the power-on instant or the load short-circuit and the PFC output voltage is lower than the input voltage, thereby preventing the danger of the PFC inductor magnetic saturation on the PFCMOS tube. It also reduces the burden on the PFC inductor and boost diode and protects it. The role of the diode can still be said to reduce the impact of the surge voltage, but mainly to reduce the threat of surge voltage on the switch tube, the shunt diode also has shunt protection, instead of protecting the filter capacitor. After the normal operation of the boot, since the right side of D2 is the B+PFC output voltage, the voltage is higher than the left side, and D2 is reverse biased off, which has no effect on the operation of the circuit. D2 can use ordinary large current that can withstand large inrush current. rectifier diode.

In some power supplies, the capacity of the capacitor behind the PFC is not large, and some do not have access to the protection diode D2. However, if a large-capacity filter capacitor is used behind the PFC, the diode cannot be reduced, which is of great significance to the safety of the circuit. .