Practical Tips | Finally understand the real role of the diode on the boost PFC inductor
Latest update time：2021-08-30 21:05
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In order to improve the power factor of the power grid and reduce interference, most power supplies for flat-panel TVs use active PFC circuits. Although the specific forms of the circuits are numerous and different, and the working modes are also different (CCM current continuous type, DCM discontinuous type, BCM critical type), the basic structure is similar, and all use the BOOST boost topology.
As shown in the figure below, this is a typical boost switching power supply. The basic idea is to separate the rectifier circuit and the large filter capacitor, and by controlling the conduction of the PFC switch tube, the input current can track the change of the input voltage, obtain the ideal power factor, reduce electromagnetic interference EMI and stabilize the working voltage of the switch tube in the switching power supply.
The figure below is a widely used boost switching power supply topology, which I believe is familiar to everyone.
In this circuit, the PFC inductor L stores energy when the MOS switch tube Q is turned on. When the switch tube is turned off, a positive voltage on the right and a negative voltage on the left are induced on the inductor L. The energy stored when it is turned on is charged to the large filter capacitor through the boost diode D1, and the energy is output.
A diode D2 is connected in parallel to the Boost boost PFC inductor L.
There are some different opinions among power engineers about the role of this diode, excerpted as follows:
Statement 1:
Reducing the impact of surge voltage on the capacitor limits the huge self-inductance potential generated by the PFC inductor L due to the surge current
at the moment of power on, thus causing circuit failure. Every time the power switch is turned on, the voltage added to the inductor can be any instantaneous value of the AC sine wave. If the moment the power switch is turned on is near the maximum peak of the sine wave, then a sudden change in voltage is applied to the inductor, which will cause a huge self-inductance potential to be generated on the inductor L. This potential is more than twice the applied voltage, and a large current is formed to charge the capacitor behind it, which may cause the fuse of the input circuit to blow, or even cause the filter capacitor and the chopper switch tube Q to break down.
After setting the protection diode D2, at the moment of power on, D2 is turned on and charges C, which greatly reduces the current flowing through the PFC inductor L, and the self-inductance potential generated is also much smaller, which may cause much less damage to the filter capacitor and the switch tube and the fuse to blow.
Statement 2:
Reduce the impact of surge voltage on the boost diode. The diode shunts part of the current in the PFC inductor and boost diode branches, thereby protecting the boost diode.
Misunderstanding analysis
The above viewpoints all mention the protective function of the diode D2, and they all make sense, but some of the above explanations are questionable.
As we all know,
the large energy storage filter capacitor C and the PFC inductor L behind the PFC circuit are connected in series. Since the current on the inductor L cannot change suddenly, the PFC inductor itself limits the surge current of the large filter capacitor C. The "capacitor charging when a large self-inductance potential is generated on the inductor L1 at the moment the power switch is turned on" mentioned in the first point of view will not occur, because the direction of the self-inductance potential is also positive on the left and negative on the right. This point of view is puzzling.
After the parallel protection shunt diode D2 is connected, the impact on the filter capacitor will be greater instead of reduced due to the lack of the limiting effect of the inductor.
Practice has also proved that after removing the diode D2, the surge impact on the capacitor C is reduced.
The second point of view, the protection of the boost tube D1, is reasonable, 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 the ordinary rectifier diode used by D1 has a strong ability to withstand surge current. For example, 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 PFC inductor L connected in series, the author believes that the main function of the protection diode D2 is not only to protect the boost tube D1.
Some information also states that the parallel connection of the diode D2 is to reduce the surge voltage during the startup process. This general statement is correct, but I think that the protection diode D2 reduces the surge impact on the PFC inductor and the boost diode on the surface, but in fact it has another important function:
protecting 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 passing through the PFC inductor is relatively large. It is possible that it is at the maximum value of the sine wave at the moment the power switch is turned on. In the process of charging the capacitor, the PFC inductor L may be magnetically saturated. 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.
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, start the PFC circuit;
another relatively simple method is to connect a bypass diode in parallel to the PFC coil and the boost diode, and provide another branch for the charging of the large capacitor at the moment of startup to prevent the large current from flowing through the PFC coil and causing saturation, and avoid the switch tube from overcurrent when the PFC circuit works, protecting the switch tube. At the same time, the protection diode D2 also shunts the current on the boost diode D1 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 when starting up, and make the PFC circuit work normally as soon as possible.
In summary, the role of diode D2 in the above circuit is to provide a charging path for the capacitor at the moment of startup or when the load is short-circuited or the PFC output voltage is lower than the input voltage in abnormal conditions, to prevent the danger of magnetic saturation of the PFC inductor to the PFCMOS tube, and at the same time to reduce the burden of the PFC inductor and the boost diode, playing a protective role.
The role of this diode can still be said to be to reduce the impact of surge voltage, but it is mainly to reduce the threat of surge voltage to the switch tube, and it also has a shunt protection effect on the boost diode, rather than protecting the filter capacitor.
After the power is turned on and working normally, since the right side of D2 is B+PFC output voltage, the voltage is higher than the left side, and D2 is in reverse bias cutoff state, which has no effect on the operation of the circuit. D2 can choose a common large current rectifier diode that can withstand large surge current.
In some power supplies, the capacitance after the PFC is not large, and some do not have a protection diode D2 connected. However, if a large-capacity filter capacitor is used after the PFC, this diode cannot be reduced, which is of great significance to the safety of the circuit.
Source: Internet compilation. If copyright is involved, please contact us to delete.
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