Two breakdown phenomenon and protective measures of power transistor in switching power supply

Two about 1 breakdown and protection
Two reasons for the 1.1 breakdown
The two time breakdown is mainly due to the high local temperature caused by the device. The reason for the increase in temperature is that the thermal imbalance is caused by the forward bias, and the reverse bias is caused by the avalanche breakdown.
Because of the thermal resistance of the transistor in the pipe internal distribution is uneven, in some weak areas, the temperature rise will be higher than other parts, forming the so-called hot, local temperature caused by the local current increases, the current increases, so that the temperature rise, so the cycle until a critical temperature, resulting in the breakdown of the tube.
The two breakdown caused by avalanche breakdown is due to the occurrence of an avalanche breakdown, at some point due to the current density is too large, the change of the electric field distribution, resulting in negative resistance effect, so that the local temperature is too high a phenomenon.
1.2 measures to avoid the two breakdown
Opening and closing loss are important factors to affect the normal operation of switching devices. Especially in the dynamic process of the transistor is easy to produce two breakdown phenomenon, this phenomenon is directly related to the switch loss, so it is necessary to reduce the switching loss of the device is the proper use of the device. To reduce the loss can be achieved through two ways to achieve:
(1) to low collector emitter voltage to turn off transistor (Vce);
(2) in the process of the rising of the emitter voltage, the transistor must be reduced to minimize the emitter current. The introduction of the buffer circuit is one of the ways to achieve the above purpose.
1.3 buffer circuit in switching power supply
In the design of the switching power supply, the following buffer circuit can be used to ensure that the transistor runs in the safe zone (SOA).
1) the use of the off buffer circuit is a kind of energy dissipation and off buffer circuit. Although it has more energy consumption, but the circuit is simple.
Two) 2 commonly used energy dissipation type open circuit.
A. has a non saturation reactance of the open circuit (Figure 2): the inductance – diode network with the transistor collector in series, the formation of a buffer circuit. When the tube is open, the Ls control current of the inductor is DT di/. When the tube is turned off, the energy 1/ 2 (LsI2m), which is stored in the inductor Ls, is continuously flowing through the diode Ds, and the energy consumption is dissipated in the resistance of the Ds and the reactor.
B. has a saturable reactor (Figure 3): the purpose of the opening of the buffer circuit is in order to make the transistor in the open collector current, the collector voltage drops to 0, in order to make the opening loss minimum. Especially for inductive load effect is more significant. Design of saturable reactor should be done: collector voltage drops to zero, the buffer reactor in saturation state; before saturation, collector voltage down to zero before, the reactor is high resistivity, magnetization current flowing through a tube is very small so as to achieve the purpose to reduce switching loss.

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