BJT collector-emitter breakdown voltage

Question

I'm developing a flyback converter with the FT838NB1 IC, and I got some doubts about the collector-emitter breakdown voltage of the integrated NPN switching transistor.

Absolute maximum ratings show that the collector to GND voltage can go to 700V:

From the internal schematic I understand that collector to GND value can be considered the collector-emitter breakdown voltage im my circuit, as the emmiter is connected to GND by a low-value resistor (and the current doesn't exceed 0.4 A).

All right so far, because the circuit is operating well, with full load and in universal input voltage range (85 V to 264 V).

But in the Electrical Characteristics section of the datasheet I found a minimum collector-emitter breakdown voltage of 400 V. So for me, reading this information, values larger than 400 V would cause trouble for the BJT.

But as you can see below, the ringing voltage on my circuit is reaching 513 V, and it's working well (I achieved this value with a good snubber; it was worse).

Can someone explain where my interpretation is wrong?

Answer 1

The 400V limit is with the base not connected to anything (Vceo - Voltage collector to emitter with base open).

The 700V rating is with the voltage rating between Collector and base with the emitter open. (Vcbo). This will be essentially the same with the emitter shorted to the base.

When a low impedance is driving the base (the driver circuit in the block diagram) it diverts any collector to base leakage at high voltages and avoids breakdown until a higher voltage is reached.

Although the 700V is the absolute maximum as noted by @Justme the datasheet has the same 700V in the electrical characteristics section implying that it is acceptable to operate at that value. Also, breakdown may be acceptable without damage to the component. This area f the datasheet is rather ambiguous.

Answer 2

You are considering the absolute maximum voltage as the breakdown voltage, but it's not. They are two completely separate things.

The breakdown voltage is defined so that the manufacturer guarantees that at least 400V or more needs to be applied to have 1mA current flowing, which is considered as the point of breakdown. It could be more, like 500, 600 or 700V before 1mA flows. It might be that current of 1mA is not reached even at 700V, the current could be less than 1mA. Or there might be 1mA breakdown at 690V and 2mA flowing at 700V.

The absolute maximum rating means that it is not rated to handle more than 700V, and beyond the 700V rating the chip can be permanently damaged immediately or will suffer long term degradation of electrical parameters and shorter lifetime.