My customer is using UC2861. Please see the schematic as below. It is used to convert DC440V input to DC24V output.
1. X1-1 and X1-4 is the main power input which is 440V rated voltage. This 440V input also powers X7-12 and X7-2 through a diode D24. X7-12 and X7-2 is the power supply for driver circuit.
2. UE2 is the transformer to do isolated driver. 1-4 is the primary side. 7-9 and 2-3 are the secondary side.
3. UE1 is the main power transformer to convert DC440V input to DC24V output. 4-6 and 3-1 are primary side. 12-10 is the secondary side.
During lab test, to make sure the system is safe, we only powered X7-12 and X7-2 with a 440VDC input. And we didn't power X1-1 and X1-4. In this way, only the driver circuit UC2861 is powered on and the main power circuit is not powered on. So we can test UC2861 performance without really driving the power circuit.
When we did the overvoltage test, we increase the voltage between X7-12 and X7-2 to 650V from 440V. In theory, UC2861 will output several pulses and then shut down the pulse output. And our test waveform is as below:
CH1=UC2861's Aout pin. CH2=UC2861's Bout pin. As you could see, Aout and Bout output 3 pulses respectively and then stops. But the first pulse between Aout and Bout has a very short dead time which is not safe to drive the MOSFET. But later the dead time becomes to normal for the second pulses and third pulses.
When we zoon in the first pulses as below picture, we can find the dead time is only 50ns which is too short.
So could you please help advise why the 1st pulse has so short dead time in the over voltage test condition? How can we adjust the circuit to make it normal? Thanks.