UC3842: start-up circuit design

kyung wook kwon

Prodigy 10 points

Part Number: UC3842

Tool/software:

when designing a flyback converter using the UC3842 IC with an input range of 200V to 1000V and a 24V output

would it be appropriate to use a 280kohm start-up resistor?

according to the datasheet, the maximum start-up current is 1 mA.

at the low end of the input voltage 200V, the current through the resistor seems to be close to this maximum value.

is it ok?

2 days ago

0 Hong Huang 1 day ago

TI__Guru 73455 points

Hi,

At 200Vin, I = (200V -16V)/280k = 0.657mA, so not able to start when Ivcc close to 1mA.

If to make all devices ok to start, the startup resistor = (200V - 17V)/1mA = 183k-ohm should be used.

0 kyung wook kwon 1 day ago in reply to Hong Huang

Prodigy 10 points

Hi, thanks for your answer.

I have one more question.

If I put a 1000V step input, then the current through the start-up resistor will go over 1mA.

In that case, can it damage the IC?

0 Hong Huang 14 hours ago in reply to kyung wook kwon

TI__Guru 73455 points

Hi,

The current will be about 5mA at 1000V if using 183k ohm startup resistor. The IC needs current about 11mA after start into work. So no problem to use 183k in terms of current into the IC.

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UC3842: start-up circuit design