Initial current surge

Asked Jan 16 '24 at 21:14

Active Jan 16 '24 at 21:14

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I designed a PCB that takes an input of 110Vdc and steps it down to 5Vdc for a microcontroller and a couple of ICs. At the input, I have a reverse protection diode, an inrush limiter circuit, and a filter:

Vin is connected to the DC-DC buck circuit to generate 5V for on-board ICs.

In normal operation, it draws about 60mA. But I noticed when I connected the PCB to the power supply, initially it drew a lot of current, and the current was even negative for about 125us:

I have a few questions and would very appreciate if someone could help me with them:

How can the current be negative?
How can I limit the magnitude of the current spikes at start-up?

asked Jan 16 '24 at 21:14

HV16

Can you repeat the measurement with a ferrite bead in series with Q1 gate pin? – Tim Williams Jan 16 '24 at 21:22
4

Your CLC filter is a very nice LC resonator. – Jonathan S. Jan 16 '24 at 21:23
Also, what is connected to Vin if anything, during the test? – Tim Williams Jan 16 '24 at 21:23
1

D1 should be rated 200V at least. – Kuba hasn't forgotten Monica Jan 16 '24 at 21:48
Where and how are you measuring the current? – vir Jan 16 '24 at 21:50
how are you generating the 110 Vdc? – jsotola Jan 16 '24 at 23:09
@TimWilliams everything is on the PCB and I don't think I have the room to add the ferrite. Vin is connected to the buck converter. – HV16 Jan 17 '24 at 17:44
@JonathanS. I swapped out the L with a 10R and had similar result – HV16 Jan 17 '24 at 17:45
@Kubahasn'tforgottenMonica yes I missed that. I have replaced the part with the proper ratings. – HV16 Jan 17 '24 at 17:46
@vir and jsotola, I have a DC power supply to power up the PCB. I use a clamp meter on the negative lead to measure the current. – HV16 Jan 17 '24 at 17:48
Please show the buck converter, at least the components on the input side; and the DC supply: at least the output circuit, or model number if a commercial unit. Regarding room, your transistor is DPAK type, the lead of which can be lifted at least, or the trace can be cut and a bead added. As you haven't added layout pictures, I will assume such steps can be taken. – Tim Williams Jan 17 '24 at 17:54

Initial current surge

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