| Electronics > Beginners |
| Adjusting an LC filter while circuit is running: bad idea ? |
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| ratatax:
Just wanted to share with you my magic smoke experience. I have a dc/dc converter i designed around a switching IC, it has input capacitors but despite that it creates a lot of noise on the input side, going to other circuits. So why not add an inductor in series before the capacitors, to make an LC filter and hope reducing the noise. I cut the VCC trace, put some solder pads on it, powered the circuit and tried different inductors while the circuit was running. Sometimes when you directly touch component's legs to pcb pads, connection is intermittent. I think that's what happened here, I heared a quick "buzz" sound then the switching IC literally exploded. I think it was a 33uH inductor and 47uF capacitor. I guess what happened here is that an inductor, when quickly switched on/off (by my unreliable contact) can create peak voltages much higher than the nominal current, and that's what killed the IC. Am i right ? Now I'm bothered by what would happen if the user plugs the power to my device, and put in only half-way creating a bad contact. Will the same thing happen ? I'm not sure, since it can only switch on/off before the inductor, not after, but i'd like your analysis on that |
| T3sl4co1l:
Probably inrush surge. What capacitors are around said inductor (input and output sides)? Tim |
| ratatax:
Sorry for the poor-looking schematic. Input caps are 4x22uF MLCC in parallel (two directly connected, two with long zigzag PCB traces in series to increase ESR (about +0.5ohm) and damp the input spike created when plugging the power supply) Output cap is just a 1uF MLCC |
| T3sl4co1l:
Yeh what's before the inductor? What regulator BTW? Tim |
| ratatax:
Before the inductor there is a DC wall adapter providing 12V The regulator is an AN_SY8205 : https://datasheet.lcsc.com/szlcsc/Silergy-Corp-SY8205FCC_C111875.pdf It can tolerate about +30V at its input and has configurable output voltage (R30 / R59 divider on the schematic). Could that means the inductor created a spike higher than 30V, or negative voltage perhaps ? |
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