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| Design of SMPS - Not sure what these spikes are being caused by. |
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| ocset:
your mosfet you say is switching unevenly. You can often correct this by judicious adjustment of the current sense filter and the fet gate series resistor. But there is no gate series resistor in your schem. I think you need one. Sometimes of course, uneven fet switching can be caused by insufficient slope compensation in current mode designs. Sometimes it can be caused by insufiicient delta current in the current signal, ie you have used too big inductance and the on state current ramp is so shallow that it easily hits the sense threshold through noise instead of in the normal way. Also, here is my pcb layout doc attached Oh by the way, a lot of the spikes i saw are just "pickup"...induced into the scope probes impedance by all the noise that an smps chucks out. There is a test to see if it is pickup....connect your scope ground clip to the probe tip and see if you see the spikes still, with the probe tip touching whichever node on the circuit. |
| Zog:
Ok.. thanks for clarifying. Rather than spending lots of money on more parts I am going to try centre taping my existing torroid and arranging it into the circuit as is without any other changes and see what sort of results I get. You mentioned an inverter transformer. But have never said which one .. did you ? |
| T3sl4co1l:
Like the one you found before, https://www.digikey.com.au/products/en/transformers/switching-converter-smps-transformers/168?k=&pkeyword=&pv1393=21&FV=1f140000%2Cffe000a8%2C15c0001b%2C15c0002d%2C15c0004b%2C15d00006&quantity=0&ColumnSort=0&page=1&stock=1&nstock=1&pageSize=500 :) Tim |
| Zog:
Gotcha, Thanks. It's of significant orders of magnitude larger in L... 1.2mH @ 65kHz .... Would that make much of a difference generally speaking ? The original design runs the minimum recommended by TI, 50kHz. Oh.. by the way ... what does SRF or LPD mean ? not Self Resonant Frequency surely ? --- Quote ---*Glances back at the OP*, oh yeah, a couple amps will be needed, won't it. Well, most of these are families, should be able to find something bigger SRF or LPD or whatever. Or that inverter transformer is looking quite good, despite its size and cost. :) --- End quote --- |
| T3sl4co1l:
--- Quote from: Zog on May 20, 2018, 12:30:33 am ---Gotcha, Thanks. It's of significant orders of magnitude larger in L... 1.2mH @ 65kHz .... Would that make much of a difference generally speaking ? The original design runs the minimum recommended by TI, 50kHz. --- End quote --- I said run it backwards. If it's 10:1, then the "secondary" is 1000 times less or 12uH. Right in the ballpark. ;) --- Quote ---Oh.. by the way ... what does SRF or LPD mean ? not Self Resonant Frequency surely ? --- End quote --- Self resonant frequency, yes, but I was referring to the part families. Tim |
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