Wierd spike during off time of mosfet - Page 1
Forum section: electronics / beginners

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: February 22, 2017, 07:33:25 am
content: I am driving 2 center tapped windings with 4 mosfets (IRF540) the CT is connected to 15V. and each mosfet is switched on sequentially like T1A, T2A, T1B, T2B. But I'm getting some wierd negative spikes during the mosfet off period.Currently I have attached the waveform of mosfet connected to T1A.

author: Ammar
author_rank: Regular Contributor
timestamp: February 22, 2017, 07:39:01 am
content: Do you have a schematic? How are you driving the high side MOSFETs?

author: capt bullshot
author_rank: Super Contributor
timestamp: February 22, 2017, 07:43:00 am
content: Just guessing because of lack of information:Looks like crosstalk from other mosfet stages switching at this moment

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: February 22, 2017, 07:43:21 am
content: There an no high side mosfets.. all 4 are low side and each driver one end of the CT transformer the CT is given to positive.

author: BrianHG
author_rank: Super Contributor
timestamp: February 22, 2017, 08:08:50 am
content: Is it possible for you to probe between the Gate and Source? Or if you must, use 2 probes, one on the Gate & other on source using the probe A-B function. I would like to know if the gate itself is being driven on with a bounce VS the bounce being just an artifact at the output.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: February 22, 2017, 02:23:00 pm
content: Quote from: BrianHG on February 22, 2017, 08:08:50 amAttached.The top is the gate, middle is A- B and bottom is the source...

author: BrianHG
author_rank: Super Contributor
timestamp: February 23, 2017, 12:32:15 am
content: Hmm, are you sure you setup your scope correctly?See my attachmentChecklist:Probes A and B are bot set to X10 to capture high frequency.Scope voltage per division for both channels A and B are set to the exact same.The A-B display also should have the same vertical resolution as well.Also, please zoom in the horizontal time base by 2x, I only want to see the first main switch pulse plus the 2-3 additional spikes during the off time.Double check your scope settings and sen me a new screenshot, with the vertical voltage resolution of all the channels set to the same voltage level / division & zoom in the horizontal time base.Your A-B display should loot exactly like channel A, same voltage, except, only the signal bounce spikes should alter their appearance. In fact, they might disappear completely. If you see only 1 spike after the switch on during the off period, then your mosfet is being turned on at that point and you need to clean up you gate drive. If there is no pulse on the A-B display, then, you main power supply rail is being over-loaded due to the alternate side mosfet switching on and you need to add low ESR decoupling capacitor to your power supply rail or increase the trace current capabilities/better rout you PCB power planes.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: February 23, 2017, 06:04:56 am
content: I'm pretty sure that I selected A-B but I will try to setup things again as you said and send a sceen-shot. BTW I've constructed this circuit on a breadboard, so would that be adding to the noisy gate drive ...

author: MrAl
author_rank: Super Contributor
timestamp: February 23, 2017, 06:08:43 am
content: Quote from: ZeroResistance on February 22, 2017, 07:33:25 amHi,There is some chance that it could be caused by oscillation due to inductance in the source circuit.The way to reduce that is of course to reduce lead length in the source circuit first, add a power supply capacitor closer to the mosfet if possible, and lastly if nothing else works add more series gate resistance but keep an eye on the rise and fall times.Also possible is an asymmetrical drive to turn on a little slower and turn off faster, or the other way around.This is only if the main problem is too much inductance in the source circuit.

author: capt bullshot
author_rank: Super Contributor
timestamp: February 23, 2017, 06:28:48 am
content: Quote from: ZeroResistance on February 23, 2017, 06:04:56 amIt ain't that easy to stay calm here ...- You don't give any idea of what you've build (no schematics, no picture, no functional description)- You show some waveforms that have an anomaly and ask what might be the reason for that anomalyUsually the reason for such kind of anomalies lies in the circuit and its construction. Many circuits that work perfect in simulation or in your mind just won't work in reality.The noisy gate drive surely is caused by breadboard construction and improper scope probing. Looking at the noise, there must be another switching path in your circuit that you didn't show us here. The shown spike is coincidental with the noisy waveform, so my guess still is crosstalk from the other switching path, propagated either by using a breadboard or magnetically coupled in some way. You didn't even tell us if T1A / T1B and T2A and T2B are driving one CT transformer (doesn't seem to make sense for me) core or maybe two independent ones with the same frequency but different duty cycle, or two windings on one core ...

author: BrianHG
author_rank: Super Contributor
timestamp: February 23, 2017, 07:41:22 am
content: Quote from: ZeroResistance on February 23, 2017, 06:04:56 amOhhh, I did not have this information before I made my requested tests. Yes, bread board switching transformers would reveal bouncing, false mosfet switching & a whole mess of issues due to the nature of a high power switching circuit. IRF540s are fairly strong...If I were to attempt such an experimental design, I would use a flat all copper clad board, all as 1 gigantic GND, with a thick copper conductor for the +15v, say 14 or 12 gauge mounted above the copper clad with multiple decoupling caps, high and low values along the surface before and after the transformer ct transformer. For the Gate driving switcher IC & control, I would mount it on wired sockets just right above the PCB, with once again decoupling caps & GND points soldered directly to the copper clad PCB.High current switching to CT transformers makes a lot of noise this would be the only hand-wired solution I would attempt with a degree of assurance that signal bounce due to GND and VCC can be contained, corrected for and give me close results to a real production PCB.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: February 23, 2017, 06:18:45 pm
content: Quote from: BrianHG on February 23, 2017, 07:41:22 amOK! so the cause of that dip during the off time could be bad layout techniques aka breadboarded circuit for high frequency switching right? Although capt bullshot says it could be crosstalk, I see that the negative going spike starting edge corresponds to the gate turnoff edge of one of the mosfets.I want to put up a circuit, but I only have a hand drawn one... I will try to put it up as soon as possible.

author: BrianHG
author_rank: Super Contributor
timestamp: February 23, 2017, 09:51:47 pm
content: With bread boards, all problems, cross-talk, ground bounce, high resistance ground reference & inductance all intermingle.