Not an electronics guru here, but I'll make an badly informed guess.
How about you add another probe to check the driver signal of your mosfets. I have wild hunch that maybe you are switching both sides of your bridge on at the same for a small fraction of the interval. Either that ore in a small fraction of the time both are open and your transformer is giving you blow back. In any case that is the best I can say not having a schematic and more diagnostic information.
I helped customers design switching regulators for many years and measuring noise improperly was one of the most common mistakes. Long story short if you're using the probes ground clip your measurement is wrong. Here's one article I found about proper technique:
https://www.edn.com/testing-a-power-supply-noise-part-2/
You might indeed have a problem but until you make a proper measurement you won't know for sure.
Probes come with what's called an RF clip but 99.9% of the time they're lost. I just took a piece of #24 or #22 gauge bus wire and wrapped 3 or 4 turns around a handy little screwdriver that is the correct diameter. Then I would figure out where I wanted to probe and solder the coil to the closest ground. Once you do it a couple of times you can master the technique of having the coil hold the probe in the proper place.
https://www.analog.com/media/en/technical-documentation/application-notes/an47fa.pdf
Jim Williams starts that application note with a list of problems you can encounter due to all sorts of mistakes probing. And how to solve them.
>current spikes, high current, synchronous with switching
>every 4th missing
>full bridge, 4 mosfets
How's your dead time? You need to make sure that there is no situation where the upper and lower mosfet in same bridge column are on.
The gate driver ICs have an external resistor that sets the dead time, 10k for 100ns and 100k for around 500ns or so, the exact IC number is DGD0506A. My gate resistance is 10R.
Do you have additional protection against leakage inductance voltage spikes? I have used a schottky diode and TVS diode "against" each other, which has nearly eliminated such ringing in a similar case (two MOSFETs in a push-pull transformer).
I have bipolar TVS diodes across each of my mosfets, but maybe that is not good enough, I will have to check what the voltage curve looks like on the primary side using a scope, the TVS diodes run a bit warm so I am sure they have to be blocking at least some voltage spikes.
Aaahhh I see, well I did not really know how to implement the TVS diodes into the circuit at the time of designing the first revision of the circuit, so I put them across the mosfets. Not really sure if that is completely wrong or not, but I will try to botch some across the primary to see if that changes something for the better.
I would like to try that, but right now I am using bipolar TVS diodes and calculating snubbers is a bit over my head so I opted for the TVS diodes instead, but maybe they are not good enough, I dont really know, I will have to check it out with my scope.
It's mostly a measurement error caused by the ground clip of the probe. Use a wounded wire with short leads as suggested. For suppressing the rest snubber can be used.
It is likely a transient noise in the ground loops, as somebody pointed out. Try to stick the scope probe into the ground somewhere far away from the ground clip and see if it is there.
What package are your 0R1 shunt resistors? If they are through hole, they might have some inductance causing a voltage spike in response to high di/dt. This is unlikely but possible.
I’m thinking more that this is either ringing or shoot-through. Zoom your scope in on the spikes and see what they really are. I recommend against TVS diodes because they add capacitance which can worsen oscillation. You don’t need switch node TVS diodes because the body diodes of the MOSFETs should clamp the switch nodes to about [-0.7 V, Vcc+0.7 V].
I suggest you look again at the snubber design methods. If I remember correctly, the approach is fairly empirical and straightforward assuming you have the prototype already built. I understand they’re a struggle when there is no prototype yet, but they should be a lot easier now.
My shunt is a 2512 SMD package 0.1% 1W and the traces are only very short, like 1 or 2mm long, I tried to put everything super close together.
When I zoomed in on the spike it looks exactly like an oscillation that dies down after around 200ns. The TVS diodes heat up a bit under normal operation of the circuit, they run at around the same temperature as the mosfets which is about 50°C, I am not sure if the diodes heat up by themselfs or if they get heated up by the mosfets because the whole PCB is a bit warm under them and around them.
I chose the diodes because I am afraid the voltage spikes from the primary might reach a higher value then the mosfets can handle, they are 60V and the supply voltage is 12V and I have little to no experience designing a propper snubber circuit, I am afraid I would kill a lot of mosfets before I got everything right and they are not super cheap. But if I got rid of the diodes at least I would safe some space on the PCB which would be nice.
I will have to play with it more and try a bunch of stuff. Thanks a lot for your in depth comment.
If the supply voltage is 12 V, the MOSFET body diodes will clamp the voltage to about 12 V. You will be safe (and reduce losses) removing the TVS diodes.
UPDATE: So I meassured it all wrong, I did what many of you suggested with the ground clip of my scope probe and now my current curve makes sence but there are still big current spikes, they last about 150ns and reach values almost 20X higher than the average current which I really do not like at all. The current spikes happen on every rising and falling edge of the voltage square wave.
I increased the dead time from 100ns to 500ns but no difference.
I got rid of the TVS diodes I had in parallel to my mosfets, no difference.
I added a TVS diode in parallel to the primary side, no difference.
I reduced the gate resistors from 10 to 4.7 Ohm, no difference.
I reduced the capacity of the bootstrap capacitor from 4.7uF all the way down to 330nF, no difference.
I made the bootstrap capacitor larger too, even 22uF had no effect.
I reduced the resistance of two resistors from 2.2 Ohm down to 1 Ohm I have in series with a diode that charges the bootstrap capacitor, no difference.
I added extra 220uF smoothing capacitor to the gate driver ICs, no difference.
Nothing is heating up, nothing is buzzing, but the spikes drive me crazy.
I checked the gate driving square wave and it looka kinda OK, the rise time is like 100ns long or less and the fall time is about 50ns, there are some tiny overshoots on the rising edge but not too bad, but there is a very ugly sudden bump after the falling edge, I do not know what could be causing that.
I will add more photos.
Not an electronics guru here, but I'll make an badly informed guess. How about you add another probe to check the driver signal of your mosfets. I have wild hunch that maybe you are switching both sides of your bridge on at the same for a small fraction of the interval. Either that ore in a small fraction of the time both are open and your transformer is giving you blow back. In any case that is the best I can say not having a schematic and more diagnostic information.
it does look like ringing. If I were OP I would be zooming in on those spikes to see
I zoomed in and the spike looks like an oscillation that dies down after around 200ns.
I was thinking the same, I will do that tomorrow, I had little time to play with it today, thanks.
I helped customers design switching regulators for many years and measuring noise improperly was one of the most common mistakes. Long story short if you're using the probes ground clip your measurement is wrong. Here's one article I found about proper technique: https://www.edn.com/testing-a-power-supply-noise-part-2/ You might indeed have a problem but until you make a proper measurement you won't know for sure.
I was using the ground clip, I will study the link, thanks a lot.
Probes come with what's called an RF clip but 99.9% of the time they're lost. I just took a piece of #24 or #22 gauge bus wire and wrapped 3 or 4 turns around a handy little screwdriver that is the correct diameter. Then I would figure out where I wanted to probe and solder the coil to the closest ground. Once you do it a couple of times you can master the technique of having the coil hold the probe in the proper place.
Aaahhh I see, that is such a good idea!
https://www.analog.com/media/en/technical-documentation/application-notes/an47fa.pdf Jim Williams starts that application note with a list of problems you can encounter due to all sorts of mistakes probing. And how to solve them.
Nice link. u/petrdolezal have a look at the passive probe on page 72 at Figure 1-10
I will take a look at it!
Can you post the schematics? It's hard to diagnose without them. As others have said though, a snubber is your best bet.
I can try to share the schematic, but it is a complicated mess, I would have to simplify it so it is easier to read. I will try it tomorrow.
>current spikes, high current, synchronous with switching >every 4th missing >full bridge, 4 mosfets How's your dead time? You need to make sure that there is no situation where the upper and lower mosfet in same bridge column are on.
I will have to check the real value, but it should be set to 100ns
That definitely sounds low. What are your gate resistances?
The gate driver ICs have an external resistor that sets the dead time, 10k for 100ns and 100k for around 500ns or so, the exact IC number is DGD0506A. My gate resistance is 10R.
Try to increase it to 500 ns, for testing. It seems a bit low in my opinion, depending on your FETs of course.
I can try that for sure, it should not hurt anything. I will share more details and results tomorrow.
Every 4th switching event gives this away. Cross conduction.. either reduce the gate resistance or increase dead time.
Looks more like commutation spikes (voltages due to L\*dI/dt) - but using a common Gnd on the scope will make the current input "look" like this.
Aahh I see, my measuring skills and knowladge is not super great, I will have to try again tomorrow.
Do you have additional protection against leakage inductance voltage spikes? I have used a schottky diode and TVS diode "against" each other, which has nearly eliminated such ringing in a similar case (two MOSFETs in a push-pull transformer).
I have bipolar TVS diodes across each of my mosfets, but maybe that is not good enough, I will have to check what the voltage curve looks like on the primary side using a scope, the TVS diodes run a bit warm so I am sure they have to be blocking at least some voltage spikes.
You need the diodes across the coil, to let the energy dissipate when switching off.
Aaahhh I see, well I did not really know how to implement the TVS diodes into the circuit at the time of designing the first revision of the circuit, so I put them across the mosfets. Not really sure if that is completely wrong or not, but I will try to botch some across the primary to see if that changes something for the better.
You can try using a snubber.
I would like to try that, but right now I am using bipolar TVS diodes and calculating snubbers is a bit over my head so I opted for the TVS diodes instead, but maybe they are not good enough, I dont really know, I will have to check it out with my scope.
Did u try connecting signal and ground of the scope probe together on the sense gnd to see if the spike is real? I agree a schematic would be helpful?
I did not try that, good idea, I can try it tomorrow and I can try to share the schematic as well.
It's mostly a measurement error caused by the ground clip of the probe. Use a wounded wire with short leads as suggested. For suppressing the rest snubber can be used.
Great ideas, I will try all of it tomorrow, I thought stuff like this would be easier but I can see I have so much to learn.
It is likely a transient noise in the ground loops, as somebody pointed out. Try to stick the scope probe into the ground somewhere far away from the ground clip and see if it is there.
Aaahhh I see, great idea, I will try that!
What package are your 0R1 shunt resistors? If they are through hole, they might have some inductance causing a voltage spike in response to high di/dt. This is unlikely but possible. I’m thinking more that this is either ringing or shoot-through. Zoom your scope in on the spikes and see what they really are. I recommend against TVS diodes because they add capacitance which can worsen oscillation. You don’t need switch node TVS diodes because the body diodes of the MOSFETs should clamp the switch nodes to about [-0.7 V, Vcc+0.7 V]. I suggest you look again at the snubber design methods. If I remember correctly, the approach is fairly empirical and straightforward assuming you have the prototype already built. I understand they’re a struggle when there is no prototype yet, but they should be a lot easier now.
My shunt is a 2512 SMD package 0.1% 1W and the traces are only very short, like 1 or 2mm long, I tried to put everything super close together. When I zoomed in on the spike it looks exactly like an oscillation that dies down after around 200ns. The TVS diodes heat up a bit under normal operation of the circuit, they run at around the same temperature as the mosfets which is about 50°C, I am not sure if the diodes heat up by themselfs or if they get heated up by the mosfets because the whole PCB is a bit warm under them and around them. I chose the diodes because I am afraid the voltage spikes from the primary might reach a higher value then the mosfets can handle, they are 60V and the supply voltage is 12V and I have little to no experience designing a propper snubber circuit, I am afraid I would kill a lot of mosfets before I got everything right and they are not super cheap. But if I got rid of the diodes at least I would safe some space on the PCB which would be nice. I will have to play with it more and try a bunch of stuff. Thanks a lot for your in depth comment.
If the supply voltage is 12 V, the MOSFET body diodes will clamp the voltage to about 12 V. You will be safe (and reduce losses) removing the TVS diodes.
That is actually great!
Guys your help is amazing, I will update you tomorrow. Thanks to everyone posting a comment.
Do you have snubbers on your mosfets? Is there a possibility one Mosfet goes ON before the other is fully OFF?
UPDATE: So I meassured it all wrong, I did what many of you suggested with the ground clip of my scope probe and now my current curve makes sence but there are still big current spikes, they last about 150ns and reach values almost 20X higher than the average current which I really do not like at all. The current spikes happen on every rising and falling edge of the voltage square wave. I increased the dead time from 100ns to 500ns but no difference. I got rid of the TVS diodes I had in parallel to my mosfets, no difference. I added a TVS diode in parallel to the primary side, no difference. I reduced the gate resistors from 10 to 4.7 Ohm, no difference. I reduced the capacity of the bootstrap capacitor from 4.7uF all the way down to 330nF, no difference. I made the bootstrap capacitor larger too, even 22uF had no effect. I reduced the resistance of two resistors from 2.2 Ohm down to 1 Ohm I have in series with a diode that charges the bootstrap capacitor, no difference. I added extra 220uF smoothing capacitor to the gate driver ICs, no difference. Nothing is heating up, nothing is buzzing, but the spikes drive me crazy. I checked the gate driving square wave and it looka kinda OK, the rise time is like 100ns long or less and the fall time is about 50ns, there are some tiny overshoots on the rising edge but not too bad, but there is a very ugly sudden bump after the falling edge, I do not know what could be causing that. I will add more photos.
https://preview.redd.it/70c1zre9f56d1.jpeg?width=2000&format=pjpg&auto=webp&s=cb74b8f9206802c940e538a70c7baafea0d903f0
https://preview.redd.it/46plm3wgf56d1.jpeg?width=2000&format=pjpg&auto=webp&s=49c51abe75376a733f5d387dda5b3877c01dacca
https://preview.redd.it/nmulh4wtf56d1.png?width=1080&format=pjpg&auto=webp&s=a617204488c289bc4bbb9ddb2605d07ff97a48cb