Mosfet gate oscillation - Page 1
Forum section: electronics / projects

author: gaminn
author_rank: Frequent Contributor
timestamp: March 10, 2023, 04:39:38 pm
content: Hi,I design a device that should deliver max 5 A @ 350 V DC to mainly inductive load via mosfet half bridge. When switching high side mosfet on there seems to be quite wild oscillation in the circuit. The only thing that seems to lower the oscillations is higher Rgate. When it is 1 kohm, there are almost no oscillations. Unfortunately then even with 250 Hz PWM switching frequency, mosfets get hot due to switching losses. Any idea what might help here? I would like to be able to lower Rgate and get a little higher PWM frequency which seems unachievable now for me.Green is high side mosfet gate, yellow is low side mosfet gate, both referenced to GND.In the schematic, please ignore bottom right part of it - it is current sensing circuitry which is not used at the moment. Rsense = 0The tracks on the PCB are 2 - max 5 cm long, all power tracks at least 5 mm wide.The level of oscillations doesn't change when increasing/decreasing output load.Explanation for pcb: red is top side, blue is bottom side. Input of the PCB is rectified AC 230V. I pour GND copper area on both top and bottom sides which I think might be problematic. pcb.png is without copper pour command executed.

author: jonpaul
author_rank: Super Contributor
timestamp: March 10, 2023, 05:23:48 pm
content: check probe ground lead, and technique for ringing.use 10..100 ohm series gate T to damp Q

author: inse
author_rank: Super Contributor
timestamp: March 10, 2023, 05:56:09 pm
content: Please also show us your layout/wiring.Keeping the gate-source loop as small as possible is crucial for proper switching behaviour.

author: johansen
author_rank: Super Contributor
timestamp: March 10, 2023, 06:25:51 pm
content: I think you have too much inductance in the dc bus and you have avalanche losses.at 250hz even at 1kohm it would probably still work, and for example, there is a 2kohm gate resistor on the P80nf550 mosfets in a apc back ups 600.. which is switching 12 volts at 40 amps, at 60hz center tapped half wave transformer. granted this is completely different but the gate charge of that mosfet is much higher than yours, but the voltage is far lower however the amps are as much as 10 times higher.1us time constant at 10 volts gate drive through a 1kohm resistor into 1nF, lets presume the mosfet is fully turned on within 2uS2uS times 5 amps times 350 volts is 3.5 mJ losses.. this occurs 250 times a second which is .875 watts. i guess you could multiply this by 4 because you have turn off losses for each mosfet as well.

author: gaminn
author_rank: Frequent Contributor
timestamp: March 10, 2023, 08:30:43 pm
content: Thanks to all. I added my PCB layout and some explanation for it to my initial post. Can you see any obvious flaws in it?

author: johansen
author_rank: Super Contributor
timestamp: March 10, 2023, 10:17:25 pm
content: Some of that oscillation may not be "real" Given that your mosfet gate waveforms do not show the usual 3 parts, which is that the voltage rises, then stays the same while the miller capacitance is discharged, followed by the voltage on the gate rising to the voltage of the gate drive. so i think something is wrong or its interference. -unless of course it is showing the miller capacitance part.. and you have a mosfet turning on at less than 1 volt (assuming your green trace is 10 volts gate drive)what are you using to measure the highside gate waveform? usually this is hard to do without a differential probe.you could use a 1:1 transformer (a common mode choke from a small power supply will work) capacitively coupled to the mosfet gate-source waveform through a rather small capacitor (you don't want to load the gate driver but you need enough capacitance to drive the oscope probe through the coil)

author: gaminn
author_rank: Frequent Contributor
timestamp: March 11, 2023, 07:30:38 am
content: I measure the high side gate, as well as the low side gate, with ordinary Agilent/Keysight 200 MHz probes. Both probes are referenced to GND of the circuit (= source of the low side mosfet). This is what you see in 2 images I attached to my initital post. I also measured only high side gate referenced to source of the high side mosfet and I got the same oscillations.The DUT itself is isolated from mains (and thus from GND of my scope) - I use autotransformer with isolation transformer to supply DUT. The level of oscillations doesn't change when power supply is 50 VAC or 230 VAC.

author: johansen
author_rank: Super Contributor
timestamp: March 11, 2023, 10:51:33 pm
content: I have found that the parasitic capacitance of the DUT to ground, makes strange waveforms due to current flowing through thr oscope probe cable, when you ground a switched node to the oscope in order to measure the highside gate waveform.

author: gaminn
author_rank: Frequent Contributor
timestamp: March 14, 2023, 09:14:54 am
content: My problem is probably bad PCB design, I guess. When I'm near my PCB or near input cables with a loop made by scope's probe I see oscillations exactly at the moment high side mosfet is turned on. And the oscillations are there even without any load connected to the halfbridge (the PCB consumes almost 0 current).Something badly designed. Any idea what?

author: johansen
author_rank: Super Contributor
timestamp: March 14, 2023, 05:05:54 pm
content: well you are hard switching the mosfets and they have a body diode.what's the frequency and magnitude of that oscillation? is it only 6 volts peak to peak for 50ns at 100mhz?I would add an RC damper on the output. if the switching losses are greatly reduced then you might have cross conduction.Also try driving both mosfets from a 1:1:1 transformer driven by a bench top signal generator. if you have no strange waveforms, and no switching losses then I believe this confirms you have cross conduction. -you'll have to stay above i'm guessing 10khz or the transformer saturates. but this isn't so high that it would have substantial switching losses, just crank up the voltage on the bus slowly and monitor the temperatures and waveforms.you can use 2, emi common mode chokes for a transformer, capacitively coupled to the signal generator, outputs get the resistor/diode treatment which you already have on your circuit. you'll have to disconnect your existing gate driver because they can't handle negative voltages.

author: Martinn
author_rank: Frequent Contributor
timestamp: March 14, 2023, 06:44:22 pm
content: Quote from: gaminn on March 14, 2023, 09:14:54 amThe probe ground connection.I use the exact probe setup to show apprentices what's wrong with probe ground wires. These are OK until a few MHz, you are clearly above that, ringing is an artifact from the probe ground loop.Please show how you were connecting the probes for the scope plot you posted.Reduce ground loop by soldering the probe in with a ground wire length of < 10 mm (don't use your newest probe for that) or use the spring ground wire provided most of the time. Always do a cross check probing the ground wire connection (should read 0).Or make a 1k Z0 probe (won't probably work for 350 V), active FET probe would also help but not with 350 V.

author: gaminn
author_rank: Frequent Contributor
timestamp: March 15, 2023, 02:44:15 pm
content: There is no problem with scope's probes, the measurements are real I think.I did more tests and when low side mosfet is removed from the circuit there are no oscillations. Even with load connected, nice clean signals. I also tried to ground the gate of the low side mosfet to ensure it never turns on. But with the gate grounded there are also oscillations.FYI information, the oscillations frequency is 135 MHz. The amplitude of the voltage depends on how close I got to the circuit with my loop made from scope's probe. If I remember correctly I got max 10 V p-p.

author: Martinn
author_rank: Frequent Contributor
timestamp: March 15, 2023, 05:35:06 pm
content: If you are using the ground lead as you showing in the picture you have a problem with the probes. This works only up to a few MHz and you are orders of magnitude above that.Of course it is still possible your circuit oscillates, you just can't tell if it's the probe or real unless you improve the probe setup (as I mentioned above).You could use an H-field probe (EMC precompliance) to check for oscillations.

author: inse
author_rank: Super Contributor
timestamp: March 15, 2023, 06:41:28 pm
content: Do you provide a freewheeling path when the lowside MOS is missing?What is its purpose anyway, wouldn’t a diode suffice?

author: gaminn
author_rank: Frequent Contributor
timestamp: March 15, 2023, 08:11:14 pm
content: I made the grounding clip of the probe smaller. Please see pictures. There is high side mosfet switching on and off in the pictures. For comparison, there is also highsideoffbigloop.jpg which shows high side mosfet off measured with normal probe with normal grounding clip forming big loop. So yes, oscillations are much lower when measured with small grounding loop. But at least there is something in the circuit that causes EMI. With a big loop formed with probe's grounding wire connected to probe's signal wire I can see 135 MHz up to 10Vp-p oscillations from my PCB even 30 cm far away. No load connected to the PCB, just pulses going to mosfet gates.So let's say the oscillations are not a problem in the circuit. Then I can't explain the pile of exploded mosfets I already have on my table. When the supply voltage is nominal (230VAC and thus DC bus is full 350 VDC) then after a while with load connected both mosfets of the halfbridge explode forming shortcircuit on the DC bus. The load is universal 230V AC motor. Not sure what is wrong. Mosfets should withstand 600 V and 18 A. The current is at least 4 times higher than what the ac motor needs at full load. No explosions when the motor is not connected (just pulses going to gates).If I'm not able to find the cause of this issue, then I will probably ground the gate of low side mosfet effectively using only body diode of the mosfet when the high side mosfet is off. I hope my PCB will be thermally OK with that modification (higher losses on the diode).

author: TimNJ
author_rank: Super Contributor
timestamp: March 16, 2023, 12:00:55 am
content: It looks like some sort of Miller induced ringing to me. Goes something like this:1. You try to turn on the MOSFET by raising the gate source voltage.2. Simultaneously the drain source voltage starts to fall, at some dv/dt3. But there is some coupling (capacitance), between drain and gate (Cgd), so as Vds falls at some dv/dt, some current is pulled out of the gate through Cgd, pulling Vgs towards zero4. *Hand waving* because there are some inductance in the drive and power loop circuits, you often get oscillation on the gate during this timeI would experiment with some ferrite beads on (probably) the source of the MOSFET(s). Try on drain or gate too, for good measure. Fair-Rite 43 material or similar. Laid 28F0121-0SR-10 is a surface mount bead you might want to try. Probably stay away from multi-layer chip beads because they saturate pretty easily. But YMMV.If you are suggesting that the low-side MOSFET drive is OK, but high-side is not, you might have an issue with the high side supply or driver. The high-side driver needs to be low enough impedance to keep the gate driven high as the drain voltage swings down. You could try increasing the value of C13 to see if that matters.While the distance between IC5 and Q2 is small, the gate drive and switch-node (VS) traces are very thin...what are they, 0.15mm? Thin traces increases the gate drive loop inductance unnecessarily. You can try running some fatter wires in parallel to see if that helps at all. Twisted pair will keep the loop inductance down.And, for the low-side MOSFET, just be wary that when you dump current into the same ground plane shared by the control IC, you can develop some voltage difference across the plane which may contribute in some way. (i.e. "ground bounce").Also, be careful with the PCB spacing if this is switching rectified 230Vac. I would go for at least 1mm between high and low voltage nets.

author: temperance
author_rank: Super Contributor
timestamp: March 16, 2023, 12:11:14 am
content: You might be interested in this app note:https://assets.nexperia.com/documents/application-note/AN90011.pdf