Power MOSFET blew up when increased duty cycle from 5% to 10%  - Page 1
Forum section: electronics / projects

author: XaviPacheco
author_rank: Regular Contributor
timestamp: September 13, 2018, 08:55:21 pm
content: First, these are the componentes I'm using on my PCB, click on them to see the datasheets:Gate DriverPower MOSFETFreewheeling diodeCurrent SensorThe schematic is attached below. Sorry if I'm not using some right symbologies. I took the schematic from the PCB software.Vbus voltage comes from rectified and filtered mains (110VAC).PWM frequency is 10 kHzThe gate driver is isolated, so GND1 is ground from rectifier, an GND2 is the digital ground.I tested the PCB with two different motors.Motor 1:130VDC25.5AMotor 2:130VDC27AThese are the steps that I followed before the transistor blew:1- I tested the circuit with a light bulb first. Everything ok.2- Then, I connected motor 1 with no load. Everything ok.3- I connected motor 2 with a slight load. It started ok at 5% of duty cycle. Then, I increased duty cycle to 10% and the transistor blew up, so the motor went to max speed uncontrollably.I haven't programmed the fast overcurrent detection from the current sensor. That way I would have probably saved the mosfet.The question is... what could be the cause of this? high dv/dt and di/dt? need an snubber? I have a lot of mysteries in mind right now.

author: xavier60
author_rank: Super Contributor
timestamp: September 13, 2018, 10:14:52 pm
content: Will you show the layout also?

author: XaviPacheco
author_rank: Regular Contributor
timestamp: September 13, 2018, 10:53:25 pm
content: Quote from: xavier60 on September 13, 2018, 10:14:52 pmHi, yes, I can show the layout. No problem with that. It's double layer PCB. I'll try to upload the them soon.

author: XaviPacheco
author_rank: Regular Contributor
timestamp: September 13, 2018, 11:00:55 pm
content: Quote from: blueskull on September 13, 2018, 10:57:51 pmD5 is the flyback diode. What do you mean by decoupling from supply of motor to IP-?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 13, 2018, 11:51:27 pm
content: D5 is almost useless without a capacitor nearby, from VBUS to ground. The diode, transistor and cap must be near each other.Think of it this way: the cause of the flyback is not the motor, it's just a passive element. While it holds the most inductance in the circuit, it's not the source of the flyback voltage. The transistor is. The drain voltage is what needs to be clamped, and if not to VBUS (with the diode and nearby capacitor), then to something else like a TVS diode (which would of course be very wasteful in a PWM circuit, but is acceptable for on-off outputs, say for driving lights, solenoids, that sort of thing).Tim

author: XaviPacheco
author_rank: Regular Contributor
timestamp: September 14, 2018, 12:00:57 am
content: Look at this fragment of the PCBCan't upload it because of the size of it.

author: XaviPacheco
author_rank: Regular Contributor
timestamp: September 14, 2018, 12:06:46 am
content: This is my first PCB involving a little of power electronics. Of course, I will have to make a better design, but if I have to add something, I want to use this PCB as a test board.

author: XaviPacheco
author_rank: Regular Contributor
timestamp: September 14, 2018, 12:13:18 am
content: Quote from: blueskull on September 14, 2018, 12:09:17 amThe datasheet for D5 is in my first post - Freewheeling diode. It's an ultrafast diode.

author: XaviPacheco
author_rank: Regular Contributor
timestamp: September 14, 2018, 12:22:04 am
content: Quote from: blueskull on September 14, 2018, 12:19:20 amM+ MLCC is connected to diode in the bottom layer, which can't be appreciated.

author: XaviPacheco
author_rank: Regular Contributor
timestamp: September 14, 2018, 01:28:03 am
content: Could it be that I connected source of the mosfet to the current sensor instead of directly to GND1 which is the reference ground, as suggested in the driver datasheet?

author: xavier60
author_rank: Super Contributor
timestamp: September 14, 2018, 03:10:58 am
content: Quote from: XaviPacheco on September 14, 2018, 01:28:03 amideally the driver IC's ground pin should connect directly to the Source but it doesn't look like you have added enough track length to the current sensor path to be a big problem.You really need to take a look at what's happening with an oscilloscope.I have only used Schottky diodes and at lower voltages. I also use a slower turn off speed for the MOSFET to reduce spikes and ringing.Silicon diodes can have a significant forward recovery time and is not always mentioned in data sheets.There are no capacitors anywhere across the motors?

author: xavier60
author_rank: Super Contributor
timestamp: September 14, 2018, 04:37:14 am
content: If you do decide to try it again, increase R29 and R30, in case there is instability in the Gate circuit.

author: boB
author_rank: Frequent Contributor
timestamp: September 14, 2018, 05:06:26 am
content: You need to check the G-S drive voltage with your scope.Looks from the PCB picture that the layout between your gate driver IC, FET gate and the return to pin 8 of the driver IC through IP+ & IP- is WAY FAR away, electrically, causing a very large loop area and inductance.I would also put the gate driver resistors, R29 and R30 closer to the gate of the FET itself.Pins 5 and 8 of the driver should have a decoupling capacitor very close to the chip also to reduce that loop area. It's hard to tell from the PCB picture how that is layed out.Other than looking at the waveforms, those are some things that you might want to change in the next revision. Gate drive can be tough. The resistor values looks OK though.What is your Vcc for the driver ? Hopefully more than 12V if possible.boB

author: XaviPacheco
author_rank: Regular Contributor
timestamp: September 14, 2018, 05:19:51 am
content: The 12V comes from an isolated unregulated 5 to 12V DC/DC converter. As it is unregulated it can go up to 15V. As I'm switching at 10 khz, the mean current provided by this converter should be small, i.e, below 10 mA. The peaks currents are handled by the 1uF decoupling capacitor I think. I had studied well this converter before using it, and I find it good for the specific application of activating the gate.

author: boB
author_rank: Frequent Contributor
timestamp: September 14, 2018, 06:41:22 am
content: If you aren't actually limiting the current from the power supply, then all bets are off.A driver with desaturation protection is probably a better way to go but that usually requires a micro to stop the drive after a few overcurrents.Use your scope and add a resistor, even if a low-ish value in series with that motor and that should help to safely figure it out.boB

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 14, 2018, 09:07:53 am
content: Desat protection is good, and not exclusive of what's been shown so far; nor a current limiter of whatever sort.Average current mode (with the current sensor in series with the load) is particularly good for loads like motors. To do that, put an error amp in front of the PWM generator, and tie -in = current sensor, +in = max current setting. Alternately, you can do peak current mode control, with the current sensor going to a comparator to a latch (the latch ensures the transistor stays off until the controller turns it back on -- just using a comparator would cause it to oscillate..).Tim

author: XaviPacheco
author_rank: Regular Contributor
timestamp: September 14, 2018, 12:08:51 pm
content: Do you think is worth trying with another stronger 12V source? Would a RC or RCD snubber be necessary in this application?

author: XaviPacheco
author_rank: Regular Contributor
timestamp: September 14, 2018, 02:41:00 pm
content: Quote from: xavier60 on September 14, 2018, 03:10:58 amDirectly across the motor I don't have capacitors. I just have the bulk capacitor and MLCC capacitor across the rectifier output.

author: AndersJ
author_rank: Frequent Contributor
timestamp: September 14, 2018, 03:40:25 pm
content: Perhaps your are PWM’ing so fast that the gate capacitance is filtering your gate driveso it averages your PWM, thus driving the mosfet into its linear region.

author: boB
author_rank: Frequent Contributor
timestamp: September 14, 2018, 04:39:30 pm
content: Quote from: AndersJ on September 14, 2018, 03:40:25 pmCould be if there is a problem with the fet driver or the gate resistors or even possibly the driver's voltage source...I would think that the driver's under-voltage lockout would be a big help there.You just can't work on this type of circuit effectively without an oscilloscope.boB

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 14, 2018, 05:52:51 pm
content: Quote from: blueskull on September 14, 2018, 12:19:20 amQuote from: boB on September 14, 2018, 04:39:30 pmdo i have to repeat this for the 3rd time in the thread? ++1..Quote from: XaviPacheco on September 14, 2018, 12:08:51 pmwithout knowing Vgs... if its me without a scope, i'll put diodes guard ring in all GD, GS, DS pins, same driver setup. if the mosfet still burnt then it maybe it, not enough Vgs. but well, thats if me shooting in the dark.

author: XaviPacheco
author_rank: Regular Contributor
timestamp: September 14, 2018, 06:09:59 pm
content: Ok. I'll try to see the waveforms in the scope. I'll report back.

author: XaviPacheco
author_rank: Regular Contributor
timestamp: September 17, 2018, 03:37:46 pm
content: Update: I've blown up a second MOSFET at startup of the motor. I've replaced the FET with this IGBT IKW50N60DTP and so far it's working well. Anyways, I will capture some waveforms soon.

author: AndersJ
author_rank: Frequent Contributor
timestamp: September 17, 2018, 04:38:58 pm
content: Don't just replace components.Measure and find out what is happening.

author: XaviPacheco
author_rank: Regular Contributor
timestamp: September 17, 2018, 04:48:00 pm
content: Quote from: AndersJ on September 17, 2018, 04:38:58 pmI know. The point is that I don't have a oscilloscope right now. I'll borrow one later. That's why I've been like shooting in the dark. But before I replaced the FET, I reviewed the IGBT characteristics, specifically the ones that I designed the circuit to work for, like switching frequency, gate charge, etc.

author: XaviPacheco
author_rank: Regular Contributor
timestamp: September 18, 2018, 03:20:59 am
content: I've got these waveforms so far, input and output of the gate driver. I haven't been able to capture Vge yet. The new IGBT hasn't failed yet. (That's why I'm now referring to Vge instead of Vgs) Seems like the MOSFET I used before didn't tolerate the kickbacks from the motor. I'll report back when I get Vge measurements.

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2018, 05:19:31 am
content: The Collector waveforms might show something also.

author: AndersJ
author_rank: Frequent Contributor
timestamp: September 18, 2018, 05:47:38 am
content: Your pwm frequency is high.The motor inductance may be so high that you do not get a square wave output.In that case the transistor is operating in its linear region, thus running hot.

author: XaviPacheco
author_rank: Regular Contributor
timestamp: September 18, 2018, 06:54:20 am
content: Why is the PWM frequency high? The oscilloscope says about 10 kHz (9.7 kHz), and that’s what I actually chose it to be.

author: AndersJ
author_rank: Frequent Contributor
timestamp: September 18, 2018, 07:14:00 am
content: 10 KHz may be high for a:- Mosfet, depending on gate capacitance and drive impedance- "Large" inductance like a motor.I'm only guessing.You are the only one that can verify this.I have pwm'ed small motors at around 300 Hz.Higher frequencies did not work well.That was 27 years ago.I don't remember the details.Why did you choose 10 KHz?

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2018, 07:53:10 am
content: For the amount of wire that DC motors have in them, the inductance can be rather small. If the PWM frequency is too low, the motor will draw large discrete current pulses which causes high I squared R losses in the motor windings and everything else in the circuit path and poor load regulation.As the frequency is increased, the current becomes continuous with lower peaks and load regulation improves. Going unnecessarily high will cause loses to increase with no real advantage.I have found the minimum to be 5Khz with the motors that I have used.I guess it depends on the motor. 300Hz might be suitable for some.

author: XaviPacheco
author_rank: Regular Contributor
timestamp: September 18, 2018, 01:53:59 pm
content: Actually you've made a good point regarding the PWM frequency. I didn't find anything that tells you the suitable PWM frequency straight forward, because it depends on many factors. I just did a kind of research and came up with 10 kHz. I've attached the nameplate of the motor I'm controlling now.

author: XaviPacheco
author_rank: Regular Contributor
timestamp: September 18, 2018, 01:57:19 pm
content: I haven't mentioned that I plan to measure speed and compensate duty cycle according to the load.

author: Wolfram
author_rank: Frequent Contributor
timestamp: September 18, 2018, 03:47:30 pm
content: How do you change the duty cycle? If you abruptly step the duty cycle by a certain amount, the motor will draw a proportional fraction of its stall current until its speed reaches the new value.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 19, 2018, 12:59:14 am
content: Indeed, the very first thing you're doing by adjusting PWM is adjusting the flux into the winding inductance. Get this wrong and current overshoots and *pop*.The best way (the only true way) to control an inductive load, is to control the current into it. A Hall affect current sensor could be used to measure motor current, and the PWM adjusted to maintain that at some set current. This inner loop is very fast (comparable to the clock frequency), so it can protect the transistor even in the event of a short circuited load!The current, in turn, is set by a second loop that controls whatever parameter is of interest: output voltage (especially for power supplies), torque (torque is very close to current, actually, but a torque servo would address bearing and windage losses), RPM (very close to voltage, except for winding DCR and magnetic leakage), etc.The two-loop architecture is very easily implemented in a microcontroller, or can be done discrete with a dual op-amp. Note that you need a relatively fast MCU to monitor current safely: ATmega won't cut it, the ADC is just too damn slow. But Xmega will, or pretty much anything else out there (dsPIC, ARM..).Tim

author: XaviPacheco
author_rank: Regular Contributor
timestamp: September 19, 2018, 01:03:27 am
content: As shown in my schematic, I'm using the TLI4970 current sensor to monitor the current. How effective it is, I still don't know. I'm working on it. I'm using a L432Kc nucleo for the project.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 19, 2018, 02:38:56 am
content: Right, but that's monitoring source current, so it doesn't sense motor current when the transistor is off.Source side peak current sensing is alright for protection, but you need quick response. Usually a comparator and flip-flop (consider UC3842's block diagram). Some MCUs have hardware configurable for this, but no way in hell you'll get away with doing it in software (e.g., analog comparator triggers interrupt, or let alone ADC polling current signal*).*Unless it were something like a dsPIC that can sample and process quite fast (single digit µs response time).Average current mode control allows you to respond more slowly, at least if you have some minimum inductance in series with the motor. Which is a good idea if the motor is on a long cable, by the way -- extra filtering, just make it a buck supply in the first place and run the motor on clean power.Since you're already using a Hall effect sensor, moving it to the motor is trivial.Tim

author: XaviPacheco
author_rank: Regular Contributor
timestamp: September 19, 2018, 02:43:25 am
content: Let me see if I get the idea.. do you mean moving the sensor to the drain/collector leg?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 19, 2018, 02:55:37 am
content: Probably between DC+ and motor, since that will have less noise.Tim

author: XaviPacheco
author_rank: Regular Contributor
timestamp: September 19, 2018, 02:57:37 am
content: Ok. I will take it into consideration for the next design, as I already have the PCB with the sensor in the source leg. Thank you.