r/AskElectronics • u/planeur31 • 1d ago
Why does my MOSFET driver burn after some random time ?
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u/Witty-Dimension 1d ago
Refer to page 8 (section 3.1) of the gate driver datasheet, where the recommended minimum value for C12 in your circuit is specified as 1.0uF, while your current value is 0.1uF. Additionally, under the heading 3.3 CMOS Push-Pull Output, it highlights peak currents of 6.0A and peak reverse currents of 1.5A in either polarity.
Moreover, on page 9, Figure 4-1 of the MOSFET driver datasheet includes an application schematic(with lots of capacitors) illustrating a test circuit. Please go through it and try to implement those.
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u/ThroneOfFarAway 1d ago edited 1d ago
I mean, sure with the capacitors. I just don’t think that’s what is happening here, personally. He’s not hitting the peak currents on the output of that TC4420 by a long shot. It looks like the FET they chose has really inadequate ratings though, only rated at <6A at 100 degrees C.
I’ll bet that unspecified motor is sinking waaay more than 6A, and that tiny FET is burning over time from the stress.
Just my two cents though, this feels like a “not enough information” multiple choice question on the FE exam.
edit: I found the wrong datasheet for the MOSFET op is using here, which op corrected. I take back my speculation.
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u/planeur31 1d ago
The FET does not fail while driving the motor. It does not heat up even if the motor stalls, drawing high current (up to 30A but not for a long time. I put the datasheet in another comment).
Maybe the FET's gate is making the driver burn ?
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u/planeur31 1d ago
I don't remember what I did at the time, but I might have only taken into account the capacitor between 5 and 8 (0.1 uF). To be honest, I don't know how I should chose those capacitors' values.
But to be honest, I'm not sure which capacitor I should use.
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u/Witty-Dimension 20h ago
u/planeur31 If you are uncertain, it is advisable to begin by utilizing the specified capacitor within the test circuit, as depicted in the datasheet. On the right side of the test circuit is a timing diagram as well, you can refer to that to tinker/upgrade anything in the future.
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u/I_fisted_a_bear_once 1d ago
Well first, as per the datasheet, you should have at least 1uF capacitance at the gate driver VDD. 100nF is not enough.
The mosfet you are using also has quite a high input capacitance of about 14'000pF. If you are driving with a PWM frequency that is relatovely high, it means a lot of current goes through the gate driver.
I would try increasing bypass capacitor at VDD to at least 1uF(Applications Information section has 4.7uF for a 2500pF load). Also, maybe your PWM frequency is higher than it needs to, try lowering that too.
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u/planeur31 1d ago
Should I use a higher capacitor value than 4.7 uF since the load is greater than 2500 pF ?
I can't really change the PWM frequency for now but will try to do so if it's necessary ...
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u/ZeroV8 1d ago
Everyone talking about the 100nF decoupling capacitor is wrong. I mean, they're right in the sense you should probably use something larger, but it isn't why your driver is failing.
You have no resistance between your driver and your FET, other than the internal resistance of your FET. The peak current to charge the 300nC (nominal, it could be up to 460nC) gate capacitance almost certainly exceeds the 6A peak current of your driver.
Put a resistor between the output of your gate driver and the FET. If you use the 2.7 Ohms the datasheet uses to characterize the fall time of the FET, your peak current would only be 3.3A, well below the datasheet limit.
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u/22OpDmtBRdOiM 1d ago
Actual gate voltage?
Is U5 powered down at some point?
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u/planeur31 1d ago
Gate voltage is +/- 20V in the datasheet. My supply power is +12V.
U5 is never powered down, only the PWM duty cycle variates 0-60%
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u/planeur31 1d ago edited 1d ago
I realize the text did not come with the image. Here is some additional info :
- The motor draws up to 30A (stall current, for a very short time before the rest of the system detects it and shuts the motor down). Here is the datasheet : https://asset.conrad.com/media10/add/160267/c1/-/en/000244503DS01/fiche-technique-244503-moteur-universel-a-balais-brushed-igarashi-2738-048-gfc-3.pdf
- PWM frequency is 2 kHz
- The MBR1060 acts as freewheeling diode across the motor's poles.
- I'm obviously making mistakes, pardon me !
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u/Tesla_freed_slaves 1d ago
What’s it doing when it’s not burning? Can you watch it with a oscilloscope?
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u/planeur31 1d ago
I don't have an oscilloscope.
When it's not burning, the motor works as it should and everything is fine. But after some random time, the driver just gives +12V to the FET's gate thus making the motor spin at full RPM.The driver also gets hot when it's dead.
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u/Tesla_freed_slaves 1d ago
Twilight Zone here; just for kicks, disconnect the MOSFET’s gate and strap it to the source. Ground the TC4420’s output pins through a 10nF capacitor. Replace C12 with a 22uF polymer-electrolytic. Let it run at 50% output and see if it gets hot.
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u/ThroneOfFarAway 1d ago
That MOSFET you’re using has an extremely low Id current rating for a motor drive. Less than 6A at 100c.
You’re missing a TON of information for us to accurately diagnose the issue, wave forms, motor part number, failure modes, etc., but I can almost guarantee you that you’re underrated with that MOSFET for your application. If you’re also missing adequate heat sinking, that current rating is going to drop even more.
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u/planeur31 1d ago
I don't understand the problem about the MOSFET : its rating should be way above the motor's 30A stall current, no ?
https://www.mouser.fr/datasheet/2/196/Infineon_IRFB7430_DataSheet_v01_01_EN-3363210.pdfAnyway I burnt quite a few drivers while the MOSFET is the same from the beginning and never heats up.
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u/ThroneOfFarAway 1d ago
Ah, you're right. I mistyped the part number from your schematic when I ran a search, so please disregard!
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u/TPIRocks 1d ago
You should be using a gate driver. If the fet is fine, but the driver burns up, it's gotta be that the gate capacitance is too high. Since your fet isn't getting warm, you can try using a series resistor between the driver and fet gate to limit the charging current, but this will slow down the rise and fall times, possibly heating the fet. I suppose I'd try 10ohms. That said, I'd use a gate driver instead.
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u/planeur31 1d ago edited 1d ago
What is the difference between a gate driver and MOSFET driver? I thought the latter would be more appropriate but I lack knowledge about the first one.
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u/TPIRocks 1d ago
I assume instead of the lower case MOSFET, you mean gate, as in gate driver. I suppose some manufacturer calls it a MOSFET driver, but in the end, they're just gate drivers. Their purpose is to be able to dump high currents into the gate to turn it on as fast as possible, and then back out at turn off.
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u/TPIRocks 1d ago
Oops, I think I confused this thread with another, where the motor controller was failing. You are using a gate driver. Are the capacitors big enough? I would expect at least a microfarad or two. FETs can have tens of thousands of picofarads of gate capacitance. The gate/MOSFET driver should have much more local capacitance.
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u/TPIRocks 1d ago
Btw, your schematic shows the MOSFET upside down giving a current path to ground through the "body diode", when it's supposed to be off.
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u/ConsiderationQuick83 1d ago
The schematic has source and drain in the wrong polarity, note the orientation of the FET body diode. IDK how the pin numbering is matched to the physical pins on that TO220, but there's a good chance your gate and drain are actually swapped so your driver is the one pulling the motor current (pin 1 is swapped with 3 physically).
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u/planeur31 1d ago
I will check tomorrow the schematics but on my pcb, the driver is definitely not powering the motor : the motor draws way more than the 6A the driver can provide (max). I guess this is a schematic error while I added the component to my library too fast.
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u/ConsiderationQuick83 1d ago
At those current levels IC driver stages can fail either open or shorted, easy enough to check the FET connectivity with an ohm meter.
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u/flyingsaxophone 21h ago
I noticed that as well, but it just seems you used the symbol for a PMOS when you're using an NMOS. Assuming you've got the right part, you're likely fine. A double check and schematic symbol correction would still be wise
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u/flyingsaxophone 20h ago edited 20h ago
The comments about adding capacitance to the input of the gate driver are underrated. The data sheet says a minimum of 1u. You really want to meet that. Make sure you're using a ceramic and not an electrolytic capacitor.
Use a smaller size part when you can, too - for the same value capacitance, smaller packages have lower ESL and therefore better high frequency response. More capacitance is technically better here, but don't go to the next physical size unless you actually have to. You should be in an 0603 or 0805 size. And make sure you get one rated for at least 25V, since the effective capacitance of an MLCC is derated with the DC voltage applied.
I'd go for 2.2uF with a 35V rating, in the smallest package you can reasonably get.
And yes, gate resistance and/or FET with less input capacitance is probably part of the solution if not the primary one
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u/erazer33 13h ago edited 13h ago
Hard to see, but it might be that the body diode is in the forward direction.
This will make all the current of the load through the body diode when the mosfet is switched off.
Nevermind: I checked the datasheet and there it is the right way up; second, you asked about the driver, not the mosfet
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u/nixiebunny 1d ago
Whenever a thing doesn’t work properly, please post a picture of that actual thing. We cannot see how you built it.
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u/Real-Entrepreneur-31 1d ago
Show the motor specs. It will give us better knowledge. R21 seems very useless if Rdson is bigger in the mosfet. You can use the mosfet for current sense.
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u/planeur31 1d ago
R21 is used as a shunt to measure the current to control the PWM duty cycle.
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u/Real-Entrepreneur-31 1d ago
Yeah just read the mosfet have 1 mohm Rdson so the shunt is necessary.
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u/Real-Entrepreneur-31 1d ago
A 1 Ohm gate resistor will take some heat from the gate driver. A fast switching diode in series with the gate resistor pointing towards the driver will also help.
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u/planeur31 1d ago
What would the diode be doing ?
How do you chose the value of the gate resistor ?1
u/Real-Entrepreneur-31 1d ago
Your gate driver have 2.5 Ohm output impedance. Anything that is enough to turn on the mosfet is enough. It depends on switching frequency, t_on and gaste charge. There is some calculators online for you but about 5-10 Ohm should be good for you.
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u/planeur31 1d ago
I'll try to find this. Thanks.
And regarding u/jacky4566 's comment, will the gate capacitance of 14,000pF be a problem with the driver's capability of 10,000 pF if I add the gate resistor ? Or these issues are not related ?
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u/Array2D 1d ago
The gate resistor will alleviate the issue. The problem with driving load capacitance directly is that it presents almost no impedance to the gate drivers output at the beginning of switching, so all the power dissipation happens in the driver ic itself.
A gate resistor moves that power dissipation outside the ic and spreads it out over time.
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u/jacky4566 1d ago
Bro... That mosfet has a gate capacitance of 14,000pF. The first page of the datasheet High Capacitive Load Drive Capability: 10,000 pF
The gate of an FET has a capacitance that you charge and discharge with each cycle. Your chosen IC are not compatablie like this.
Options:
- Add a gate resistor, usually the cheapest solution but adds slope and limits maximum frequency.
- Use a different mosfet, try IRFB7446PBF
- Use a more powerful driver