Author Topic: Transient positive voltage spikes killing MOSFET drivers (Read 26210 times)

MarkM · « **on:** July 21, 2015, 06:38:13 pm »

I'm switching two of these N channel MOSFETs in parallel with a TC4422a driver @ 60-100hz using an Atmega328 PWM pin on a custom PCB on battery power(2s-3s lipo pack). I started with the TC4427 driver and they died randomly. Now the TC4422a is dying randomly. My buddy used his scope and noticed something very strange. Hoping someone can shed some light on it. The load is resistive(30a for this test or so), but it has some inductive properties, as it's a coil of resistance wire. Flyback diode is in place for the load.

The issue I believe is the positive voltage spikes that are high enough to go over the max of the driver(18v), but only on the 12v rail from the battery. The usual spikes are around 20v and as high as 30v. The output from the driver to the gate looks great as well as the actual load itself. I attached pics so you could see them. The 12v input is the battery going from loaded to unloaded due to switching. It looks to me like the positive spike is where it goes from loaded to unloaded.

The 12v input from the batteries also powers a linear regulator that is working perfectly even while switching. The rest of the board is functioning as it should as well.

It would seem that a TVS diode would work, but my required clamping voltage(ideally around 15v) is too close to the max operating voltage. Is there any other solution? Would really like to figure out what's going on here.

Please let me know if you need more info.

MOSFETS:(2 in parallel)
http://www.digikey.com/product-detail/en/0/568-11557-6-ND

DRIVER:(0.1uf and 4.7uf bypass caps)
http://www.digikey.com/product-detail/en/TC4427ACOA713/TC4427ACOA713CT-ND/3622383

Simon · « **Reply #1 on:** July 21, 2015, 06:50:19 pm »

Sounds a bit odd. Try a zener at a couple of volts over the battery voltage. You could also try a nice bypass capacitor or a few in parallel, That might help deal with sudden load changes in the system

T3sl4co1l · « **Reply #2 on:** July 21, 2015, 06:52:45 pm »

Two problems:

1. Add series gate resistors. This controls the risetime at turnoff, which limits how much trouble you get into with inductances (increasing switching loss instead). At 100Hz, you probably don't need a proper gate driver at all; a level translator would suffice (e.g., just a few BJTs to switch 12V instead of 5 -- or use logic level FETs?). That is to say, by the time you're looking at 1kohm gate resistors, your peak gate current can only ever be 10mA or so, and a freaking several-ampere driver becomes absurd.

That TC4xxx thingy can drive gates in the tens of nanoseconds. That's over a million times faster than your waveform! It's a wonder you saw a spike at all on that scope shot -- the transients might be < 0.001 of a pixel wide, so it's very lucky to see them at all.

The random appearance or absence of spikes is a consequence of aliasing on the scope -- quite simply, most of the time, it doesn't happen to sample the input voltage at the exact moment the peak is its tallest. To correctly resolve them, you must zoom in on the rising edge, as tightly as possible, probably with a timebase well below 1us/div. Then you will see all the spikes and ringing in their full, horrendous glory.

2. You don't show the complete clamp circuit.
(2a. Good on you to put in a clamp circuit at all!)

With an even slightly inductive load, the instant the transistor(s) turn off, drain voltage rises, and keeps on rising (past whatever +V is on the far side of the load). The diode must be placed as close to the transistor as possible, so it can do its job as soon as possible. But a diode alone doesn't solve anything; +V needs to be stable, too. So you also need bypass caps, as close as possible to the diode and transistor. For something in the power switching range, you might be looking at an electrolytic >= 100uF.

And one possible gotcha:
3. TC4xxx are only rated to 18V, so any supply spikes are a hazard. I recommend a TVS (at least P6KE or SMAJ size, unidirectional, 12 or 15V nominal rating) and good filtering (you have 4.7uF among others, so that's not too bad; some electrolytic bulk might be handy, but may not be necessary).

Note that, when a transistor dies, it typically goes three-way-shorted, which means drain voltage getting to the gate. Which usually destroys the driver. If it's not dying, the overshoot and spikes, and ground bounce, and stuff, can cause problems, not just to the drivers but the rest of the circuit. Layout is a concern; ground loop; etc.

Tim

MarkM · « **Reply #3 on:** July 21, 2015, 07:03:38 pm »

Hi, thanks for the replies. My buddy did add an extra bypass cap and it seems to help supress the spike a bit, but just one of those spikes above 18v and the driver is destroyed. Would the zener be fast enough?

I don't have any type of clamp on it now. It's a wonder the board has been working as long as it has been.

It is a very low frequency and I really appreciate that you think the driver isn't needed, as I didn't think so either. My original design used a simple current limiting resistor from the MCU pwm pin to the gate of a MOSFET, but I switched to a 3.3v system from 5v and Rds(on) is just too high. My original thought was to use some kind of N channel -> P channel MOSFET array as a buffer with current limiting resistors on the PWM pin from the MCU and the gate pin on the MOSFETs. I found one earlier this morning.

http://www.digikey.com/product-detail/en/SI4559ADY-T1-E3/SI4559ADY-T1-E3CT-ND/1656955

This will definitely handle the spikes.

Do you think a series gate resistor will help using the TC4xxx driver now? It's probably worth a shot.

Simon · « **Reply #4 on:** July 21, 2015, 07:06:54 pm »

putting a resistor in series with the gate as Tim suggested will probably help a lot. You can try a proper TVS but as you have pointed out the manoeuvring room may be limited.

PSR B1257 · « **Reply #5 on:** July 21, 2015, 07:07:32 pm »

Quote

With an even slightly inductive load,

And/or layout. Depending how inductive* the GND-loop is and how close the driver is to the MOSFETs it leads to more or less trouble.

*) At high frequencies (i.e. fast transients) the ohmic component of a wire link becomes less and less dominant over the inductive component.

Simon · « **Reply #6 on:** July 21, 2015, 07:16:23 pm »

Quote from: PSR B1257 on July 21, 2015, 07:07:32 pm

*) At high frequencies (i.e. fast transients) the ohmic component of a wire link becomes less and less dominant over the inductive component.

As I found when I made a basic switchmode supply of my own, it was so bad the mosfet oscilated and was less efficient than a linear regulator...

Zero999 · « **Reply #7 on:** July 21, 2015, 08:08:17 pm »

Quote from: T3sl4co1l on July 21, 2015, 06:52:45 pm

Two problems:

1. Add series gate resistors. This controls the risetime at turnoff, which limits how much trouble you get into with inductances (increasing switching loss instead). At 100Hz, you probably don't need a proper gate driver at all; a level translator would suffice (e.g., just a few BJTs to switch 12V instead of 5 -- or use logic level FETs?). That is to say, by the time you're looking at 1kohm gate resistors, your peak gate current can only ever be 10mA or so, and a freaking several-ampere driver becomes absurd.

That TC4xxx thingy can drive gates in the tens of nanoseconds. That's over a million times faster than your waveform! It's a wonder you saw a spike at all on that scope shot -- the transients might be < 0.001 of a pixel wide, so it's very lucky to see them at all.
Tim

I agree. At such low frequencies, the old 4504 could be used to drive the MOSFETs directly.
http://www.onsemi.com/pub_link/Collateral/MC14504B-D.PDF

MarkM · « **Reply #8 on:** July 21, 2015, 08:12:27 pm »

My buddy added a 1k series gate resistor and the spikes only go to about battery voltage + 1v! Not bad at all! He's also using one 0.1uf and 2x 10uf in parallel for bypass caps close to the driver now. I still wouldn't be comfortable using it like this without some sort of clamp, though.

The other option is using that MOSFET array I linked to instead of the driver. It wouldn't need to be clamped. Any thoughts?

I'd rather not use a large electrolytic cap if I can avoid it, but a stable system would make me happy.

EDIT:I should've said that I'm really trying to get Rds(on) as low as possible, hence the reason for battery voltage at the gates.

krish2487 · « **Reply #9 on:** July 21, 2015, 09:00:43 pm »

Please dont such random values like 1K.
A gate drive design is an interesting topic in and of itself.

Long story short - The series gate resistance, the input gate capacitance and the driver supply voltage decide (for oversimplification purposes) decide how fast the FET turns on, turns off and what the rise time slope will be like.

If you are only operating in the 100 Hz range, then it is a lesser concern but a concern nevertheless. The risetime of the gate decides how long the FET behaves as a resistor - the shorter the time in this region the less power the FET dissipates as wasted power. (Again gross oversimplification but makes the point). The FETs hardly dissipate any power during on time (very low RDSon) and during off time (no drain current except leakage current). Hence any wasted power in the form of heat is during the high-low and low-high transition.

Decide how much time you would like the transistor to spend in the transition stage (generally 2-5 % of the period is a good point to start), appropriately size the resistor to charge the input gate capacitance in the calculated time and choose the nearest value.

Secondly, Were the measurements taken in 1x or 1:10 attenuation? For measurements like this the probe should always be in 1:10 mode.
The scope probe capacitance and the long, flying crocodile clip of GND are good enough reasons to cause the FETs to blow up. The probe is directly parallel to the gate - source terminals -> It is a load for the driver.

MarkM · « **Reply #10 on:** July 21, 2015, 09:10:09 pm »

The 1k value would limit the current at 12v to 12ma. The value will end up being an issue, as this board is designed to work with 7v-12.6v input, but it looks great at 12ma. Here is what the output looks like with a 1k resistor. Even with the right value resistor, the real issue to me is still clamping the driver VDD to 15v or so. Not sure about the best way to do that.

I'm not sure what he had the probe set to. I'll ask him.

krish2487 · « **Reply #11 on:** July 21, 2015, 09:22:19 pm »

Hmm, Actually the reason that a spike occurs on the drain of the FET is during the off time, for the reasons you mentioned.

The inductive load present will not allow the electric field to collapse instantly when the drive is removed. Hence the voltage build up on the drain
aka Spike. The faster you try to turn off the device, the larger the spike on the drain. The 1k resistance is proof. The 12V, 1K, 4nF capacitance charged and discharged the gate at such a slow rate that the inductive spike is heavily damped.

Now, it also means that we can now use a seperate resistance - diode combination for turn on and turn off.
Something like this

-----|>---/\/\/\/----
-------- ------------
-----<|---/\/\/\/----

in the place of the 1K. The top branch is for turn-on and the bottom for the turn-off.
You can start with 22E for the turn on and 47/100E for the turn off. Then add a snubber capacitor directly across the Drain Source of the mosfet, as close as possible. Say something like a 1uF or a 2.2uF. That should damp the spikes considerably.

MarkM · « **Reply #12 on:** July 21, 2015, 09:36:06 pm »

I have a flyback diode in parallel with the load. There's no spikes on the output or between the driver output and MOSFET gates even without the 1k resistor(which is odd, because the driver isn't clamped). The problem is the battery voltage going into the driver. The issue is the positive spikes, I believe. VDD was seeing spikes of up to +30V and killing them. I'm also using two of these MOSFETs in parallel.

Here's a pic of what the battery voltage going to the driver VDD looks like now. I should've posted it before. If you compare it to the last one, there's a massive difference.

krish2487 · « **Reply #13 on:** July 21, 2015, 09:48:27 pm »

I apologize, I misread that you were blowing up the FETs themselves.

Try decoupling the Battery using a diode in series between the battery and the driver VDD pin. Any low forward voltage drop diode should do.

Then after the diode use a ferrite bead in series and then a tank cap (10 uF Electrolytic) || filter cap (0.1uF MLC) across the supply pins, as close as possible. That should clamp the voltage appearing across the driver. Adjust the filter caps as required 0.1/0.2/0.47 etc.

PS : The spikes never occur in between the driver output and the gate. And yes, use the series gate resistor between the driver and the FET. 10/22/47 E preferably.

MarkM · « **Reply #14 on:** July 21, 2015, 09:59:59 pm »

Thanks. I'm still new to electronics, so I might not be very clear on things sometimes.

What does the diode and ferrite bead do? I was hoping for some kind of solution like a TVS diode, but there's not one that exists that meets my requirements. Someone said you can use a zener diode to supress the spikes as well.

krish2487 · « **Reply #15 on:** July 21, 2015, 10:07:17 pm »

The diode "decouples" the battery from the driver. I.E it still supplies the voltage and current required but any noise on the driver is not "loaded" onto the battery and vice versa (to a certain extent).

The ferrite bead smoothens out any such noise and ripples on the supply. Similar to a inductor but a very low value inductor.

Actually a TVS is a zener meant for power applications. And no, a zener is a wrong solution.

T3sl4co1l · « **Reply #16 on:** July 21, 2015, 10:14:19 pm »

A TVS is a perfectly legitimate way to clamp spikes; the question is whether it's best for the task (and given other constraints, like cost or size).

If you already have a well-bypassed +V rail on your circuit, then a clamp diode is far easier, cheaper and more reliable. If you don't, a TVS may well be the better option.

Such is the case with my electronic fuse:
http://seventransistorlabs.com/Fuse/
A TVS absorbs spikes from even the most violent disconnect events: peak currents up to 110A, for microseconds at a time. With an undefined load voltage, the clamp is simply chosen to be somewhat above the nominal rating, so it doesn't interfere with normal operation but protects the transistor even in fault conditions.

Tim

krish2487 · « **Reply #17 on:** July 21, 2015, 10:16:06 pm »

Duh moments!!!

TVS / Zener is a wrong solution for this case.
Not as a generalization.

MarkM · « **Reply #18 on:** July 21, 2015, 10:30:40 pm »

@Tom

What would be considered as a well bypassed voltage rail? There's a 10uf MLCC on the input for the 3.3v regulator and we tested with these MLCC caps on VDD of the driver:

0.1uf + 4.7uf
0.1uf + 10uf
0.1uf + 2 10uf in parallel

Is the clamp diode what krish2487 is talking about?

@krish2487 Thanks. I'll keep that in mind. Cool stuff.

PSR B1257 · « **Reply #19 on:** July 22, 2015, 04:35:52 pm »

Quote

What would be considered as a well bypassed voltage rail?

First of all, you need a proper layout, as I mentioned.
If you have an inappropriate layout, which is causing voltage spikes, it is more or less meaningless to try to clamp these spikes.
It is in general not a good idea to medicate (so to speak) the effect, rather than the reason.

So, what does your layout look like?

However this

Quote

He's also using one 0.1uf and 2x 10uf in parallel for bypass caps close to the driver now.

is how it should be done. But if the return path to the battery and/or the connection to the FETs are inappropriate, it dosen't help a lot.

Regarding the oscilloscope:
To verify the amplitude spikes you should set the trigger above the supply voltage, set it to a fast timebase (i.e. high sampling rate) and have it in single shot mode.

MarkM · « **Reply #20 on:** July 22, 2015, 08:46:39 pm »

I absolutely agree with you. I would much rather have a properly designed board. The design that's having problems is most likely the layout, but I didn't think about that before. I've designed some very basic PCB's before without issues, but as you'll see these have issues. I'd actually rather have someone that knows what they're doing design the PCB if you know anyone. Would rather have it done right the first time instead of these issues. If you could give me some tips though, that would be very much appreciated as well. I have the firmware 99% done for the project and it usually works great until the driver dies. Works for up to a week sometimes.

First off, the huge pads are all stitched, but you can't see in this 3d render. You can see there is a perfect place to put the driver between the battey + and - pads in the old design. That was my beginner mistake and being in a rush, as I widened the space between them to move the 3.3v regulator on the bottom of the board when I could've also moved the driver closer to the FETs. I also didn't even think about the driver return path. The gate trace is also way too long and could've been shortened in the old design. Weird thing is there's no ringing or any other issues at the gates. Maybe that's due to the low frequency?

The new design is one that I sent off 4-5 days ago and is untested. I switched to the tag connect footprint instead of the ICSP header, switched to another driver(which wasn't needed) , added pads on the board for SMD switches, switched some passive components to 0603(also wasn't needed) and moved the driver closer to the FETs. The new design is untested.

Thanks for your time.

MarkM · « **Reply #21 on:** July 22, 2015, 09:25:31 pm »

I've been searching a bit more, and I'm starting to think the inductance from the battery wires could be the issue or at least add to it. The spike is random though, so that doesn't make much sense. I attached a pic from another driver datasheet.

My battery wires are 14g and about 5 inches long total.

nuno · « **Reply #22 on:** July 22, 2015, 10:48:48 pm »

Do you have a schematic?

krish2487 · « **Reply #23 on:** July 23, 2015, 12:06:15 am »

Cable inductances as you mentioned are mostly an issue with a switching power supply. With a battery, it is not really a issue.
Besides, proper LC filtering and decoupling near the Driver IC is more than sufficient to offset and stray transmission inductances.

[size=0px]'ve been searching a bit more, and I'm starting to think the inductance from the battery wires could be the issue or at least add to it. The spike is random though, so that doesn't make much sense. I attached a pic from another driver datasheet. [/size][/size][size=0px]My battery wires are 14g and about 5 inches long total.[/size]

PSR B1257 · « **Reply #24 on:** July 23, 2015, 03:22:06 pm »

Well, the layout without all designators is a bit odd...

Quote

The spike is random though, so that doesn't make much sense.

What doesn't make much sense? My advise regarding the oscilloscope? It makes perfect sence, BECAUSE it is (at lest appears to be) random. As already mentionend by T3sl4co1l, the oscilloscope could give you absolutely wrong readings (if it runs in continiuos mode).


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Author Topic: Transient positive voltage spikes killing MOSFET drivers (Read 26210 times)

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