Eliminating Optocoupler Turn-On Spike

[PDDD]

Hi guys, I'm new to this site.

I'm using optocouplers as gate drives of power mosfets in an H-Bridge. When power is applied to the optocouplers they put out a simultaneous pulse to all the mosfet gates, and POOF - even though power to the drains are off. I would sure like to know why this happens and also if a low pass filter in series with the optocoupler output and the mosfet gate will solve my problem.

Thanks for your response.

 

[MaxHeadRoom78]

Can you show your schematic?
Max.

 

[ronsimpson]

if a low pass filter in series with the optocoupler output and the mosfet gate will solve my problem.

NO NO
Yes it will probably help your problem BUT the MOSFETs will turn on/off slow causing heat and "POOF".

 

[PDDD]

Thanks guys. There's not much to show if I simplify the question. Single N-Channel Mosfet with gate resistor to ground, power on the drain, source tied to ground, and square wave signal coming from the optocoupler output to the gate. So my question is how to prevent the optocoupler turn-on spike from reaching the Mosfet gate without compromising the performance of the mosfet.

 

[ronsimpson]

Can you show your schematic?

No I can not!

optocoupler

What optocoupler? There are many many different kinds! Some should not be used in power supplies.
I know you are a new member......but if you can't give information then you need to see a mind reader.

 

[tomizett]

I'm having difficulty visualising your circuit too, I'm afraid. If you could draw up a rough schematic it would really help.

In addition, could you clarify:
1) What sort of switching frequencies are you working at? Is this an SMPS aplication, or something low-frequency like reversing a motor?
2) What's the part number of the optocoupler, and how is it driven.
3)

even though power to the drains are off

Does this mean that the MOSFETs are failing before you've even applied power to the bridge? In that case, you must be over-driving the gates.

Cheers!

 

[PDDD]

Hi Tom,
Thank you for your kind reply! I don't know why the other guys have such an attitude...
The optocouplers are AVAGO HCPL-J312-000E. The output pin goes straight to the gate of the N-Channel Mosfet. When the opto is turned on, it sends a 5uS pulse to the Gate. The voltage does not exceed the Mosfet specs and the current is limited by a 1K resistor to ground. No power on the drains. Source to ground. But when both sides of the bridge receive this spike at the same time, it appears to cause the Mosfets to fail, even though no drain voltage present.
I realize I haven't uploaded a schematic yet, but its just a straight line between the opto output and the FET gate. The issue doesn't involve switching frequency, unless you're thinking to suggest a low pass filter. In that case, my switching frequency is in the Hz range while the nuisance spike would presumably be in the KHz range.
I appreciate your interest in helping.
Cheers to you also!

 

[MikeMl]

What is the source of the of the signals that drive the opto?

Get off your dime and post a schematic...

 

[alec_t]

a 1K resistor to ground

If you mean between the gate and ground, then that won't limit the gate current. As requested above, if you post a schematic we'll have a better chance of solving your problem.

 

[PDDD]

Hi Alec,
Thanks for your kind reply. That's two for two from across the pond. It seems North America thinks too highly of themselves.
Yes, I was referring to the gate pull down resistor which passes current from the opto output, since mosfet gates do not pass current.
Here's the schematic as requested. As I said, the only issue is a pulse on the opto output when it is powered. Would prefer no output pulse without an input pulse...

 

[ronsimpson]

Thank you for the PDF!
Need more information.
Is Vin really 0V, or just at power up?
Where does Vout go to?
What voltage do you expect at Vout?
I think you have a H bridge. Are there 4 of these?

I think you are not using the gate drive opto-isolator right but I need more information.

It usually takes 10 to 15 posts to get a schematic and then at post #50 we find out something important.

 

[ronsimpson]

Here is a H configuration.
I do not have the IC you are using, loaded in to my computer so I am using one that is different.
Pin-5 must go to the MOSFET-S!
Pin-6 goes to MOSFET-G.

 

[PDDD]

Ron, Thank you for your post. This seems like a good way to control the H-Bridge, but would you help me understand how this keeps all four Mosfets from turning on simultaneously when the opto fires its output pulse during turn-on? Also, what type of capacitor is your C3 symbol, and is there no need for a C4 on OK1? Also, I see how this works with Pin 5 of OK2 going to the Mosfet -S, but can you help me understand why it must? Couldn't it go straight to ground as it does on OK1?
Thanks again, Ron.

 

[ronsimpson]

I do H bridges at 300 to 400 volts. Some people put the "top" gate driver on ground. They don't understand that the gate and source MUST be with in +/-20 volts. With the top MOSFET open and the bottom MOSFET closed. The top gate is at 0 volts. When it is time to have the top MOSFET on the driver lifts to 12V or 20V. This turns on the top MOSFET. Now the gate is at 20V and the source is at 300V. MOSFET and Driver are dead. Typical error.

I did not read the data sheet that will.......
I can't believe any company would make a gate driver that pulls high at power up.
I would remove the MOSFET and try it driving "nothing".
30 volt supply is all the opto-isolator is good for. Most MOSFETs can only take +/-20V on the gate. (G to S) So the 30V might kill the MOSFET.
Next this is a IGBT driver not a MOSFET driver but that is probably OK.
How long is the pulse?

I can't find it in the data sheet but most gate drives "power up low".
----edited----
Attached is a bridge driver, is old but good. (no isolation) I use it at 400 volts many times. A quick read is good.

 

[PDDD]

Thank you, Ron! Now I see why you have so many likes! Actually, I'm glad you mentioned 300 to 400 Volt H-bridges. Are you using enhancement mode MOSFETS or something else? I wanted to run higher VDD, but I can never get more than the Gate voltage through the MOSFET!

The opto gate driver spikes to VDD for 5 to 15uS at power up and power down, in the same way a monostable multivibrator does if the reset is not held low during power up. So turning off the opto causes the same problem as turning it on.

According to the data sheet, the opto is ideally suited for IGBTs and MOSFETs. It has a Vgs rating of 30V. I had some relays that were staging power to various parts of the circuit and thought the spike was coming from the relay coils, but as I said, it repeated when the opto was alone on the breadboard.

How do I "like" your posts on this forum?

Thanks again.

 

[KeepItSimpleStupid]

See the icons under "quote" at the bottom right of a post? You have about 7 choices.

 

[PDDD]

Great! Did that. Sure hope you will tell me how you get more than 20V through the MOSFET...

 

[tomizett]

I agree with Ron in #14: 30V is a lot to put on the gate of a MOSFET, it's right on the edge of the device's rating. The MOSFET R(on) is rated at 10V G-S, so a supply of 15-20V should be plenty. Surely this is the only way that you could damage the FET with no drain voltage applied?

I also

can't believe any company would make a gate driver that pulls high at power up.

... it seems like something else might be wrong here.

 

[ronsimpson]

Before I suggest a change; how fast (often) do you switch the MOSFETs?
I switch at 200khz. If you are just reversing a motor then we can do it another way. (switching every second?)

 

[PDDD]

Tom, Thanks for joining in. I need to pass 24V through the Mosfet. In my experience, I can only pass whatever voltage is at the Gate. So if I were to put 15V at the gate I would expect to only get about 13V through the Mosfet. How do I go about passing 300V at the drain with a 20Vgs FET?

Ron, Right now I have an 8mS pulse that alternates through the H-Bridge. So, if I'm right, I'm swithing the MOSFETS: 1/.008 = 125Hz?