Hello,
I am working on a transmitter board, Which uses MOSFET for HV output. I am using CPLD and gate driver MD1213 to drive 100V Mosfet. I am seeing output at coupling capacitor placed between Mosfet and gate driver but My Mosfets are not opening.I think the problem is at the Zener diode that I placed between gate and Source.
Can you someone please give me some feedback regarding this design?
Thank you!
Hey @naik, interesting problem here! What are the voltage rails set at? (vpp2 and vnn2)
Also, what is the reverse voltage of the zener supposed to be? do you have a part number?
Hi @ChrisGammell, Thank you for your quick reply.
VDD2 = 10V
Vpp2 = +100V
Vnn2 = -100V
Zener volatage = 15V
Mosfet part no: (Part no is from digikey)
P type = FQD7P20TMCT-ND
N type = FDD7N20TMCT-ND
Zener Diode = SMAZ15-FDICT-ND
Thank you!
So normally I expect to see people driving the gates of a FET directly with DC current. In your case it looks like you have the capacitor there to block the higher voltage. Is that correct? If not, could you explain the caps?
Also, do you have any scope traces at various points in the circuit? Have you measured it while operating?
That is correct, Those caps are just a coupling caps between driver IC and gate of the MOSFET.
I have tested circuit at various point. Until C17 and C18 Everythings works as excepted. I think the problem is with the zener diode (D5, D6) and resister(R12, R13) that I placed between gate and source terminal as a protection to the MOSFET.
Could you share where you borrowed this circuit from?
I expect if you have caps in line with the driver line, you will only see the impulses passed through to the gates of the FETs.
I got it from the MD1213 datasheet, Please have a look at page no 3.
http://ww1.microchip.com/downloads/en/DeviceDoc/20005713A.pdf
I have replaced TC6320 with different MOSFET.
Ah ha, cool. So you’re trying to drive high voltage, high frequency signals out through the mosfet. That sounds good. What is the frequency of the signal? And what’s happening on the other side of those caps?
As far as the zener, I believe those are the built in ones. You see even on your diagram there is a zener in the FETs diagram. Have you tried it with the original part? What made you decide to switch it out?
yes, I am using Mosfet to drive high voltage. The frequency is variable at the moment, But I’ve program CPLD to generate the frequency of 500KHz(for testing).
TC6320 has built-in zener diode but I am using individual Mosfet. I haven’t tried that exact circuit. The reason is I am aiming to drive transducer( which is complex RC load, it can go higher up to 50 Ohm) and My driving voltages are +/-100V which requires 4A current and TC6320 don’t have that capacity.
So I am wondering if that zener you have included is necessary.
Normally I wouldn’t recommend people jump to simulation right away but with higher voltages that is my default. Have you tried simulating? Doesn’t need to be the exact parts you’ve included, but it could be useful to match your physical model.
I think it is necessary to protect Mosfet from input transient.
http://www.semtech.com/images/datasheet/transient_voltage_protection_of_mosfets.pdf
(But I haven’t done this before so looking for any suggestion/advice that might help to troubleshoot)
I haven’t simulated the design yet, was trying to avoid it, But I think now I have to do it.
Your resistors are too small, try something in the range 50k - 100k.
How about some feedback as to where you are at with this? The zeners are unlikely to be a problem, if you still have issues after increasing the resistors then I would suspect the capacitors. You haven’t mentioned what capacitors you are using but if they are regular ceramic caps then that could be a problem. I would use some good polypropylene film caps.
Whatever you do, do not bypass the caps and drive the fets directly.
Hi @1.21Gigawatts, I am waiting for some resistors in 50k -100k range. Don’t have them in stock. I should be able to test them by end of the day.
The caps are ceramic at the moment but I will try to change them to polypropylene film caps.
Thanks for the suggestion.
Thanks for the update, I look forward to hearing how it turns out.
By the way, make sure those caps have a voltage rating significantly higher than 100V, such as 150 - 200V.
Hi,
I’ve looked at this a couple times and I’ve come to believe the problem is not your diodes but rather the values of R12 and R13 are too low. Since the gate of a MOSFET is voltage controlled and not current, I believe the are essentially shorting away any voltage passed though C17 and C18. I would try increasing the value of R12 & R13 to at least 1 Meg and perhaps even higher.
Hello,
I’ve tried to change that resistor with 97K. One more thing, Yes, those resistors ended up shorting my gate and source pin, I will change those resistors with 2M, Thanks!
You’re wasting your time. 2M or even 1M is a ridiculous value. I told you what to do if the resistors alone did not fix the problem.
Any progress with your driver?
Not yet, But I did a quick simulation and from it looks like,
A. I am driving those MOSFET with the 2MHz sampling frequency, Somehow Those mosfet requires lower gate driving frequency, Reducing it to 1.25MHz does the job.
B. 90K resistor is fine, doesn’t look like to increase its value further.
Haven’t got a chance to implement this change on actual board.
Thanks!
