Linear Mode 1000w Discharger prototype keeps popping fets

kubeek

Joined Sep 20, 2005
5,587
I added a .1uf and 10uf capacitor on the gate of the FET
why would you try to slow down the fet even more?:confused: What purpose does increasing the gate capacitance serve? Are you sure you read the original circuit correctly?
 

Thread Starter

magudaman

Joined Feb 27, 2012
31
I agree. The gate resistor and capacitor add a pole in the loop, in addition the the op amp's pole. This is why it was unstable. Adding the 100nF cap pushes the external pole to a lower frequency, but probably not far enough to overwhelm the internal pole.
Try changing the gate resistor to 100Ω, and the resistor in the feedback path to 10k. Add 100pF from op amp output to inverting input.

EDIT: I didn't make it clear that you need to omit the cap from gate to GND.
Ron your amazing your setup worked. I did add a variable to the mix though since I used a different FET and it is logic level. I dropped the circuit's input to 5V instead of 10V. I really don't think that will make a difference once my new fets show up. Your setup created the first FET to survive the full 60 volts at 1.3 amps out of the 5 or 6 I had tried. This particular fet wasn't rated as well so I didn't want to push it to the full 1.6 amps but it made it past where I have been so far! Thanks for modeling that out and figuring this out whole thing out.

Thanks everyone for all the input.

Maybe I can start an actual build thread for my full device.
 

Thread Starter

magudaman

Joined Feb 27, 2012
31
why would you try to slow down the fet even more?:confused: What purpose does increasing the gate capacitance serve? Are you sure you read the original circuit correctly?
Meant to do this all in one message. I am novice to electronics and was trying to model after a product that I knew worked. I now understand how the capacitors can slow these loops down but didn't know that going into this.
 

Thread Starter

magudaman

Joined Feb 27, 2012
31
So I decided not to go the CNC route and instead tried for the first time making a board from scratch with the Toner Transfer method. Considering its my first board it turned out pretty good. In fact I made all the traces way too small for this method but they all ended up working. This is 2oz board. It was a struggle to solder up correctly since there is no silk screen and everything is extremely tight, I ended up with a bunch of bridges I had to find and clean up.






There was only one mistake that reared itself today and it turns out I had it wrong in my schematic. It was an easy fix on the prototyped board and my next batch it will already be fixed. I also got and attached the new 200v FET to my circuit and by golly it works beautifully!

Here is a little video of it at 100w at around 60V:

http://www.youtube.com/watch?v=Y4-440Fybew&feature=youtu.be

So now I should be able to make my 10 boards and order all of the components from digikey.
 

Ron H

Joined Apr 14, 2005
7,012
Ron your amazing your setup worked. I did add a variable to the mix though since I used a different FET and it is logic level. I dropped the circuit's input to 5V instead of 10V. I really don't think that will make a difference once my new fets show up. Your setup created the first FET to survive the full 60 volts at 1.3 amps out of the 5 or 6 I had tried. This particular fet wasn't rated as well so I didn't want to push it to the full 1.6 amps but it made it past where I have been so far! Thanks for modeling that out and figuring this out whole thing out.

Thanks everyone for all the input.

Maybe I can start an actual build thread for my full device.
Excellent! The 100pF cap, BTW, actually "speeds up" the loop, in that it provides a high frequency path that bypasses the lag caused by the MOSFET gate capacitance
 

Thread Starter

magudaman

Joined Feb 27, 2012
31
I know this resurrects this thread but I again just wanted to say thank you to everyone who helped me out on this thread. While the final discharger hardware had been done for a little while I have finally got it in a presentable state.

I built 10 more of those boards shown above and got them mounted to the heat sink:





After than I worked on bread boarding the controller board and a fan PWM filter which was a very basic buck switcher board. The switch board on the left and the picaxe controller on the right:



I put all my stuff into autocad and then spent way too much time cutting a case out of acrylic on a laser cutter. Handles 1300w, fans only run when needed and are heat dependent speed, 300a max, 100v max, Here it is all completed:







So again guys thank you so much for the help, made my project happen!
 

#12

Joined Nov 30, 2010
18,076
This site has a, "Completed Projects" section. If you want to go through the work of documenting all this, you can post it there.
 

THE_RB

Joined Feb 11, 2008
5,430
Thank you for taking the time to show people the finished project!

I wish everyone who starts a thread asking for project help was as courteous as yourself and showed their final result. :D

It's also good to see a serious heatsink and fans, that always separates the men from the boys. ;)

Now you just need to turn those 2 fans over so they extract air from the heatsink instead of trying to push air in, you will get a much higher airflow and improved air efficiency as you will eliminate cavitation caused by the fans trying to blow air into a higher pressure (resistance).
 

#12

Joined Nov 30, 2010
18,076
Just a note on air direction: If you were using a heatsink with tall, closely spaced fins, (which you aren't) the velocity of the air impacting the fins would be important. (That means, "blow toward the heatsink"). People often steal an MCU heatsink and natural convection or mass drift doesn't work well with them.

ps, thanks for the excellent pics.
 
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