Just had a thought! If I am running PWM at 10,000 or 20,000 should I be able to actually see an incandescant lamp flickering? I don't just mean a corner of the eye flicker either, its a fully visible flicker. Could my pic or program be running too slow?

Al

 

That thought had crossed my mind, you should not see flicker, you don't see any on 50/60Hz.!!
The pic clock should not be an issue, even if the update time is relatively slow, the out put should be continuous.
Max.

 

Now that I think on it the LED that I have connected directly to the PWM output pin which feeds the FET driver IC does NOT flicker at all. Maybe I am missing something with the driver or something around it? But what?

Al

Am I right with - the IC I am driving the FET with is made for that purpose only so I assumed that it should be directly connected through a 10r resistor to the FET gate, and since I have it provided with its required voltage (12v) and I am using a 10r resistor, what else can be wrong?

Al

 

How do you have it mocked up? bread board or perf board? or?
Max.

 

Hi Max, Its on VeroBoard as most of my stuff is unless I need small or a lot of them.

Al

 

I am just going to join an LED to the output of the driver and see if that flickers.

Al

Not a flicker from the LED between driver output and GND just smoothe ramp. Baffling.
Al

 

Another issue may be the ground plane, you really have to make sure the 300v DC although common to the PIC and driver is sourced direct to the input of the Mosfet Source and Drain.
IOW keep it at one end of the circuit.
I have a qty of IRFP460's if you want to try a couple, gratis.
If these blow then there is more to it.
They are also easily paralleled.
Max.

 

Hi Max, Thanks for the offer mate, the last FET I blew was 600v 20A and the IGBT before that was 80A 600v. Its a pity you can't series FETs. I have loads of the sub 200v ones. I just don't know what the hell is going on. Just wish I still had my scope. Just so hard trying to do something like this without one.

Things look grim for my other projects, the Interosseter and the matter transporter. lol

Is there a sure fire (safe for the FET) way of testing the motor with the FET that I can try?
Al

 

There is a few ways such as a 555 PWM generator to drive the gate direct.
Max.

 

If the lamp has flicker, then it indicates a problem. Fix this first!
Are you sure both the inputs on the MAX627 are connected and one is not floating? You may not need to run both drivers in parallel but if you do only use one, make sure the input on the other driver is grounded and not left floating.
Good bypassing is a must, and also your layout. If there is a path for high currents that can cause induces voltages you may be making an "oscillator".
There needs to be only one path for the return current from the motor to flow, and it needs to be connected directly to the IGBT "E" and not allow any motor current to flow anywhere near the MAX627 of the PIC. If induced voltage spikes can get to the control and driver circuits you will have problems.
Short heavy wires will help but do carefully check your layout.

 

Hi Max, I have been trying to put a 555 circuit together all day but been babysitting too, somehow the two don't seem compatible. lol, Always tomorrow.

Al

dendad, Both inputs and outputs are connected and paralleled as was indicated in the sheet so should be ok.

As to the return from the motor. Are you saying I should NOT have the return connected to the other grounds in the circuit? Wouldn't that mean the gate voltage would have no reference? Am I misunderstanding this?

Al

 

They have to be common if no method of isolation is used, but the common for the motor voltage must be kept physically separate as possible from the logic common, although they are not electrically separate, if that makes sense.
I have used a circuit such as http://www.discovercircuits.com/DJ-Circuits/simplepwm2.htm.
Except I used 15v for the 555 and used a resistor in the gate of the Mosfet.
This is the basis for the $5.00 ones you get on ebay.
Max.

 

The ground wiring must be setup so that no motor current flows through the grounds that connect the controller to the FET. What can happen is there can be a voltage drop across the connections that adds to the gate voltage and turn the FET on but not under the micro's control.

Here is quick drawing of what I mean.

I hope this helps.

 

Success! I built the small 555 circuit and powered it directly from my supply to the FET Driver (removed and pic removed too) then put its output to the FET gate and it works perfectly!

Next I will try its output to the driver and see what happens. Hopefully I can rule the driver out as a problem which means it can only be the pic or my programming at fault.

Watch this space for more news. And thanks for all the valuable help.

Al

 

Doh! Another FET bites the dust.

All I did was swap the PWM from the drivers output(the gate through a resistor) to the input of the driver IC and POP!

This is just too weird for me to understand. I am using the "proper" part that is designed to drive a FET and it kills them every time but a six component board suggested by Max works flawlessly. Baffled.

Al

 

It is obviously something to do with the driver IC, may be a time for a re-think and use a different method.
Looking at the common denominator the MAX627, I am wondering if that voltage is on the low side at 12v, I would have tended to use at Least 15v, or even 20vdc to supply this IC, the gate voltage may be on the low side once it exits the MAX627.
Max.

 

Hi Max, Funnily enough at first I had 20v supplying the IC and after the first few blow ups I swapped the transformer for a 12v one and stabilized it. As I understood the D/S I thought the output was very close to the supply level. Wish I had my scope!

Edit. Just checked and it says within milivolts of the suppl rail.

Is it worth making a driver with a normal transistor instead of using a dedicated one? Any advice welcome.

Al

 

I have used the one in fig1 in the past, this is from an engineer from TI.
I used the 2n2222 & 2n2907 recognized as the best pair.
If trying it, I would do a few tests without the Mosfet first!
http://www.eetimes.com/author.asp?doc_id=1279277

http://tahmidmc.blogspot.ca/2012/12/low-side-mosfet-drive-circuits-and_23.html
Max.

 

Hi Max, I got the discrete driver made and it worked perfectly with the 555 PWM, smoothe and slick. Then I unplugged the 555 and replaced the pic hoping beyond reason that it would work but no! With the pic running it its back to flickering again, it adjusts as always but flickers with the lamp load. I daren't try the motor, but at least the lamp never blows the FETs.

I have to rethink the pic prog but its hard when it works ok when in the pic dev board with LEDs as output.

Al

 

That sounds like your PIC is going into reset. Can you program the pic to light up a led for a second right after it boots up, and then start doing the pwm?