Hey guys, im using a mosfet to power a 150Watt load at 24V.

This is the mosfet in question.

Im using a full wave rectified 25V supply in conjunction with a voltage divider to drive the gate and the load.
The resistances of the divider are 4.7K and 2.2K - Input V is 34V (peak) and the output is held around 10V with a 220uF cap.
25 * root 2 = 35 roughly around 34 with diode drop
34V * (2.2K /( 2.2K + 4.7K)) = 10.84V

The gate voltage is beyond the level required to pass 40A through the drain.
The load takes no more than 7 A(steady state) and even if the inrush from the cold load is ten times this - the mosfet is rated for 80A pulses.

However, when i feed the rectified 25V through the drain with my load spanning the source to ground.
I end up with smoke and a very very hot mosfet.
Also, when i view the source on my scope (with 10+V at the gate) i can see a vast difference from what is going in at the drain.

So in summary - what am i missing here?
My Voltage divider dissipates 170mA (i have 1/4W resistors) and 4.9mA of current flows - Is the gate driving current maybe too low?

I just though, i might of had the drain and source hooked up the wrong way round - i just quickly reversed them and now i get good conduction and the mosfet is no longer dissipating a load of power - however the mosfet conducts with no gate voltage.

Im sure it is a simple error that i am making, but an extra set of eyes will be greatly appreciated!

EDIT: initially, i had drain and source backwards.

 

You're using an N-channel as a high side switch. Move it to the low side, source to ground. When you reverse it, you send current through the body diode no matter the state of the gate.

 

You're using an N-channel as a high side switch. Move it to the low side, source to ground. When you reverse it, you send current through the body diode no matter the state of the gate.

I see - so source to ground and drain to Vcc?
This stops latch up - but i still get smoking when i drive the gate at 10V

 

Put the load between +24V and the drain.

Bob

 

http://forum.allaboutcircuits.com/attachments/tran21-gif.25142/

Like this?

That does work, however i canont seem to make my PWMed optocoupler short the gate voltage to ground.
If i move the jumper from 10V to ground manually, the mosfet switches on or off.
With my 500Hz pwm output shorting the gate voltage to ground via an optocoupler - i do not get any control.

Thank for the assistance! I have made progress!

 

We're not clairvoyant.
Post your schematic.

 

We're not clairvoyant.
Post your schematic.

i do apologize!
Let me get on it!

 

This awful paint rendition shows what's going on.
Also - the 25 pulsed dc produces around 34V peak to which the cap charges.
This dc level is maintained at the top of the v divider for driving the gate.

 

The 4k7Ω resistor provides over 7mA of current from the 34V supply. Can the opto carry that much current to ground?

 

The 4k7Ω resistor provides over 7mA of current from the 34V supply. Can the opto carry that much current to ground?

yes, the max output is 500mA
Thanks

 

Maybe you should cut off the wire from the common of 4.7K and 2.2K to G of mosfet, you just connecting the wire from the common of 4.7K and 2.2K to the optocouple, and the pwm signal connecting to the G of mosfet.

There is a question here is that if the vg must >= 10V, then the pwm input also need to increasing the voltage up 10V.

The current from 4.7K and 2.2K maybe too less for the optocouple, you need to reducing the values of resistor.

 

The opto needs a good amount of current for the input diode, to make the output transistor conduct and goto saturation. What is the opto used and how much is the current to the diode? Schematics again!

 

yes, the max output is 500mA

But how much input current does that require?

 

The opto needs a good amount of current for the input diode, to make the output transistor conduct and goto saturation. What is the opto used and how much is the current to the diode? Schematics again!

The input for the opto is configured correctly. I have already used this configuration to PWM the gate of a IGBT with success.

But how much input current does that require?

As in how much is dropped across the opto output? I am not sure.

However - i havent got to the point of using the PWM to control the gate as i cant manage to get it working with 10V straight up.

 

Maybe you should cut off the wire from the common of 4.7K and 2.2K to G of mosfet, you just connecting the wire from the common of 4.7K and 2.2K to the optocouple, and the pwm signal connecting to the G of mosfet.

There is a question here is that if the vg must >= 10V, then the pwm input also need to increasing the voltage up 10V.

The current from 4.7K and 2.2K maybe too less for the optocouple, you need to reducing the values of resistor.

Your first point relates to passing the gate voltage with the optocoupler rather than sinking it? I did try this but the datasheet shows it being sunk as an example.

The aim of the PWM is to drive the gate to 10V and then to zero rapidly - rather than to average out a voltage over a time period.

I think you may be on to something with the gate current !

 

Your first point relates to passing the gate voltage with the optocoupler rather than sinking it? I did try this but the datasheet shows it being sunk as an example.

The aim of the PWM is to drive the gate to 10V and then to zero rapidly - rather than to average out a voltage over a time period.

I think you may be on to something with the gate current !

I attached the circuit as below, you can check where should to modify.

 

First - let me clarify i can understand the circuit - given that it is more than a slight alteration.

The zener and R1 provide a 12V voltage for the gate - with the capacitor in parallel holding that level.
R2 limits the current into the opto.
The PWM output of my microcontroller will short the input to the opto to ground when high via a transistor Q1.
The 12V gate voltage is passed by the opto when the PWM output is low.
22k Gate resistor to stop ringing.

Now i am afraid i have a few questions...
Why the zener rather than a Vdivider? (i also do not have any zener diodes)
Why is the additional transistor necessary? The optocoupler i am using (4n25) seems to operate just fine with my 5V output from my microcontroller - and worked in this manner for my high voltage bulb testing.

I am still struggling to get my mosfets to conduct reliably with a straight 12V applied to the gate - i am not at the point of using the opto and PWM for rapid switching.

 

This is what i am using at the moment.
I am simply trying to get the 10+V at the gate to switch the mosfet so that pulsed DC can flow through the bulb.
EDIT: fixed an issue - with no gate voltage - the mosfet smoked and the bulb was very dim.
I added a 700K resistor from the gate to ground and now it works as expected when applying the gate voltage and removing it!

Now to try and PWM through the opto!

If anyone has an idea of what is going on without that gate resistor to ground - im intrigued to find out.

 

One issue sorted - the optocoupler does not vary from zero to full.
When the PWM output is at 100% (shorting the gate to ground, therefore should be zero V) i can see that it is only managing to drop to half its max before rising again.

This has to do with the PWM frequency and the optocoupler maybe.

 

Now i am afraid i have a few questions...
Why the zener rather than a Vdivider? (i also do not have any zener diodes)

If you like then you can still using the resistor.

Why is the additional transistor necessary?

I just drawn the normal driver circuit, If you want then you could take the bjt away, and using the port of uC to drive the LED.