Need help with PWM mosfet

[rapenia]

I got the resistors wrong Sorry

R8, R9, R10 are 0.22 ohm not .022
Sorry.

 

[indulis]

Well, with 3, .22 ohm resitors in parallel you'll only get ~9A pk.

 

[rapenia]

I changed R8, R9, R10 to .15 ohms 10watt and it was running at about 12.5A
The MOSFET got really hot so I put on a big cpu heatsink with a fan

The MOSFET is
BUK9535-55A TrenchMOS transistor logic level FET
I waiting for
HUF75343P3 Fairchild ULTRA-FET POWER MOSFET 75A 55V
I got this in thinking that if the amperage rating was higher, the MOSFET would not be under so much stress.

The truth is I don't really know what to look for in choosing the MOSFET, I'm looking mainly at Vds Id and Idm and I think I understand what the Rds(on) is.

The Voltage is variable, but I'll just stick to 12v it will make things simpler.
If I lift the resistance of R2 would that help with the temp increase in T1

 

[Chippie]

The truth is I don't really know what to look for in choosing the MOSFET, I'm looking mainly at Vds Id and Idm and I think I understand what the Rds(on) is.

The Voltage is variable, but I'll just stick to 12v it will make things simpler.
If I lift the resistance of R2 would that help with the temp increase in T1

You need a mosfet with the lowest Rds(on) value to minimise the voltage drop across D-S hence lowering the overall device dissipation...

Changing the value of R2 will do nothing....other than I quote, "R2 and C4 are a filter to get rid of the spike that will be there at turn-on. "

The power dissipation in the mosfet is goverened by load current and supply voltage even when the mosfet is turned hard on...

What are you actually trying to achieve?

 

[rapenia]

I would like to get the PWM to run steady at about 28Amps.

But I would like to learn as much as possible as well, if I'm going to make changes, I would like to have some kind of reasonable idea want I'm doing.

This is my first project.

The HUF75343P3 has a much less Rds on 0.007 to 0.009
BUK9535-55A is 0.024 to 0.032

 

[indulis]

At what frequency are you running the PWM? You also have to be a little careful changing MOSFET’s. As Rds on goes down, you will find that total gate charge goes up, which basically means that you’ll need more current to switch the MOSFET at the same speed. If you don't, switching losses will go up. You may want to consider using a totem-pole driver for the FET.

 

[Chippie]

At what frequency are you running the PWM? You also have to be a little careful changing MOSFET’s. As Rds on goes down, you will find that total gate charge goes up, which basically means that you’ll need more current to switch the MOSFET at the same speed. If you don't, switching losses will go up. You may want to consider using a totem-pole driver for the FET.

I think he is attempting to drive a motor, PWM frequency, motor inductance and supply voltage all go into the mix as I'm sure you know....I was under the impression that the SG3525 had Totem pole outputs ?

Whether 28 amps is achievable is a bit of a challenge....imho

 

[indulis]

I was under the impression that the SG3525 had Totem pole outputs ?

Yes, but your going thru diodes on both outputs, so they don't help for turn-off. Also, the SG3525 is only good for ~200mA (500mA max).

28 amps shouldn't be that big of a problem with the right MOSFET's and a proper driver.

 

[rapenia]

I would like to keep the frequency variable. It's adjustable from 100hz to 5000hz, will this be a problem.
I'm still waiting for HUF75343P3 I bought them online so have to wait for delivery. I got 3 just incase.
Totem Pole ??
I did a search and it looks like the gate is connected between 2 transistors.
I think I understand, it supplys a larger peak current to help charge and discharge.
Is this part of the chip or will I have to add this.

Cheers

 

[Chippie]

Try Googling a tc4427 which is a mosfet driver.......

 

[rapenia]

I got the HUF75343P3 today.
Dropped the sense resistors to 0.1 ohms
The Amps started at 0 then started to rise slowly untill it got to 7.4A.
Amperage dropped when I turned up the frequency.
The MOSFET got REALLY hot.

Not sure what I'm going to do next, getting a MOSFET round here is pretty hard.
We don't have the access in the UK.

 

[mneary]

Your turn off time is slow, and power is being dissipated by the MOSFET whenever it's not full on or full off.

You can use each side of the SG3525 to help turn the other side off. Remove R1. Put a 100 ohm 1/2 watt resistor in parallel with each diode. You might even try values as low as 47 ohms, but this might make the SG3525 get warm.

This will speed your turn off time by at least a factor of four or five, and the MOSFET will tolerate 5x the frequency that it does now.

(Analysis: The diodes provide full voltage to the gate when one output is high. The other output is low, so it loads the gate with a 100 ohm load. At 12V this is a 120mA load. When both are low, the two resistors are in parallel, +4x better load than the old 220 ohm R1. If you use lower values it gets better, but eventually the SG3525 runs out of drive current.)

 

[Chippie]

You can use each side of the SG3525 to help turn the other side off. Remove R1. Put a 100 ohm 1/2 watt resistor in parallel with each diode. You might even try values as low as 47 ohms, but this might make the SG3525 get warm.
QUOTE]

You could always add a DIL heatsink to the chip to aid cooling...A dab of cyano will secure it to the chip