MosFet Problems

[GregK]

Hi

I have used IR logic level power fets (H-bridge) with Pics and a driver chip with no failures. I'm now trying to use these fets as a linear load for a dc motor and the fets are failing with a gate to drain short (usually within 15 min). The setup consists of a 12 volt motor driving a 12v motor that is loaded by the fet. The fet is rated at 100 amps, I'm drawing 26 amps, the fet is well heat sinked and fan cooled. The duty is 25 to 50%, not PWM'd. A scope from the gate to source doesn't show any large spikes.

What am I missing? Any help will be greatly appreciated.


Greg

[Melanie]

Makes me think you've exceeded the maximum Gate/Drain voltage, or your voltages are going negative polarity. Are the FETs protected in any way with Diodes, VDR's, Transorbs or similar?

[mister_e]

Wich Mosfet do you use ???

[GregK]

Hi Melanie and mister_e

I tried using a 100 ohm gate resistor. I also tried a 1000 ohm gate resistor with a gate to source reverse biased diode, this also caused the gate to short out. I also tried a separate power supply to bias the gate (grounds tied together) with the same results. The gate spec is +-16 volts.
I am using the IRL2505 and then the IRL3103D1. I looked at the spec's again on the IRL2505. I think I know what's wrong. The continuous current is rated at 105A I am drawing 24A. The unloaded gen output is 11.6v, I need to check the loaded output across the Mosfet, I believe its around 8V, this puts me right at the Power dis. rating. I will try 2 fets in parallel, I hope thats the problem.

Thanks for your responses.

GregK

[mister_e]

Hi GregK, what about if you use an Zener reversed bias on gate. Gate resistor about 200 ohms should work. As i can guess maybe PIC do not source enough current or voltage on Gate/source is below 2.5volt. It apear in some case that PIC are better to sink than source current. sp maybe buffer/driver might be insert before MOSFET gate.

Wich PIC do you use and on wich pin MOSFET are attach to.

24A load at 12V=288Watt. IRL2505 Pd=200w

[GregK]

Hi mister_e

I placed 2 MosFets in parallel on 2 of the 3 motor/ gen. setups and set the loads for 12A on one and 27A on the other. I have run them for 8 hrs with no failures, so it looks like I was just trying to dissipate too much wattage through them. This project is controlled by 3 Zworld micro's and power relays. The Zworld micro's and relays turn on the motors which are connected directly to the generators. The generators output is connected across the drain and source of the fet. The gate bias is obtained by a 10K pot also across the gen. output with the wiper going to the gate through a 100 ohm resistor.
One of the other projects controls the speed of a linear actuator by counting pulses from an optical encoder and adjusting the HPWM up or down based on the error. This project uses the 16F628 and a driver chip and 4 IRL2505's and a few other components.

Thanks for your replys.

Greg

[mister_e]

good news Greg. This what i supposed when at the end of my last post iwrite down

24A load at 12V=288Watt. IRL2505 Pd=200w

great datasheet...


good luck for the rest of your project

regards