I need to pass between 10 - 150 Amps at 12V through a semi resistive/capacitive load. The 28x1 works quite well, BUT ! (always but...) My mosfets keep frying !
description:
I use from 4 to 8 IRF3205's(55V/75A/TO-220) in parallel to carry the load. I drive a PN2222A npn transistor (with PWM at 4kHz, duty between 10% and 100%), when "on", the collector grounds ( 0V ) the bases of 4 to 8 BC327's(pnp) each which in turn switches regulated 12V onto the Gates of the Mosfets. Gate power is via 47R resistors to the gates. The mosfets are well heatsinked with air cooling via a 12V 92mm fan. I clamp the mosfets down with a piece of flat bar across the face, thus pinching the fets between heatsink at back and 5mm metal at front. I use heat transfer compound on both sides. My dual channel oscilloscope shows VERY FAST "on" switching ( < 2uSec) and quite good "off" switch times (about 5 uSec on very slight slope, with curved gradient the last few mV's at about another 5 uSec). I do not use the pnp's to short the gates to 0V but to actually drive the gates positive (at +- 7.5V to +- 11V Gv). This is the best driver I have had to date.
Problem:
At 100% duty (gates fully "on", low RDS +- 8 mOhm) the mosfets run beautifully....: at close to ambient temp. of +- 38 deg C.
At anything less than 93% duty, the temp starts rising dramatically ! At 85% today, at 55A via 5x mosfets (only 11A each), temp. went up to 88 - 96 deg C on the "gate side" on the face of the fet. Thats when the "snap/crackle/pop" starts....
To date I have "popped", "smoked", "cracked" and sublimated a round total of +- 100 fets !!
What am I doing wrong ? I am not a professional electronics expert, just a very serious hobbyist. Am I driving the gates too hard ? The spec sheets all show gates driven at 10V ??
Any help out there ?
Manie
description:
I use from 4 to 8 IRF3205's(55V/75A/TO-220) in parallel to carry the load. I drive a PN2222A npn transistor (with PWM at 4kHz, duty between 10% and 100%), when "on", the collector grounds ( 0V ) the bases of 4 to 8 BC327's(pnp) each which in turn switches regulated 12V onto the Gates of the Mosfets. Gate power is via 47R resistors to the gates. The mosfets are well heatsinked with air cooling via a 12V 92mm fan. I clamp the mosfets down with a piece of flat bar across the face, thus pinching the fets between heatsink at back and 5mm metal at front. I use heat transfer compound on both sides. My dual channel oscilloscope shows VERY FAST "on" switching ( < 2uSec) and quite good "off" switch times (about 5 uSec on very slight slope, with curved gradient the last few mV's at about another 5 uSec). I do not use the pnp's to short the gates to 0V but to actually drive the gates positive (at +- 7.5V to +- 11V Gv). This is the best driver I have had to date.
Problem:
At 100% duty (gates fully "on", low RDS +- 8 mOhm) the mosfets run beautifully....: at close to ambient temp. of +- 38 deg C.
At anything less than 93% duty, the temp starts rising dramatically ! At 85% today, at 55A via 5x mosfets (only 11A each), temp. went up to 88 - 96 deg C on the "gate side" on the face of the fet. Thats when the "snap/crackle/pop" starts....
To date I have "popped", "smoked", "cracked" and sublimated a round total of +- 100 fets !!
What am I doing wrong ? I am not a professional electronics expert, just a very serious hobbyist. Am I driving the gates too hard ? The spec sheets all show gates driven at 10V ??
Any help out there ?
Manie