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MOSFETs and their differences

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zzz_julian_zzz

Thu Dec 26 2013, 08:46AM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

Hello everyone... :)

I'm just wondering, why my small table top SST coil behaves differently when I change my Half-Bridge's switches(MOSFETS) to different kind of MOSFET considering that the replacement is more superior in terms of ampacity.

MOSFET #1: IRFP460A Rds:0.27 Id:20 Amps Vdss: 500v

Coil performance: Nice sparks (corona) - means, it follows the interrupter "Go" signal from 4bps up to 60bps

MOSFET #2: 44N50 Rds:0.12 Id:44 Amps Vdss: 500v

Coil performance: No corona sparks coming out, Experience corona ONLY at higher BPS 40hz to 60hz

-----
spark lengths are just the same for both MOSFETS, their thermals are also both good..
My theory is that the Problem in the MOSFET 2 is insufficient heating/conduction of the output in the air. So that at higher BPS rates the sparks sustain conductivity of the air.

My Question is, how can i solve this, and what is the relativity of this theory on the MOSFETS? are there any specific characteristics of these MOSFET i should be more concerned about to avoid getting this problem?

Thanks!

Sulaiman

Thu Dec 26 2013, 09:06AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

the ixys device has 2.7 x input capacitance of the irf device
and about 2.6 x gate charge

probably your gate driver is not 'strong' enough to drive the ixys devices hard enough.

Sigurthr

Thu Dec 26 2013, 09:12AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

MOSFET #2 is much slower, and has much higher input and output capacitances.

What is your switching frequency? I'm willing to bet insufficient gate drive is a major factor.

Edit; sulaiman beat me to it, haha. Darn slow typing on my phone!

zzz_julian_zzz

Thu Dec 26 2013, 09:56AM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

hahaha thanks very much guys! :) in terms of Drivers capacity, i used the same topology to drive my CM300s with no problem at all (using P channel and N channel mosfets on the output stage) BUT with smaller size of GDT.. Fres of my coil is somewhere 350Khz.. I doubt the speed of these 44N50 is the reason why I'm getting this, because I used to drive this coil using IRG4PC50ud which is slower than 44n50.. :) Thanks!

Sigurthr

Thu Dec 26 2013, 10:03AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

What is your sstc's feedback method? Antenna feedback is finicky on pulsed coils.

Dr. Dark Current

Thu Dec 26 2013, 10:04AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Or maybe it has something to do with your breakout point temperature/wear. If a switch change results in output change, then something is horribly wrong.

zzz_julian_zzz

Thu Dec 26 2013, 10:25AM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

Its feedback is thru a ~ 40 turns CT... I doubt there is something wrong with the circuit logics , feedback and etc., because I'm not changing or touching the circuit at all... just replace the isolated bridge module... :) when IRFP460A is installed, there are no problems at any of BPS setting, but when i try to replace it/use the 44N50s then the problem is re-occurring.. :) thanks!

Sigurthr

Thu Dec 26 2013, 12:05PM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Hmm, you said bridge "module", is layout different between the two?

Just switching fets shouldn't change coil performance, thermal aspects aside.

zzz_julian_zzz

Fri Dec 27 2013, 01:28AM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

The 2 FETS are different in Packing.. so yes, their traces are different from one another but they're designed to have the minimal inductance on its lay out.. If no other factor that cause this error,
Then I guess I have to check if drivers + GDT is driving those FETS very well..
I'll replace the GDT will larger diameter first, then check, then post the updates..

Thanks! :)

Sulaiman

Fri Dec 27 2013, 10:19AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

If your gate drive transformer could handle the volt.seconds required to operate the irf's
then it will not change due to using the ixys transistors,
since the drive voltage and frequency will not change significantly,
and the resistance of the gdt windings are negligible.
so
I'm fairly sure it's not the gdt, more likely the gdt driver (ic or discrete)
or the resistor(s) effectively in series with the gates
(any primary or secondary series resistor) are too high value.

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