Good gate driver for MOSFET

[1843]

Hi all,
I've recently made two PIs with totem-pole gate driver as follow... But both burned their MOSFETs after 2-3 hours working.
I there any problem in this circuit ?


Is it better to use the gate driver of Goldscan-4 ?


Thanks in advance,
1843

[porkluvr]

pulse duration?

Maybe you just need to lower the pulse width or frequency. I compared your totem pole drive circuit next to a Goldscan 4 (in LTspice) and yours (after about 1.5us propogation delay) switches faster than the Goldscan. All other things equal, it should be superior. I thought possibly your -5V trigger would be a problem, but it seems to work fine.

If you think you need long tx pulses, think about paralleling 2 FETs or adding a heatsink. I'll maybe do some more comparisons and post some pics a little later but I've got to go for now.

[1843]

Hi porkluvr,
Frequency is 100Hz and pulsewidth is 300us.The MOSFETs remained cool during the bench tests(about 5 minutes).

Thanks for your time and attention.


Regards,
1843

[porkluvr]

I must retract what I said about a 1.5us delay - I don't know what I was doing to cause that but it's gone now. It scare me sometimes
when crazy things happen. Gotta' watch the "controls" to maintain consistency.

Even knowing what your pulse train is, I would still like to know which FET you are using, your coil parameters,
and if you use a series drain resistor. The coil setup in my "test", here, is similar to the Goldscan IV, as are the RC timing
components - except that I increased R7 to give me about 300us instead of the 220us normally found in GSIV.

I have tried to make the three circuits below have identical trigger pulse, coils, damping resistance, power supply, FETs, etc. Unless there's
something else I overlooked, any difference in waveforms will be due to the individual FET driver circuits.
I have taken the liberty of adding RC filters (which makes it easier to monitor input current), and also a drain resistances.
The fourth circuit is same as #3 except has a different drain resistor.

On the left is Goldscan IV. In the middle is my best creation - using a few extraneous components in attempting
to minimize power supply transients. On the right is your "totem pole" circuit. For simplicity and for having a smart waveform,
I think your circuit wins.

Look at the time that the FET is in breakdown. I don't want to try and quantitize power dissipation based on this impromptu simulation,
but multiply current through the coil times voltage across the FET to get some idea how much instantaneous power
the device must dissipate!!

I can't explain why your PIs seem to run cool during test and then break under operation, and your 300us @100pps doesn't exactly
put your PI over the top so far as power output goes (GSIV does about 220us @ 500PPS) - but I do think you should reconsider the need
for 300us pulse.

I'll retest when I know more about your coil and FET, and series resistor (if any). Will send you my files (via email) after I get some rest.

[porkluvr]

Got your info. 1mH coil at 1.8ohms.
I tweaked the 555 timer RC components to give approximately 100pps @300us (the 100pps is unimportant excpt to measure battery drain).
I see the FET in breakdown at about 390V for about 5.5us.

The coil current at that time averages about 1.35A (approx. 2.5A at the start, dropping to 0.4A when the trailing edge begins).
Assuming that the coil current is being sourced by the FET (!?!), that's about 500W, or, a whole lot of instantaneous power at the TX peak!
(Of course, this is only a virtual simulation.)

I have not run experiments myself ( ...still trying to hammer out my monster of an IB, [gaaa] wanting to finish that before I work up a PI),
but there are two schools of thought concerning your pulse power. I think. One school says that more power is better.
The other one says that even though you may get more overall depth penetration; if you over-saturate the ground,
smaller targets (shorter tc) may be obscurred by background noise. Just a thought. Maybe that's blowing smoke, but I seem
to remember reading that bit of info.

I'll get you some timing circuits material in a day or so. I made things difficult when I modified the R and C symbols (but I like things small).

[1843]

if you over-saturate the ground, smaller targets (shorter tc) may be obscured by background noise. Just a thought.

I think that it's a good reason for why most PIs have a max pulse duration about 250us.
Even in deep seeking PIs we see that 24V batteries are used instead of lengthening the pulse width.

Best regards,
1843

[Tinkerer]

Mosfet driver

These drive circuits look good, but why not just use a Mosfet driver like the TC4427?

Tinkerer

[Ferric Toes]

I use the totem pole emitter follower arrangement to drive a mosfet, but with one difference - I limit the drive to the gate to 10V w.r.t. the source. This is done by using a potential divider in the collector of the transistor feeding the bases of the totem pole. At the moment I am driving a IRFP250 with a 25V supply and pulsing 20A peak in a 600uH coil. The pulse width is 1500uS and the prf is 270 per sec. No reliability problem at all, although the coil temperature hits 75 degC after about half an hour. Good job it's an underwater application, as that will cool things down.

Eric.

[joecoin]

The MOSFET on my Hammer Head heats up after about 5 minutes when the coil is attached, it runs cool to the touch with out the coil. Still trying to figure it out. I'm using a surplus AN/PSS12 coil.

[vasvas]

The MOSFET on my Hammer Head heats up after about 5 minutes when the coil is attached, it runs cool to the touch with out the coil. Still trying to figure it out. I'm using a surplus AN/PSS12 coil.

Propably too much inductance and low resistance. Try to lower it and see

[6666]

porkluvr do you still read this forum ?