Re: LF: Protecting a MOSFET driver chip from MOSFET failure?

[Andy Talbot <andy.g4jnt@gmail.com>]

No, capacitive coupling with diode restoration ends up with near-enough  the same voltage applied to the gate as you had in thr first place.

A 0/12v output from the driver chips ends up at -0.7 /+ 11.3 V (assuming 0.7V diode drop)

As for the question of the dual drive with resistors.    The resistors may as well say in, just to smooth off edges and reduce dissipation in the driver.     i doubt you'll notice any difference.

Andy  G4JNT

On 10 August 2015 at 15:35, Rik Strobbe <Rik.Strobbe@fys.kuleuven.be> wrote:

Maybe also worth to mention:  Capacitive coupling of the driver to the gate will reduce the peak voltage at the gate by half (assuming 50% duty cycle).

The IC driver voltage should be at least 12V in order to ensure proper switching of the MOSFET.

If you use a transformer (as suggested by Stefan) one can fiddle with the transformer ratio to get the proper voltage at the gate.

 

73, Rik  ON7YD - OR7T

 

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Van: owner-rsgb_lf_group@blacksheep.org [owner-rsgb_lf_group@blacksheep.org] namens Andy Talbot [andy.g4jnt@gmail.com]
Verzonden: maandag 10 augustus 2015 15:25
Aan: rsgb_lf_group@blacksheep.org
Onderwerp: Re: LF: Protecting a MOSFET driver chip from MOSFET failure?

Forgot to mention, although I hope it is more than self-evident to readers here, the diode is orientated so it points 'upwards';  allowing a positive drive voltage to appear on the gate and clamping the negative  to ground.

Andy  G4JNT

On 10 August 2015 at 13:31, Andy Talbot <andy.g4jnt@gmail.com> wrote:

Yes there is.  Or , are: several.

Don't drive the MOSFET directly from the IC.  Instead use a capacitor and DC restorer circuit.  That guards against both  the driver staying high permanently if drive is removed, and also against IC damage if the FET fails

Capacitor (typically 100nF for 137/475kHz) from IC output to gate

Diode like IN914 , 1N4148 etc from gate to ground.  Shunt the diode with a resistor of a few kohms to stop it floating

The capacitor decouples the chip output at DC.  The diode clamps the waveform at the cap output / gate so it's most negative excursion is forced to sit near enough at ground.

Or use transformer coupling, as in my QRO 137kHz transmitter.  http://www.g4jnt.com/137tx.pdf

Andy  G4JNT

On 10 August 2015 at 13:09, Chris Wilson <chris@chriswilson.tv> wrote:

10 August 2015


Is there a way of protecting a typical MOSFET driver IC from being
taken out by a MOSFET failure in a Class E LF amp?

I was experimenting with drive level into my G3YXM 1kW 136 kHz amp and
managed to blow 2 of the 4 IRFP450 MOSFET's and half the TC4426 driver
chip.

I also came to realize that I did not follow my car engineering
knowledge, and built the thing with little regard to servicing it,
changing the devices was surgically challenging (read a PITA...) and I
should perhaps have put them on PCB screw type connector blocks in a
more accessible position. If I build another that would be my main
change, making repair access far easier!


As an aside why would reducing drive to the amp perhaps cause MOSFET
failure when TX'ing into a dummy load?


I am still trying to find a way to set the over current trip on this,
it's by means of a Hall effect sensor looking at the south pole of the
ferrite choke CH1. The thing draws 12.5 amps tops when TX'ing into a
dummy load, and even with the Hall sensor against the face of the
ferrite (and I am sure I have the south pole end, and the correct face
of the Hall sensor together), it doesn't trip, diagram at
http://www.chriswilson.tv/schematic_modified_2.jpg

Am I going to have to put some stonking big resistor on the output end
of the choke and try and make it draw more current momentarily?

Thanks, and thanks for a most entertaining "LF Weekend" sadly I was
not able to get an antenna sorted in time.




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Best regards,
 Chris                            mailto:chris@chriswilson.tv