Transformer mosfet gate drive?

[Hammy]

I'm trying to restore some of my gate signal using an isolated gate
drive. I've tested a Murata 2mH pulse transformer, part# 78601/1C,
1:1, ET 20VuS, 49pf capacitance.

With a twelve volt signal and 50% duty cycle @ 100kHz my Vgs is only a
little over 6V. This isn't enough to fully enhance my mosfet FQP27N25.
So I looked into some DC restorer circuits and found one from IR ;
AN-950.Shown here

http://i43.tinypic.com/bfkyza.png

But it's not working I cranked my primary side fet driver up to 16V
and I'm still only getting +7.6Vgs and an equal -Vgs.

Theses are my actual waveforms.CH1 is the Mosfets Vgs. CH2 is the
primary input after Z1 which is 0.2uf cap and 3.3oHm resistor. I'm
using an irlm2803 fet for the rectifier.

Magnified few one period

http://i44.tinypic.com/14b1ac3.jpg

Several cycles

http://i41.tinypic.com/2yuxdo7.jpg


I'm useing a mic4416 driver on the primary which can sink/source 1.5A.

 

[Jim Thompson]

Looks tricky. You can usually get away with just a series cap and a
resistor||diode from source to gate. That (almost) always applies the
transformer p-p voltage to the gate as a unidirectional pulse.

John

I did that very same scheme for a satellite "spin launcher" (Sperry
Satellite Systems) around 1990... except I used bipolars for the DC
restore section to allow for an accurate and very wide PWM range.

One has to be a little more elegant if you need things to work 100% of
the time ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
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| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
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Isn't the definition of "totalitarian" when the government owns
significant manufacturing; and controls the major media outlets?

 

[Hammy]

Looks tricky. You can usually get away with just a series cap and a
resistor||diode from source to gate. That (almost) always applies the
transformer p-p voltage to the gate as a unidirectional pulse.

John

John what you recommended was the first result from a search, and then
I read this from Ridley.

"The circuit works well during steadystate operation (a gate load
resistor is Recommended), but when the PWM controller shuts off, the
dc blocking capacitor Is connected across the gate drive transformer
for an indefinite period. This can lead to saturation of the
transformer, as shown in Fig. 6b. When the transformer
Saturates, the secondary is a short circuit, and the secondary
capacitor can turn on the FET. The saturation can be avoided with a
gapped core, and smaller value of capacitor, but this will increase
the reactive current needed from the gate driver, and may produce
other problems."

Fig. 6b
http://i42.tinypic.com/a1nau.png

The controllers I'm using tl3844 (flyback) and ucc25600 (llc half
bridge) are limited to 50% duty max I knew my Vgs would reduce at 50%
duty I was just surprised it reduced as much as did. So I must be
doing something wrong.

I'll do some more testing with the input impedance and see if I can't
get the Vgs unidirectional. If not I'll try the other restorer that
John recommends.