Re: G08's and inductive kick

Al Kossow wrote:
>
> Would it be possible to design a new deflection circuit using 2
> N-channel MOSFETS as the drivers?"
>
> ---
>
> I suspect the designs you're mentioning are actually PWM (class D)
> designs. These designs weren't practical before the invention of fast
> high current MOSFET drivers, so the deflection amps of the early 80's
> used class AB output drivers. Most of the power in the output stage
> is dissipated as heat in the deflection amp output transistors (and
> the fairly high resistance of the current to voltage conversion
> resistor in the return path of the yoke. Modern designs use a higher
> gain error amplifier and a must lower value sense resistor.
>
> Considering the voltages and currents involved (especially the G08)
> at high input voltages the deflection transistors must dump over 100
> watts of power into the heat sinks.
>
> Zonn and I were exchanging email last week and we suspect what actually
> is taking out the output transistors is base-emitter breakdown from
> the inductive kickback of the yoke when the direction of current flow
> is reversed, which was why I wanted to try to set up a deflection amp
> on the bench to see what happens to the yoke voltages at fast switching
> rates.

But I think newer mosfets with faster switching and a lower rDS specs
should work. Getting the correct impedence matching to the yoke will
be a little tricky. Do you know the slew rates on the x/y deflection
circuits? Maybe I will find a little time to do some experiments with
this as well. You guys have my brain cooking now. Thanks. Let me know
what you find on the counter electromotive force on the yoke. Other
things I would like to know:
1) The resonance frequency of the yoke.
2) Phasing difference of voltage and current. In the yoke, the voltage
   should preceed the current by 90 degrees. But, what happens to this
   as you approach resonance and at what frequency? You should see the
   phase angle inverse I would think when it switches from one
   transistor to the other. In other words when the current is reversed.
   This could cause the other transistor to get over biased and
   saturate.
Received on Mon Sep 1 11:05:01 1997