Re: LV supply design

At 05:02 PM 9/4/97 -0700, you wrote:
>
>Here are a couple of things I was thinking of / wondering about on the LV
>supply:
>
>1) why is only one of the pass transistor drivers on the WG a power transistor?
> does the HV supply draw a lot more positive than negative current?

I believe it's the negative side with the power transistor (I could be
wrong). But if it is the case it's probably because PNP transistors
typically have less gain than there NPN counter parts, so the pre-driver
would have to pass more current to drive the PNP pass transistor.
>
>2) the LM3xx daughter board got me thinking it would be fairly easy to build
> two little PC boards that you could mount on a heat sink to add a MOV or
> commutation diodes and current limiting to the deflection drivers.

I've wondered how much current limiting on the deflection drivers would help.
On the monitors that have this (Sega and Cinematronics) I don't see any less
"blowing out" of deflection transistors. I'm still suffering from the belief
that voltage transients are blowing these things out. Some higher speed
protection diodes (instead of the 1n400x's I've seen used) might be in order
though. Once the transistor is fried, current limit doesn't do a bit of good.

>3) foldback current limiting on the LV pass transistors would be nice
> that way, if a driver shorts (which shouldn't happen with #2 above..)
> you aren't toasting the LV pass transistors.

This is probably a better place to put in current protection. If you
current protect at a current slightly higher than the fuses, if and when a
deflection transistor were to short, the current would be maintained well
within the specs of the complimentary transistor and any rectifying diodes
until the fuse blew. This would minimize the damage to the single source of
the problem.

>I'm wondering in a 723 (or modern equivalent) with built in current
>limiting would make more sense than the three terminal parts.

The 3 pin regulators do a better job than the 723. If your designing a high
current power supply (more than 1.5 amps) then the 723 is probably cheaper
in mass quantities. But it's harder to use, and needs a lot more glue logic.

-Zonn
Received on Thu Sep 4 17:38:17 1997