Re: Hybrid linear/switching supply

john jardine wrote:
"Gibbo" <gibbo@xxxxxxxxxxxxxxxx> wrote in message
news:52n129F1oca16U1@xxxxxxxxxxxxxxxxxxxxx
Fred Bartoli wrote:
Gibbo a écrit :
R2 and R3 are the loads. R2 load always between 5mA and 15mA. R3 load
not always on.

Acceptable for the +10V rail to drop down to +7V on low input voltage.
5V rail must always be at 5V

Needs to operate from 9V to 34V input. But it also needs to survive
ISO 7637-2. That dictates upto -600V spikes with rise times of 1uS or
so and repetive +200V spikes with similar rise times.

<...>
Dissipation in Q2 is about 1/3 an equivalent linear reg at max input
(34V)

Nothing like optimised, just testing the idea. Anyone done anything
similar? Any better ideas?

Did something similar with a power mosfet, a TVS, and few other
components as a voltage limiter in front of a PFC (not the boost one
which happily withstands the main pulses, but a sepic one).
Worked like a charm.

Now for your case I'd go with a depletion mosfet, a zener and one or two
resistors:

[snip]

You can even strengthen this further with a front R+TVS.

Yes I saw that trick on the "suriving automotive load dump" thread. Neat
idea but the exisiting circuit is already immune to load dumpt etc and
it does nothing to help dissipation in Q2 which is the main idea.

But thanks for looking.


--
Gibbo

This email address isn't real.

Without looking too deep, seems a faster startup and less dissipation if
you lose R5, D1, C3 and (oddly) D6.
john




But I've just looked at it again and I think you might be right. Remove the zener and associated crap and just limit the drive current to Q1 so the inductor can't go ballistic. I think there's enough of a gap (with hfe spread) between required minimum drive and maximum allowed to limit the current in the inductor. I think. I'll have a look tomorrow when the gin has worn off.

--
Gibbo

This email address isn't real.
.

Relevant Pages

  • Re: Hybrid linear/switching supply
    ... Acceptable for the +10V rail to drop down to +7V on low input voltage. ... Dissipation in Q2 is about 1/3 an equivalent linear reg at max input ... Yes I saw that trick on the "suriving automotive load dump" thread. ...
    (sci.electronics.design)
  • Re: Hybrid linear/switching supply
    ... Acceptable for the +10V rail to drop down to +7V on low input voltage. ... That dictates upto -600V spikes with rise times of 1uS or so and repetive +200V spikes with similar rise times. ... Did something similar with a power mosfet, a TVS, and few other components as a voltage limiter in front of a PFC. ...
    (sci.electronics.design)
  • Re: Hybrid linear/switching supply
    ... Acceptable for the +10V rail to drop down to +7V on low input voltage. ... That dictates upto -600V spikes with rise times of 1uS or so and repetive +200V spikes with similar rise times. ... Now for your case I'd go with a depletion mosfet, a zener and one or two resistors: ...
    (sci.electronics.design)
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