DEC H744 +5 supply

[Noel Chiappa]

    > From: Brent Hilpert

    > With the given 20-30VAC input (do I have that right?)

Yup.

    > A glance at the schematic ... you might think it's just a linear
    > regulator

And the writeup in the maint manual gives that impression too, which didn't
help! (Hence my assumtion that it was acting in the way a plain linear
regulator might, in terms energy efficiency.)

    > Diode D5 provides the current path for L1 to supply energy to the load
    > when the source is switched off.

Right. What is the role of the pair of big caps, C8/C9. Is that just to
filter ripple, or do they play a role in the provision of current when the
supply is switched off (by Q2)?

(My guess would be only the former, since unlike the energy stored in L1,
which can be used provide electrons when Q2 is off, capacitors only store
electrons, so they can't play much of a role in the conversion of V1I1 to
V2I2, which requires 'creation' of more electrons when I2>I1. Oh, reading the
maint manual, when Q2 is on, they store some of the current coming through
L1. So I guess they have a peripheral role in the overall operation.)


    > The subtle thing about designs like this is where does the switching
    > oscillation come from?, as there is no obvious oscillator present.

The maint manual does cover that. It more or less says that as the output
voltage rises through 5.05V, the voltage regulator turns off Q2, and as
it falls through 4.95, it turns it back on. (Presumably when the whole
works is first turned on, the output voltage is less than 4.95V, so Q2
stays on until it gets to the turn-off voltage.)

Q7 is part of the over-current sensing, it says.


    > the switching is taking place after the transformer rather than
    > straight off the mains, this allows the switcher design to be simpler
    > and get away with using much lower voltage semiconductors.

Ah, I was wondering about why they did it that way.

    > The transformer is nonetheless much smaller than it would be in a
    > straight linear regulator design because the secondary current it has
    > to supply is several factors lower than for a comparable linear reg.

That's because of the higher efficiency of this circuit, as opposed to a
straight linear regulator, which would need more mains power in to produce an
equivalent power out?


    > Q5 is functioning as a common-base stage in the driver chain ...
    > It is not part of the +15V supply to the 723, that is provided by
    > R2, zener D2, C2.

I was confused by the maint manual, which says "D2 is used with Q5 and R2
to provide +15V to E1".


    > There are a thousand configurations for power supplies possible
    > depending on what needs to be accomplished ... It's useful to keep in
    > mind that regulation and EI conversion are different objectives but
    > they can be achieved either separately or in concert.

Got it.

    > bipolars have a fixed MINIMUM drop, which can be used in a switching
    > supply to as much advantage as possible with bipolars, but have a
    > varying drop in a linear regulator

Right.


OK, I think I've got a decent grip on all this now - although I still
wouldn't want to try and _repair_ one... :-)

	Noel