DEC H744 +5 supply

Brent Hilpert bhilpert at
Fri Sep 21 18:03:21 CDT 2018

On 2018-Sep-21, at 1:03 PM, Noel Chiappa via cctalk wrote:
> Oh, one thing I forgot to include:
>> a lot of the incoming power in that 30V AC has to be thrown away, in
>> producing +5V.
> So, if my understanding is correct, the 'switching' H744 really isn't much
> better than a classic linear supply. It still wastes a very large amount of
> the input power, and it still has a massively heavy transformer in it. Yes?

No, it's much better than a linear supply.
With the given 20-30VAC input (do I have that right?) - lets take a conservative 25VDC after rectification and filter - if input to a linear regulator producing 25A @ 5V,
that's a loss of (25-5)V * 25A = 500W, to produce a usable 125W.
That heatsink isn't sinking 500W and DEC wouldn't be producing a supply that inefficient.

A glance at the schematic (as Eric provided the ref to) you might think it's just a linear regulator: AC input, rectification, cap filters, pass transistors, filter choke, with a 723 IC regulator controlling it, all the standard elements are there.

But it's not: L1 is the bucking inductor - it's not just a filter choke.
Diode D5 provides the current path for L1 to supply energy to the load when the source is switched off.

The subtle thing about designs like this is where does the switching oscillation come from?, as there is no obvious oscillator present.
I think what's going on is:
	- Q1/R4 are sensing current from the source via voltage drop across R4.
	- As that current goes up Q1 starts to conduct, eventually tripping SCR Q7 hard on.
	- This raises the voltage at E1.4, the error amp sense input.
	- This fools the regulator into thinking it's seeing way too high an output voltage so it hard shuts down the Q5-4-3-2 chain,
	   disconnecting the source from L1 and the output, and shutting off the source current.
	   E1.4 is now back to sensing the output voltage via the V-SENSE feedback.
	- With source current off, L1 now starts supplying energy to the load through D5, as it's magnetic field starts to collapse rather than build.
	   As that energy peters out and the output voltage droops, E1.4 via the V-SENSE line now sees too low an output voltage and turns
	   Q5-4-3-2 back on so the source can supply current, and energy into L1 and the load.
	- repeat. Hence oscillation.

There is still a sizeable 60Hz transformer (step down to 20-30VAC) because 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.
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.

> So I wonder what exactly the advantage was in going to the switching approach?

Improved power efficiency.
Improved material economics.

> Yes, it keeps the output voltage steadier then a pure linear supply could -
> but I'll bet there are analog approaches that can do the same. (They'd need
> something that can produce a steady reference voltage, but the switching
> approach needs, and has, the same thing.)

In addition to reference stability, the basic error factor in a regulator is the loop gain of the error amplifier, and applies to both linear and switching regs.
Neither produces a perfect output, it just reduces the the variation by some factor (the gain factor).
If you think about it, if the output were perfectly stable how would the regulator know to provide correction?

> Maybe the main output transistors
> are happier being full-on or full-off, or something like that?

Yes, that is one of the benefits of switching regulator over a linear regulator. It keeps the in-regulator losses to a minimum.

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