DEC H744 +5 supply

[Brent Hilpert]

On 2018-Sep-22, at 11:54 AM, Noel Chiappa via cctalk wrote:
>> From: Brent Hilpert
> 
>> 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.)

Your conceptualisation around the role of electrons is perhaps a little off.
Displacement of electrons is the generation of potential (voltage), current is a RATE of electron flow.
You don't need 'more' electrons to generate a higher current, you just need to 'expend them' more quickly -
using words like 'more' and 'expend' loosely, as it's not about moving a quantity of electrons from source to load, or creating or consuming them.
Energy transfer is not equivalent to electron flow, or, electron flow does not correspond to (or theoretically even imply) energy transfer.

Filtering ripple and supplying current are not mutually exclusive functions.
Capacitors very much play a role in supplying current to the load.
Both the L & C play a role as energy reservoirs.

You can charge two identical capacitors with identical charges (number of electrons displaced); discharging one of them into a load in 1mS will generate some peak current, discharging the other in 10mS will generate a much smaller peak current, with the same energy expended in both cases.


>> 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.)

OK, my analysis of the operation may have been partly off then, I didn't think they'd be trying to look at just slight voltage variation of the regulated output to generate the switching/oscillation so thought it might be done via input current and Q7 providing a sharper switch.
Not  clear to me whether the whole is working as an on-off switcher relying on high enough gain in the 723/driver chain to make it a comparator or whether it is more like a phase-delay oscillator with the amp/semis operating in the linear region.


> 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?

The higher efficiency means less power expended in toto, so that's a partial influence, but primarily it's because it's still a higher voltage by several multiples than the eventual target output voltage and thus the current that the transformer secondary has to deal with is the same factor lower to deliver the same output power, meaning less copper for the secondary and less iron for the core.
The EI conversion in this supply is taking place in two stages, firstly in the transformer step-down from mains to 20-30VAC, then further in the subsequent switcher(s).


>> 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".

I don't know why they would phrase it that way but I haven't seen/read this TofOp.
E1.12 is the 723 supply pin (being supplied with +15V from D2), E1.11 is the control signal out.


>> 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... :-)