22 Sep
2018
22 Sep
'18
11:54 a.m.
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
23 Sep
23 Sep
1:45 p.m.
On 2018-Sep-22, at 11:54 AM, Noel Chiappa via cctalk wrote:
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.
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.
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.
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.) 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 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).
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... :-)
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2283
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bhilpert＠shaw.ca -
jnc＠mercury.lcs.mit.edu