SMPS oddities
 

Hi all,

not exactly reapir related but, I think, interesting.

I've been studying SMPSs for a while out of personal interest, and I stumbled
upon an old schematic for a discrete component-based SMPS:

http://bitman3.altervista.org/SMPS.png

As I understand it, this is a self-oscillating SMPS operating in current mode.
T302 is the main switcher, Th301 is the turn-off SCR, T301 acts as the voltage
error amplifier whose output controls the maximum current on the primary coil.
In particular, the cathode of Th301 is subject to a sawtooth negative voltage
due to the primary coil current flowing through R317, while the gate voltage
is controlled by the error voltage amplifier T301; the main switcher is turned
off on a cycle-by-cycle basis when Vgk on Th301 reaches the turn-on threshold,
and this determes the duration of Ton.

The general principle of current mode operation is that a lower than expected
voltage on the secondary should result in a longer Ton. My problem is that in
this case it seems that the opposite is true! In fact, the secondary rectified
voltage drives the base of T301: a higher secondary voltage results in a lower
current on R304 and thus in a lower Vg on Th301. This, in turn, means that a
higher magnitude of primary coil current is needed on R317 to turn on Th301,
resulting in a longer Ton.
In short: the higher the secondary voltage the longer the Ton, positive
feedback, BOOM!

I haven't got the real thing on hand so I cannot check the waveforms. I assume
it's been built and it works, so I'm obviously missing something here.

Can anyone shed some light?

Thanks in advance,
-- bitman

SMPS oddities
 

Bitman wrote in message
...
Hi all,

not exactly reapir related but, I think, interesting.

I've been studying SMPSs for a while out of personal interest, and I
stumbled
upon an old schematic for a discrete component-based SMPS:

http://bitman3.altervista.org/SMPS.png

As I understand it, this is a self-oscillating SMPS operating in current
mode.
T302 is the main switcher, Th301 is the turn-off SCR, T301 acts as the
voltage
error amplifier whose output controls the maximum current on the primary
coil.
In particular, the cathode of Th301 is subject to a sawtooth negative
voltage
due to the primary coil current flowing through R317, while the gate
voltage
is controlled by the error voltage amplifier T301; the main switcher is
turned
off on a cycle-by-cycle basis when Vgk on Th301 reaches the turn-on
threshold,
and this determes the duration of Ton.

The general principle of current mode operation is that a lower than
expected
voltage on the secondary should result in a longer Ton. My problem is that
in
this case it seems that the opposite is true! In fact, the secondary
rectified
voltage drives the base of T301: a higher secondary voltage results in a
lower
current on R304 and thus in a lower Vg on Th301. This, in turn, means that
a
higher magnitude of primary coil current is needed on R317 to turn on
Th301,
resulting in a longer Ton.
In short: the higher the secondary voltage the longer the Ton, positive
feedback, BOOM!

I haven't got the real thing on hand so I cannot check the waveforms. I
assume
it's been built and it works, so I'm obviously missing something here.

Can anyone shed some light?

Thanks in advance,
-- bitman


Try sci.electronics.design.
I got some useful advice , posting about a SMPS problem a week or two back

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/