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Thread: Design challenge - switching regulator - update
picon face BY : Russell McMahon email (remove spam text)

Progress to date and then a QUESTION below

Several people provided useful input which was mostly along lines of thought
I had considered.
Nobody missed the point which is much better than usual!

Peter's idea of a small startup smps to power the larger smps which won't
run on low voltage is potentially OK BUT still problematic as there is
sometimes NO low voltage supply at first and the small smps would have to
run on the full HV supply (0 - 130 volts) or shut down when it was no longer
needed. Not too hard to do but extra parts required.

I've tried Roman's idea of going back to a simple fixed cycle PWM modulator
with on/off control - I'd been "unkeen" to try this due to the potential for
disaster which modern dual loop current mode controllers help prevent.

I've resisted (so far) going all the way back to a single transistor self
oscillating ringing choke (I built my first one of these almost 30 years ago
! :-) ).
They have too much potential for problems especially with my vast input
voltage swing.(4 to 130 volts if possible - even lower would be nice).

I've modified the simple fixed cycle PWM to alter its mark-space as input
voltage increases. Improves ability to handle voltage range well.
Still just using a packet of hex Schmidt inverters. Not very pretty though
and passives are starting to proliferate.

The UCC3803 (grandson of UC3843) seems a near ideal solution EXCEPT for its
price of $US1.50 ish in 1000 volume.
This is a very nice IC with 4.1 volt startup (very precise :-) ), current
mode, much elimination of passives from original circuit, soft start,
current sense noise blanking etc and can be used for buck, boost,
buck/boost, flyback, forward, Sepik, .... (I'm talking myself into using
this IC at this rate ! )

I'm now looking at old and horrible smps controllers in the hopes that
something cheap and nasty will also be OK and give me the <= 5 volts startup
Some which are not meant to (eg ref voltage about 5 volts) MAY behave
properly in all cases but this is of course a risky path to take.
(Once running full operating voltage is immediately available).
Candidates include eg TL494 (US0.50/1000), UC3524 (cheap), even just maybe
LM723! (v cheap, v horrible)

BUT - I'm getting keener on the UCC3803 every time I rethink it :-)
Overkill, but ...


Can anyone suggest an IC I may not have thought of that will do most of the
job that the UCC3803 will do but at rather lower cost.
Many ICs with a more modern origin cost the same or more (eg various MAXIM
Power out is very modest (600mA at 9v or so, 1,5A startup at somewhat lower
voltage) but recall the 4 volts or so desired start and run voltage and the
130 volts maximum input voltage.

I'll tidy up the hex inverter design to see how simple it can be made but
Maybe even dig out my vv old 1 transistor ringing choke designs?


           Russell McMahon

     Russell McMahon

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See also: www.piclist.com/techref/power/priswitch.htm?key=switching
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Subject (change) Design challenge - switching regulator - update

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