piclist 2001\01\18\075308a >
Thread: Design challenge - switching regulator
www.piclist.com/techref/power/priswitch.htm?key=switching
picon face BY : Russell McMahon email (remove spam text)



A "design challenge" seems to be the best way to get input - hopefully :-)

I need to implement a switching regulator of modest power output but
somewhat outside the scope of the normal "off the shelf" simple-switchers or
their ilk.

At present I'm prototyping using the time honoured and capable UC3845 or
equivalent (3842/3/4/5 family) but it's minimum startup voltage of around 9
volts is annoying.
I'd be interested in anyone's suggestions on a design which at least meets
the following specification.
When I ask this sort of question here the answers are usually interesting
and often enough add to my general body of knowledge BUT they often also
fail to address the actual problem as proposed. To be useful to me the
design MUST meet all the vital specs and as many of the "useful" specs as
possible.

The 100 volt  (occasionally 130 volt) input requirement eliminates most
integrated solutions.

VITAL REQUIREMENTS

1.    Operates from an input of typically 10v to 100v DC but input MAY rise
to 130 volt DC for up to 2 seconds (and ideally will operate on a much lower
voltage than 10 volts).

2.    Mean maximum output 600 mA DC at 9 volts (5.5 watts) BUT must be able
to supply up to 1.5A at 7.5 volts for 1 second.

       OR

       6.5v at 600 mA plus 1.5A as above AND a separate 9 volt 75 mA output


3.    Must start with worst case load applied with an input voltage of 10
volts or more.

4.    As cheap as reasonably possible.

5.    Acceptable EMC performance in a stand alone battery operated
equipment.


DESIRABLE REQUIREMENTS


5    Very low cost of manufacture including components, magnetics and
assembly.

6.    Isolated or semi-isolated design so that failure of the switching
supply will (probably) not destroy the powered circuitry
    ie a buck regulator would probably not be a nice solution as failure of
the pass transistor exposes the output circuitry to 100v plus.

7    Start and run from 4 volt input



=================================================================

The UC3845 which I am presently using provides current limiting and voltage
regulation.
High voltage operation is possible due to use of an external pass FET.
A flyback configuration provides semi-isolation (ground common but not
direct path from 100 volts input to the output).
The IC is cheap (well under $US1).
The power FET is cheap (under $US1).
Magnetics are not certain yet - powdered iron toroid or small ferrite U core
both have advantages.
Startup voltage is 10 volts plus but once started it will operate down to a
few volts input.


Your ideas?


regards

               Russell McMahon

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