Thanks for the suggestions & comments - as I noted, it works OK as a relay
driver (hence the high inductance) but I was too busy last week to try any
changes to make it a straightforward buck regulator.
Richard P




                    Dave Dilatush
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                    Sent by: pic          Subject:     Re: [EE]: Design Challenge - low power step down switching              regulator
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                    27/08/01 11:14
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Richard Prosser wrote...

>How about something like this.
>The circuit attached is used to drive a 5V(~15mA) relay off a supply
>ranging from about 20 to 70V.
>I can't guarantee all the component values but they are about right. The
>coil is the relay coil and the current is sensed by R7. If R7 became the
>load and D1 became a zener at about 5.6V then it should work as a buck
>converter. Transistor types are BF422/BF423 or BSR19A/BSR20A for the smt
>version. Similarly the IN4148s are actually BAX12 or BAV99.  D2 stabilises
>the feedback so that the hysterisis is stable with supply voltage. Input
>voltage is limited mainly by transistor & diode ratings.

I think this circuit is going to end up the winner.

After setting this thing up in SPICE and making a few tweaks (see
below), I got some really good efficiency results.  At 30V in and
5V/20mA out, efficiency was over 75%.  With that high an input/output
differential, and that low an output current, that's a pretty
impressive figure.  The efficiency holds up well even at Iout = 10 mA,
which is downright amazing.

As much as I liked my own design, this one has it beat both in cost
and efficiency.  For a micropower switcher, it's going to be hard to
do any better than this.

The changes I made while fooling around with it are as follows:

1.  Added a 100uF filter cap at the output, across the load (you have
    to have one, otherwise it won't likely oscillate);

2.  Changed L1 to 2.2mH to reduce cost/size;

3.  Reduced R5 to 47K to increase Q2's base drive a little;

4.  Eliminated R1 and C2, tying Q1's emitter directly to the output;

5.  Eliminated R8 and D2, and tied the right-hand end or R9 directly
    to the top of L1.

6.  Made D1 a 1N4734 (5.6V) Zener diode and flipped it over; and

7.  Rearranged D1, R10 and R2, by connecting R10 directly to the (+)
    end of D1 and moving R2 to a position between that junction and
    the junction of R9 and Q1's base (this made a big improvement
    in line regulation).

Nice circuit.

Dave

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