I'm pretty sure that without the resistive hysterisis the regulator entered
a "linear" mode. Possibly with the cap. accross the load the current kick
would be enough to start it unless the supply came up slowly. Certainly as
a relay driver it needs the feedback.

Richard P




Appears to be a very nice circuit.
Similar in concept to mine but eliminates a transistor by emitter driving
the control transistor.
And the zener is a true reference so can have its current controlled
independently of feedback.
The high side drive is no better (in fact no different) than in my circuit
and would benefit from the same optimisations if a FET was used as high
side
switch. (eg a bipolar high side FET driver).

As well as Dave's suggested changes It would be worth trying removing R9
(replace with O/C) and R2 (replace with S/C).
This would remove the resistive hysteresis and end up with the same
hysteresis system as in "my" design.

Also, for higher input voltages consider driving D1 (now a zener as per
Dave's changes) from the OUTPUT via a resistor and tapping the emitter of
Q1
off a resistive divider off the output. Now a high value of resistor from
Vin to D1 will initially bias the zener on and once the system starts,
zener
current will be provided from Vout. This reduces the otherwise not
insignificant power taken by the zener bias current.

I realise that anyone who hasn't looked at and modified the circuit
diagrams
will be unable to make any sense of this. I'll try to get to post a GIF of
this 'shortly" unless someone else beats me to it. Dave?




            Russell McMahon







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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