Ken said (offlist)

> I like it (but feel am somewhat miffed as this is largely along the 
> lines
> that I suggested some time ago  - namely that the brake should be
> implemented using the H-bridge itself).

All input always much appreciated, but, to be fair to all, I'm  afraid 
I can only allow a tip of the hat in your direction on this occasion 
:-).
The aim has always been to provide braking and voltage regulation 
using only 4 transistors, less if possible :-).

ie yes, you did suggest both braking and voltage regulation within the 
bridge and I had of course wanted to achieve both of these as well 
both before and after you said it.

Others also suggested FETs and of course I wondered about FETs. 
Despite their great advantages I sought to avoid them due to the 
potential problems with getting the people doing the work to acquire 
them. Not an inconsiderable problem potentially.

BUT what made me take the leap was the concept of having BOTH bottom 
FETs always on in the inactive state with no drive current required. 
Measuring current on a 200 uA range there is no reading at rest! 
Removing the two gate drive diodes adds about 30+ uA quiescent when 
the top bipolars are reverse biased when turned off.

Even in retrospect using the two lower on-when-off FETs is not quite a 
wholly obvious idea. Just an obviously excellent one. It may well be 
standard practice elsewhere but I have never encountered it before. 
One more thing to add to my armoury.

The low side FET drive state relative to input drive is similar to the 
high side case when bipolar emitter followers are used, as was the 
case in the original. In the original the high side drivers are turned 
on when there is no drive and the unused high side driver is turned 
off when drive is applied leaving the other to remain biased on. This 
is what contributes to the original very poor voltage drop as the high 
side driver must not only drop 1 Vbe (as it is an emitter follower) 
but also the extra voltage required across its base drive resistor, 
which increases until it is high enough to provide 1/beta-th of the 
load current. The result is about 1.5 volts drop across the high side 
driver, which is unacceptable with flattish batteries.

The relevant low side FET is also "passively" turned on under drive 
but the full battery supply is available to bias it into full 
saturation (enhancement) and there is no gate drive current required, 
allowing a high value resistor to be used.

"Logic" FETs are required due to the need to operate down to under 4 
volts supply.


        Russell McMahon

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