> I had a look at the design, and figured if you are using MOSFETs, 
> why not put them in the H bridge?

I looked at the design quite carefully at the time, I thought, but 
missed it's key point. It's brilliant. I don't know if it's workable, 
but brilliant things don't have to work. I looked at it just now, saw 
it didn't do what you said (I thought) started to explain why, and 
then realised I'd misunderstood it. The FETs being turned hard on in 
the both-inputs-off state is of course the key.

**** NOTE ****     Anyone intetrested DO look at Michael's circuit. 
The key is that BOTH FETS are on as of right when the bridge is at 
rest. This is acheieved with no quiescent current (other than FET gate 
leakage of typically << 1 uA). This provides an "automatic" motor 
brake. The voltage regulator may work :-). Q5, Q6 will need base 
pulldowns (based on my experiences with this sort of regulator).

__________

I'm sure it will have problems as is (life's like that) BUT it has the 
makings of an excellent solution.

I'll go and simulate it in the only way that works when you are using 
things like motors (hard silicon) and see what happens. Expect a 
report and quite possibly a slightly modified real world design 
"soon".

> I haven't simulated this design, my worry would be making sure
> the MOSFETs turn off before the PNP switches on
> ( which I guess may not be possible without a few more components ).

A small amount of "shoot though" may be tolerable in this case, 
although I'll try to avoid it.

> However, it does solve the motor braking problem without any 
> additional components,

It does !!!!!!!

> and without a high quiescent current.

Yes.

> Using two zeners does feel a bit clunky, I guess you
> could use steering diodes, and one zener and turn on
> Q5 and Q6 at the same time with no problems.

Probably. but you will probably need to retain both R8 and R9 to allow 
'spreading' of  Q5 & Q6.

___

I've just seen a potential problem, similar to what I had when I used 
a similar arrangement of the bridge to limit current rather than 
voltage. As you start to switch off eg Q2 to limit current you'll also 
start to turn on M1. This is a way to limit motor voltage but at the 
expense of dissipation in M1. Fortunately the Vth of the FET helps us 
here (for a change). As long as M1 gate is below pinchoff voltage it 
wont shunt current from the motor. Suitable design of R7/R4 can 
arrange that Vth is never reached during normal voltage regulating 
operation.

Also - with my design the brake and FETs are an add on option. With 
yours the FETs (and brake) are integral to the bridge operation. This 
is not terrible but is worth noting.

> Anyway, that's my coffee break gone, back to work!

Brilliant. Have a virtual coffee on me at this stage pending 
investigation.
At this stage the gains look moderately modest over my working design 
but very worthwhile if they can be made to work in the real world. I 
have 2 x FET and  10 bipolars. You have 2 FETS and 6 bipolars. Glue to 
be decided.

The latest console turned up by Courier from Taiwan today and has been 
taken away to courier's not-at-home-must-sign-see-you-tomorrow-land. 
So I have a day to get this right :-).


                Russell McMahon




Cheers

Mike

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