Newsgroups: rec.models.rc
Subject: Re: RE:ESCs - brake transistors - why?
From: kaiser@elec.canterbury.ac.nz (Chris Kaiser)
Date: 24 Aug 92 12:10:45 +1200
Distribution: world
Organization: Electrical Engineering, University of Canterbury, New Zealand

In article <1992Aug21.013910.3184@proton.llumc.edu>, borodin@hope.llumc.edu (Sergi Borodin) writes: 
> kaiser@elec.canterbury.ac.nz (Chris Kaiser) writes:
> 
> >>In article <1992Aug19.213712.25560@proton.llumc.edu>, borodin@hope.llumc.edu (Sergi Borodin) writes:
> 
> >The problem with this is that when braking one of the transistors will be 
> >reverse
> >biased, i.e. you're trying to force current through it in the wrong direction.
> >e.g. 
> 
>   In fact, it is not a problem here. All power MOSFETs have an intrinsic 
>   rectifier, or body diode, a junction rectifier whose current rating is
>   the same as that of transistor. Since the direction of the diode is the
>   opposite (e.g. cathode of the diode is connected to the drain and the
>   anode - to the source) as that of transistor, it is IMPOSSIBLE to 
>   reverse bias it. Besides, if those diodes would not exist we should invent
>   them anyway.

I've got a circuit somewhere that uses BJT's in a bridge connection as you described
for forward/reverse speed control. This uses flyback diodes connected across each
transistor (reverse direction of course). Presumably if I used MOSFET's I wouldn't
need these diodes?
 
> >>  For instance, it could be bad (I don't know) to apply
> >>  full brakes and a small resistor (to acheive lighter braking) might
> >>  be desirable.
> >
> >This would also be a reason - for speed control the transistors would be
> >pulsed between full on and full off. When braking however, you want the
> >current to be continous but variable - varying the brake current of load
> >varies the amount of braking effect.
> 
>   How can one vary the braking current if there is a constant resistor?
>   In other words, you can't adjust the braking effect.

I think most brakes on ESC's use a MOSFET as a voltage controlled resistor, i.e.
operating in their linear region. This allows the braking effect to be controlled
by the driver. What I was trying to say above was that it may be easier to use
a separate MOSFET for this rather than having two drive circuits to the bridge
MOSFET's (one pulsed full on/off for speed control and the other linear for braking).

 
> >Do you know the on-resistance of these MOSFET's? Motorola MTP50N05's and
> >IRFZ 40's (both cheap and readily available) are about 0.028 ohms,
> 
>  The transistors I'm talking about are IRFZ40.
> 
> > In fact most aircraft
> >speed controls use up to 6 (or more) of these (or similar)  MOSFET's in 
> >parallel!
> 
>   It leads us up to 30 MOSFETS in total, doesn't it? Hard to beleive.
>   I read in RCM that (well, I didn't touch an ESC, though) there are
>   mostly like a couple of transistors in parallel.

One article I read said that paralleling up more than 3 IRFZ 40's was pointless
because their combined on-reistance (about 9 milliohms) was less than that of a
single nicad cell! However the drive for 101% efficency demands more MOSFET's!

The aircraft speed controls don't have reverse, so they only need a single set
of switches instead of a bridge. This allows them to use large numbers of MOSFET's
paralleled together.

You could also do tricks like using more MOSFET's for forward than for reverse,
since presumably you wouldn't use reverse that much.

Actually, now that I think about it, do ESC's with reverse also have brakes?
I imagine that that you could brake pretty well by going into reverse!


Chris Kaiser

Postgrad, Elec. Eng. Dept.
University of Canterbury
Christchurch, NEW ZEALAND