Newsgroups: rec.models.rc
Subject: Re: ESCs - brake transistors - why?
From: kaiser@elec.canterbury.ac.nz (Chris Kaiser)
Date: 21 Aug 92 10:59:29 +1200
Distribution: world
Organization: Electrical Engineering, University of Canterbury, New Zealand

In article <1992Aug19.213712.25560@proton.llumc.edu>, borodin@hope.llumc.edu (Sergi Borodin) writes:
> 
>  When I was trying to understand (I want to built one) the principles
>  of ESCs, I realized that they don't need special (dedicated for this
>  purpose) brake transistors. Since the motor is driven by a full bridge
>  MOSFET amplifier, this is the matter of control logic how to make the
>  motor to stop. Imagine the four switches (A,B,C,D) and the motor (M):
>  
>     A   B
>       M
>     C   D
>  To turn the motor in one direction we close (let's say) A & D, in 
>  opposite - B & C.  

So far so good...

>  Obviously, in order to brake we would need to close either A & B or
>  C & D.

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. Assume A & D were conducting for forward rotation, then switch on A & B to
brake - A is now reverse biased since the current under braking flows out of the
motor. If C & D were used for breaking then D would be reverse biased. Likewise
if you started with B & C conducting, then when you breaked either B or C would
be reverse biased.

>          Since the law of the conservation of energy still applies, 
>  the current supplied by the motor cannot exceed the current had been
>  applied to it by the drivers, e.g A&D or B&C. So transistors cannot
>  be damaged (besides the pulse current is much higher). The whole 
>  circuit will be much cheaper also.

I don't think the reverse biasing will do them much good...

>   I understand that the reason of using brake transistors might be
>  outside of technical ideas, and might be dictated by the desirable
>  performance. 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.
  
>   So, experts, WHY ???

Not an expert by any means :-), but hopefully I've sort of explained why.
  
>   I was able to locate (DGK) 50 Amps n-channel MOSFETS ($2.50 apiece),

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, so I wouldn't use anything
with significantly higher resistance unless you parallel them up. In fact most aircraft
speed controls use up to 6 (or more) of these (or similar)  MOSFET's in parallel!
When combined with high switching frequencies, this gives extremely high efficencies.

>   but I couldn't find p-channels with the same current ratings -
>   anybody has the idea?

I think p-channel MOSFET's are much more difficult to make - something to do with
the silicon substrate being p or n only, requiring different constructions for
the two types of FET's.

>   S.B.
> 
>  
Newsgroups: rec.models.rc