As I read up more on h-bridges, I understand better why you say this=20
(about switching time and linear region).  I chose 50khz just thinking=20
it's well outside the upper audible range) but that's not a critical=20
parameter.  AIUI, if I drop to 20Khz, the drive requirements get much=20
simpler.

However, I just discovered this seemingly neat device -- VNH5019. At=20
US$8 with minimal other support components required, is there any reason=20
I should really be trying to engineer my own?  From what I see of=20
discrete-component versions, I'm very much in the same price range already.

Cheers,
-Neil.


On 12/17/2014 5:11 AM, IVP wrote:
>> Usually you don't need vast average gate drive mA - you need
>> around an amp while the FET is switching but most of the time
>> the gate does not require current to hold it on or off
> AIUI the current is needed mostly to charge the gate cap quickly to
> keep the switching time low (ie keep out of the linear region as much
> as possible). And once turned on, the FET is voltage, rather than
> current, driven
>
> I PWM a 1/2HP 12V motor with 3 paralleled FETs using a BC337/
> BC327 push-pull pair and that works fine, even though the BCs are
> rated at only 500-800mA. Stall / start current is many times the run
> current so I do tend to try and remember to soft start
>
> Neil, useable speed / torque might not kick in until the PWM is quite
> high. I think mine is around 30%. Below that the energy simply heats
> the windings without the motor turning usefully. This may or may not
> be a problem. For example if there's fan cooling on the shaft
>
> My scrollsaw has a *proper* variable-frequency speed control
> for the induction motor and can be run slowly enough (with good
> torque, so it's tempting to use it too long) to cause damage if forced
> cooling isn't used to assist the internal fan
>
> Joe
>
>
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