Re: DC Motor Control: H-Bridge +5A, 48v

On 3 May 2005 21:13:40 -0700, "Ethan" <idethan@xxxxxxxxx> wrote:

>My problem was more due to the EMC susceptability that the ground
>bounce from switching transients. The HIP4082 has logic internal that
>is supposed to make it impossible for the top and bottom FETs from
>turning on at the same time. The example circuit in the data ***
>shows the input logic holding the high side FETs on all the time and
>relying on the guts of the IC to take care of everything.
>Unfortunately the internal logic would screw up when exposed to just
>the right (wrong) RF signal, and the fireworks begin.

In a previous life I worked for a electric forklift manufacturer.
We designed our own motor controllers. It could PWM about 700A
at 36V into each of the lift and traction motors. Fun stuff,
where interesting things happen when things fail. The first
version used the HIP4081, but it was later redesigned to use the
HIP4082 to save some money.

Now it's been a while so you might want to check the datasheets
to make sure I know what I'm talking about (I am on a slow dialup
connection at the moment or I'd check myself), but I seem to
remember one huge difference between the two chips.

The 4081 has built in oscillator for the charge pump circuit.
That means you can turn on the upper FETs and just leave them on.
The 4082 doesn't have the oscillator. It just uses a cap and
diode to power the upper FETs. So that means that it needs
constant PWM of the upper FETs to keep the cap charged. We just
changed the software to put a maximum PWM of 0xFE out of 0xFF,
and it worked fine.

With the 4082, if you just leave the upper FETs on at 100%, the
cap will quickly discharge and the gate voltage will start to
drop and bad things will happen. :-)

Carl Smith
.

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