# HELP : Question about DC-motor driver

Discussion in 'Electronic Design' started by bamse, Jan 25, 2004.

2. ### bamseGuest

And why use MOSFET transistors instead of other transistors?

4. ### John PopelishGuest

I was just about to post and ask you if you had read the data sheets.

The gate resistors are there to spoil the very highest frequency
response of the fets to discourage ringing oscillations every time
they switch that might keep going and destroy the fets with excessive
losses.

As to how the IR2104 works, start with the block diagram on page 4 of
the data sheet. What there does not make sense to you?

5. ### MacGuest

The resistors are there to dampen the ringing which would otherwise occur
at the FET gates.

FET's are good for driving high current loads because they dissipate very
little power when they are on, and almost none when they are off.

The main drawback to discrete power FET's as switches is that they have a
lot of gate capacitance, so switching them fast requires a lot of drive
current.

There's lots more you should know about FET's before you use them. Start
by going over the datasheet for one and trying to understand all the
absolute maximum limits.

Mac
--

6. ### John SmithGuest

Your question about the resistors has been answered.

As for the driver, if /SD is held high, the upper FET is off and the lower
FET is on when IN is low. The upper FET is driven on and the lower FET is
driven off when IN is low.

When /SD is low, both FETs are off.

One end of the motor is connected to the FETs. Assuming the other end of the
motor is connected to common: when the upper FET is on, the lower FET is off
and source voltage is applied to the motor. When the lower FET is on, the
upper FET is off and the motor is shunted by the FET. This latter condition
will brake the motor.

If you don't need braking, you can use one logic-level FET and eliminate the
IR2104. In this case, attach one end of the motor to the + supply and the
other end to the FET drain.

HTH

John

7. ### John SmithGuest

RATS! I meant the upper FET is driven on and the lower FET is driven off
when IN is HIGH.

Sorry.

8. ### Bill SlomanGuest

At Cambridge Instruments we were reputred to have had a couple of
cases where the MOSFETs oscillated at a couple of hundred MHz if you
left off the resistors - the ringing was infinitely sustained (at
least until MOSFETs got hot enough to blow up).
The data sheets for most MOSFETs, including the RFP40N10, include data
on the charge that has to be shifted into or out of the gate to get
the device to turn on or off.

9. ### Winfield HillGuest

Bill Sloman wrote...
Not surprising with open gates. Power MOSFETs, especially high-
voltage ones, love to oscillate when biased in their linear mode
at currents of say above 5% of Imax. An open-input FET gate can
certainly find itself at the right voltage for such action. You
were referring to omitting the resistors on a PCB which had them?

Thanks,
- Win

whill_at_picovolt-dot-com

10. ### Bill SlomanGuest

More probably seen on bread-board lash-ups. I was reporting hearsay
evidence - hence the "reputed" above - from a time where we tended to
bread-board our prototypes, or cut and link them onto printed circuit
boards laid out for older circuits.

11. ### Ben BradleyGuest

Depending on the circuit (things like switching speed and the
amount of power involved), the FET's are not neccesarily destroyed.
The circuit may actually work okay, right up until the time the FCC
comes around looking for whatever it is that's interfering with your
neighbor's broadcast TV reception (there's always one in every
neighborhood who still watches off-the-air local broadcasts instead of
cable/dish/DVD's).
Did I ever fix a legacy design that used mosfets as switches with
no resistors in the gates that worked fine except for grossly
unacceptable RF emissions when this part of the circuit was activated?
No comment...

This circuit isn't used to drive the motor for rotating a TV
antenna, is it? If the circuit is made without the gate resistors, I
could imagine the oscillation being powerful enough to damage the RF
front end of the TV tuner. You certainly wouldn't receive a TV signal
while rotating the antenna.

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