Re: Determine switching frequency of photocoupler

There really isn't a need to isolate them. I just liked the fact that
the optocoupler was a single IC chip as opposed to multiple
transistors. I just thought it would be fast enough!

At any rate, I guess I will switch to no isolation with a design like
Fred suggested.

I have a few more questions though, if anyone is still following...

This logic driving MOSFETs is for an H-bridge motor driver. There is a
Motorola HCS12 microprocessor providing the logic PWM driving signal,
and the MOSFETS are IRF1405 (large), digikey part IRF1405-nd.

The remainder of this h-bridge design is simple 4 of these MOSFETs set
up in a typical h-bridge. Two drains are tied to 24V, their two
sources are tied to the drains of the other two MOSFETs, whose sources
are 24V ground (system ground for 5V, 12V, and 24V). The motor output
is taken between the common source-drain connections of the MOSFETs.
These MOSFETs do have built in recovery diodes.

The logic is continually driving all four MOSFETs. One of 3 scenarios
is possible. The upper left and bottom right on (motor "forward") in
this case the upper right and bottom left are off by driving zero pwm
signal, the upper right and bottom left are on (motor "backward") in
this case the upper left and bottom right are off by driving zero pwm
signal, or all four MOSFETs are off by driving zero pwm signal. There
is one PWM signal and one "direction" bit from the processor, which
other hardware assists in creating the PWM inputs to each gate. The
opposite corner gates are tied together (upper right and bottom left,
and upper left and bottom right).

I have been testing this circuit using the original optocoupler with a
3k pulldown into the gates of the MOSFETs. The signal into the
optocoupler is clean, the signal out (going into the gates) is more
sine wave like than square, and is the part I will be cleaning up with
Fred's suggestions, but even with a sine wave it will turn a small
unloaded motor in each direction properly. Everything looked pretty
good.

When I hooked the driver up with the same setup into a bigger motor,
which had a small load on it, everytime I had the processor send the
signals to turn the motor in one direction or another, the processor
would reset, and the driver would not output the correct signal. I
haven't had a chance (yet) to do further testing to find out why it is
resetting, but I'm having some trouble coming up with initial guesses.

One problem that I did notice when looking at the gates of the MOSFETs
when driving them, is that the MOSFETs that *should* be off (by driving
zero PWM signal into the optocoupler diode) are seeing some noise (up
to nearly 5V) at the 10kHz frequency that the opposite MOSFETs are
being driven at. The gates of these "off" MOSFETs are connected to the
collector of the optotransistor (which should not be conducting), and a
3k resistor to ground. This should mean they are grounded, yet they
are seeing this noise. This made me think that for some reason
something is going on with my ground, and this could potentially cause
the processor to resetting. I did not see resetting on testing the
small motor with no load, but on the larger motor, I did.

What could the problem be with my design? Unfortunately the 5V, 12V,
and 24V commons cannot be isolated, and up to this point with many
other components, there has not been any issues.

An additional problem with the h bridge design is that when the MOSFETs
are suddenly turned off when the motor is on and moving a load, the
motor's back EMF will need to be handled. I have no determined exactly
what the effects of this voltage are in this design, but I will need to
in the future. First thing is to get the processor to stop resetting.

Sorry for such a long post, and for asking some silly questions (or
having silly problems, if they are), but I am pretty new to the EE
scene and many "tricks of the trade" I have yet to learn :)

.

Relevant Pages

  • Re: Determine switching frequency of photocoupler
    ... Two drains are tied to 24V, their two sources are tied to the drains of the other two MOSFETs, whose sources ... The upper left and bottom right on (motor "forward") in ... this case the upper right and bottom left are off by driving zero pwm ... signals to turn the motor in one direction or another, ...
    (sci.electronics.design)
  • Re: Why did my drive blow up?
    ... the MOSFETs literally exploded. ... since my friend built a drive from the same design and tested it ... completely with a small motor. ... voltages that the MOSFET zener diodes did not suppress. ...
    (comp.robotics.misc)
  • Re: Driving Dual MOSFETs From A NPN Transistor
    ... I'm working on a circuit that uses a NPN transistor to drive (i ... the right word) two MOSFETs. ... A 50 amp, 36V motor. ...
    (sci.electronics.basics)
  • Re: Voltage Gain Switch (Design Help)
    ... so I will have to use a decoder to drive the MOSFETs accordingly. ... From what I gather, more VGS voltage better ... The obvious solution needs two CD4002 quad 2-input NOR gate packages, ... signals at 1.6 kHz will be down by 100x, ...
    (sci.electronics.design)