> I need to build an H bridge to drive a 1/8 HP DC 220 volt motor....I started
>trying with mosfets from International Rectifier (IR350)...I'm
>driving the transistor with 15 volt in the gate, but somehow I can't get the
>voltage drop between collector and emitter
                      ^
        [I'll assume you meant "drain and source"]

>to go under 12 volts, even when it's
>only 35 volts I am using to power a small 12V permanent magnet motor with no
>load,and I keep having this drop even if I'm turning a led on through a 2.2
>kohm resistor.
> You can imagine how hot the mosfet gets with that voltage drop and the
>inductive load of the motor.

Are you using N-Channel MOSFETs for switches in the H-bridge? Including the
"upper" switches? If so, then you need to drive the gates of the two
"high-side" switches with a voltage that is _higher_ than the Drain
voltage. What counts is the gate to source voltage. If, as it sounds like
you're doing, you are applying 15V to the gates of the high-side switches
then your high-side MOSFETs can never turn fully ON. You would need to
apply 35V PLUS the gate turn-on voltage (~5 V for logic level MOSFETs, 12 V
for others). The low-side switches are not a problem. It's the high-side
MOSFETs that are hard to drive.

There are several ways to drive high-side MOSFETs. The most straightforward
is to use optocouplers to do level translation so you can use a
ground-referred logic level to switch an auxiliary voltage level (35 V + 12
V) at the high-side gate. Usually this is inconvenient, though.

Another way is to use P-Channel MOSFETs for your high-side switches. Wire
these up Common-Drain (i.e. with their Sources connected to the supply
voltage), and their Drains connected to the motor terminals, a mirror image
of the low-side N-channel MOSFETs. You can then connect the gates of the
"left" side of the H-bridge together, and likewise those of the right. The
disadvantage of this method is that P-Channel MOSFETs are more expensive
than equivalent N-Channel devices, by about a factor of three.

Finally, you can use any of several "bootstrap" methods to do level
translation and provide the high-side drive voltage. These invlove either
using inductors to provide an inductive "kick" to charge up the high-side
gates when desired, or a flying capacitor arrangement situated between the
motor leads and the high-side gate switching circuitry (a charge pump, in
effect).


> Can you show me how the circuit should be like? I intend to build an H
>bridge for driving 220 volts of DC current.The only components I have are some
>opto-photo-darlingtons (the optocouplers work fine) which opto isolate the
>microprocessor signal from the mosfets,a 16c74,and the mosfets of course.
> How should I build that bridge?

I can e-mail you GIFs of the relevant circuit topologies.


> There is a fifth mosfet,between the bridge and ground,which is the only one
>working at 10 kherz (doing the Pulse width modulation).In that way,the bridge
>conducts pulsing current either on one direction  or the other,to make
>the motor run clock or counterclockwise at the desired speed.

Why do this? Why not just switch one of the low-side MOSFETs at 10 KHz?
That would be simpler and more efficient. Using a single MOSFET in the way
you describe is more appropriate for a relay H-bridge circuit where you use
the relay to change motor direction and the MOSFET to do the PWM.

--BN

----------------------------------------------------------------------------
   Country Robot            "Modular robot components
   69 S. Fremont Ave. # 2     for education and industry"
   Pittsburgh, PA 15202
----------------------------------------------------------------------------