I recently experimented with the Dual Serial Motor Control PIC driving a discrete MOSFET H Bridge. The H Bridge was made of two IPS0551 and two IPS5551 MOSFET switches. The only other parts used were four 1k ohm resistors, one on each MOSFET input. A BS2 was used to create the Serial Out commands to the PIC. Here is what I found:
-
The PIC outputs float during the serial communications.
-
Two pins (13, 14) on my BS2 are burned out.
-
The PIC doesn’t respond at Baudmode = 32 (19.23 Kb/s), but works flawlessly at Baudmode = 33 (18.87 Kb/s) or Baudmode = 34 (18.52 Kb/s).
-
Turning the motor on, off or reverse at 12, 24, or 36 volts works great.
-
I tried the example speed up/speed down/reverse speed up/reverse speed down routine. The motor that I was using is a new, 12 volt DC automotive power window motor with worm drive. The motor “sings” at any speed less than about 80% of max. During the “singing” the MOSFET switches generate a lot of heat.
-
The thermal shutdown on the MOSFET switches works!!! They reset during the standard PWM “off” states or when the BS2 sending data to the PIC.
-
The “singing” of the motor is the result of the relatively low PWM frequency of 600 Hz for the PIC.
-
I tried a smaller motor at 24 volts. The speed up/speed down routine didn’t generate any “singing” but resulted in a very smooth speed transition
-
Conclusion: smaller motors will work better with the 600 Hz PWM frequency than larger motors.
-
I tried our big sumo motors (Peg Perego). They “sang” at speeds under 10% of max. Using 12 volt power, the Peg Perego motor drew 2.30 amps and combined high side and low side H Bridge voltage drop was .024 volts. Using 24 volt power, the motor drew 4.00 amps and combined high side and low side H Bridge voltage drop was .044 volts. Using 36 volt power, the motor drew 6.20 amps and there was .070 volts total drop in the H Bridge. This calculates out to about .005 to .006 ohms per MOSFET switch. This correlates nicely with the MOSFET specifications.
Rick Brooks