H-bridge help

[calhau0]

hello

i'm trying to make an H-brige to control a car with a arduino

in order to avoid having to drive the top N-fets i replaced them by P-Fets (M5 and M6) , and used a Nmos inverter to invert the signal (M1 and M2)

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the H-bridge is working fine except the fact that it has little current flowing to the motor,
it works fine while the wheels are up in the air, as soon as i drop the car it stops or goes very slow

any ideias why this is happening?
for the N-fets i'm using IRFBC40 for the P-fets IRF9612 (both handle a pretty decent current)

thanks

 

[ronv]

I'm guessing your micro operates on 5 volts, so you need to use logic level NFETs or it won't turn them on well.

 

[dr pepper]

Yes as above the fets might need more drive volatage, your car motor is probably only getting a few volts, the hi side looks ok, the issue will be the low side.

You could try 2 more fets on the n channel part of the bridge, and use a pullup resistor to 12v on the gates the same as you've done with the + side, if you do dont forget to invert the drive from the micro as the gate drive fets will invert the on state, ie hi = fets off.

You can get automotive rated fets that will turn on fully at 5v, I've never used such devices.

Another way to get that circuit to work is to use gate drive transformers and rectifier caps on the gates of the fets, the xformer steps up the drive voltage and isolates the gates, so the + side fets can be n channels without extra gate drive devices.
There is a little software overhead, depending on your pwm freq you need to keep pulsing the fets to keep the rectifier cap charged or discharged.

 

[calhau0]

well i applied manually 15v to N-fets gate and got the same result, so i guess the output voltage of the micro isn't the problem

 

[ronv]

Both the top & bottom ones? You should also add clamp diodes while your at it.

 

[calhau0]

i measured the current avaible to the motor (by locking the wheels) and i got about 0.9 amps

then i build a half bridge with a N-channel mosfet and got 2.3 amps (wich is pretty much the maximum i can get from my voltage source)

then i build another half bridge with the P-channel mosfet and got 1.1 amps (but the data sheet says it can handle 1.5 amps?) i will try replacing the P-channel fets

Both the top & bottom ones? You should also add clamp diodes while your at it.

yes both top and bottom ones

sorry for my begginer question but what is a "clamp diode"?

 

[dr pepper]

If you have 15v supply and your gate drive is 15v the fets should be fully on and only present rds on which more likely is less tha an ohm, something else might be up with your circuit.

Clamp diodes are diodes connected in reverse from each side of the motor to the load and ground, the idea that the back emf from the motor is restricted from going higher or lower than the supply.
You fets might be dead for this reason mos can be sensitive to overvoltage.

 

[alec_t]

100k seems too high a value for the inverter resistor. If it fails to pull up the M5/M6 PFET gate high enough to turn the FET fully off, then some of your motor current will be lost to the PFET when the N3/N4 NFET below it turns on. Try 10k, or even 1k, instead of 100k.

 

[ronv]

If your motor only draws .9 amps it sounds like both fets on the same side of the H are on (1.1 amps). The smaller resistor is a good idea, but make sure it is top right and bottom left or top left and bottom right that are on.
Ground on the gate of P FETs will turn them on, +12 on the gate of the NFETs.
Here's a picture of clamp diodes.