H-Bridge Problem

[bilalbajwa]

My motors are very slow when i drive them via H-Bridge. OR i rather say they have not enough power to make my Robo move. When i lift my Robo The tire spins. I am been thinking about this thing for a while. And yes i did short out my R6,R7.
I separatly drived the motors with the batteries and they are just fine and have a lot of torque. But, with H-Bridge, torque decreased a lot.

[waltr]

Well R6 & R7 seem to be way too large. I would think maybe 10 Ohms for these.

What is the PWM duty cycle?
What is the PWM frequency?
Both have large effects (I am assuming you are driving the H-bridge with PWM to control speed).

Try just taking one input high and the other low. This should turn the motor full on. Then measure the voltage drops across each MOSFET that is on and R5, R6. This should tell you where the power is going.

Do any of the parts get warm?
I can't read the MOSFET part numbers, what are they?
What is the MOSFETs Rdson?

Do have an O'scope? If so, are the top and bottom MOSFET on one side both on at any time?
It is very possible that either the top or bottom MOSFETs are not turning all the way off.

H-bridges are simple in principle but there are some details that can really affect how well they work.

[bilalbajwa]

Our teacher told us to short out R6 & R7.

For now we are not using any PWM. We are just turning motors on and off.

None of the parts are getting warm.

MOSFET Part Numbers from Digikey: PMV40UN and PMV65XP.

 
Will you please tell me how to modify this circuit to get max performance. Or same torque as i get from direct battery connection.
 

 All of these components are surface mount.

[bilalbajwa]

So i guess no one knows what to do. I asked my teacher and he is also blank. Though he designed the circuit.

[guru]

I imagine you aren't the only one that cant get it to work. It imagine you arent getting enough voltage or charge to the gates of the MOSFETS. All your resistors are 10k. The bottom N channel is going to charge through a 20k ohm impedance, that's too high. Your top P channel will charge through a 10k ohm and discharge through a 10k ohm, thats too high too.

What voltage is Vcc? Most MOSFETS, except the TTL type, require voltages higher than 5v to switch. Perhaps you have the TTL type I dont know. Ever with the TTL I doubt those high impedances will be enough to switch those transistors complete on.

Your transistors arent hot now because they are only passing a little bit of current. A MOSFET is like a variable resistor when the gates is not fully on or off. But as you lower your impedances but the transistor is still not fully on or off (i.e. it is in the resistive zone) you can expect those mosfets too heat up a lot. This will depend on the current rating of the mosfet compared to the motor but believe me a small motor can heat up a large mosfet if the mosfet is spending much of it's time in the resistive zone.

Look up the turn-on voltage for the mosfets you are using. Then measure the voltage on the gate and see if it is high enough.

I just designed my own mosfet h-bridge and it's no small task taking all the part characteristics into consideration. I started with a resistor-like circuit like you see here. Even though the author I got it from said it worked, I doubt it. I couldnt get it to work with all the tweaking. Any change helped one thing but affected another. The solution was to redo it all and provide proper mosfet drivers for each mosfet. This means two 2n2222 or similar hooked up in totem-pole configuration so that on a high or low input a dedicated general purpose (GP) transistor is driving the gate of the mosfet high or low. You can also find these drivers in a package as a "mosfet driver" IC. For applicatons where the motors are small under an amp, the mosfet driver can drive the motors directly. For greater than an amp, use mosfets and drive them with the "mosfet driver".

Sorry, I think this circuit is doomed to fail....but I do not call myself a transistor expert.

C

[waltr]

Ok, those are logic level MOSFETs so you do need the Gate-Source voltage (Vgs) to be 4.5V to fully turn on the MOSFETs.

How much current does the motors draw when run directly from the battery?
What is the battery voltage? (same question guru posted)

I've also designed and built several H-bridges and did get them to work well but its not an easy task but it is a very good learning experience so keep at it.

Do you have an O'scope?
Are you using a SPICE program to model the circuit? This can be very helpful. Linear Tech has a free one available to download called LTspice.

Will you please tell me how to modify this circuit to get max performance.

You will have to make measurements and then determine what to change. Maybe start with just the high side MOSFET and get it to turn on fully. Then do the low side MOSFET working. After that combine just one each of the high and low side MOSFETs an get them working together. Lastly do the full H-bridge.

This is the DIVIDE and CONQUER method as it simplifies the problem into solvable pieces. Once each piece works then combine them into the finial circuit. I use this method in the professional realm in designing hi-tech instrumentation.

Good luch and be sure to have fun learning.

[guru]

Good point waltr about divide and conquor. I started with the half H-bridge, so one lower N channel and the opposing P channel. I did this because my mosfets were working in the resistive zone at first and causing shoot-through between the two H-bridge halves...very bad! Once I got the one side working then the other side was a piece of cake. However, I didnt really get it to work to satisfaction until I implemented the mosfet drivers.

I've included the schematics to my h-bridge. I also have some lecture notes that helped me out (they are not written by me.)

[guru]

Note the Larry Barello pdf above the pic, its kind of obscured.

Also, I simulated mine too like waltr suggested. It's much easier to test and see what is going on.

[waltr]

guru,
The transistor drives in your circuit is close to what I ended up using.

[guru]

I used the left hand one. That wasnt my pic btw, I got that somewhere off the net. I simulated both of them and I got better timing on the left one too. Also, using the same transistor allows me to get my parts in the higher bulk discount bracket.

Here is my transient analysis simulated with spice.

C

[Abdulla M.A.]

I think using one of the H-bridge IC is much better from wasting time in design a full bridge,
so I think L293, or L298 is very good IC.

Abdulla

[waltr]

I think using one of the H-bridge IC is much better from wasting time in design a full bridge,
so I think L293, or L298 is very good IC.

Abdulla

I now use a motor driver chip when I just need to control some motors but building your own is a very good electronics learning experience. I did learn a lot about MOSFETs when I built a few H-bridges. I'd say it would depend on your person goals. If you just want to build a bot so you can write code then use a chip. If you really want to learn electronics then build an H-bridge.

[guru]

The L293 or L298 are made from BicMOS technology (correct me if I am wrong). They will never get the performance or high-current control that a discrete mosfet h-bridge will. However, using the L293 or 298 to drive larger mosfets, that works just great. If the L293/8 is within your operating parameters than that is definately a cheaper=better solution too.

I agree too waltr. I learned a lot about mosfets by building a high power h-bridge too. It was worth the enterprise.

Funny that I should join this thread a few days ago and then blow two of my mosfets last night. I went from a wall wart 12v to 20v Lipo battery and it blew right away (a small jitter from the motors). The transistors are definately rated for it but I must have some shoot-thru transients or something going on. Under 12v operation these mosfets are cool without a heatsink so I am not sinking too much current on average. I have a big honkin' heat-sink on there now coupled with heat paste. They have got to be blowing because of transients.

I use a CPLD to control the hbridge. I have a "sanity" module (like a filter) that makes sure the signals to the h-bridge are ..well.. never insane! If the signals are wrong, then the sanity module outputs a default pattern (off pattern) and a fault signal. I have delays inserted in between switching directions (as P channels take longer to turn off than N channels). However, I wasnt changing directions at the time it blew, but PWM was toggling. I really need to buy a usb scope and see what's going on.

I think using one of the H-bridge IC is much better from wasting time in design a full bridge,
so I think L293, or L298 is very good IC.

Abdulla

I now use a motor driver chip when I just need to control some motors but building your own is a very good electronics learning experience. I did learn a lot about MOSFETs when I built a few H-bridges. I'd say it would depend on your person goals. If you just want to build a bot so you can write code then use a chip. If you really want to learn electronics then build an H-bridge.