Pwm controller design help needed

[kallileo]

I'm control/automation engineer so I'm not very experienced with electronic circuit desing.
I need to control two dc motors using a PLC. Unfortunately the PLC has only one PWM channel so I can't control the speed of the second motor.
I decided to design (using my limited knowledge and sample cirtuits) a custom voltage controlled PWM controller which will be powered from the analog output 1 of the PLC and the PWM will be send from analog output 2. The PLC and both motors are powered by two 12V battery connected in series (24V).
The circuit is as in the picture attached.
I build the circuit on breadboard and seems to operate correctly but I think it needs some additions like as noise reduction capacitors and some circuit protection components.
The output current of the analog output is 10mA Max so while LM324 consumes only 2-3mA it seems that it's not enough for the optocoupler as at 100% duty cycle I measured around 15mA.
Would a low power quad op-amp and optocoupler help with this limitation?

[fcb]

I would (be tempted to) redesign the LM324 circuit to run on 5v, and use a low drop-out regulator to on the switched OP.

Also, as you point out, try using an low-power opto-coupler - there are other devices out there that do the same job and possibly use a lot less power.

Another thing - watch the Vge max on that IGBT - your hitting it with 24V.

If you don't need fast PWM, then you might also want to try adding some gain on the photodiode side, you can then cope with a lower transfer ratio on the opto.

That said - if the design works, then stick with it!

[kallileo]

I want the circuit to be as simple and effective as possible. I'm also trying to learn as much as possible about electronic circuit design an this project is a good start.

Can you elaborate more the 5v design. What would be the gains of using a 5v voltage regulator?

You right about the voltage on the gate of the mosfet... Fortunately I haven't yet connect it to the 24v source.
The pwm frequency will be around 1KHz.

[nickm]

Assuming this is a high noise environment be sure to have good bypass caps from pins 4 to 11.  A 1-10uF capacitor in parallel with a 0.1-0.01uF X7R cap should work. Place a capacitor from pin 3 to ground to stabilize your reference. 1uF should keep it solid.  Put 100pF caps to ground on pins 5, and 12 to suppress noise without affecting cirucit operation.

How does the duty cycle control work? Is 0V supposed to get 0% duty cycle and 10V get 100% duty cycle? If so your triangle is between 1 and 2Vpp so 0 to 4V will give you 0% and 6-10V will give you 100% so your entire linear range will be compressed in the 4-6V range.  To get back the linear range put a resistor in between the duty cycle level and pin 12, and another resistor 4 times bigger from pin 12 to virtual ground.  Adjust the ratio to maximize linear range but still allow 0% and 100% duty cycle.

A gate drive transformer might be a better choice than the opto because you won't have to connect it to 24V and risk blowing your FET.

[DakLak]

Not sure why you are needing fuses in both positive and negative supply lines.

I would remove the negative supply line fuse and insert it in to the output positive line of the battery OR in the feed to the motor. Batteries tend to be very unforgiving when it comes to shorts/overloads - and result in fires.

If you are designing a PCB toughen up the tracks where there are high currents. Nothing wrong with running a heavy gauge wire in parallel to PCB tracks to carry the load. Just add holes either end at end of each track for the connections.

I agree with the others who suggest adding capacitors. Capacitors for the IC should be physically adjacent to it for maximum effect. Ditto for the lines to the motor.

I looked in our corporate library and found many circuits you would be interested in - if there is a way of sending them to you.

[johansen]

you need a big capacitor across the input of your pwm controller, because the input current stored in the supply side inductance gets dumped into the mosfet everytime it turns off. this is the cause behind about 50% of the reasons why many noob's H-bridges "gets warm"

[kallileo]

Thank you guys for your comments.
I have replaced the LM324N with a low power TL064CN and the maximum current of the whole circuit is around 8-9mA.
I have also taken into account your suggestion on adding extra capacitor for better stability.
I have also noticed that there as small DC component on both 2nd and 3rd outputs.

@Johansen
How big the capacitor should be?

@DakLak
The whole system will be portable. So there is a 24Vdc, 26A, 500W motor while will be responsible for the transportation of the device. It has its own sophisticated speed controller and control joystick. I have an ON/OFF switch cutting both positive and negative lines as well fuses installed on both positive and negative lines. It's possible that fuses only on the positive side are enough but for better protection I have installed on both.
I will PM you my email for the sample circuits.

@nickm
Yes it works the way you have describe. I will test your suggestions.

[johansen]

@Johansen
How big the capacitor should be?

So there is a 24Vdc, 26A, 500W motor while will be responsible for the transportation of the device.

depends on the supply side inductance.
the issue is energy stored.
suppose you have 10 uH, and the motor is pulling 30 amps
The 4.5mJ of energy stored in the thin air between the battery and the h bridge will continue to go somewhere when the h bridge turns off.
if the pwm frequency is 20,000 hz, that's 90 watts of power... again, it will find some place to go.

suppose you have a 10uF capacitor across the H bridge.
when the fets turn off, it will be charged from 24 volts to about 42 volts.
this isn't normally a problem for say, 60 volt mosfets, but 50% voltage ripple is insane, and will cause small electrolytic caps elsewhere on the circuit to explode.

generally, the issue isn't how much capacitance do you need, but rather, can they handle the ripple current.
100uF would be enough in this case, which is trivial for an electrolytic, but they need to be able to handle the ripple current, which for instructional purposes only, is equal to the peak motor current you expect to pull.

note however that the distance between the H bridge and the capacitors is critical, the wires must be as short as possible.