H-Bridge Design

[audioguru]

Yes, the bottom clamp diodes are backwards.

 

[ronv]

Oopps! Yes the bottom ones are backwards.

 

[ronv]

Flavin, There is one other thing we should address. This driver requires that the duty cycle not be held at 100% because the boost cap needs to be recharged. So you need to limit the duty cycle in your micro to like 99%. Don't know if that causes a problem or not.

 

[Falven]

Yes, the bottom clamp diodes are backwards.

neither have I some learn by releasing the magic smoke for themselves i guess...

I've never seen an h-bridge with the diodes forward biased in the bottom quadrants. Is that by design ronv? Looks to me like they would short out.

Oopps! Yes the bottom ones are backwards.

Thanks for the catch, I reversed them.

Flavin, There is one other thing we should address. This driver requires that the duty cycle not be held at 100% because the boost cap needs to be recharged. So you need to limit the duty cycle in your micro to like 99%. Don't know if that causes a problem or not.

This shouldn't be a problem, I will put like a 1second delay on the uC.

Parts are ordered, I will try and work on this on the weekend, and I will post back with progress and/or errors.
Thanks for all of the help!
-Falven

 

[Falven]

Hello,
So I assembled the H-Bridge. It was REALLY cramped on this small circuit board. The Diagram on the second page of the Driver's Data Sheet lists the pins in the wrong places, I found the correct way on page 5:
View attachment 64168

View attachment 64164

View attachment 64165

When I first connected it to the power supply, I got some of that magic smoke Turns out I placed one of the diodes backwards, so I replaced the Diode and the Transistor that smoked. I reconnected the H-Bridge to my Arduino, a multimeter, and the power supply. When I wrote "HIGH" (Passed a voltage) through both Driver's IN pins, It measured about 3.5V, and when I write "LOW" it measures 12.30V.

View attachment 64166

View attachment 64167

I tried connecting it to the Motor, but nothing happens. I am not sure on how to debug such a complex circuit, but I did do the basics such as checking if there are shorts anywhere, and making sure everything is properly connected, I could have overlooked something, or maybe another component fried form that first time I connected it. How can I go about debugging this. Thanks for the help!
-Falven

 

[ronv]

Program the input on one side to a low (0 volts) and pulse the input on the other side at say 50% duty cycle at 1KHz. You should then see about 6 volts across the motor.

Are you going to run it from a battery or a power supply in the final version? The reason I ask is that if you run from a power supply you need to be careful that the motor is stopped before you reverse direction or it will "pump up" the supply and may cause problems.

 

[ronv]

picture

Here is a picture of how it will work. The input on the left is low which turns on the lower left FET.

The input on the right is pulsing which turns on and off the top right FET. The voltage across the motor is proportional to the duty cycle on the right.

 

[Falven]

Are you going to run it from a battery or a power supply in the final version? The reason I ask is that if you run from a power supply you need to be careful that the motor is stopped before you reverse direction or it will "pump up" the supply and may cause problems.

Power supply. Do you mean cut off the current? What do you mean by stopping the motor? The linear actuator has limiting switches, by the way.

I managed to get it to extend the actuator once, and now the power supply won't turn on when connected to the bridge. Is there a short somewhere? How can I find it? My multimeter doesn't have that capability where it beeps if power is going from the + to - pins...

 

[ronv]

You should be ok with the limit switches.
You should be able to disconnect the power supply and measure from the top of the bridge to ground with the ohm meter to see if there is a short.
How do you have it hooked up now? Are you pulsing the one side of the bridge? Can you make a schematic of the motor and limit switches?

What I was saying is that the actuator needs to be stopped before you reverse it. With the limit switches I'm thinking that will happen.

 

[Falven]

You should be ok with the limit switches.
You should be able to disconnect the power supply and measure from the top of the bridge to ground with the ohm meter to see if there is a short.

View attachment 64184

How do you have it hooked up now? Are you pulsing the one side of the bridge? Can you make a schematic of the motor and limit switches?

This is the actuator:
https://www.firgelliauto.com/product_info.php?cPath=114&products_id=234

At the moment, I only have the H-Bridge connected to the PSU, as I am trying to investigate the "Short"; why the power supply turns off when I have the H-Bridge connected.

What I was saying is that the actuator needs to be stopped before you reverse it. With the limit switches I'm thinking that will happen.

Yeah, from my knowledge the limiting switches cut power to the actuator when it reaches its peak length.
Sorry to sound so naive with the troubleshooting, as I said at the beginning, this is the biggest circuit i have assembled, and honestly like the 2nd or third time I have debugged a circuit :3 they usually always work for me on first try.
Cheers,
-Falven

 

[ronv]

Seems to be shorted from the picture. So now measure from drain to source of each FET. They should all be open.

 

[Falven]

Seems to be shorted from the picture. So now measure from drain to source of each FET. They should all be open.

On the 2000 ohm dial, they each measure

View attachment 64187

When measuring from the drain (+) to source (-) of each.

I pulled out the bottom right diode, and measured the resistence, that diode is fine. Maybe the MOSFET's burned up? Either that or the other Diode?

 

[ronv]

Measure with the probes in both directions (red on drain black on source and vice versa.

 

[Falven]

Measure with the probes in both directions (red on drain black on source and vice versa.

Left showed ~ 276 for both FETS both prove directions, right ones showed 0 for both FETS both prove directions.

 

[ronv]

So both of the right ones are shorted. Usually that happens when both are turned on at the same time but the driver should prevent that. Any thing unusual happen?

I hate to leave now but I must go.

 

[Falven]

So both of the right ones are shorted. Usually that happens when both are turned on at the same time but the driver should prevent that. Any thing unusual happen?

I hate to leave now but I must go.

I don't see how they both could have been turned on at the same time provided I don't have them connected to the uC at all. But possibly the other transistor burned up with the initial diode incident unknowingly to me, and that caused it to short the new one I added? So I must replace both of these? No problem sir, thanks for all the help thus far, until next time you're on?

 

[ronv]

I'm concerned aboubreaking the circuit with the limit switches. I'll start a thread for some input.

 

[johansen]

You need to install a capacitor of about 1-4.7nF per amp of motor current, and in this case (12v/5 amps) a 2 ohm resistor in series, across the H bridge terminals as close to the Mosfets as is practical (a couple inches away is ok)

you should also have at least 100uF per ampere of motor current connected directly across the 12v supply rails as close to the mosfets as is practical.
this capacitance is in addition to any bulk capacitance for the purpose of holding down the 12v rail when using the motor as a generator and returning power, and its only purpose is to avoid 50+ volt spikes during the switching transitions.

i can't really help you with the problems of shorted out mosfets other than to say you should get yourself in the habit of connecting things to current limited power supplies and then using your fingers to find hot spots.

 

[ronv]

H Bridge

I think we need to make a course correction. Take a look at the relay implementation. I think using the FETs will be a problem even with the suggestions made. So lets take the best of both worlds.
In this circuit the actuator will retract unless told to extend by the micro. I think tou can get everything at Radio Shack.

Input from others.
https://www.electro-tech-online.com/threads/limit-switches.127421/

 

[ChrisP58]

I think there is another issue with the choice of mosfet driver for the application.

As I understand it, the H-bridge is to be used as an on/off and direction control switch. So it may stay in one state for more than a few seconds at a time. This type of high side driver uses a boost capacitor to create a high side gate drive signal that is more positive than the input voltage. This capacitor needs to be 'pumped' relatively often to make it work. How often will depend on a few things, but I would think at least once per second, maybe a few hundred times per second.

This issue was touched on back in post 23. While duty cycle is important, period is also important.

When using a mosfet for a high side static switch, you need to either use a p-channel mosfet, or make a super positive gate voltage in some other way. One gate driver that does so is:
**broken link removed**