H Bridge forward/reverse power.

[2camjohn]

Hello people, Long time no cheese.


I want to design a H-Bridge which can apply alot of power to a motor in one direction, it must also go in reverse but in reverse there will be less load and speed isnt as important.

Can I just use two big fets/transistors for forward and two smaller ones for reverse? Or am I missing something?

Also I want current sensing (ie an analog voltage I can interpret with a micro) when the motor is going forward, what do you guys think the best way of acheiving that is? Ive read about vurrent sensing mosfets and current sensing ICs, but im not sure where these would go in the circuit?




Any other tips on this kind of circuit, or weblinks would be greatly appreciated. Ive googled up on regular H-Bridges, but not on modifying them in the way I want.



John

 

[dknguyen]

Yes you can pair up transistors like that if you wanted to. For current sensing you can use a current sensor and current sense resistor from Zetex. It measures the voltage across a resistor in series with the H-bridge on the high-side and converts it to a current which you can run through a resistor into an ADC to measure the current being used.

If you use two different kinds of transistors (even if you just use one) beware of shoot-through and deadtime. Transistors turn off at a different speed than they turn on, and its even more different for two different kinds. You never want both left or right transistor pairs to be on due to short-circuit.

 

[2camjohn]

Thanks for the help dknguyen.

I have seen these current sensors, but I cant figure out where to put it in the H-Bridge? For example this one looks as though it can only go in paralell with the motor, and the motor must be connected to ground? Is this the right one, if so how do I combine it with my H-Bridge?


Thanks for the hint about switching off time of the transistors. In this application I can easily wait 50ms on changeover between forward and reverse, so I can make certain that isnt a problem.

 

[audioguru]

MStechca shows up on some forums at Electronics-Lab. He has a new name. I recognized him right away.

You should stop the motor before you slam it into reverse or its current will be a lot higher than you think.

 

[Hero999]

If you use two different kinds of transistors (even if you just use one) beware of shoot-through and deadtime. Transistors turn off at a different speed than they turn on, and its even more different for two different kinds. You never want both left or right transistor pairs to be on due to short-circuit.

A choke in series with the power supply will solve that problem.

Anyway, connect the current sense resistor in series with the negitive supply to the h-bridge.

 

[Styx]

A choke in series with the power supply will solve that problem.

Anyway, connect the current sense resistor in series with the negitive supply to the h-bridge.

err no, an inductor in series with the PSU will make it worse
That inductor will try to keep any current you have established in the load flowing. When you try to turn-off the switch there will be a bigger over-shoot at turn-off and possible reverse breakdown

a H-bridge (if setup as a voltage source inverter) needs to be connected to an ideal voltage source, the best that can be done is a nice big capacitor right on the DC-rail connections of the H-bridge

 

[Hero999]

The diodes across the MOSFETs should protect against that. The inductor will prevent current surges if both transistors at either side of the bridge get turned on simultaniously for short periods, If you're worried about back emf from the inductor, a zenner wired in reverse parallel with absorb it.

 

[JRoque]

Hello. You might be able to use smaller FETs for the high-side ends of the H bridge if the goal is to reduce cost. But the difference will be minimal for a one-of.

Depending on your current and voltage requirements, you could also use an all-in-one chip to drive the motor directly. These include the MOSFETs and their drivers and have shoot-through deadtime guards, over-voltage/current/temp, under-voltage lockouts. I have used the **broken link removed** with great results. Unfortunally, TI is sunsetting that product.

As for where to put your current sensor, as mentioned here you can place it in series with your motor but you can also put it in series with the power supply. If you put it in series with the motor - on the MOSFET outputs - then you can use a current sensor amp and reference up to, say, 2.5V. So if no current flows, the output is 2.5V. When the motor moves forward, the output voltage is >2.5V. If the motor is reversed, the output of the sense amp is <2.5V. When I did my H-bridge a couple of years ago, I used a current sensor chip but still connected it in series with the power supply to minimize the code needed on the uC. After all, you will likely know if the motor is moving fwd or rev and just need to know what's the A load.

I'm posting the design I used back then as a possible reference, though it can use a tweak or two to improve it. In my experience, the HIP4081A is finicky about how you lay it out on the PCB so proceed with caution. The datasheet and design notes have good info on that topic.

Regards,
JR

 

[Styx]

The diodes across the MOSFETs should protect against that. The inductor will prevent current surges if both transistors at either side of the bridge get turned on simultaniously for short periods, If you're worried about back emf from the inductor, a zenner wired in reverse parallel with absorb it.

This is not to do with the BackEMF (which the diode will provide a freewheel path) and thus is not to do with extra inductance in the load (since you did say in the supply)

Extra inductance in the supply is BAD!, I spend days ensuring our busbars are as lamina and as close to the DC-link cap as possible, as a result we get 100V of extra overshoot on a 600V DC-link and that 100V is purely from the inductance we cannot remove (internal to the modules, connections to the modules and other strays).

Extra inductance in the DC-link will force the turn-off peak overshoot to get higher, this will
1) increase yr turn-off losses
2) cause a load of EMI/EMF/EMC issues
3) over-volt yr module

all three are bad. Snubbers are added purly as a band-aid for a poor designed power-stage. Sure if you want to advocate the use of a snubber go ahead, but please be aware a better designed power stage (and that means NO EXTRA INDUCTANCE BETWEEN THE BRIDGE AND THE DC-LINK CAP!!!) will perform and last longer