Help designing a dual H-Bridge controller

[chadj2]

I am trying to build a circuit to control 2 H-bridges. The H-bridges outputs must be 90 degrees out of phase with each other. I was thinking about using a sg3524 signal to control one H-Bridge and using the same signal as an input to 2 556 timers acting as a delay and trigger signal to the other H-bridge. The H-Bridges will be driving coils. I don't believe I have the skills to use a microcontroller unless I am thinking it is more complicated than it is. If anyone has any ideas as to how I can control this setup more effectively please give me some guidance.

Chad

 

[OperaHouse]

Why not use two 3524. Anything but a 555!!!!

 

[chadj2]

How do I ensure that the 3524s stay 90 degrees out of phase?

 

[IRstuff]

The brute force and ignorance solution is to start with a 4x version of the control signal and divide by 4, and decode the output quadrature signals

TTFN

FAQ731-376

 

[sreid]

I've got to ask. Is it possible that the 90 degree requirement refers to the relationship of the current through the coils? If the coils are at right angles, the current drive often has the relatioship of sine in one coil and cosine in the other coil so that a magnetic vector can be rotated.

 

[chadj2]

Thanks for the response. I am performing an experiment on magnetic field interaction. I need the first B field to be at full north while the second B field is at neutral.

 

[itsmoked]

Ah man... Just get one of these.

http://www.parallax.com/Default.aspx?tabid=295

Then you need a laptop or PC and about an hour of time and you can have it any way you want it. You can also adjust things with a few taps of the keyboard.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Madcow]

http://focus.ti.com/lit/an/slua143/slua143.pdf

Figure 22 page 3-61. Did something similar a while ago.
I used 2 uc3846 chips and a flip/flop to an external
clock. The f/f gives a square wave 180 out. The resultant
outputs out of the chips where 90 degrees out.
Should work the same for a uc3524 chip.
Now, I would agree with itsmoked and use a micro.

 

[sreid]

Since you are doing an experiment, I'd sugest that you purchase amplifiers to drive current into the coils. Varedan

http://varedan.com/

makes some nice linear current mode amplifiers that can be factory configured to drive voice coils. Linear means no PWM switching noise. The current is proportional to the command signal and is bidirectional. You would also need a servo chassis to power the amplifiers.

 

[chadj2]

Once again, thanks for taking the time to give me some guidance on the best way to tackle this problem. Unfortunately, I think it will take me quite a while to learn how to use a microcontroller (I dont even know any computer languages). I will continue to study the ideas that you guys have provided.

Thanks again
Chad

 

[itsmoked]

Lay hands on one of those Stamps and you will find that some are trivial to program.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[DHambley]

I would create an independent clock at twice the frequency of either H-brige, divide it by two with a flip-flop, then reshape them into two pulses. Send these to the synchronous clock inputs to the PWM IC.

Now you've got your 90 out-of-phase.

 

[itsmoked]

Yes and how do you prevent shoot-thru due to turn off delay?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[DHambley]

It's odd that people are suggesting a micro to produce two simple pulse trains. Most of the PWM IC application notes will show you how to sycn them with a clock. Producing two staggered clocks would just take a few parts. Your two H-bridges will run independently from the two clocks.

 

[ScottyUK]

You haven't answered the previous question about preventing DC shoot-through when the off-going switches are still conducting as the on-coming switches begin to conduct. You can't just use a pair of antiphase square wave drives - there has to be a short 'dead time' when all gate drives are 'off' to allow the off-going switches to reach a blocking state before the other pair switch on. Following your advice will lead to DC shoot-through and destruction of the switches.


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If we learn from our mistakes I'm getting a great education!

 

[benta]

Scotty, using off-the-shelf H-bridge gate drivers solves this problem, most of them have built-in dead-time circuits.

Regards,

Benta.

 

[ScottyUK]

I know - that's why the comment about antiphase squarewaves to drive the gates seems strange! On re-reading I think I know what hgldr is getting at - using a quadrature pair of squarewaves to synchronise two driver ICs which take care of the dead time etc for each bridge.



----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[DHambley]

About the two staggered clocks. The leading edge of each square wave would go into a simple pulse forming circuit, about 500ns width would do. These are what are sent to each PWM IC sync input, not square waves.
For an IC like the 3524 The sycn pulse current would be sent to the ramp capactior and must be higher than the ramp-discharge current. This isn't specified in some IC's so it has to be high to make sure it works. I'd use a 3525 PWM instead of the 3524. The sync methods are specified and more straigtforward.

 

[ScottyUK]

Thanks hgldr.

 

[fsmyth]

Use a TL494 or equivalent. Adjustable timing, adjustable
dead time, bi-phase outputs, dual feedback inputs, etc.

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