PWM chip that can drive an H bridge?

Don't forget to look at IR's 2110 half-bridge driver and friends. Two of them are actually cheaper than the HIP full-bridge driver, and they go to 600V. The HIPs have some nice features though, like UV lockout, charge pumps, shoot-through protection, and higher speed.

robert

Ignoramus, this is about the time you'd better start telling us what it is that you're trying to accomplish, because as the thread wears on you're getting more and more off track.

If your problem falls into the typical range of PWM applications (motor/solenoid control or power conversion) there is a host of good PWM control chips out there, and the XR2206 isn't one of them.

robert

I think I've seen a 555 set up ( or was it a 565) I think it was the daul timer chip, the first timer was a VCO and the other did duty cycle, I think it was in a radio shack book on powersupplies. Pat

I am trying to build a tig inverter. To that end, I want to have gate driver[s] controlling a full H bridge. To send signals to gate drivers, I want to find an appropriate chip that would send square wave signal to gate drivers and have its frequency and duty cycle separately settable. Frequency should vary between 30 and 1000 Hz, at least, and duty cycle between 15% and 85%, or better.

Do you have any suggestions, such as specific chips.

Would you have a specific suggestion for the application that I described?

That would be most appreciated.

accurate, high-frequency

external components. The

No, their output will be the logic input for the gate driver chips. That's my plan. In light of this, do you have any specific suggestions, thanks.

Okay, I'd call that symmetry, since the load is 100% on, just the polarity swaps around. In any case.. .

You can probably do this acceptably well with a CMOS 555 and a CMOS comparator and a couple of pots.

Alternatively, if you can live with switch selected frequencies (like a rotary switch with a number of set approximate frequencies) and a pot for duty cycle you could just use a single TLC555. The switch would select capacitors.

Best regards, Spehro Pefhany

Using the two-chip method,. the TLC555 would set the timebase and generate a somewhat nonlinear triangular waveform. The pot setting would be linear with period (1/frequency) and completely independent of symmetry.

The symmetry would be set by the comparator threshold (completely independently of frequency) and would be somewhat nonlinear.

A single TLC555 could do this if you switch-select the capacitor and adjust the symmetry with a pot and a couple of switching diodes. That's the really easy way to go if you don't need a lot of resolution on the frequency, or would like to increase it in a series like

Best regards, Spehro Pefhany

accurate, high-frequency

external components. The

EXAR !

You're missing the point.

These chips aren't designed as PWM controllers.

No dead time for starters.

Graham

A UC3525 appropriately configured will do that.

Graham

Duty cycle is a term used in PWM chips. It means, in the context of square wave signal, the time that the signal is ON, as percentage of the total oscillation period.

For example, if the period is 0.1 seconds (10 Hz), and the signal is ON for 0.07 seconds (and therefore OFF for 0.03 seconds), that this is a 70% duty cycle signal.

Our normal symmetrical square waves have, by definition, 50% duty cycle.

In tig welding, especially aluminum, it helps to vary the duty cycle and vary the time of electrode negative and electrode positive time. That lets the welder balance the cleaning action of electrode positive with deeper penetration of electrode negative.

Well, yes, you are correct, but it greatly helps to also vary the EN and EP times as percentage of every cycle.

Thanks Graham... They have only less than 50% duty cycle, whereas I would like to see a greater variation. Nevertheless, it is a possibility. I will check out tig welding books, maybe I can get away with less tan 50% duty cycle. I appreciate your input.

If I read the app note correctly, the input to the output inverter is assumed to have significant series inductance. In this situation, catch diodes are not necessarily helpful.

Best regards, Spehro Pefhany

Yes. The problem is that regulation of frequency vs. duty cycle is very screwy. You have two pots, R1 and R2, and frequency and duty cycle are functions of R1 and R2. You basically have to solve a linear equation system every time you want to set frequency and duty cycle, to find out what pot settings to use. Very counterintuitive.

On Fri, 07 Oct 2005 11:13:30 GMT, Ignoramus30105 wrote in Msg.

LM393 (dual comparator). Get the data sheet from National.com. The first example they show is the "square wave generator". Replace the top 100k resistor with a trimpot and resistor in series and get an appropriate capacitor to cover the desired frequency range.

This circuit will produce a somewhat triangularish waveform at the capacitor terminal whose frequency is settable by the trimpot.

Now connect this node to the + input of the other comparator. Wire up another trimpot as adjustable voltage divider and connect the wiper to the - input. Now the output of the other comparator will go high whenever the sawtooth voltage exceeds the pot voltage. High pot voltage

-> only the tips of the sawtooth icebergs will stick out -> low duty cycle. Low pot voltage -> sawtooth voltage exceeds pot voltage most of the time -> high duty cycle. Limit adjustment range of duty cycle pot by appropriate resistors in the top and bottom leg of pot.

Not that since the sawtooth isn't a real triangle wave (it doesn't have straight edges) the duty cycle percentage isn't exactly linear with pot setting.

If you have an oscilloscope you won't even have to do any math to tailor this circuit to your needs.

robert

On Fri, 07 Oct 2005 12:29:37 +0100, Pooh Bear wrote in Msg.

It took me a long time to realize this, but his application isn't exactly the target of PWM controllers. He's really looking for a variable frequency, variable duty cycle oscillator.

Dead time is another matter -- he'll have to ensure that there's no shoot-through, but I think the HIP4081A will do that for him.

robert

that's correct.

If you are referring to controlling the H bridge, then the gate drivers will take care of shoot through. All I want from the oscillator chip is a square wave 5V signal that I can properly control and that can assume proper frequency and duty cycle range.

I kind of agree, but duty cycle is an accepted term for pwm chips.

See my another answer, I could, but control is cumbersome. You basically have to solve a 2x2 linear system to figure out proper pot positions for any given frequency/duty cycle combination.

Maybe I am missing something.

Look at my other post -- you can do that with a single LM393.

As for dead time -- in "normal" PWM applications, the dead time is a life saver for the IGBT bridge. In your application, as the IRF app note mentions, you need some shoot-through due to the inductive nature of your supply (I just realized this). In fact if you had dead time in your application, your bridge would indeed be dead right away.

So you CANNOT use a gate driver that "takes care" of this, because they're all designed to avoid cross-conduction.

What is the max current/max voltage of your TIG supply?

robert

Yes, because the supply is inductive. If the IGBT bridge were to switch off altogether (as in any "normal" PWM design), there would be IGBT bits all over the place.

robert