Measuring the Output Impedance of a (Large Capacity) Voltage Source

I had such things in the past also (but with less power). You may observe the transient (step) response of the supply by switching a large low inductance resistor with a mosfet or fast IGBT. It was strange to see that some power supplies were close to instability.

You could also use small signal measurement, but the large signal response may be different from the small signal response.

Somewhat OT: I think it is good to have sufficient decoupling close to the H-bridge so that the DC power supply doesn't "see" the switching transients.

If you mean what I mean by "large capacity", why aren't you using IGBTs?

How are you loading the H Bridge? Motor?

If there's any inductance in the load, you need freewheeling diodes across the FETs, don't trust the parasitic substrate diodes to do the job.

Firstly, look at the waveform at the power supply. It should be DC, with no more than 20% P-P ripple.

Measure its open circuit voltage, then resistively load it for, say 1 amp, and measure the voltage again. Calculate the source resistance from the reduction in voltage, and the value of your load resistor. It's just a resistive voltage divider.

Describe, or preferably show us, the waveforms.

Slap a scope across it and see how it reacts to the actual load.

Consider getting an isolated-input scope like a Tek TPS-series unit. With that, you can measure the voltage drops and waveforms across every element and really understand what's going on.

The one you're seeing unacceptable behavior is most likely a switching supply or one with a very tight regulator in it. The ringing or even sudden changes of current loads to no loads for example is making it a little jumpy.

inductive ringing can generate more voltage than your supply and it's also possible you are hitting a safety shut down because the caps being used in the supply are small, since they are a switcher at most likely much higher frequency than the ring you have, it makes it easy for your H-bridge to upset it..

Try hanging some large low ESR caps on the output side of the supply or load it down with non inductive load.

Jamie

to

off

unless you disable the parasitic diodes with a series diode I don't see external diodes helping much

It seems most power fets have similar rating for the diode as for the fet but they can be slow

maybe that is the problem, shoot-through triggering a current limit in the supply

-Lasse

That's the point.

Add a series low ESR capacitance to the output (high voltage type). Use a n etwork analyzer to sweep the impedance coupled into the capacitor (if you d o not have an impedance analyzer, apply a function generator with series re sistance, and plot values a regular frequencies.

If the load is really low impedance, use a HiFi amplifier as the source. If that proves to feed to little current into the output to measure the imped ance, add a transformer to boost the current.

Sub 10mohm impedance measurements can be done this way

Cheers

Klaus

(if you do not have an impedance analyzer, apply a function generator with series resistance, and plot values a regular frequencies.

If that proves to feed to little current into the output to measure the impedance, add a transformer to boost the current.

Seems to me you should to meditate a bit about charging this series cap. The instantaneous connection would put the full 400v into the network analyzer. Perhaps a precharge is in order.

The first thing to consider that the power supply isn't linear. If it is a switcher, then for sure it isn't linear. If it is a "linear" supply, well linear circuits have slew limitations, and if you are slewing, you are not linear. Further, there is usually protection circuitry in the loop. Linear supplies have multiple control loops.

Thus you probably have to whack the supply with a load pulser and observe the voltage via a scope with probe AC connected. But then the next issue is are you testing the power supply or the load pulser. Most electronic loads are pretty crappy, so you are watching the electronic load settle as well as the power supply.

For datasheets, where you want to be 100% sure all the ugliness (and hopefully lack thereof) is due to the device under test, you use as passive of a load pulser as possible. Typically you go from no load or in some cases minimum load current (i.e. high value resistor) to a low value resistor switching it in by a power mosfet. I've built these pulsers using PCB strips to solder a number of parallel carbon film resistors in order to make a low inductance resistor. Generally one mosfet is enough. You just make sure the on resistance is low compared to the resistor array.

Use this somewhat passive load pulser to perturb your supply and see what happens.

Precharge would be a good idea for this voltage. I have used it for lower voltage (30V), and the inrush surge current is manageable at that voltage

Cheers

Klaus

We have a MOSFET that can handle 20A and 1200V. That was large, atleast for me.

No, we currently have only lamp loads (50 * 100W incandescent bulbs).

Noted.

Surprisingly, the voltage, atleast as seen at the input of the H-bridge PCB shows only a DC voltage.

I am not too sure this will hold good when we are using the DC to produce a switching output (as in a PWM sine or rectangular wave).

Will try.

Thanks for replying and wish you a happy new year.

Regards, Anand

That's small to medium capacity, to me. I'd still use IGBTs, though.

IGBTs don't have parasitic diodes, BTW.

Lamps have very low resistance cold. You'd be better using 5kW worth of heating elements. 50 hundred watt lamps, in parallel, will be a fraction of an ohm cold.

Never start an investigation thinking you know the answer. That's what police do. Not engineers.

Under load or open circuit?

Measure at different load currents, up to maximum. Plot the results.

You need a pulsed load to go any further, so you can see rise times. Maybe use your H bridge as the load.

Only at 25C otherwise, it's a 20 amp unit. and also, they do have diodes.

Jamie

With an infinite heatsink.

With an infinite heatsink. Good luck with any practical cooling to

Good grief. ...and cool that with LN2?

Dissipation is dependent on duty cycle.

Typical VCEsat 1.75V at IC=20A, TC=125C That's 35W at 100% duty. PDmax is 348W at TC=100C

Note temperature quoted as TC, not TJ.

The OP is presently using 20A MOSFETS.

I hadn't noticed the diode. Silly me.

Sorry, that should be 174W at 100C