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

[Anand P. Paralkar]

Hi everyone,

I am trying to test a MOSFET H-bridge at around 400V DC input. I need
to observe the MOSFET Drain-Source waveforms for ringing during the on
to off state (and vice versa) switching.

I observe that the drain to source waveform is affected by the DC source
that I use to supply in the input DC voltage to the MOSFET H-bridge. We
have three different sources (including one that I built using a simple
variac, rectifier and huge capacitor bank) and I feel that there is a
particular DC source (bought from a vendor) that does not work well
(gives horrible drain to source waveforms).

In order to be sure, I would like to see that the one that gives a bad
waveform has something in its output impedance that makes it misbehave
while the other two have something "good" about their output impedance
that make them give good switching waveforms.

Is there a (simple) method to measure the output impedance of a 600V,
15A DC voltage source?

I will use the suggested method to evaluate the three sources that I
have and hopefully, I will find the evidence to prove that the source
that gives the bad waveforms has something "bad" in its output
impedance.

Regards,
Anand

 

[Wimpie]

El 29-12-13 10:55, Anand P. Paralkar escribió:

Hi everyone,

I am trying to test a MOSFET H-bridge at around 400V DC input. I need
to observe the MOSFET Drain-Source waveforms for ringing during the on
to off state (and vice versa) switching.

I observe that the drain to source waveform is affected by the DC
source that I use to supply in the input DC voltage to the MOSFET
H-bridge. We have three different sources (including one that I built
using a simple variac, rectifier and huge capacitor bank) and I feel
that there is a particular DC source (bought from a vendor) that does
not work well (gives horrible drain to source waveforms).

In order to be sure, I would like to see that the one that gives a bad
waveform has something in its output impedance that makes it misbehave
while the other two have something "good" about their output impedance
that make them give good switching waveforms.

Is there a (simple) method to measure the output impedance of a 600V,
15A DC voltage source?

I will use the suggested method to evaluate the three sources that I
have and hopefully, I will find the evidence to prove that the source
that gives the bad waveforms has something "bad" in its output
impedance.

Regards,
Anand

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.

 

[Fred Abse]

Hi everyone,

I am trying to test a MOSFET H-bridge at around 400V DC input. I need to
observe the MOSFET Drain-Source waveforms for ringing during the on to off
state (and vice versa) switching.

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

I observe that the drain to source waveform is affected by the DC source
that I use to supply in the input DC voltage to the MOSFET H-bridge. We
have three different sources (including one that I built using a simple
variac, rectifier and huge capacitor bank) and I feel that there is a
particular DC source (bought from a vendor) that does not work well
(gives horrible drain to source waveforms).

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.

In order to be sure, I would like to see that the one that gives a bad
waveform has something in its output impedance that makes it misbehave
while the other two have something "good" about their output impedance
that make them give good switching waveforms.

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

Is there a (simple) method to measure the output impedance of a 600V,
15A DC voltage source?

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.

I will use the suggested method to evaluate the three sources that I
have and hopefully, I will find the evidence to prove that the source
that gives the bad waveforms has something "bad" in its output
impedance.

Describe, or preferably show us, the waveforms.

 

[Maynard A. Philbrook Jr.]

Hi everyone,

I am trying to test a MOSFET H-bridge at around 400V DC input. I need
to observe the MOSFET Drain-Source waveforms for ringing during the on
to off state (and vice versa) switching.

I observe that the drain to source waveform is affected by the DC source
that I use to supply in the input DC voltage to the MOSFET H-bridge. We
have three different sources (including one that I built using a simple
variac, rectifier and huge capacitor bank) and I feel that there is a
particular DC source (bought from a vendor) that does not work well
(gives horrible drain to source waveforms).

In order to be sure, I would like to see that the one that gives a bad
waveform has something in its output impedance that makes it misbehave
while the other two have something "good" about their output impedance
that make them give good switching waveforms.

Is there a (simple) method to measure the output impedance of a 600V,
15A DC voltage source?

I will use the suggested method to evaluate the three sources that I
have and hopefully, I will find the evidence to prove that the source
that gives the bad waveforms has something "bad" in its output
impedance.

Regards,
Anand

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

 

[Lasse Langwadt Christensen]

Den søndag den 29. december 2013 16.08.54 UTC+1 skrev Fred Abse:

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.

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

 

[Fred Abse]

but they can be slow

That's the point.

 

[Klaus Kragelund]

Hi everyone,



I am trying to test a MOSFET H-bridge at around 400V DC input. I need

to observe the MOSFET Drain-Source waveforms for ringing during the on

to off state (and vice versa) switching.



I observe that the drain to source waveform is affected by the DC source

that I use to supply in the input DC voltage to the MOSFET H-bridge. We

have three different sources (including one that I built using a simple

variac, rectifier and huge capacitor bank) and I feel that there is a

particular DC source (bought from a vendor) that does not work well

(gives horrible drain to source waveforms).



In order to be sure, I would like to see that the one that gives a bad

waveform has something in its output impedance that makes it misbehave

while the other two have something "good" about their output impedance

that make them give good switching waveforms.



Is there a (simple) method to measure the output impedance of a 600V,

15A DC voltage source?



I will use the suggested method to evaluate the three sources that I

have and hopefully, I will find the evidence to prove that the source

that gives the bad waveforms has something "bad" in its output

impedance.

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

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

Sub 10mohm impedance measurements can be done this way

Cheers

Klaus

 

[miso]

Add a series low ESR capacitance to the output (high voltage type). Use a network analyzer to sweep the impedance coupled into the capacitor

(if you do not have an impedance analyzer, apply a function generator
with series resistance, 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
impedance, add a transformer to boost the current.

Sub 10mohm impedance measurements can be done this way

Cheers

Klaus

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.

 

[miso]

Hi everyone,

I am trying to test a MOSFET H-bridge at around 400V DC input. I need
to observe the MOSFET Drain-Source waveforms for ringing during the on
to off state (and vice versa) switching.

I observe that the drain to source waveform is affected by the DC source
that I use to supply in the input DC voltage to the MOSFET H-bridge. We
have three different sources (including one that I built using a simple
variac, rectifier and huge capacitor bank) and I feel that there is a
particular DC source (bought from a vendor) that does not work well
(gives horrible drain to source waveforms).

In order to be sure, I would like to see that the one that gives a bad
waveform has something in its output impedance that makes it misbehave
while the other two have something "good" about their output impedance
that make them give good switching waveforms.

Is there a (simple) method to measure the output impedance of a 600V,
15A DC voltage source?

I will use the suggested method to evaluate the three sources that I
have and hopefully, I will find the evidence to prove that the source
that gives the bad waveforms has something "bad" in its output
impedance.

Regards,
Anand

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.

 

[Klaus Kragelund]

(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.

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

 

[Anand P. Paralkar]

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

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

How are you loading the H Bridge? Motor?

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

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.
Noted.

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

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

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.

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).

Describe, or preferably show us, the waveforms.

Will try.

Thanks for replying and wish you a happy new year.

Regards,
Anand

 

[Fred Abse]

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

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

40A, 1200V IGBT FGA20S120M $2.24 at Newark.

IGBTs don't have parasitic diodes, BTW.

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

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.

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

Under load or open circuit?

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).

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.

 

[Maynard A. Philbrook Jr.]

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

40A, 1200V IGBT FGA20S120M $2.24 at Newark.

IGBTs don't have parasitic diodes, BTW.

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

https://www.fairchildsemi.com/ds/FG/FGA20S120M.pdf

Jamie

 

[[email protected]]

Only at 25C

With an infinite heatsink.

otherwise, it's a 20 amp unit.

With an infinite heatsink. Good luck with any practical cooling to
25C while dissipating 30W.

 

[[email protected]]

Good grief. ...and cool that with LN2? <sheesh!>

 

[Fred Abse]

With an infinite heatsink. Good luck with any practical cooling to 25C
while dissipating 30W.

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.

 

[Fred Abse]

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

https://www.fairchildsemi.com/ds/FG/FGA20S120M.pdf

The OP is presently using 20A MOSFETS.

I hadn't noticed the diode. Silly me.

 

[Fred Abse]

PDmax is
348W at TC=100C

Sorry, that should be 174W at 100C

 

[[email protected]]

Sorry, that should be 174W at 100C

Sorry, try doing that in the real world.

 

[Lasse Langwadt Christensen]

Den onsdag den 1. januar 2014 21.32.10 UTC+1 skrev John Fields:

On Wed, 01 Jan 2014 12:54:24 -0500, [email protected] wrote:

On Wed, 1 Jan 2014 12:17:27 -0500, "Maynard A. Philbrook Jr."

peltiers are terrible when you need to move more than maybe a few watt
they just more the problem and makes it much bigger


-Lasse