Forums

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

Started by Anand P. Paralkar December 29, 2013
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
El 29-12-13 10:55, Anand P. Paralkar escribi�:
> 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. -- Wim PA3DJS www.tetech.nl Please remove abc first in case of PM
On Sun, 29 Dec 2013 15:25:44 +0530, Anand P. Paralkar wrote:

> 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. -- "Design is the reverse of analysis" (R.D. Middlebrook)
On Sun, 29 Dec 2013 15:25:44 +0530, "Anand P. Paralkar"
<anand.paralkar@gnospammale.com> wrote:

>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?
Slap a scope across it and see how it reacts to the actual load.
> >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
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.
In article <bia9t7Fjmj8U1@mid.individual.net>, 
anand.paralkar@gnospammale.com says...
> > 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
Den s=F8ndag den 29. december 2013 16.08.54 UTC+1 skrev Fred Abse:
> On Sun, 29 Dec 2013 15:25:44 +0530, Anand P. Paralkar wrote: >=20 >=20 >=20 > > Hi everyone, >=20 > >=20 >=20 > > I am trying to test a MOSFET H-bridge at around 400V DC input. I need =
to
>=20 > > observe the MOSFET Drain-Source waveforms for ringing during the on to =
off
>=20 > > state (and vice versa) switching. >=20 >=20 >=20 > If you mean what I mean by "large capacity", why aren't >=20 > you using IGBTs? >=20 >=20 >=20 > >=20 >=20 > > I observe that the drain to source waveform is affected by the DC sourc=
e
>=20 > > that I use to supply in the input DC voltage to the MOSFET H-bridge. W=
e
>=20 > > have three different sources (including one that I built using a simple >=20 > > variac, rectifier and huge capacitor bank) and I feel that there is a >=20 > > particular DC source (bought from a vendor) that does not work well >=20 > > (gives horrible drain to source waveforms). >=20 >=20 >=20 > How are you loading the H Bridge? Motor? >=20 >=20 >=20 > If there's any inductance in the load, you need freewheeling diodes acros=
s
>=20 > the FETs, don't trust the parasitic substrate diodes to do the job. >=20
unless you disable the parasitic diodes with a series diode I don't see=20 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
On Sun, 29 Dec 2013 11:12:42 -0800, Lasse Langwadt Christensen wrote:

> but they can be slow
That's the point. -- "Design is the reverse of analysis" (R.D. Middlebrook)
On Sunday, December 29, 2013 10:55:44 AM UTC+1, Anand P. Paralkar wrote:
> Hi everyone, >=20 >=20 >=20 > I am trying to test a MOSFET H-bridge at around 400V DC input. I need=20 >=20 > to observe the MOSFET Drain-Source waveforms for ringing during the on=20 >=20 > to off state (and vice versa) switching. >=20 >=20 >=20 > I observe that the drain to source waveform is affected by the DC source=
=20
>=20 > that I use to supply in the input DC voltage to the MOSFET H-bridge. We=
=20
>=20 > have three different sources (including one that I built using a simple=
=20
>=20 > variac, rectifier and huge capacitor bank) and I feel that there is a=20 >=20 > particular DC source (bought from a vendor) that does not work well=20 >=20 > (gives horrible drain to source waveforms). >=20 >=20 >=20 > In order to be sure, I would like to see that the one that gives a bad=20 >=20 > waveform has something in its output impedance that makes it misbehave=20 >=20 > while the other two have something "good" about their output impedance=20 >=20 > that make them give good switching waveforms. >=20 >=20 >=20 > Is there a (simple) method to measure the output impedance of a 600V,=20 >=20 > 15A DC voltage source? >=20 >=20 >=20 > I will use the suggested method to evaluate the three sources that I=20 >=20 > have and hopefully, I will find the evidence to prove that the source=20 >=20 > that gives the bad waveforms has something "bad" in its output=20 >=20 > impedance. :) >=20
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=20 Cheers Klaus
On 12/29/2013 1:55 PM, Klaus Kragelund wrote:

>> > > 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.
On 12/29/2013 1:55 AM, Anand P. Paralkar wrote:
> 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.