Resistor pulse handling?

Greetings:

I wish to sample a raw 75V 125kHz H-bridge output. The two H outputs will go to a differential amplifer/attenuator to create a single ended, low-level sample of the output voltage. Trouble is, I don't want the raw signals running along my PCB from the high powered department to the low level zone and radiating spikes all over the place. Thus, I'd like to slow the edges just a bit before routing them long distances.

So use an RC. Trouble is, for instance a 1.25MHz cutoff filter with a

500 ohm resistor could potentially dissipate 11.25W in the ressitor on transitions, even though the RMS current is only 17.7mA.

The question is, can a simple 0.25W 1210 resistor handle the transients?

If I make the resistors larger, then they contribute to errors in the differential amp gain by being close to its input resistance. Well just make them a part of the input resistors you say? Ok, but then doesn't the AC CMRR suffer if the caps aren't matched?

Looking at the Panasonic ERJ 1% series, they allow only 2.5x rated power for short term overload. But I suspect this isn't really the applicable spec for this situation. There is nothing in the datato help one figure out the device's tolerance to repetitive VERY short term overload, where average power is still well within the continuous rating.

Comments appreciated.

Good day!

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Christopher R. Carlen
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Chris Carlen
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Chris,

Why not use a passive divider to get down to 50 ohms, and skip the diff amp? You might miss some important stuff with a low-pass filter.

This allows you to look at either output individually to see if one side has a problem such as distortion, oscillation, risetime, etc. You can connect both outputs to the scope and use the diff amp in the scope and add them together.

A 10k resistor into 50 ohms will give plenty of bandwidth to see oscillations well into the VHF region. With 50% duty cycle, you would only need 1/2 watt.

I find every time I throw information away, it turns out to be just the piece I need to solve a problem:)

Mike Monett

Reply to
Mike Monett

Chris,

Another thought - with an H bridge, you definitely don't want both sides on at the same time. It would be difficult to monitor this with a single diff amp, but easy with separate outputs.

Mike Monett

Reply to
Mike Monett

Since you definitely are not considering carbon composition resistors, then decades of experience with them is not useful. Perhaps the following compromise can be made.

1) start with a 100 ohm resistor from the source to node 1 that has a (filter) capacitor to ground; set the capacitor value to ten percent of the original calculated value. Peak pulse dissipation is fairly low, as there is less energy in the harmonics. 2) from node 1 to node 2 use a 200 ohm resistor and at node 2 use a capacitor that is 40 percent of the original calculated value. Do not know energy (spectrum) content of what flows thru resistor thru capacitor, but some whiz could do the math. I would dare say that energy may be in the same ballpark. 3) from node 2 to node 3 use a 200 ohm resistor and at node 3 use a capacitor that is 80 percent of the original calculated value. Do not know energy (spectrum) content of what flows thru resistor thru capacitor, but some whiz again could do the math. Guess in similar ballpark. The idea is to dissipate the fastest stuff first, and do the rest later; spreading the energy in steps. The values may need adjustment, but the idea is given for evaluation, not as an exact solution. ** Alternate method: use a ferrite bead in series with the source to the bypass node; ther aer SMT versions as well as leaded versions.
Reply to
Robert Baer

Christopher, If you can tolerate a through-hole mounted part, a wirewound resistor can easily handle this situation. If inductance is a problem, there are several manufacturers that offer non-inductively wound wirewounds (but not in surface mount packages). I think that you will find that the tolerance to high crest factors will vary from manufacturer to manufacturer, and that you will have difficulty finding one that will guarantee high crest factor performance. Regards, Jon

Reply to
Jon

You

Perhaps I should have explained more. I agree with you that a 10k:50 ohm divider would provide a nice diagnostic port. But the purpose of the diff amp is specifically differential to single ended conversion in order to implement a voltage mode control loop around the PWM amplifier H bridge.

Indeed.

I

Thanks for the input Mike.

Good day!

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Christopher R. Carlen
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Chris Carlen

I am aware of their pulse handling capability. Trouble is tolerance.

Hmm. Perhaps it would work anyway, since the carbon component of the total resistance into the diff amp would be a very small one.

However, I have discovered that my fears regarding screwing the performance of the diff amp are unwarranted, and that I can use a larger SMT resistance that is still within it's surge rating.

Interesting method. Can't afford the space for a lot of parts though.

Good to keep in mind as well.

Thanks for the reply.

Good day!

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Christopher R. Carlen
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Chris Carlen

Sounds fine to me.

John

Reply to
John Larkin
[...]

Ha! That's a completely different problem:)

You may have mentioned it in another post - have separate RC filters on each side going to ground, that feed the inputs of the op amp. Then slow the loop bandwidth down enough to follow the average output.

This is similar to the problem of handling fast pulses in pll error amplifiers.

[...]

Thanks for the explanation!

Mike Monett

Reply to
Mike Monett

I would recommend the THS4502 or AD8132 true differential amplifiers. I used one in a similar application and they are fast enough to follow the rise and fall times of a H-Bridge. The inverted output can come very handy for delta-sigma or integrator stages of higher order or any other analog signal processing. The differential output can also drive a high-speed ADC directly.

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ciao Ban
Bordighera, Italy
Reply to
Ban

I know the following link is not exactly what you're looking for, but I hope is some information you could use. Vishay uses a MELF-style resistor for an example in the document but they go on to say it is applicable to other styles and in carbon film.

See the PDF document at the bottom of the page titled "Pulse Load Handling for Fixed Linear Resistors."

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Good luck.

John

Reply to
John - KD5YI

Hi Chris,

see if you can find a copy of Philips ACM2 databook, they have a lot of pretty pictures of peak pulse power rating for various smt resistors. an RC01 1206 resistor can handle a single 10W pulse 4us long, and repetitive 3W pulses 1us long and 1ms apart. A PRC201 1218 will do 10W for 10us every 10ms, continuously.

IRC make a range of seriously impressive parts, with a Joule rating - stay within the Joule rating, and the average power rating, and you are OK. CHP1/8 would eat your task, as would any MMA0204 part (roughly 1206 sized MELF-like package)

the problem is simply thermal. smt resistors have bugger all thermal mass, and so cant reliably take large pulses.

not all wirewound resistors are created equal, either :). Vitrohm make great parts. I have done a lot of peak pulse power testing of WW R's, the cheap ones have a wound former sitting in a U channel in the ceramic case, with stuff poured over the top. Large pulses can be seen as a red glow through the back of the ceramic - air gap = localised hotspot = sudden/gradual failure. Better parts ensure the wound former is fully encapsulated, removing air gaps and extracting the heat better.

Cheers Terry

Reply to
Terry Given

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