polymer aluminum cap

You might want to solder some in series. It makes the measured values larger and provides a better estimate of the average component. Obviously divide by N.

You didn't zoom in on the spike?

I've seen both schools of thought, i.e. the person that like to flog the component, and the person that just builds the circuit and sees how it performs. I'm more inclined to do the latter since it is real life. Of course, if the component doesn't work well in the circuit, the person that did the flogging has a better idea of why.

The scope is max'd out at 2 mV/cm.

I like to understand parts, especially their undocumented and destructive limits. The polymer caps seem to suddenly fail shorted at about 2x rated voltage, with no change in leakage current as a warning... like a film cap.

The quality of the test pulse is important. A common home brew device in a lab is a low accuracy but extremely clean pulser. Basically you make a very low inductance load resistor by paralleling up carbon comp resistors with strips of PCB. Then the load is pulsed with a power fet. This voltage step isn't all that accurate, but you can measure it. The idea is the pulse is very clean. Much cleaner than the commercial load pulsers.

In your case, it makes far more sense just to try the component in the actual circuit, but it you want to test components, you near a very low glitch test source.

For ESR, I just used a B&K function generator. I set it to make a 5 volt pulse, which will make a 100 mA step.

Thank you, John. Very informative and educational.

You really need that home brew pulser. A function generator, being a driver with feedback, is probably not putting a clean pulse into the cap. Most LDO test scope photos are done with the design I described. It is a good low tech (translate: cheap) scheme to put seriously fast edged into capacitors.

Jim Williams has app notes on power pulse generators that are more complexe, but the scheme I mentioned is used by a few semis. The key is the low inductance resistor. I used a tape reel of free resistors.

Why not? It's a pulse generator with a 50 ohm output impedance. The risetime is obviously adequate.

Most LDO test scope photos are done with the design I described. It

I could have used our P400 digital delay generator, but it wouldn't have made any difference.

The pulsers I've built are in the ohm range source impedance. You really need to whack the cap to evaluate it. In the case of LDOs, you need to insure that a blazing fast input won't make it latch or over-voltage. When the goal (mind set) is to sell something by the millions, you need to be very thorough in evaluation.

If you don't want to evaluate the cap, just build the circuit. That at least gets the product out the door.

Gee, it would have been nice if EDN provided the schematic.

The paper does describe building the power resistor.

In my example, the fet is used as a source follower since the goal is to create a very low impedance pulse generator rather than a dynamic load.

There may be commercial loads these days that do the same job, but these home brew pulsers are common in semi test labs. Nobody testing components wants the settling time of the source (which uses feedback for accuracy) to be part of the component test.

Why? I measured 15 mohms to engineering accuracy, good enough for most purposes, and it only took a few minutes to set up.

In the case of LDOs, you need to

I did evaluate the cap, for ESL and ESR. I like to understand my parts and circuits.

I've done mosfet based pulsers when I wanted a lot of current, but I didn't need that to measure this ESR.

The B&K makes a fast step with a 50 ohm source impedance. It sure behaves like an amplifier followed by a real 50 ohm resistor, which it probably is.

There are nonlinearities when you really whack a cap.

The goal is to avoid the op amp with the 50 ohm series resistor. That is not remotely how the cap will be used in the application.

Your eval circuit doesn't reflect the application, thus testing the part in the actual application is the way to go if you don't want to be bothered building a pulser.

Is ESR nonlinear? Is ESL nonlinear? How?

Your goal maybe. Certainly not mine. My goal was to put a current step into the cap and scope the resulting voltage step.

That is

Current is current. The cap could be used in all sorts of applications. Will ESR change as a function of nearby components?

Testing the part in the application gives me no baseline for design and simulation, tells me nothing about margins, makes it hard to optimize after the parts are selected and soldered to the board.

The tests that I did gave me a pretty good idea about the ESL and ESR of these caps. Those are not things I'd prefer to be ignorant of.

I simply can't get through to you. Let me try this one more time. You DO NO, I repeat DO NOT, want a circuit with feedback driving the cap. Feedback circuits have rather complicated settling characteristics. You want to whack the cap with a very simple source follower using a low inductance resistor.

I = c * dv/dt.

You need a small dt to get a significant I.

You seem to think I'm pulling this out of my ass, but I gave you links to National's app note. It even mentions building the resistor by paralleling resistor rather than using the traditional wirewound power resistor. Even ADI references the National article. This is how the gurus do it. You can chose your own route.

I say to try the application circuit because, let me repeat this again, your test circuit does not replicate the application.

Do you think I made up putting devices in series to get more measurable data? That is how it is done.

I think I'm beginning to see why Thompson has issues with you.

Here we go again. Can't you folks understand the difference between an argument and a quarrel?

You guys are talking past each other. John is using a pulse generator, not a function generator. Pulse generators IME almost all have a very low impedance output amplifier driving a real genuine physical 50-ohm resistor. They work fine open or shorted, or with any arbitrary cable reflection you like. They really really look like a 50-ohm source.

Do you have a pulse generator that isn't like that? Which one?

Cheers

Phil Hobbs

That's what *you* don't want. Beats me why.

I can't get through to you. The B&K makes a nice clean, fast pulse with a 50 ohm source impedance, no complicated settling that I've ever seen. I don't know or much care what's inside the box. I wanted a 100 mA current step, and I got it.

Look at my waveform. It has an obvious fast step (Ipulse * ESR) and an obvious ramp (Ipulse integrated into 180 uF.) The information to calculate ESR is in plain sight. Note that the generator pulse is 5 volts open-circuit, and the scope is 2 mV/cm.

Of course not. My test circuit measures ESR.

Well, he is a redneck asshole. I bet he has trouble with lots of people. At least you haven't insulted my wife, so far.

I measured a part. I posted the results for public use. The numbers are good. Polymer aluminum caps are great.

Hey, it's still set up over on my bench. I blasted the cap with freeze spray and it looks like ESR went *down* a bit. Try that with a regular 'lytic cap!

The B&K 4003A is a classic analog function generator, but it does have a "pulse" setting. It sure behaves like a voltage source behind a 50 ohm resistor.

Geez, all I wanted was a 100 mA current step. The B&K can sure manage that. The waveform that I posted looks right.

A 50 ohm source is not low enough to jam current into a cap. Consider the impedance associated with a switch mode power supply fet and you will come to the same conclusion.

Hence those in the biz build these drivers.

Did somebody repeal Ohm's Law? Did I fake this waveform?

--

John Larkin Highland Technology, Inc

jlarkin at highlandtechnology dot com

Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation

Of course you faked it. Paint shop goes a long ways. :)

Jamie

On reconsideration, it looks more like 12 milliohms.