Misleading specifications

I've been learning more about solar cell testing over the past year.  I have a couple of test systems I'm developing, I've taken data, and it's been interesting.  It's also been frustrating.

As an example let's consider I-V sweeping, the bread and butter of semiconductor testing.  Most of the basic values of a solar cell can be determined from a quick IV check.  One of those values, the short circuit current (I_sc), helps determine the maximum power a cell could theoretically produce.  A high I_sc is nice to have.

When I first looked at measuring IVs for solar cells last year, I tried to re-purpose my National Instruments PXI chassis to this task.  I knew that the source measure unit (SMU) in the chassis (PXI-4130) could only go up to about 1A, but there was an auxiliary supply that could plug right in to the SMU: the PXI-4130 Power SMU.  On the NI page for the aux the output specs are listed at "12 VDC, up to 5 A, up to 60 W, 0 to 55 °C" (note the "5A" limit).  So I bought the aux supply and got started testing.

But as I tested larger and better cells, with more total current output, I ran into limitations with the NI approach.  I couldn't measure an I_sc above 2 amps.  This confused me somewhat, since I had seen the the five amp limit on the NI page.  Adding insult to injury, I didn't even need the SMU to output that current - the solar cell was generating the current.  

I then considered connecting multiple SMUs.  There is a "knowledge base" article on the NI website that supposedly answers the question, "What is the Maximum Voltage and Current that the NI DC Power Supplies can Source when Cascading Outputs?"  But nowhere on that page does it list a maximum current.  I even found a Photovoltaic Solar Cell I-V Characterization Bundle page that says you can "add more SMUs to get three times the voltage or current."  I struggled with this problem for a month.  

Well, I finally got an answer from NI last week.  The SMU cannot measure above 2A current because the PXI chassis has to dissipate the power, regardless of where the current source is.  The chassis has "a hard limit on the amount of power consumed by each of the SMUs."  In other words, National Instruments has some cleaning to do on their website.

------------------------

So what's the moral of this rant?  First, I'm going to get a different power supply (maybe Agilent).  Second, you cannot always trust the hardware verbage you read online.

Test it until it works

After graduate school, I worked for an aerospace subcontractor firm as my first real job. The company’s products were split 50/50 between the military and commercial fields. On my first manufacturing project I started to run behind on shipments because the yield was slipping. When that happened, I was told by more seasoned engineers (who had initially started this project) to go through the "marginally failed" units and "test them until they work."

The rationale behind this statement was that:

1. The spec was extraordinarily tight for the product (blame was placed on sales & marketing).
2. We were up against the accuracy limits of the system.
3. It didn't really matter if the positioning of the cannon was off by a couple arc-seconds. They had redundant systems in place.

Thinking that this is how it must be done in the "real world," I did what he told me to do and got back on schedule. Granted, I figured out some other things to do to correct the yield, but going through the marginal failures was a contributing factor. But it always bothered me.

Stepping aside from the questionable moral grounds of this situation, let's look at that rationale list from a test engineer's perspective.

1. Spec too tight. Marketing should certainly know to what tolerance the product can be tested. If they don't, then it is the job of test engineering to inform them. If marketing plays the word game of "it is guaranteed by design" then it should not need to be tested, now should it?
   Of course, if marketing knows the limits and chooses to ignore them, then you have much bigger problems...
   
2. Limited test accuracy. If you are trying to test to a spec that is at the limit of what you can measure, then you have serious problems. Buy a more accurate tester, build one if you can't buy it, or do sufficient test system qualification to verify your accuracy. You have no business being anywhere near those limits. Test equipment manufacturers themselves can play "specsmanship" games, so you cannot always trust their numbers.
   
3. The customer doesn't really need that accuracy. I'm sure it's possible that the customer has over-specified what they need. They may have other backup systems in place if the accuracy is not there, they may have an over-tight spec because they don't entirely trust the product (or the company). Or they may just be clueless. But you can't get into the game of second-guessing the customer. That'll get you in deep trouble, somewhere down the line.
   

So, did I really screw up as a test engineer (although I wasn't called a test engineer back then), or was I just doing what I was ordered to do? I think I will just plead 'youthful transgression' and try not to let that happen again.