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On Saturday, September 28, 2019 at 6:31:18 PM UTC-4, blo...@columbus.rr.com wrote:
no... this is not my stuff for sale and this has not been a lead in to sell these items. But I do make a great recommendation for these bad boys
On Saturday, September 28, 2019 at 7:24:13 AM UTC-4, blo...@columbus.rr.com wrote:
Over the last few years the RF design work in my company has dried up and I have been frequently assigned to power supply testing (and some design) per MIL-STD-704 and DO-160.
Here goes.....
Item 1.
Q. What do all of these test boil down to?
A. Aside from working over the specified input voltages and conditions ("normal operation" - which have a few subtleties) the important thing is that if 28V (typical nominal voltage for my category of equipment) is present on the input the box better be working. It is surprising how difficult that simple requirement is to achieve. The testing regiment requires the unit to be put through many types of over/under voltage tests that will take the box down and then the requirement is that the unit re-powers up without user intervention and works correctly. This is not a trivial requirement. Much of the testing really revolves around the standard's notion that this is not a trivial requirement.
---As an aside , one aircraft manufacturer requires additional tests above and beyond those in DO-160. In one case they require 8 waveforms that impose glitches on the startup voltage when powering up. Sure enough, we ran these waveforms and one of their sequences locked our unit up (power was applied to the terminals and the unit was latched up and required user intervention to power it off and back on to get it to work).
Many (most) of our products require a write to flash memory on power down.....oh boy , this requires a lot of attention to get it right.
Item 2.... Just run the waveforms.....OK, I have had to work real hard, at times, to educate my management that most of the time I find a problem it is in the setting up of the required waveforms. Say I have an undervoltage requirement of 12 Volts and as I am setting it up I apply 11 volts and the unit locks up. The first thing a manager tries to do (until properly educated....fortunately my mangers do try to do the right thing....most of the time) is to ask if the unit works with the specified waveform and I have to answer yes.....but.....The real purpose of the test is to make you look at the unit 100 different ways to Sunday to see if the unit latches up and part of the process is the setup variations that will frequently be where the sweet (sour) spot is. So I have to go back and remind them that if I can have 28V appllied to the box and it is latched up it really does not matter what I did to the unit before ---the box not adequate for an aircraft. (of course this whatever you do are within reasonable guidelines....undervoltages, short duration overvoltages and some ripple on the input voltage). When a manager tries to tell me to just run the waveforms I ask him if we can have this discussion with the customer to get clarification.....99% of the time that forces the required (and painful) design changes.
Item 3. Power input is a big deal to aircaft manufacturers. If you think MIL-STD-704/DO-160 is rigorous you should see what one well known aircraft manufacturer in France requires in addition to DO-160. It takes weeks to run the tests.....They do not want a box to latch up on their aircraft
Item 4. Normal operation.... These are the voltage variations that are applied that the unit is expected to work through. The aircraft manufacturers HATE HATE HATE latchups, but they also do not like the unit temporarily shutting off when it should not shut off. These normal operation requirements revolve around3 basic things 1. the unit will stay powered up for 200 ms (this can vary) when power drops below a specified voltage (hold-up cap required) 2. The unit will stay powered up when over voltages are less than 200 ms 3. The unit will operate through various audio frequencies imposed on top of the DC voltage. 4. The unit will operate from typically 16or18V to approximately 29-33 volts (each unit will have these nailed down exactly in a spec)
Item 5. All the other stresses are abnormal conditions and they should not cause the unit to latch up (per above) and also not damage the unit (OK thats obvious, but it is explicitly stated and required).
Item 6. Ideal diodes, they are not so ideal. I have seen designs where an Ideal diode (smart FET) is used instead of a schottky diode on the input. Yeah they work for negative voltages....but About those Audio frequencies imposed on the DC lines..... , they walk right through the ideal diode.. I am of the persuasion to use a schottky diode with a large capacitor after it. This will not allow internal resonances to develop which can cause many amps of current to circulate through the input circuitry. Take the 0..5V hit on efficiency. The real diode is so much nicer (do note I am coming at this for units that are 30W or less, usually less).
Item 7. Application of transients. MIL-STD-704 typically requires 5 applications of a particular interrupt (for instance) waveform. I insist on doing each one manually and slowly. Apply and observe before the next application of the transient. I have seen where people try to automate these transient applications and wind up making a burst of 5. I think this breaks the spirit of what is expected. I have seen the automatic test blow through them so fast that you cannot even tell what happened. I insist on manual application of transient waveforms.
Item 8 Some things can be automated. For instance , the slow sweep and dwell of audio frequncy ripple is OK if you are still able to properly monitor the equipment.
Item 9. In MIL-STD-704 people get confused because the first battery of tests are not about how the unit operates with voltage corruptions, but rather it is an assessment of how your unit might affect the power bus. For low power equipment always fight for these things to be characterized but not specified. EVERY TIME I test any of these items to a spec it fails....Especially inrush current. Just characterize it!
Item 10. Inrush current. On low power units fight, FIGHT, FIGHT against a hard spec. It will fail. Fight to characterize it. Low power stuff is not going to take down the aircraft power bus and your internal capacitors have to get charged up and yes, your EMI caps theoretically take infinite current for short durations to charge up. When you design around inrush you start to take away internal capacitance that you really want. The aircraft has to tranfer energy into your unit on power up. I think the aircraft manufacturers get that but the designers see a spec as a challenge and try to meet it (at the expense of inadequate internal capacitance or being in the crappy position of telling your management that you failed at test time)...Actually, the customer will probably give you relief on it (not on anything else though)so fight up front to avoid the headaches.
Item 11.....OK I guess I am done for now...Bye
Item 17: The link here :
https://www.ebay.com/c/21030870708
shows the picker gradient amplifier that I use to generate all kind of MIL-STD-704 compliant waveforms. These things are amazing! +/- 100 ish V @ 20 amp output and 200Khz BW. They are indestructible. you can drive this with a 2K AWG. If you have a calibrated scope and calibrated power meter (throw in required transformer for another 1K) you can be doing a lot of these tests for credit instead of going to a 3-5K/day test lab. And you can do all your own dry runs. You can also show your power supply designer where additional work is required
no... this is not my stuff for sale and this has not been a lead in to sell these items. But I do make a great recommendation for these bad boys