Ramblings about Airborne Power supply design and testing

blocher · 2019-09-28T11:24:07+00:00

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) pe r 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 ("no rmal 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 t o 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 use r intervention and works correctly. This is not a trivial requirement. Mu ch 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 impo se glitches on the startup voltage when powering up. Sure enough, we ran t hese waveforms and one of their sequences locked our unit up (power was app lied to the terminals and the unit was latched up and required user interve ntion 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 i s 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 educa ted....fortunately my mangers do try to do the right thing....most of the t ime) is to...