Measure the DUT conduction

Measure the DUT conduction Welcome to a Laptop AC Adapter specialist of the Agilent Battery

The other day Linda from Purchasing came to me with a problem: Lou from Operations needed to source a replacement for a shorted diode on a switching power supply. The darned thing was marked with a strange part number that no amount of Googling could decipher.

There was a recognizable logo marking, but that manufacturer could not provide a data sheet. The part number was from a previously acquired company and was unique to a specific customer. We were on our own.

Fortunately there was a second identical unit in for repair with such as Tektronix Y350C Battery, Tektronix Y400 Battery, Rohde Schwarz FSH626 Battery, Rohde Schwarz FSH-Z32 Battery, Rohde Schwarz FSH6 Battery, Rohde Schwarz FSH18 Battery, Rohde Schwarz FSH3 Battery, Biocare ECG-9803G Battery, Biocare HYLB-114A Battery, Aspect 185-0152 Battery, Aspect 186-0208 Battery, Aspect VTI 14564 Battery, and Lou was able to provide me with a good diode of the same type. Now all I had to do was figure out what it was. A standard rectifier diode? A zener? Schottky? Reverse voltage breakdown rating? Junction capacitance? Recovery time?

From the DO-41 package size it was easy to deduce that the rating was a watt. It was also easy to inject various currents and measure the forward voltage drop to determine that it was not a Schottky diode. Hooking up a few power supplies in series and gradually increasing the reverse voltage (with adequate series current limiting resistance in case a zener threshold was reached) proved it was not a zener diode – at least not below 200 volts.

The required PIV rating could be resolved by initially substituting a test diode with a high voltage rating, and scoping later.

This left only the unknown junction capacitance, Cj, and reverse recovery time, Trr. This is the time that a diode remains conducting when suddenly switched from forward to reverse biased. I had to figure out a way to measure these parameters. Not with exotic equipment; just enough to get in the ballpark – in other words, a function generator with a falltime of 40ns, and a 100MHz scope, which was all I had to work with.

The test setup was easy: Drive the diode-under-test (DUT) with a 5V pulse – DC offset set mostly negative to bias the diode on only during positive peaks. Scope both sides of the DUT and trigger on the turn-off edge. Varying the DC voltage offset controls the DUT forward voltage and conduction current. Measure the DUT conduction current as the voltage dropped across its series 50Ω resistor.