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Testing reverse-recovery behaviour of diodes

19 May 2016  | Louis Vlemincq

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Testing the reverse-recovery behaviour of diodes normally call for complex testing gear. You must be able to establish the forward-conduction conditions, the blocking state, and the transition between the two. You also need a means of extracting the characteristics from the resulting waveform. In short, a specialist should handle this complex job; it is not something you routinely control in the field. This fact explains why engineers generally prefer to rely on published data.

Checking the reverse-recovery time yourself could be advantageous, however, if testing were simple and straightforward. Such a setup would enable you to compare devices from different manufacturers under identical conditions and test devices having no such specification, such as substrate diodes of driver ICs, zener diodes, and standard rectifiers. (Because of the number of combinations of the test parameters, a direct comparison of the data is rarely possible.) Note that shorter reverse-recovery time is not necessarily better. Slow diodes can be useful, too. They can generate small dead times, improve the efficiency of converters, and provide other benefits (Reference 1).

This Design Idea presents a tester that, using only a handful of inexpensive, standard components, allows you to check reverse-recovery time. The test conditions are fixed for simplicity, to normalise the tests and to provide a common standard for comparison purposes. These conditions are compatible with 99% of the devices susceptible to test. The tester's forward current is just low enough to be safe with small switching diodes but high enough to overcome the capacitive effects in larger devices.

A diode-resistor AND gate lies at the heart of the circuit; the gate's diode is the DUT (device under test, figure). IC1 buffers flip-flop IC2A, which derives the antiphase square waves that drive this gate. R35 sets the DUT's forward current to approximately 75 mA. With an ideal diode, the gate's output would always stay low, because one of the inputs is always low. But a real diode remains conductive after the transition, generating a positive pulse across R35. Instead of using the brute-force approach of directly measuring this pulse width, the circuit uses a subtler scheme. The R19/C15 network averages the pulse and amplifies and displays the resulting voltage. Because the measurement frequency is fixed at 50kHz, a correct scaling factor is all that is necessary.

Figure: This diode-recovery test setup allows you to compare devices from different manufacturers under identical conditions.

A real diode also has a forward voltage, which you would average with the result. Q3 takes care of this problem by sampling this forward voltage through IC4A and subtracting it from the output voltage through R32. Varying the gain of amplifier IC4C sets the various ranges. In this case, the ranges are in a 1, 2.5, 5 sequence, which suit the salvaged galvanometer this circuit uses as an indicating device. You could easily create other ranges by adapting the values of R8 through R22. The big advantage of this measuring method is that it handles only dc or low-frequency signals, requiring no fast comparators or samplers, yet it can resolve a few hundreds of picoseconds.

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