In a recent project I had to use a Transient Voltage Suppressor Diode (TVS) to protect my power supply input against any disturbance from mains. The unit was powered by a 18V power supply, but in order to pass certification, it had to incorporate several measures of safety against fire/explosion.
I had used TVS’s in the past in many instances, for both low and high voltage circuit protection, but never did a destructive bench power test of the diodes. This post explains the procedure and results of one such test.
Below is the simulated set up for the tests performed:
The test was performed using an isolation transformer for safety. A variac was also used to control the voltage used during the initial system checking. A digital capture oscilloscope was used to allow the results to be easily stored.
The fuse and transient suppressor diode are of type:
- TVS – Transorb SMBJ30A 30V 600W (Digikey part number SMBJ30ABTR-ND)
- FUSE – Bel fuse inc. 0685-4000-S1, 4A-SlowBlow (Digikey part number 507-1193-2-ND)
The test was performed as follows:
- Set the oscilloscope to at least 20 V/Div at DC coupling.
- Open the switch and apply power to the input.
- Ensure the oscilloscope triggers and captures.
- Turn off power
- Close switch and re-set oscilloscope to single shot trigger.
- Apply power and capture trace.
- Store results as a screen capture
- Substitute Fuse and repeat
The scope captures below describe the tests.
The centre peak is the first clamping and the next on the right is the clamp where the fuse blows.
This is a zoom of the first clamping peak area:
This is a zoom of the two clamping peaks. Note the clamping voltage reaching ~50V:
On all tests performed, the TVS diode did its job never failing. I substituted the Fuse for each test, but the TVS was always the same. It did not show any signs of visual/electrical degradation. On a real equipment would be wise to substitute the TVS diode for a new one, due to the possible internal degradation of the one that had to clamp mains power.
It can be confirmed the effectiveness of this diodes and the speed they operate. The resulting clamping voltage was within datasheet parameters.