We have one project with following E-fuse test requirement:
1. Power up the card and complete the power sequencing.
2. Setup and connect the E-load on the device’s output Vout_+12V (Net +12VA).
3. Validate the Vin_+12V and Vout_+12V.
4. Apply an E-load value to create a 3A load and validate Vin_+12V (Net +12V) and Vout_+12V (Net +12VA) Values.
5. Read by I2C the Peak current monitored.
6. Repeat the step 4 and 5 for every Amps step of 500mA.
7. To meet the SPEC, the E-fuse device should trip between 4.02A to 6.04A. If it trips out of the low and high SPEC, the card is bad.
During debugging, when the E-fuse tripped, the observed voltage of Vin_+12V on the scope would be over 35V, which exceed the voltage limit, MAX 20V. Refer to Figure 1. When trips, Spike happens on power input +12V. Output +12VA voltage level would reduce slowly.
The first way to reduce the voltage spike level is to add a regulator circuit (like Zener diode or else) that would limit the voltage around 15V.
Another option is to increase bulk input capacitance by adding a big capacitor (with low ESR) at Vin_+12V to reduce/avoid voltage spikes, because the capacitor reacts to the sudden change in voltage with a lower impedance during the spike itself, soaking up the energy.
Increase bulk input capacitance on Vin_+12V, a total 8mF equivalent capacitor was added to parallel with Vin_+12V. When the E-fuse tripped, the spike voltage peak value was about 28V, which was still higher than the 20V limit.
Placing a Transient Voltage Suppressor (TVS diode) right at the power input Vin_+12V and rated for the maximum clamping voltage required to protect any downstream components.
A TVS diode operates by shunting excess current when the induced voltage exceeds the avalanche breakdown potential. It is a clamping device, suppressing all overvoltage above its breakdown voltage. It automatically resets when the overvoltage goes away but absorbs much more of the transient energy internally than most other similarly rated device. A TVS diode can respond to over-voltages faster than other common over-voltage protection components such as varistors. The actual clamping occurs in roughly one picosecond, this makes TVS diodes useful for protection against very fast and often damaging voltage transients. Hence, on a low voltages circuit, a TVS diode rather than a MOV is the standard choice.
In this fixture, TVS diode SA14A was selected to add into the fixture, paralleled with board power input Vin_+12V. SA14A has the Breakdown Voltage (VBR) Min = 15.6V, Max = 17.2V.
After adding SA14A, it will suppress the input voltage level on Vin_+12V to about 19V, which is below the 20V threshold. Refer to Figure 3.
When trips, Spike happens on power input +12V. Output +12VA voltage level would reduce slowly.
In ICT fixture, TVS diode can be used on power rail spike protection, it also can be added on normal net to suppress the voltage spike, protect the DUT circuit and furthermore prevent the tester's pin card from damage.
In Keysight 3070 tester, the pin card relay SPEC shows 200V on the Max DC/Peak AC Resist. In some DUT circuits, e.g. the transformer related net may have voltage spike higher than 200V which results the pin card relay stuck. In this case, a TVS diode may be added in the fixture to avoid removing probe and coverage loss.