FEC200 VBO Test
The "B" version of the FEC200 tester is needed to do this test. The B version has a "Peak" detecting circuit in addition to the usual Track/Hold circuit. It also uses a differential pre-amplifier for all VZ or VF measurements on scales up to 640V full-scale.
Please note that the power supply compliance is NOT 640V.
The test is similar to a standard VZ test except that the test current is high enough to turn on the Thyristor, that is, greater than IBO. Also the peak detector is turned on.
Here is a typical test:
- TEST(n) VBO 200MA >300V <375V
The power supply is turned on at the programmed current but of course the voltage must ramp up at about 100V/mS to reach the VBO and IBO before the part turns on. This test does not confirm that the part actually turns on, so be sure to program enough current to ensure that it does. Also, please see separate document about turning on thyristors with the FEC200.
Since the peak detector will record the highest voltage seen, it will record VBO. 6mS is allowed for the whole test but the on time is less depending on the time to ramp up to VBO at about 100V/mS.
TURNING ON THYRISTORS WITH THE FEC200 DIODE TESTER
For thyristors with VBO greater than about 20V the 500V 20Watt reverse test supply is needed to turn the part on.
Since the steady state power rating of the supply (20W) is less than the peak power needed to turn on some thyristors, special arrangements are needed.
Even with the circuit described below, there may be some parts that will not turn on.
The user should be alert to this possibility and be sure to program so as to detect such parts. The VT test should do this.
HOW IT WORKS
A 1µF capacitor on the B507 reverse test supply control board supply, allows it to supply a peak current of at least 1.5A but NOT FOR LONG.
This capacitor is charged to nearly 600V at the beginning of the test, but will be rapidly discharged as the supply ramps up at about 100V/mS from VBR to VBO. The profile of the current vs time waveform during this period can only be roughly approximated. Therefore the amount of compliance loss is uncertain.
The negative supply, which is driving the other end of the part helps in two ways. First it can supply plenty of current to keep the device turned on. A large rectifier in the circuit passes this current around the B507.
Also since it is very fast (compared to the B507) it can supply a "boost" of at least 20V to the applied voltage in order to get the part closer to VBO in a hurry (before the capacitor discharges).
Please also be aware that as soon as the part turns on the two supplies mentioned, it will deliver a fairly large pulse of current for some tens of microseconds before the supplies regain control.
Note regarding that "about" 100V/mS. We need to know for other reasons that the supply is at least that fast, so it should not be slower but it may be moderately faster, typically 105V/mS.