Thursday, May 29, 2014

Semi Conductor Tester - SemTest Part 4

Testing was done in part 3, this last part is consist of following:
Final assembly
The front panel assembly can now be lowered down onto the case. Make sure that the three ribbon cables are folded neatly into the space above the lower PCB and not caught between the edges of the case or lid. Fasten the case together with four M4 screws into the corner holes, then fit the knobs to the rotary switch and the pot and the assembly is complete. 

Testing the HV crowbar
It’s now necessary to check that the HV crowbar circuit is working correctly. To do this, power up the unit, wait a few seconds and then press the Menu Select button. You will get a display like this:
Device to Test:▲
Press Enter and then the Up button. The display will then show:
Test parameter:▲
Press Enter again. Set the Device Operating Voltage to 25V, using the right-hand knob. Then press the Test On/Off button to start the test.
Now carefully measure the voltage across the top and bottom A and K terminals in the diodes and LEDs section of the test socket. You should get a reading close to 25V. If it’s much lower (say, 12V) then either the crowbar circuit has triggered prematurely or there is a fault in the DC/DC converter circuit. You will need to switch off, open up the unit and check the crowbar and converter circuits for faults, such as incorrectly oriented components.

If you get a much higher reading than 25V, there is a problem with the DC/DC converter section. Switch off and measure the voltage across the A and K terminals until it drops to a safe level. Then open the unit up and look for the source of the problem.

Assuming all is well, press the Test On/Off button to terminate the test. You can now do a high-voltage test. The procedure is similar to before, except you want to do an IREV(BV) test. So when you get to this stage:
Device to Test:▲
press enter twice and start the test.

Carefully measure the voltage across the A and K terminals again. It should be several hundred volts and it will rise to close to 600V after a number of seconds. Now press the Test On/Off button again to terminate the test while monitoring the voltage between the A and K terminals. It should immediately fall to just a few volts when the test is terminated.

If it remains high and only decreases slowly, the crowbar has failed to operate and you will need to wait for the capacitors to discharge before opening the unit up and checking for faults.

If the crowbar is not working (eg, if it fails), a warning will be displayed on the LCD immediately after performing a high-voltage test. This indicates that there is still a high voltage present at the test socket. If you get this warning then you should open the unit up and repair the crowbar circuit.

An add-on crowbar module instantly kills the high voltage applied to the ZIF socket at the conclusion of any breakdown voltage test. As a further safety measure, it also kills the high voltage in the event that the SemTest is inadvertently turned off before a test has properly concluded.

This minimises the chance of the user getting a shock from the test terminals when removing the DUT or a possible breakdown of the DUT itself when the power is inadvertently removed.

The crowbar module is wired to three points on the main (lower) SemTest PCB. On our prototype, these wires have been soldered to specific component leads, but the final
SemTest PCB has pads for these wires.

The crowbar board senses the 11.4V supply rail to the MC34063 DC/DC converter IC1. This drops very quickly to around 6V when a test finishes or more slowly if the unit is switched off during a test. Either way, this is the trigger for the crowbar to discharge the capacitor bank from 600V to a few volts in around 20ms.

Circuit description
The full crowbar circuit is shown in Fig.7. It could potentially be used in other devices, but for use with the SemTest, link LK1 is installed, to short Vin (the sense input) and V+ (its power supply) together. 
The +HV and GND terminals at CON1 are connected across the SemTest’s high voltage capacitor bank. Fig.8 shows a fragment of the SemTest circuit and demonstrates how the crowbar module is connected. The V+ terminal goes to pin 6 of IC1, which is at around +11.4V when the DC/DC converter is running and drops to 0V when it is switched off.

While the DC/DC converter is running, current flows from this rail, through diode D1, charging the 100μF capacitor. As this capacitor charges, the gate (AG) of programmable
unijunction transistor PUT1 is pulled up too, via the 10kΩ and 100kΩ resistors.

At the same time, the anode (A) is pulled up via a 330Ω resistor. The 10nF capacitor between PUT1’s anode and gate is initially discharged and this helps to keep the gate at
anode potential, preventing false triggering if there are any initial glitches in IC1’s power supply (eg, due to relay contact bounce).

A PUT is essentially a small anodegate SCR. While a conventional SCR is turned on when its gate is pulled above its cathode (K), a PUT turns on when its gate is pulled below its anode, sinking current from the gate. Both SCRs and PUTs remain on once triggered, until their anode-cathode current flow drops below the ‘holding’ current, in this case much less than a milliamp.

As long as V+/Vin are held at around 11.4V, the crowbar circuit remains deactivated. But once Vin drops precipitously, the 10nF capacitor begins to charge while the 100μF capacitor retains its charge, by virtue of diode D1.

Once Vin drops below the ~6V threshold, sufficient current flows from PUT1’s gate to trigger it on. It then dumps the charge in the 100μF capacitor into SCR1’s gate (KG), via the 330Ω current-limiting resistor. This happens in less than 100μs if Vin drops fast, as when a test ends normally.
The 330Ω resistor limits the current into SCR1’s gate to around 25 to 30mA, enough to trigger it reliably. SCR1 then rapidly discharges the high voltage capacitor bank through
the 100Ω resistor. The peak discharge current is 600V/100Ω = 6A.

PUT1 switches off as soon as it has finished dumping the charge of the 100μF cap into SCR1’s gate. But SCR1 stays on until the current through it drops below 40mA (its holding current) so the capacitor bank discharges to around 4V.

The specified TYN816 SCR is rated for 800V and 16A. Do not use an SCR with lower ratings.

Construction and testing
Refer to the overlay diagram, Fig.9. Fit the two small resistors first, followed by diode D1, with its cathode stripe towards the right side of the board. Use a lead off-cut for link LK1 and solder it in place. Then install the two 1W resistors.
Wiggle the middle lead of SCR1 back and forth until it snaps off. If there is any lead remaining, remove it with side-cutters. Bend the remaining two leads down and insert
them through the holes on the PCB, then use the machine screw to attach the metal tab with a shakeproof washer, both under the screw head and under the nut. Do it up tightly since the screw conducts the current when the crowbar activates. Then solder the two pins.

Fit the 10nF capacitor and then PUT1, bending its leads out with pliers to suit the pad spacing. Push it down as far as it will go before soldering and trimming the leads. Next, mount the 100μF capacitor, with its longer (positive) lead towards the left side of the PCB.
Bend its leads so that it lays down flat on the board before soldering them – see photo.

Don’t fit a terminal block for CON1, since we have limited clearance to fit the unit into the SemTest. Instead, solder a red wire to HV, a yellow wire to V+ and a black wire to 0V. Make sure there are no stray copper strands.

Wire the unit up to the SemTest as shown in the main overlay diagram (previous parts). Trim each lead so that you don’t have a lot of extra length. The photos show the best place to fit it.

Once it’s wired up, slip the crowbar module into the heatshrink tubing and apply gentle heat. Make sure there is no exposed metal when you are finished. Some silicone sealant can then be used to hold the unit in place, so it doesn’t rattle around inside the case.

Once the SemTest unit itself is complete, the HV crowbar must now be tested for correct operation, as described in the main article.

Using the SemTest
The SemTest is used as follows:
STEP 1: place DUT in ZIF socket and switch on.
STEP 2: Press Menu Select.
STEP 3: Use Up/Down buttons to select device type and press Enter.
STEP 4: Use Up/Down buttons to select test and press Enter.
STEP 5: For OPV tests, use right-hand knob to select test voltage.
STEP 6: Press Test On/Off to start test (red LED on) and read result.
STEP 7: Press Test On/Off again to finish test (red LED out).
STEP 8: check red LED is out and there is no high voltage warning on the LCD before removing DUT.

Exercise caution when testing components for high-voltage breakdown. Up to 600V DC is present on the device leads during such tests, so be careful not to touch them!

The biggest problem in using the SemTest is knowing the various lead configurations of the devices it can test. To that end, we have prepared a connections chart showing commonly used diodes, LEDs, BJTs, MOSFETs, SCRs and PUTs. It can be stuck on a wall or to the underside of the SemTest case for easy reference.
For less common devices, you’ll need to look up the connections in a data book or by downloading a data sheet from the manufacturer’s website.

Finally, here are a few tips to guide you when you’re doing some of the more specific tests:
  • When reading the forward voltage drop VF of a diode or LED, or the voltage drop VAK of an SCR when it’s conducting, be aware that the accuracy of this measurement is not very high due to measuring circuit limitations. So if you need to make really accurate measurements of VF or VAK, you’ll need to use an external DMM with its leads connected across the device’s ‘A’ and ‘K’ leads. 
  • Remember that during the same tests, it’s OK to increase the device operating voltage to a higher setting in order to see the voltage drop at higher current levels.
  • When you want to measure the hFE of a BJT, start on the setting with the lowest IBIAS level (ie, 20μA), because this is the setting with the highest hFE range. Only swing down to one of the  higher IBIAS settings if the hFE reading you get is very low (ie, below 300). This should only be necessary with medium-to-higher power devices, which often have their ‘peak’ hFE at higher currents.
  • When you want to measure the IDS vs VGS characteristic of a MOSFET to get an idea of its transconductance or ‘gm’, start by selecting the highest device operating voltage which will not exceed the device’s VDS ratings. That’s because the VGS bias voltage (adjusted via VR10) is derived from  he actual device operating voltage, which inevitably tends to drop once the device begins to draw drainsource current (due to voltage drop in the current-limiting resistors).
If you don’t set the switch for a reasonably high voltage to start with, you’ll find that it won’t be possible to provide much VGS once the device starts to conduct.

Actually, although you need to set the operating voltage within the device ratings when you start this test, it’s OK to increase the setting to 100V during the test itself, if you need to do so in order to achieve a higher VGS.

This won’t cause any problems if you only increase the voltage setting once the device is conducting.
Downloads: PCB n code files


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