Test & Measurement - Top tips for cabling and test fixture safety

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In general, test cabling and test connections must all be designed to minimise resistance (R), capacitance (C), and inductance (L) between the device under test (DUT) and the used source-measure unit (SMU) explains the applications engineering team at Keithley Instruments

To minimise resistance, use heavy gauge wire wherever possible, and definitely within the test fixture itself. The gauge required will depend on the level of current being carried; for example, for cabling that must carry 40A, a 12 gauge cable is probably necessary. For guidance on choosing cabling for higher current levels, refer to construction industry wire gauge tables, such as the one available at: www.powerstream.com/Wire_Size.htm. Check the ‘Maximum amps for chassis wiring" column to find the wire gauge needed to carry the level of current involved.

Low-resistance cabling is critical to preventing instrument damage. Choose cables with resistances of less than 30 milliohms/meter or lower for 10A pulses. Keep cable lengths as short as possible and always use low-inductance cables (such as twisted-pair or low-impedance coax types), heavy gauge cable in order to limit the voltage drop across the leads. Ensure the voltage drop won't be excessive by checking the SMU's Voltage Output Headroom spec. For example, if you were using a Keithley Model 2602A (pictured above) SMU to output 20V, the test leads should have no more than 3V of voltage drop across them to avoid inaccurate results or instrument damage. It is specified for a maximum voltage of 3V between the HI and SENSE HI terminals and a maximum voltage of 3V between LO and SENSE LO. 

Although many believe guarding can minimise the effects of cable charging, this is typically more of a concern for high voltage testing than for high current testing. Four-wire Kelvin connections must be kept as close to the DUT as possible; every millimetre makes a difference.

Also, it should be noted 0voltage readback should be done with the SMU that's forcing voltage, because the current-sourcing SMU's voltage readings will all vary quite a bit due to the connections, and will differ from what is actually seen at the DUT.

The jacks used on the test fixture should be of known high quality. For example, some red jacks use high amounts of ferrous content to produce the red colouring, which can lead to unacceptably high levels of leakage due to conduction. The resistance between the plugs to the case should be as high as possible and in all cases >1010 ohms.

Many published test setups recommend adding a resistor between the SMU and the device's gate when testing a FET or IGBT. When pulsing large amounts of current through these kinds of devices, they tend to oscillate. Inserting a resistor on the gate will dampen these oscillations, thereby stabilising the measurements; because the gate does not draw much current, the resistor does not cause a significant voltage drop.

If voltages in excess of 40V will be used during the test sequence, the test fixture and SMUs must have the proper interlock installed and be operated in accordance with normal safety procedures.

Many electrical test systems or instruments are capable of measuring or sourcing hazardous voltage and power levels. It's also possible, under single fault conditions (e.g., a programming error or an instrument failure), to output hazardous levels even when the system indicates no hazard is present. These high levels make it essential to protect operators from any of these hazards at all times. Protection methods include:
- Verify the operation of the test setup carefully before it is put into service.
- Design test fixtures to prevent operator contact with any hazardous circuit.
- Make sure the device under test is fully enclosed to protect the operator from any flying debris.
- Double insulate all electrical connections that an operator could touch. Double insulation ensures the operator is still protected, even if one insulation layer fails.
- Use high reliability, fail-safe interlock switches to disconnect power sources when a test fixture cover is opened.
- Where possible, use automated handlers so operators do not require access to the inside of the test fixture or have a need to open guards.
- Provide proper training to all users of the system so they understand all potential hazards and know how to protect themselves from injury. It's the responsibility of the test system designers, integrators, and installer to make sure operator and maintenance personnel protection is in place and effective.

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