This board forms the heart of an equipment upgrade for a new test mode now incorporated into production line test equipment for discrete semiconductors. This design was done for one of our customers, a holder of a Queen's Award for Enterprise.

With the requirement for very low voltage, high current power supplies for the latest processors in computers, power FETs are paralleled to make high efficiency, and thus cool, switchmode voltage regulators. The FETs have to be matched for on-resistance and switching speed. The switching speed of these power FETs is determined by their capacitance and intrinsic gate series resistance, which is typically 10 ohms or less. The normal technique of measuring these parameters is with a precision LCR bridge. Tests with these instruments take between 200msec and one second. This is far too slow for production line testing of these devices as the full datasheet set of tests needs to be done in less than 200ms. The bridge instruments are also very expensive as only a small part of their capability is used for this particular test.

Our design uses a high speed analogue interface to a DSP which forces a low voltage sinewave onto the gate of the device under test. It then measures the the gate-source voltage and the current into the gate. The resulting magnitude and phase of these measurements is used to compute the series resistance and gate capacitance in under 5ms. The result is then sent to the tester through a high speed serial link.

We have recently implemented new algorithms for the compensating calculations which try to remove the effect of parasitic components in the connections to the handler probes. This further improves the accuracy of both the device capacitance and series resistance (Rg) measurements.

This board was initially fitted into the high voltage or the combined high and low voltage test heads on a device handler. It is now also available in three variants of a stand-alone test head.