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DEVICE TESTING Figure 3. Load-line effect


through the drain to increase. As this current increases, the voltage output from the curve tracer will start to drop due to the load-line effect. If the peak voltage was not set high enough, the drain


to source voltage will drop below the minimum VDS level required for valid results.


Let’s consider the difference the choice of instrument will make for a test in which gfs


been specified at a VDS of 10V and an IDS has of 30A. If


the resistance in the output of the curve tracer is 75mΩ, then at 30A, we can expect a voltage drop within the supply of 2.25V. This means the peak voltage knob of the supply must be set to 12.25 V in order to get 10V at the DUT at 30A. In reality though, extracting gfs


requires sweeping past 30A


of current in order to place the function line cursor tangent with the trace, so the peak voltage knob actually needs to be set even higher.


though the curve tracer sweeps up to the same programmed peak value in every curve, the maximum voltage that actually reaches the device is different. If one were to draw a line across the ends of these curves, the line would be straight. This represents the load-line. When the slope of this line is measured, it is equal to the negative of the resistance in the output.


In contrast with the curve tracer, a pair of high current SMUs wired together in parallel can output up to 40V at up to 100A. Unlike the curve tracer, they do not experience the load-line effect because they use an active feedback loop to monitor their output rather than a series resistor. This loop only pulls back the voltage when it senses the programmed current limit has been reached. This allows all curves to reach the same peak voltage despite the increasing current. That allows them to output their maximum current all the way up to their maximum voltage, thereby providing more coverage in the power envelope. It’s also worth noting that the power envelope of the SMU extends into quadrants II and IV, also known as the sink quadrants.


In these quadrants, power from the device is transferred into the SMU and the SMU acts as an electronic load. This expands the use of SMUs to testing additional devices like DC-DC converters and solar cells. Curve tracers have long been used to perform forward transconductance (gfs


)


tests (Figure 4). SMUs’ lack of load-line effect is invaluable for this test, in which the drain to source voltage (VDS


) must remain higher than a minimum level throughout the test to get valid results. In this test, the gate voltage is swept, causing the current


In contrast, when using an SMU, there’s no load-line effect to worry about, which simplifies the testing procedure. The VDS


can simple be set at


exactly the specified voltage and that voltage will stay the same even as the drain current increases. Programmable current limits are one of the advantages SMUs have over the curve tracer that make them a better fit for testing modern power semiconductor devices. On an SMU, when voltage is swept across the device, the current can be limited to avoid causing device damage. On the curve tracer, the only current-limiting device is the series output sense resistor, which helps protect only the power supply, not the device.


SMUs have another major advantage over traditional curve tracers: their ability to source current or voltage, which is particularly important when making ON-state resistance (Rds(on)


)


measurements. Manufacturers are always trying to reduce the ON-state resistance of devices because less resistance means less power consumed by the device, increasing the efficiency of power delivery by the device.


Today, devices with an Rds(on) of less than


10mΩ are common. When testing devices with resistances this low, a very minor change in voltage can cause a major change in current.


For example, for a device with an Rds(on) of 2mΩ,


a 10mV change in voltage will create a 5A current change in current. Considering that the curve tracer’s high current supply only has a programming resolution of 30mV, if one ignores the load-line effect, that would mean a 15A change in current for each click of the knob!


Although curve tracers can only source voltage, SMUs have the ability to source both voltage and current. Sourcing current allows SMUs to make extremely accurate Rds(on)


measurements 44 www.siliconsemiconductor.net Issue IV 2013


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