Long-term stress testing of silicon carbide (SiC) semiconductor devices is required to determine suitability for power electronics applications. During testing, preventable catastrophic failures can occur due to drift in steady-state operation or transients that shift the device outside of its safe operating range. Both steady-state and transient drift are easily monitored values including temperature, on-state resistance, voltage, and current, as well as others. By measuring and reacting to shifts in these values, device damage can be minimized.

Figure 1. The complete, unisolated Threshold Detector System with the probe board (left), relay board (right), and a shared ±15 V DC LEM power supply.
These values can be converted to a voltage allowing a variety of threshold detectors to determine when the measured values fall outside of safe limits. This prevents damage by providing an interlock signal if any monitored values vary outside a preset limit. A threshold detector/safety control module has been developed that is versatile, simple, reliable, and rugged. Figure 1 displays the complete detector module.

This threshold detector has applicability to a wide variety of test applications through the following capabilities:

  1. Upper and lower window thresholds adjustable between ±l3 V, accommodating a wide range of probe or transducer output ranges and offsets.
  2. Latching with 10 mV*60 ns sensitivity.
  3. Channels can be set for window compare or single threshold detect on.
  4. Four inputs combined by logical AND functions.
  5. Low hysteresis (typically or less).
  6. Low noise.
  7. High common-mode rejection (between input and threshold references).
  8. Most stages are designed to fail in a safe mode.
  9. Optical isolation provides safety, prevents ground loops, and provides the ability to float the output stage at any voltage differential between references.
  10. Powered using either DC power supplies or batteries.
  11. Highly reliable through simple design and construction.
  12. Easy to troubleshoot.

Figure 2. The system Signal Flow Diagram. The signals at gate U3’s inputs are discrete digital outputs from each of the comparators.
The threshold detector consists of two different electronics boards. The probe board consists of the window comparators and simple multiplexing circuitry. The relay board provides the control signals to external relays or interlocks. The comparator channels on the probe board utilize two window comparators, each to compare measured values within preset limits. The four channels of the window comparator can be adjusted to accommodate a wide range of voltage inputs (–13 V thru +13 V), allowing use of many different types of probe and transducer outputs.

Each input channel has a user-set upper and lower threshold reference. The window comparator compares the input value to its corresponding threshold limit. In most cases, the inputs need to be impedance-matched to the source due to the high input impedance of the comparators (AD-790s).

The comparator outputs latch low during threshold events, but they can also be set to not latch onto transient events. If the detection input to U1 and U2 surpasses either reference provided by R1 or R2, the respective output of U1 or U2 will be pulled low, resulting in a low at the output of U3. The signals at U3’s inputs are discrete digital outputs from each of the comparators (see Figure 2). A low output state from the comparators indicates either a failure or an exceeded threshold.

The complementary metal oxide semiconducting field-effect transistor (CMOS) AND gate U3 provides the output signal by combining the signals from each pair of comparators. Due to the CMOS AND gate-limited current output, a high-input impedance amplifier is used to drive the optical transmitter and the light emitting diode (LED), both of which are on during a non-failure condition. The output of U3 drives the gate of a junction field effect transistor (JFET) amplifier that controls the current for a optical transmitter. This optical transmitter allows an optical link between the probe board and the relay board, providing the ability to apply different reference voltages to each board. The differential voltage level between boards is only limited by power supply isolation.

As demonstrated in a die attach power cycling experiment, the threshold detector has proven to be a reliable safety switch. The power cycling test consists of several thousand on/off cycles with durations of 5 to 30 s each. During this test, increasing diode temperature caused by device degradation or current transients causes damage to the device and die attach materials. The threshold detector prevented catastrophic failure of the SiC diodes caused by the increasing device temperature. In this experiment, the gradual temperature drift caused a drift in the forward current and forward voltage of a group of series-connected diodes. The threshold detector monitored the anode voltage of each diode using differential probes (one diode per channel). If the voltage drifted beyond a predetermined threshold, the test was shut down to prevent damage due to excessive temperature rise.

The threshold detector’s use in the power cycling experiment is one example of the potential of this circuit. This device can limit the voltage, current, and temperature of the test item. The detector can also be controlled with an external timer to allow timed testing. Scalable and optically isolated inputs and outputs make this threshold detector applicable for many tests where monitoring and automatic shutdown are required.

This work was done by Mark R. Morgenstern of the Army Research Laboratory. ARL-0073

This Brief includes a Technical Support Package (TSP).
Detector Module for Testing Silicon Carbide Semiconductor Devices

(reference ARL-0073) is currently available for download from the TSP library.

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This article first appeared in the August, 2009 issue of Defense Tech Briefs Magazine.

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