Tech Briefs

This device prevents damage during testing by preventing catastrophic threshold breaches.

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).