Testing Silicon Carbide Using the Avalanche Test and the Double Pulse Test
Switching Device Characterization Tests
There are two common tests for characterizing the performance of switching devices, the Avalanche test and the Double Pulse test.
Avalanche Test
- Purpose: The Avalanche test evaluates the device's ability to withstand unclamped inductive switching (IUS) events, that can occur during operation. It helps determine the device's robustness under high-stress conditions.
- Explanation: The Avalanche test subjects the device to conditions where it must absorb and dissipate energy from an inductive load without external clamping. This test is crucial for understanding the device's reliability and durability in real-world applications.
Double Pulse Test
- Purpose: This test characterizes the switching losses of the device, specifically focusing on turn-on and turn-off losses (EON and EOFF).
- Explanation: The Double Pulse test involves applying two consecutive pulses to the device and measuring the energy losses during switching transitions. It provides valuable insights into the device's efficiency during dynamic operation. It is important to note that this test does not measure conduction losses, which are the losses incurred when the device is in the on-state.
By conducting these tests, engineers can understand the performance characteristics of SiC devices, ensuring they meet the necessary requirements for high-efficiency and high-reliability applications.
Avalanche Testing
Avalanche testing is crucial for evaluating a MOSFET's ability to withstand unclamped inductive switching events. These events occur when the maximum drain-source voltage is exceeded, causing the device to enter a breakdown state where current flows from the drain to the source. This current flow dissipates as thermal energy within the die, and excessive energy can lead to a significant temperature rise, potentially damaging the device.
This phenomenon, known as an avalanche event, is simulated during avalanche testing to characterize how much energy the device can absorb without failure. The test helps determine the robustness and reliability of the MOSFET under high-stress conditions.
In Figure 1, you can see an example of an avalanche test setup from our lab in Austin, TX. This setup is designed to rigorously test the energy absorption capabilities of MOSFETs, ensuring they meet the necessary standards for durability and performance in real-world applications.
Figure 1: Avalanche Testing Setup
Avalanche Test Steps
The Avalanche test involves several critical steps to evaluate a MOSFET's robustness under high-stress conditions summarized in Figure 2:
Energy Storage in the Inductor
Current is allowed to flow through an inductor, storing energy in the form of 1/2 x L x i2, where ( L ) is the inductance and ( I ) is the current.
Turning Off the MOSFET
The MOSFET is then turned off, causing the drain-source voltage to rise rapidly until it reaches the breakdown voltage of the device under test (DUT).
Current Flow Through the Body Diode
At this breakdown voltage, current flows through the body diode of the DUT, dissipating the energy stored in the inductor.
Incremental Step Testing
For incremental step testing, the drain current is gradually increased in successive tests until the device fails. Failures typically manifest as either leaky gates or shorts from drain to source.
Determining Avalanche Energy Capability
The highest energy level at which the device can operate without failure is considered its avalanche energy capability. This metric is crucial for understanding the device's ability to handle real-world inductive switching events without damage.
By following these steps, engineers can accurately characterize the energy absorption capacity and overall robustness of MOSFETs, ensuring they are suitable for high-stress applications.
Figure 2: Avalanche Test Steps
Avalanche Test Pass
An example waveform from an avalanche test where the device successfully passed is shown in Figure 3. A long pulse is applied to the inductor in a typical avalanche test, allowing current to flow and ramp up. Once the desired current level is reached, the MOSFET is turned off, causing the drain-source voltage to increase rapidly until it exceeds the device's breakdown voltage. At this point, current begins to flow through the body diode of the device under test (DUT).
In the waveform, you can see the pink trace representing the current flow. This current continues until all the energy stored in the inductor is dissipated through the device. In this particular test, the energy released from the inductor was 200 microjoules, and the device did not fail, indicating a successful pass. The MOSFET was able to handle the energy without any damage, demonstrating its robustness and reliability under high-stress conditions.
Figure 3: Avalanche Testing - Pass
Avalanche Test Fail
In Figure 4, we observe a waveform from an avalanche test where the device failed. During the breakdown period, current flows through the device as expected. However, at a certain point, the device fails, causing the current to spike dramatically. This sudden increase in current indicates that the device has shorted internally.
Following the failure, the current stabilizes at just under 60 amps, clearly showing that the device can no longer regulate the current flow and has entered a short-circuit state. This waveform highlights the critical moment of failure, providing valuable insights into the device's limitations and the conditions under which it can no longer operate reliably.
Figure 4: Avalanche Testing - Fail
Double Pulse Testing
Next, we'll discuss the Double Pulse Test (DPT), a standard method for characterizing switching losses in power devices. DPT measures the energy dissipated as heat during turn-on and turn-off events, providing crucial insights into the device's efficiency and performance.
Switching losses are influenced by various factors, including device characteristics, operating conditions, gate drive parameters, parasitics, and more. These losses are highly dependent on the real-life application of the device. Mathematically, switching loss can be represented as the power dissipated through the device, calculated by integrating the product of current and voltage over the switching period.
In Figure 5 on the right, this period is shown from time ( ta ) to time ( tb ). The total power dissipated in the device due to switching is the sum of the turn-on energy and turn-off energy, multiplied by the switching frequency. This comprehensive approach allows engineers to accurately estimate the switching losses and optimize the device for specific applications.
Figure 5: Double Pulse Testing
Double Pulst Testing Steps
During the Double Pulse Test (DPT), the first pulse is applied to the gate of the device under test (DUT) to establish a soak time. This soak time determines how high the initial current ramps, following the relationship ( V = L x di/dt ), where ( V ) is the bus voltage, ( L ) is the inductance, and (di/dt) is the rate of current change.
To achieve the desired test current, the pulse time ( t1 ) is calculated using the formula:
In Figure 6 on the right, you can see the first pulse causing the current to climb with a slope of ( V\L ) until the pulse time ( t1 ) is reached. At this point, the device is switched off.
Figure 6: DPT Operation
During the off period following the first pulse, the switch remains off, and the current freewheels through the catch diode. As shown in the diagram, this period is crucial for calculating the turn-off energy. The turn-off energy is determined right at the beginning of this off period, as indicated in the bottom right of Figure 7.
Figure 7: DPT Operation
Then the gate is turned back on and this is the point at which the turn on energy is measured (shown in Figure 8). Note here that this energy is not only the current that is flowing through the inductor but also the energy that comes from the reverse recovery of either body diode or Schottky barrier diode depending on the architecture of the converter.
Figure 8: DPT Operation
Why do we do Double Pulse Testing?
The dynamic characteristics of switching are heavily influenced by the operating conditions of the switch. Performing Double Pulse Testing (DPT) under real-life operating conditions provides a much more accurate picture of switching losses compared to relying solely on datasheet values.
Figure 9 illustrates this point by comparing the datasheet values for a silicon IGBT and a Silicon Carbide MOSFET. As shown, the operating conditions specified in the are not identical and rarely match the specific application you are targeting with the device. This discrepancy means that datasheet values may not be as meaningful or accurate as in-situ measurements obtained through DPT.
By conducting DPT, you can capture the actual switching losses under the specific conditions relevant to your application. This approach ensures that the data you gather is directly applicable and provides a more reliable basis for optimizing the performance and efficiency of your power devices.
Figure 9: Why do DPT?