For power-electronics engineers, power assemblies are our design objects, and IGBTs are widely used in power assemblies due to their excellent characteristics. The efficiency, protection functions, EMC performance, and other aspects of power assemblies are closely related to the application design of IGBTs, and different IGBT technologies also produce IGBT products with different performance.
To optimize and verify assembly performance and to verify the performance of different IGBTs, we introduced the double-pulse test method; with this tool we can achieve the following specific functions:

Detailed understanding of various IGBT parameters, such as Eon, Eoff, Tdon, Tdoff, Tr, Tf, chip switching characteristics, etc. The significance of this test is to verify the parameter values in the datasheet and evaluate the performance of different modules.

Evaluation of gate-driver circuit design. The selection of Rg, Cg and the gate cable affect the gate waveform, whether oscillation occurs, dv/dt, di/dt, and other characteristics. In addition, Miller clamp, gate clamp, active clamp, and soft turn-off designs can all be evaluated in this test.

Thermal design parameter verification. Conventional thermal design is based on Eon and Eoff in the datasheet; these given values are provided under specific test conditions. Due to requirements for optimizing and balancing module performance, actual Eon and Eoff are often inconsistent with theoretical values. For example, due to EMC requirements we desire lower dv/dt and di/dt, which will lead to larger Eon and Eoff. With this test we can obtain the true Eon and Eoff values, making simulation more realistic.

System stray inductance: The operating voltage of the module under all conditions must be less than the rated voltage; at low temperatures, derating must also be considered. The IGBT voltage spike also depends on system stray inductance, turn-off di/dt, DC-bus voltage, etc. This test can evaluate the maximum stray inductance allowed by the system and compare the stray inductance of different busbars.

Snubber circuit verification: When the turn-off spike is too large, we need a snubber circuit to reduce the spike voltage. The type of snubber circuit, the capacitance of the snubber components, the internal inductance, and the placement all affect the magnitude of the spike voltage.

Overcurrent and short-circuit protection circuit verification: In this experiment, the turn-off spike voltage, gate waveform during turn-off, short-circuit current, protection circuit response time, presence or absence of oscillation, etc. need to be verified.

Busbar and AC busbar design for parallel operation: When modules need to operate in parallel, the module junction temperature is related to the degree of module current sharing, which determines whether the modules can operate reliably and their lifetime; the lifetime of the entire system depends on the module under the harshest operating condition. Moreover, paralleling multiple modules also poses challenges to the design of overcurrent and short-circuit protection.

Basic method of double-pulse testing

As shown in the figure above, the double-pulse test system consists of a DC power supply, DC-bus support capacitor, snubber capacitor, gate-driver circuit, IGBT under test, load inductor, oscilloscope, voltage probe, and Rogowski coil. Because IGBTs usually operate at high temperatures and their performance differs from that at low temperatures, we also need a hot plate to understand IGBT characteristics at different temperatures.

In the figure above, during the initial pulse the high-side IGBT is blocked and the low-side IGBT receives the pulse and turns on; the DC-bus voltage forms a loop through the load inductor L and the low-side IGBT. The voltage across the inductor causes the current to rise, and the inductor current can be calculated by di=dt*(U/L).

At the same time, by adjusting the DC-bus voltage, inductance, and conduction time, different peak currents can be obtained. At the end of the first pulse, the IGBT turns off, and the inductor current freewheels through the high-side diode. Due to the presence of stray inductance, the current cannot change abruptly, causing a spike voltage at turn-off; the turn-off spike voltage can be calculated by U=L*di/dt.

Before the second pulse is issued, the high-side diode is in the freewheeling conduction state. After the second pulse is issued, the high-side diode undergoes reverse recovery, and this current will flow through the low-side IGBT while the inductor current continues to rise. At the turn-off instant, a larger voltage spike will appear in the low-side IGBT. If the reverse-recovery characteristics of the diode need to be observed, the waveform at the turn-on instant of this pulse must be observed.

The figure below shows the measured double-pulse waveform:

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Double-pulse test equipment

Users can purchase or build their own double-pulse test platform, which includes the following parts:

DC power supply:
The voltage of the configured DC power supply must be greater than the rated voltage of the module. Because the load is a short-duration pulse, a DC power supply of about 1KW is usually sufficient.

Low-voltage gate-driver power supply:
If the gate-driver core has isolation and voltage regulation, an adjustable regulated power supply with 24 V input can be selected; the current is around 1 A, and the power supply output is adjusted according to the requirements of the gate-driver core. If the gate-driver circuit does not have isolation and voltage regulation, the low-voltage power supply should provide these functions; the isolation voltage must be greater than twice the module rated voltage, and the voltage output accuracy must be within ±0.2 V.

DC-bus support capacitor:
During IGBT switching, the energy is supplied by the capacitor. Typically, the IGBT module under test ranges from a few amperes to about 2000 A, and the corresponding capacitance is selected approximately linearly between several hundred microfarads and several millifarads.

DC busbar:
For platform test equipment targeting different power assemblies, the busbar inductance of each assembly differs. To emulate the busbar inductance of different assemblies, the DC busbar in the platform can be constructed as an adjustable inductor. A better method is to make the length of the terminals contacting the module adjustable.

Inductor:
The inductor in the system has two functions: one is to act as a load to adjust the output current and observe module switching at different currents, and the other is to emulate different operating conditions, such as long and short cables during short circuit. This inductor must not saturate at high current and is generally made as an air-core tapped adjustable inductor. The adjustable inductance range is typically from tens of nH to a few µH.

Hot plate:
The switching characteristics of the module are temperature-dependent; for example, the switching loss at 125°C is 1.38 times that at 25°C. For a detailed explanation, refer to the Semikron Application Manual, English edition, page 279, or Chinese edition, page 292. When verifying IGBT performance at different temperatures, a digital multimeter can be used to measure the internal NTC or PTC temperature to confirm that the die has reached the required temperature.

Double-pulse generator:
The double-pulse generator can provide test signals for the test system and is required to independently control the pulse width and pulse interval.

Enclosed cabinet and discharge device:
Protective function.

Gate-driver circuit:
Select the gate-driver core actually used by the module under test whenever possible. Because the push-pull circuit performance of different gate-driver cores differs, different gate characteristics may occur even with the same gate configuration.

Oscilloscopes and probes:
Differential low-voltage probe for measuring gate voltage, differential high-voltage probe for measuring collector-emitter peak voltage, and Rogowski coil for current measurement.

Using actual power assemblies as the test platform

The standard double-pulse test platform is more suitable for comparing performance parameters of different modules, while the IGBT actually operates in an assembly. The system stray inductance, gate-driver circuit, and other aspects differ from those of the standard test platform. To evaluate the actual performance of the IGBT in the assembly, double-pulse testing can be performed in the actual assembly, so the test results are closer to actual conditions. The user only needs to modify the software to implement this function.

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Test Precautions

Wear an antistatic wrist strap and confirm that it is effective when installing the test system or handling IGBT modules.

Select an appropriate test platform according to the test objective; for example, testing Eon and Eoff in the assembly is preferable because the system stray inductance, etc., is closer to actual operating conditions.

To obtain accurate test data, test equipment must be periodically calibrated by a professional testing organization, especially test probes.

Accurate testing requires preheating the oscilloscope for 10–20 minutes so that the sampled data are accurate.

During testing, note that interference may cause inaccurate results. Probe placement has a major influence: the probe can be oriented perpendicular to the DUT and away from large di/dt and dv/dt noise; during testing, probe leads should be twisted in pairs to reduce common-mode noise interference.

Test pulse width should account for the narrowest pulse allowed by the IGBT. An excessively narrow pulse prevents internal carriers from stabilizing, causing abnormal phenomena such as module oscillation; narrow pulses are usually limited to above 4 us.

Inactive IGBTs in the DUT should be clamped to a negative voltage or have their gates shorted to their emitters; otherwise, noise interference or the Miller effect, etc., may falsely turn on the module. As shown in the figure, a large dv/dt causes current to flow through the Miller capacitance to the gate, generating a voltage drop across the gate and emitter impedance and thereby possibly turning on the IGBT.

When testing high-power modules, the required current is relatively large. In this case, pay attention to bus voltage drop and increase the bus input voltage to compensate for the voltage drop during pulse testing so as not to affect test results.

Due to stray inductance in the module, the auxiliary terminals are closer to the actual chip peak voltage when measuring the IGBT turn-off spike.

For short-circuit testing, the pulse width should not exceed 10 us; thus, even if the protection circuit does not turn off the module within 10 us, the double-pulse test equipment will also shut down. If the peak voltage across the IGBT C–E terminals is less than the maximum allowable voltage and gate voltage clamping is reliable, the module will generally not be damaged.

Gate cable length affects test results: it causes changes in the gate waveform, and the gate waveform affects parameters such as losses and short-circuit current. Therefore, cable length during testing should be close to actual operating conditions.

The maximum allowable operating peak current of the IGBT is twice the rated current. Except for short-circuit testing, the current under test should not exceed twice the rated current of the module.

Because different internal layouts of IGBTs lead to different stray parameters in the high-side and low-side switches, these differences are reflected in inconsistent characteristics of the high-side and low-side switches. Therefore, the high-side and low-side switches should be tested separately during double-pulse testing.

Regarding whether to add a snubber capacitor: when verifying module performance, it is not necessary to add a snubber capacitor for testing. When verifying assembly performance, the module is placed in the assembly for testing; in this case, ensure that the setup is the same as the actual assembly setup. If the assembly actually operates with a snubber capacitor, it must be added.

Because strong interference and different reference potentials exist in the system, optical fiber should be used as the interface between the double-pulse generator and the gate driver whenever possible.

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Safety Precautions

Because high voltage exists in the system, test personnel must be trained before they are permitted to perform testing.

Test personnel must be equipped with protective equipment, such as insulated shoes and protective goggles.

Double-pulse testing requires at least two people to be performed simultaneously.

The double-pulse platform shall be equipped with an emergency power-off device.

The double-pulse test platform cabinet, high-voltage power supply, and oscilloscope shall be reliably grounded; otherwise, there is a risk of electric shock.

The capacitor bank shall be equipped with a forced-discharge device.

Each time the test platform is opened to adjust settings or remove a module, a multimeter shall be used to verify that the system has been fully discharged.

Before charging the bus voltage, power up the low-voltage system first and issue test pulses to confirm that the gate-driver board and the double-pulse test equipment are operating normally. After confirmation without error, adjust the output voltage from low to high and the output pulse width from short to long; after each test is completed, reset the high-voltage power supply voltage and pulse width to 0.

Before testing, test personnel shall cross-check whether the wiring is correct.

When powering up, the gate driver shall be powered up first; when powering down, the main power shall be powered down first and drop to 0 before the gate driver is allowed to power down; otherwise, the gate may turn on in an uncontrolled state.

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References:

A. Wintrich, U.Nicolai, W. Tursky, T. Reimann, “Application Manual Power Semiconductors”, 2ndedition, ISLE Verlag 2015, ISBN 978-3-938843-83-3