IGBT and SiC-MOSFET, both power semiconductor devices, are used in various fields such as electronic products, industrial equipment, aerospace, military equipment, railway systems, energy applications, smart grids, electric vehicles, etc. They are frequently used in high-power conversion, large-current power conversion and power control circuits, which may have an operating voltage with several kilo-volts. Because they will be switched ON and OFF very frequently, there is a PWM (Pulse-Width Modulation) high voltage difference across between the gate and collector or between the gate and drain of the module, and as well as between the module and heatsink. When the high voltage is across the insulation materials that contain voids, air gaps, or cracks, there is a higher likelihood of PD occurring. After long-term operation, the insulation material will be gradually degraded, which will eventually lead to insulation failure of the material and cause product damage.

▲ [Figure 1] Circuit diagram of a motor driver

Additionally, the operating bias voltage (threshold voltage) between the Gate and Emitter or between the Gate and Source of each module may be provided by individual transformers. These transformers may have a high voltage difference with high frequency between their primary and secondary sides. When the insulation capability between the primary and secondary sides of a transformer is insufficient, the surges of continuous abnormal discharge may interfere with the digital control operations and result in transistor failure.

Although the wires used in the transformers may have sufficient withstand voltage capabilities (e.g., 3000V) by themselves, when the coils/wires on the primary and secondary sides are very close to each other, it may appear that the wires can withstand a relatively high voltage (e.g., 6000V). However, in fact, after operating a period under the typical voltage (e.g., 1000V), insulation failure may occur. Because the permittivity of the general wire's insulation layer is much greater than that of air, the voltage division ratio across the air gaps between the wires is relatively high. When the voltage across the air gap between the wires is >350V (the discharge inception voltage for the shortest air distance at 1 atm), PD begins to occur on the partial surface between the wires. Because the insulation layer of the wires does not deteriorate or get damaged immediately, the insulation layer gradually carbonizes after continuous use over a period of time, ultimately leading to a short circuit on the primary and secondary sides of the transformer (Figure 4).

▲ [Figure 4] PD occurs between the wires of primary and secondary coils of a transformer

Photocouplers and digital isolators are used in various environments that require isolation. When the isolation voltage is applied across insulation materials that contain voids or cracks, the voltage across the voids or cracks may be sufficiently high to cause PD. After long-term degradation, the voltage isolation fails due to the insulation failure of the insulation material (Figure 5).

▲ [Figure 5] PD occurs when void is present inside a photocoupler

Partial Discharge Calibration

The PD tester is used for measuring and determining a tiny discharge quantity (in pC). Because this discharge signal is very small and rapid, the PD tester has to be precisely calibrated for ensuring the high frequency discharge signal can be accurately measured when the PD occurs.

HV Module (A195005 & A195004)

The HV modules offer precise and wide-range PD measurement. Different high-voltage modules can be selected based on the capacitance of the DUT and the requirements of PD measurement.

• The A195005 has four PD measurement Ranges (1 - 4), with a total PD measurement range from 1pC to 6000pC.
• The A195004 has two PD measurement levels, with a PD measurement range from 1pC to 2000pC.

After the test is completed, the mainframe will intuitively display the data and measurement results on the screen, aiding in the judgement and analysis of the discharge quantity.

The shape of the A195004 HV module has been designed as an isosceles trapezoid for the purpose of automation integration. This design allows multiple HV modules to be arranged in a fan-shaped configuration on the automated test system, which optimizes space usage. Additionally, the plug-in connection design brings the A195004 closer to the test socket, reducing test cable length (minimizing the impact of the cable on the test result). Moreover, the plug-in design makes insertion and removal of the HV module easier, and improves the convenience of maintenance and repair.

Anti-interference design

Because there may be unavoidable high-frequency noise in the test environment, test & measurement" and "operation & display" need to be separated. The HV modules are designed for the purpose of test & measurement, and the mainframe is designed for operation & display. This design allows the HV module to perform the measurement as close as possible to the DUT, which minimizes interference on the test cable by high-frequency noise from the surrounding environment. The measurement circuit is designed to isolate the noise signal, and makes the measurement circuit as short as possible. Additionally, the low voltage connection uses a coaxial cable to isolate the interference of environmental noise, and prevent the noise interference with the measurement circuit. Furthermore, a grounded metal shielding can be used to isolate the DUT from environmental noise and thereby ensure measurement accuracy.

Regulations require/suggest PD test for specific products and provide a reference formula for test voltage (Vpd) for the PD test. The PD test usually requires the maximum working isolation voltage or the maximum repetitive peak isolation voltage (whichever is higher) multiplied by 1.875 for the test voltage. The test voltage formula of PD test (Vpd):

3 Regulation-Compliant Test Methods

Chroma 19501 has built-in the regulation test methods (1), (2), and (3) as stipulated in the regulations. The bottom of the program page displays the diagram of the selected test method to assist users editing the parameters and selecting the test method, which allows them to get started easily and quickly.

3-Stages Test Methods

Chroma 19501 has built-in 3-stage voltage test methods. This enables manufacturers to not only meet the regulation's PD test requirements, but also the requirements of quality improvement by using a test voltage that is higher than the regulation's PD test requirements. This test method starts with a withstand voltage test (1st test stage), then uses the test voltage that is higher than the regulation's PD test requirements for a PD test (2nd test stage), and finally uses the test voltage (Vpd) which meets the regulation's PD test requirements for a PD test (3rd test stage). This test method not only meets the regulation's requirements but also improves product quality.

Application of High Frequency Voltage (A195004 Only)

High-frequency voltage has three main advantages:
(1) Increasing the frequency of output voltage increases the occurrence frequency of peak voltage, which can shorten the PD test time and improve productivity.
(2) Because the original insulation condition of the DUT may contain unstable factors (Ex. moisture, dust, burrs, etc.), which may lead to unstable PDIV results, the high frequency and high voltage can be used to pre-process the unstable factors of the DUT (pretreatment) in order to obtain more stable PDIV results. Higher voltage can be used to eliminate the unstable factors, and the higher frequency can be used for shortening the time cost of eliminating the unstable factors.
(3) When the weakness of the insulation quality needs to be analyzed or the location of the weaknesses needs to be identified, high frequency and high voltage can be used to deteriorate the insulation of the DUT. The higher voltage (> PDIV) can gradually degrade the insulation of the DUT, and the higher frequency can shorten the degradation time.

PD Count Judgement

When PD occurs, its signal is very small, and its measurement is easily interfered with by high-frequency noise from the surrounding environment. This is why the purpose of PD count judgement is to reduce misjudgments of test results caused by interference from external noise, and to confirm that the PD indeed happens on the DUT instead of merely being a one-time interference event from the environment. The PD occurring on/in the DUT is usually periodically due to the cyclical variation of voltage. Therefore, compared to high-frequency noise from the environment, the PD is relatively stable.

The Chroma 19501 is designed to count the occurrences of PD that persist over the Q Max limit for every half-cycle of the test voltage. There must be more than two occurrences of PD over the Q Max limit within every 5 consecutive half-cycles in order to accumulate the PD count. After the first PD exceeds the Q Max limit, there must be at least one additional PD over the Q Max limit within the subsequent 4 half-cycles (2 cycles) in order to accumulate the PD count. If not, the count will reset to zero and start over. When the PD count reaches/exceeds the set limit, it will judge the DUT as PD fail.

Example: PD count judgement is set to 5 P.D.

(1) Condition met: PD Count 5

(2) Condition not met: PD Count < 5

▲ PD > Q Max (ex. 5pC) doesn't occur more than once within every 5 half cycles. If PD > Q Max (ex. 5pC) doesn't occur within every 4 half cycles, PD count resets to 0.

HVCC (High Voltage Contact Check)

It is very important to perform a contact check on high insulation components before the high voltage output. Chroma's unique HVCC (High Voltage Contact Check) function uses Kelvin measurement to perform a contact check on the high insulation components before the high voltage output, which enhances the test reliability and productivity. The wiring circuit diagram is shown below.