Power Device Dynamic Parameter Test System Selection Guide to Avoid Pitfalls

According to the package type of the device under test, power device dynamic parameter test systems are divided into two categories: for discrete devices and power modules. For a long time, the test system for power modules occupies the majority of the market share, and the demand for the test system for discrete devices is less and the choice is very limited. With China's power device localization process accelerated, power device manufacturers and system application enterprises also pay more and more attention to the dynamic parameters of power devices test, especially for discrete devices test system put forward more and more demand.

" Dynamic characteristics is an important characteristic of power devices, in device development, system applications and academic research and other aspects play a very important role. Therefore, testing the dynamic parameters of power devices is a necessary part of the related work, which is mainly carried out using double pulse testing."

According to the package type of the device under test, the power device dynamic parameters test system is divided into two categories for discrete devices and power modules. For a long time, the test system for power modules occupies the majority of the market share, and the test system for discrete devices has less demand and limited choice. With China's power device localization process accelerated, power device manufacturers and system application enterprises also pay more and more attention to power device dynamic parameter testing, especially for the discrete device test system has put forward more and more demand.

Throughout the market at this stage can provide the power device dynamic parameter test system, its technology and service levels are uneven. Very easy to appear the actual test effect can not meet the specifications of the situation, and even some test systems do not even have the basic test function, making the user spend money, but also a lot of time and effort wasted.

In order to avoid the above-mentioned problems occurring again in the majority of engineers, this article will lead you to see how to avoid pitfalls in the selection of dynamic parameter test system for power devices.

01: The test voltage and current range to meet

When we choose a test system, the first problem we face is the voltage and current range of the device that the test system can test. Test system specifications will generally be marked "maximum xxxV / xxxA", but such a way of marking is far from enough, there will be less than the maximum voltage when the maximum current situation.

Let the test load inductance is L, bus capacitance is C, test voltage is V, test current is I, then the double pulse the first pulse width τ, the first pulse width at the end of the bus voltage dip ratio is less than Kv need to meet.

It can be seen that τ is used to make the current reach I. τ increases with the increase of I and L, and decreases with the increase of V. In the actual test, the time of τ should not be too long, responsible for making the device heat up seriously affect the test results. At the same time, C needs to be greater than a certain value to ensure that its bus voltage dip at the end of the first pulse width proportional to less than Kv, so as to ensure that the bus voltage dip in the second pulse in the acceptable range, responsible for the double pulse test of the turn-on and turn-off voltage inconsistency. c increases with the increase of I and L, and decreases with the increase of V and Kv. In testing, C and L are determined by the hardware conditions, V is given by the test conditions, while there is an upper limit requirement for τ. Together, these parameters determine the test current that can be achieved.

For high-voltage devices, it is assumed that C=40uF, capacitor withstand voltage value of 1100V, L=10uH/50uH/100uH, and the upper limit of τmax<20us; for low-voltage devices, it is assumed that C=3000uF, capacitor withstand voltage value of 200V, L=10uH/50uH/100uH, and τ<20us. According to the above equation, we can list the maximum current that can be achieved at this time The maximum current is shown below.

It can be seen that τ is used to make the current reach I. τ increases with the increase of I and L, and decreases with the increase of V. In the actual test, the time of τ should not be too long, responsible for making the device heat up seriously affect the test results. At the same time, C needs to be greater than a certain value to ensure that its bus voltage dip at the end of the first pulse width proportional to less than Kv, so as to ensure that the bus voltage dip in the second pulse in the acceptable range, responsible for the double pulse test of the turn-on and turn-off voltage inconsistency. c increases with the increase of I and L, and decreases with the increase of V and Kv. In testing, C and L are determined by the hardware conditions, V is given by the test conditions, while there is an upper limit requirement for τ. Together, these parameters determine the test current that can be achieved.

For high-voltage devices, it is assumed that C=40uF, capacitor withstand voltage value of 1100V, L=10uH/50uH/100uH, and the upper limit of τmax<20us; for low-voltage devices, it is assumed that C=3000uF, capacitor withstand voltage value of 200V, L=10uH/50uH/100uH, and τ<20us. According to the above equation, we can list the maximum current that can be achieved at this time The maximum current is shown below.

03: Oscilloscope, probe measurement capability

Suitable measurement instruments are the basis for the test system to obtain accurate test results, mainly including oscilloscopes, voltage probes, current probes. We can see that some test systems have big problems in the selection of measurement instruments, for example

Using basic oscilloscopes to measure high-speed MOSFETs, high-speed IGBTs, SiC MOSFETs, due to a serious lack of bandwidth and sampling rate leading to large deviations between test results and actual values

The use of ADC bits for 8bit oscilloscope measurement of high voltage, high current devices, due to low resolution leads to poor accuracy of the measured value

Using high differential probes to measure drive waveforms, resulting in large waveform noise and serious oscillation

Using a Roche coil to measure the end current of SiC MOSFETs leads to large deviations between test results and actual values due to severe lack of bandwidth

Tektronix chose the appropriate measurement instrument to enhance the accuracy of the test results for the characteristics of the measured signal in the power device dynamic parameter test system DPT1000A. The oscilloscope selected MSO5B series, with bandwidth up to 2GHz, record length up to 500M and 12-bit ADC, can meet the bandwidth requirements of high-speed switching and has a higher sampling rate, lower noise and higher vertical resolution. The gate waveform measurement uses a passive probe with a bandwidth up to 1GHz, a small attenuation multiplier, and an MMCX interface to accurately measure the drive voltage of the lower tube and reduce the impact of ground wires.

End voltage measurement using high-voltage differential probe, to meet the wide voltage measurement range at the same time has a larger input impedance, providing a safe test security. The end current test uses shunt resistors, which have a bandwidth of more than 1GHz and can meet the bandwidth requirements of high-speed devices.

04: Upper tube test capability

Double pulse test using a half-bridge inductive load circuit, sometimes it will be necessary to measure the upper bridge arm devices. Many test systems use high-voltage differential probes to test the upper bridge arm device drive signal, the measured waveform often has very serious oscillations, and the situation is even more serious when testing high-speed MOSFETs, high-speed IGBTs, SiC MOSFETs. This situation is caused by the common mode rejection ratio of the high-voltage differential probe is seriously reduced at high frequencies, when the test system actually does not have the ability to test the upper bridge arm devices.

In the dynamic parameter test system DPT1000A, Tektronix's IsoVu optical isolation probe is selected for the test of the upper bridge arm. Such excellent features ensure the testing capability of the upper bridge arm devices.

05: Main Circuit and Driver Circuit Loop Inductance

There are two key circuits in the test circuit, namely the main circuit circuit and the drive circuit circuit, which have a great impact on the dynamic characteristics of the device and are the key indicators for judging the performance of the test circuit. The switching speed of traditional power devices is slow, and the parasitic inductance of the above two circuits is not high. However, with the emergence of high-speed MOSFETs, high-speed IGBTs, SiC MOSFETs, the original power device dynamic parameters test system circuit inductance of large problems are exposed.

Specifically, when the main circuit loop inductance is too large, it will cause the device's turn-off voltage drop too high, when it exceeds the device withstand voltage value, it may lead to device over-voltage damage. When the drive circuit loop inductance is too large, it will lead to serious oscillation of the drive waveform, while the drive circuit is also susceptible to high di/dt interference generated by the device in the switching process, further aggravating the oscillation, which may lead to device gate overvoltage breakdown, device misconductor leading to bridge arm through.

The Dynamic Parametric Test System DPT1000A addresses this issue by optimizing the test circuit parameters to enable it to measure high-speed devices including SiC MOSFETs. The drive circuit is close to the device under test and linked by PCB wiring to minimize the drive circuit loop inductance. At the same time, the bus capacitance selection, PCB wiring, and current sampling method are optimized to further reduce the main circuit loop inductance.

Editor：Challey