Zth-measurement (Si) - Zth Messtechnik - transient thermal impedance of power semiconductors - thermal-electrical simulation

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Zth-measurement (Si)

Principle of Zth-measurement technology:

Components such as diodes, MOSFETs (SiC), IGBTs or thyristors have a pn junction internally. This pn junction is temperature-dependent and serves as a heat source as well as a temperature sensor in the Zth-measurement.

To measure the transient thermal impedance, the DUT is heated to a steady state with a defined load current. At the end of the heating phase load current and forward voltage for the power loss Pv are determined. After switching off the load current, a defined measuring current is fed in the cooling phase and the resulting forward voltage is recorded. The "virtual junction temperature" can then be determined from the measured forward voltage by means of a calibration curve. The comparative measurement with the liquid crystal analysis proves that the junction temperature corresponds relatively accurately to the mean temperature distribution in the semiconductor chip. The following figures show the schematic structure for measuring the transient thermal resistance, a characteristic curve for calibration and the conduction characteristic of a diode at two different temperatures.

Example: Zth-measurement on a 900A/1200V half-bridge module

Zth control protocol (1000A Zth measuring station)

Date: 03.09.2017  time: 10:18:12

Zth-measurement on a 900A/1200V IGBT half-bridge module

Heating: low-side-IGBT
Measure up: low-side-IGBT

Heat sink: closed water cooler
Tightening torque module: 5Nm
Thermal compound: silicone-free, thickness: ~ 70µm
Cooling medium: domestic cooling water

File: 900A-IGBT.dat

Heating up sec: 100sec     cooling down sec: 100sec

Pv = 1992,010W     Uload = 2,213V     Iload = 900,14A

Imeas = 0,29746A   Ug = 15,002V

Cooling down phase:
Channel 1: pressure flow water = 1,86bar
Channel 2: pressure return water = 0,68bar
Channel 3: temperature flow water = 7,32°C
Channel 4: temperature return water = 7,40°C
Channel 5: water flow = 20,16l/min

Heating up phase:
Channel 1: pressure flow water = 1,88bar
Channel 2: pressure return water = 0,67bar
Channel 3: temperature flow water = 7,37°C
Channel 4: temperature return water = 8,85°C
Channel 5: water flow = 20,05l/min

Calibration value: T = 7,40°C       U = 0,5594V
Calibration value: T = 63,36°C     U = 0,4287V
Calibration value: T = 95,69°C     U = 0,3517V

Zth evaluation protocol

Date: 03.09.2017  time: 10:29:03

Curve from file: 900A-IGBT.dat

Voltage/temperature conversion:
Table of file: 900A-IGBT.pk1

Thermal power loss (thermal equilibrium) = 1992,010W

Ta(meas curve) = 7,39°C           Ta(eval curve) = 7,40°C        
Tj(meas curve) = 113,00°C        Tj(eval curve) = 113,55°C      

Rthja(meas curve) = 0,053020K/W      Rthja(eval curve) = 0,053288K/W

First measurement point = 2,0000e-05s   last meas. point = 1,0000e02s

Partial fraction network (Foster):
Rj, Cj, and Tau values from t(0)
Rj (K/W)             Cj (Ws/K)           Tau (s)
0,001238282      0,081634856      0,000101087
0,001658596      0,459951021      0,000762873
0,003451200      3,057068614      0,010550555
0,015215485      3,537202080      0,053820244
0,009498891      21,56445427      0,204838408
0,015009741      46,65171253      0,700230130
0,006463940      369,0026996      2,385211271
0,000757191      26449,71712      20,02748770

equivalent continues fraction network (Cauer):
Rj, Cj, and Tau values from t(0)
Rj (K/W)             Cj (Ws/K)           Tau (s)
0,001838832      0,066206795      0,000121743
0,002392428      0,367834798      0,000880018
0,012005141      1,179961977      0,014165610
0,013985544      2,650966841      0,037075214
0,012660052      19,39961793      0,245600177
0,007353640      72,43003162      0,532624347
0,002527141      828,4847185      2,093697492
0,000530548      36696,99001      19,46950219
Log file generated by the control program 900A-IGBT.pk1                                              Log file generated by the evaluation program 900A-IGBT.pk2
The recorded Uce curve after switching off the load current at a measuring current of 300mA

From the Uce curve and the calibration values ​​(file 900A-IGBT.pk1) converted T (junction ambient) cooling curve

Measured cooling curve ΔTja (measured curve blue) in comparison to the evaluation result (evaluation curve red) from the partial fraction network table
(file 900A-IGBT.pk2)

Screenshot (evaluation program) from the last R/C value extraction in the time range 20μs - 220μs after switching off the load current. The R/Ʈ line (red line)
corresponds approximately to the thermal properties of the module layers silicon, aluminum, bondfoot and chip solder. This correct determination at short
times is crucial for accurately calculating the thermal behavior in the simulation, for example in the lifetime calculation.

Zth-diagram: Zth=f(tp), parameter D=tp/T

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