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Posted on 05 July 2019

Rectifier Integration Opens Door for High Power Density Intelligent Power Modules

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The temperature increase of the case is moderate

High integration of power electronic systems requires optimized packages. This paper presents an innovative, fully-molded intelligent power module for small power three phase drives in a dual-in-line package. The package itself offers the possibility of integrating additionally the input bridge rectifier. The article discusses the thermal performance of the module concept by working out the impact of the rectifier losses in respect of case temperatures. The overall losses balance is calculated and the result is compared with measurements while operating the module with and without input rectifier.

By D. Chung, J. Lee, J. Song, LS Power Semitech Korea and W. Frank, Infineon Technologies Germany

 

Inverters are increasingly used in consumer appliances such as refrigerators, washing machines and heating, ventilation and air conditioning (HVAC) systems in order to improve efficiency, reliability, and controllability of the system. While the sales volume of these applications increases steadily, the price expectations are rather decreasing. It is therefore a continuous optimization process ongoing in the industry in order to align with this trend.

The trend covers on one hand the optimization of the functionality of the end product. Refrigerators not only cool down and conserve food. Meanwhile, they also produce ice cubes, they contain technologies, which make defrost-cycles obsolete or they offer various sections with different temperature inside the space optimized cabinet. On the other hand, the space, which is available for the electric drive system for the compressor, or the fan, or the washing machine drum, is smaller and smaller due to the higher space utilization of modern appliances. We can buy washing machines with a drum size of 12 kg or even more today. There are the same trends in refrigeration and freezer equipment, which offer today a larger cabinet size for the same external form factor. More and more functions of the drive have to be integrated, therefore.

This is the main motivation for converting even small power drives from discrete semiconductor systems towards a setup with intelligent power modules.

Package Concept

Most intelligent power modules (IPM) use a highly thermally conductive interface material to contact the power transistors to the heat sink. This is usually quite expensive and requires a complex assembly technology as shown in [1], [5] and [6]. It is the target of the proposed IPM to use a lead frame construction only, which is supported by a PCB substrate. The module is therefore overall molded, so that the generated heat must pass an ideally thin mould compound layer to get to the heat sink. It is easy to understand, that an extremely thin layer of mould compound between the lead frame and the heat sink is necessary in order to avoid excessive heating of the IGBT.

Figure 1 shows its cross-sectional structure where all the power components are isolated each other from the heat sink. Clearance and creepage distances of pin-to-pin are 2.6mm and over 3.0mm respectively. In case of pin-to-heat sink, clearance distances are 1.6mm. The low power components such as the gate drive IC and thermistor are assembled on an internal printed circuit board (PCB). PCB technology allows flexible design and very easy routing with EDA tools as they are commonly available. The lead frame style package is produced by a simple module assembly process and then the dual-inline structure can be soldered directly into the drive design board like a through-hole integrated circuit.

Cross-sectional structure

The new CIPOS-mini in a fully molded package offers the smallest module size (36 x 21 mm²) while providing high power density from 4 A, 600 V up to 30 A, 600 V by employing the RCD-IGBT and 6-channel gate driver. Furthermore, it offers enough space to implement an input rectifier bridge for the current rating of 6 A as shown in figure 2. This specific module (IGCM06B60HA), which contains the rectifier, is discussed more in detail.

Internal schematics of IGCM06B60HA including three phase inverter (green shaded) and input rectifier bridge (blue shaded)

Effect of Rectifier Losses in Application

Calculation of losses

The loss balance consists of three major contributions:
- conduction losses of three phase inverter
- switching losses of three phase inverter
- conduction losses of input bridge rectifier

It is a well known technique to approximate the output characteristics of IGBT and the forward characteristics of the freewheeling diode by piecewise linear curves, which simplifies the calculation with acceptable loss of precision. The characteristic is therefore given by the threshold voltage VCE0 and VF0 and the differential resistance rCE and rdD of the transistor and diode, respectively. All values can be derived out of the characteristics given in the datasheet. We can calculate now the conduction loss per one pair of IGBT and diode of a continuous sine wave modulation with the modulation index m and the power factor cos φ:

Equation 1

where Ipk is the peak value of the motor phase current.

The switching losses can also be derived from datasheet characteristics by assuming their linear dependency on switched current:

Equation 2

where EON is the turn on energy of the IGBT per Ampere [Joule/Ampere], EOFF is the turn-off energy of the IGBT per Ampere, and EREC is the recovery energy per ampere of the diode. The losses are scaled by the ratio between the actual DC bus voltage VDC and the nominal DC bus voltage of the datasheet test condition Vnom.

The calculation of the conduction losses of the bridge rectifier is similar to the calculation of conduction losses of the IGBT. The forward characteristics is approximated piecewise, which results in parameters VF0,rect and rd,rect for the threshold voltage and the differential resistance of the rectifier diodes. The integrated rectifier diode has therefore a threshold voltage VF0,rect = 0.8 V and a differential resistor rd,rect = 0.027 Ω at a junction temperature of TJ = 25°C. The conduction losses of a rectifier diode is therefore

Equation 3

where Iin,avg is the average value and Iin,rms is the rms value of the input rectifier current. The capacitive load of the rectifier by means of the DC bus capacitor leads non sinusoidal input current in respect to the input voltage. It can be shown analytically, that the rms value of the input current equals to

Equation 4

where Iin,pk is the measured peak current, t1 is the conduction period of the input rectifier and T is the input voltage period. The average current equals to

Equation 5

We investigate a commercial refrigeration application case with the following conditions:
- AC input voltage Vin,ac = 220 V
- line frequency f = 60 Hz
- ambient temperature Tamb = 26 °C
- thermal resistance of heat sink Rth,h-a= 5 K/W

The thermal resistance of the heat sink is given by the application itself and is verified by measurements under application related conditions.

The different behaviour of the two configurations can now be defined as the additional power dissipation caused by the rectifier. The dissipation of the inverter part is the same in the same application and must not be considered for a relative comparison of both configurations.

Figure 3 shows the correlated wave forms of the application and the following data are derived:
- duration t1 = 2.4 ms
- average DC voltage under full load VDC = 300 V
- 3 phase inverter power Iout,inv = 0.5 A
- switching frequency fP = 3.8 kHz

Rectifier and inverter wave forms

Therefore the rms value of the input current Iin,rms is

Equation 6

The average current results in 0.33 A. The overall losses are giving with equation (3) a total of 1.1 W for the rectifier. The contribution of the input rectifier is very small.

The higher power dissipation leads accordingly to a proportionally higher case temperature of .

Equation 7

Verification and measurement

The module is first operated by using the inverter only. The rectifier is outside the module. Additionally, a measurement was done by using also the rectifier part. A reference measurement with a commercial low cost module is also performed. The commercial module does not contain a bridge rectifier. It utilises a larger package (44 mm x 26.8mm), which allows a better heat flow to the heat sink.

The lead frame is here designed in a way that the heating of all power semiconductors, i.e. rectifier diodes and RCD-IGBT, is almost equal during full load. The application requires a single shunt only, which is sufficient for simpler control methods, such as v/f-control or trapezoidal waveform control of BLDC motors.

Figure 4 shows the test setup for the verification measurements. The tested module is mounted on a heat sink. A spring presses the temperature sense onto the case with a defined and constant pressure. This ensures the reliability of the contact conditions during the measurement process. The module and the heat sink are assembled on a test PCB as shown in figure 5. The test conditions are set to values, which are mentioned before.

Mechanical setup for temperature measurements

Practical setup for temperature measurements in application

The relevant parameters for the measurement are the ambient temperature Tamb and the case temperature TC. Table 1 shows the peak values of these parameters during an initial start-up procedure.

Measurement Results

The difference between both operating modes is 18.0°C – 11.9°C = 6.1°C. This is slightly higher than expected, which was 5.5°C. However, the mismatch of 0.6 °C between measurement and calculation is acceptable.

The measurement confirms that the temperature adder in respect of the case temperature TC for operating an input rectifier bridge inside an intelligent power module is moderate. This makes the module highly applicable for all systems, with a power rating below 300 W.

Conclusion

A new concept of intelligent power modules for low cost home appliances has been discussed. The module includes an input bridge rectifier for single phase input. Measurements under typical application conditions show, that the temperature increase of the case is moderate while using also the rectifier part. Therefore, the module offers advantage for all space sensitive applications, that the rectifier is connected in best way to the heat sink, because it shares the same heat sink. The overall space consumption is less and the module provides the highest power density.

 

References:

[1] W. Frank, J. Oehmen, A. Arens, D. Chung, J. Lee, “A new intelligent power module for home appliances”, Power electronics, intelligent motion, power quality, Conference proceedings, May 2009.
[2] D. Chung, S. Sul, “Minimum-loss strategy for three-phase PWM rectifier”, IEEE Trans. on Industrial Electronics, Vol.46, No.3, Jun, 1999.
[3] K. Ammous, S. Abid, A. Ammous, “Thermal modeling of semiconductor devices in power modules”, in Microelectronics International, vol. 24 issue 3, Emerald Group Publishing Limited 2007, pp.46.
[4] T Kojima, Y Yamada, M Ciappa, M Chiavarini, A novel electrothermal simulation approach to power IGBT modules for automotive traction applications, vol. 39 no. 4, R&D Review of Toyota CRDL, Toyota, 2004.
[5] M. Kato, T. Nagahara, H. Kawafuji, T. Nakano, M. Honsberg: “New Transfer Molding PFC series with Compact Package”, Power electronics, intelligent motion, power quality, Conference proceedings, May 2009.
[6] K. Satoh, T. Iwagami, H. Kawafuji, S. Shirakawa, M. Honsberg, E. Thal: “A new 3A/600V transfer mold IPM with RC(Reverse Conducting) –IGBT”, Power electronics, intelligent motion, power quality, Conference proceedings, May 2006.

 

 

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