Thermal simulation analysis of power module of combined converter for metro vehicle

In order to reduce the occupancy space and equipment cost of the on-board converter, a combined traction-auxiliary converter for subway vehicles is proposed in this paper. The power loss of the main traction converter module (MCM) and the auxiliary converter module (ACM) was calculated according to the speed and power variation of the vehicle running between five consecutive stations, and the flow rate of cooling air was calculated through the heat balance theory. The air volume of MCM and ACM and the plate temperature of the radiator are analysed by step method. Then, according to the instantaneous maximum temperature of the radiator obtained by simulation and the thermal parameters of the IGBT module, the junction temperature of the IGBT and diode chips is calculated by means of the junction temperature equivalent circuit. The results show that the plate temperature of the heat sink and junction temperature meet the design requirements. The design and thermal simulation method in this paper provide theoretical reference for the design of railway vehicle converters.


Preface
With the development of electronic technology, power converters have been widely used in railway electric locomotive, electric vehicle, new energy power generation and other fields [1][2][3].Locomotive on-board converter is different from industrial inverter, because it is installed under the car body and the working environment is very harsh, so it has higher requirements for sealing.On-board converters contain a variety of electrical components that generate different types of thermal loss during operation.Due to the compact structure and high integration of the box, how to dissipate the heat generated by each electrical component smoothly is the focus of the converter design [4].
On-board converters are divided into traction converter that provides power to the train and auxiliary power supply for the vehicle [5,6].Usually the traction converter and auxiliary converter are designed separately and run independently.Such a scheme not only takes up a large space for the converter, but also requires two sets of independent cooling systems, which increases the cost.In order to reduce the space occupied and the equipment cost, this paper proposes a traction-auxiliary combined converter, calculates and analyzes the heat flow field of the radiator and junction temperature of the IGBT power module with the help of CFD thermal flow coupling simulation.

The combined converter
The combined converter is composed of the main converter module (MCM), the auxiliary converter module (ACM) and the central chamber, and adopts forced air cooling.The fan, transformer, threephase capacitor and reactor are installed in the central chamber, as shown in Figure 1.The cooling air flows into the MCM radiator and ACM radiator from the upper inlet respectively, gathers at the bottom of the central chamber and enters the centrifugal fan, then flows through the transformer, electric reactor and three-phase capacitor successively, and is finally discharged by the outlet.See Figure 2 for the duct and the air circulation.The IGBT module layout of MCM and ACM converter is shown in Figure 3.Each IGBT module of MCM contains 24 IGBT chips and 12 anti-parallel diode chips, while each IGBT module of ACM contains 16 IGBT chips and 8 diode chips.The arrangement of chips is shown in Figure 4.The equivalent thermal resistance (thermal impedance) circuit of IGBT power module is shown in Figure 5 [7].Junction temperature T J,I of IGBT chip and junction temperature T J,D of diode chip in IGBT module can be expressed as:

MEIE-2023
, , , -Thermal resistance between IGBT junctions and case; , -Thermal resistance between diode junctions and case; -Contact thermal resistance between case and fin; -Thermal resistance between fin and ambient; -Thermal loss of IGBT; -Thermal loss of diode; -Temperature of fin plate; -Temperature of ambient.

Thermal loss of the power module
Power loss in a converter system refers to the electrical energy absorbed by the electronic device.Since there are reverse parallel diodes in the IGBT module, it is necessary to consider both the IGBT loss and the diode loss.The power loss of IGBT mainly includes on-state loss and switching loss, and the power loss of diode mainly includes on-state loss and Turn-off loss.The calculation method can be found in References [4,8].
The main traction converter is used to provide traction power for vehicles.Due to the short distance between subway stations and frequent acceleration and deceleration, the power and thermal loss of the converter vary with the speed of vehicles.The data of speed and traction power between five stations of Changzhou Metro Line 1 were selected to calculate the thermal loss of the main traction converter.Figure 6 shows the change of vehicle speed between five stations, and Figure 7 shows the thermal loss of single IGBT chip and single diode chip in the MCM module.The auxiliary converter is mainly used to provide power for train lighting, air conditioning, signals, etc., so the auxiliary converter power is relatively stable and can be considered in terms of constant power.The thermal loss of single IGBT chip and single diode chip in the auxiliary converter ACM is 32.2W and 31.25Wrespectively, and the total loss of ACM is 4590W.

Air volume of the fan
A centrifugal fan is used for heat dissipation, and the fan must have enough air volume to take away the heat generated.The heat balance equation: / ∆ (3) Where: -minimum air volume of the fan, m3/s; -total heat, W; -Specific heat capacity of air, J/(Kg•°C); ρ-density of air, kg/m 3 ; ΔT-air temperature difference between outlet and inlet, K.The maximum operating temperature of centrifugal fans is generally 60 °C.In order to ensure long-term reliable operation of the fans, when the ambient temperature is 40 °C, the temperature of the outlet is set to 55 °C, that is, the temperature difference between the outlet and the inlet is ΔT=15 °C.The air parameters and calculation results of fan air volume are shown in Table 1 ( is multiplied by the reliability factor 1.1 to consider the leakage loss of air volume and improve the reliability of heat dissipation).Therefore, a centrifugal fan with rated air volume of 1 m 3 /s is selected as the cooling fan.

Fluid simulation
The hydrothermal coupling simulation method based on fluid dynamics (CFD) is an effective method to design the heat dissipation of converter [3][4][5].However, due to the large number of electronic components and complex structure of the system, if a complete hydrothermal coupling simulation model is adopted, it will not only require high configuration of the computer, but also greatly increase the calculation time.Therefore, this paper adopts the step analysis method.Firstly, the converter flow field is analyzed, and the air flow through MCM and ACMD is calculated.Then, the MCM and ACM are taken as independent models for the hydrothermal coupling simulation.
In fluid simulation, transformers, reactor and capacitor have no influence on the air flow distribution of MCM and ACM, so they are not taken into account in modelling.Figure 8 shows the fluid simulation model of the converter, with 7711012 elements.The body and internal fluid of the radiator adopt a structured grid.The mixing chamber and the inlet and outlet of the fan adopt a tetrahedral grid with a minimum mesh size of 0.5mm.
The air inlet is set as the pressure outlet boundary, the gauge pressure is the resistance of the air filter screen, the air temperature of the air inlet is set at 40 °C, the outlet is the speed inlet, and the inlet speed is -1m3/s.The flow state of air in the duct is assumed to be completely turbulent, and the standard k-ε turbulence model is adopted.As shown in Figure 9, since the area of MCM air duct is larger than that of ACM, the air speed in MCM air duct is significantly higher than that of ACM.The air flows in the mixing chamber as spiral     By the design requirements, the surface temperature of the board under the power module should not be higher than 95 °C, and the maximum temperature of the IGBT/DIODE chip should not be higher than 115 °C.According to the simulation results, the temperature of the heat sink reached to be stable during the operation of the vehicle between five stations, as shown in Figure 10.The highest temperature of the MCM radiator was 69.9 °C, and the highest temperature of the ACM radiator was 86.4 °C, both of which were lower than the design limits.Figure 11 shows the temperature map of the heat sinks.

Junction temperature
According to the maximum instantaneous plate temperature obtained by simulation and IGBT module thermal parameters, the chips junction temperature was calculated by using Equations ( 1) and ( 2), shown in Table 3.The results show that during the operation of the vehicle between the five stations, the maximum junction temperature of the MCM is 98 °C, and the maximum junction temperature of the ACM is 98 °C, both of which are lower than the specified design limit.

Conclusions
For the combined subway converter proposed in this paper, the power loss of the main traction converter and the auxiliary converter is calculated according to the speed and power variation of the vehicle running between five stations, and the air volume of the fan is calculated through the heat balance theory.The air volume of MCM and ACM and the plate temperature of the radiator were calculated by step analysis method.Then, according to the instantaneous maximum temperature of the radiator obtained by simulation and the thermal parameters of the IGBT module, the junction temperature of the IGBT and diode chip was calculated by means of the junction temperature equivalent circuit.The calculation results tell that the plate temperature of the heat sink and the junction temperature of the power module meet the design requirements.

Figure 1 .
Figure 1.The structure of the converter box.

Figure 4 .
Figure 4. Chip layout of the power module, MCM (left) and ACM (right).

Figure 5 .
Figure 5.The equivalent thermal resistance circuit.

Figure 6 .
Figure 6.The vehicle speed.Figure 7. Chip power loss of MCM.

Figure 7 .
Figure 6.The vehicle speed.Figure 7. Chip power loss of MCM.
MEIE-2023 Journal of Physics: Conference Series 2591 (2023) 012062 cross flow and enters the centrifugal fan after mixing.The cross-sectional area, air volume flow, inlet speed of MCM and ACM are shown in Table2.

Figure 11 .
Figure 11.The temperature map of the heat sink.

Table 1 .
Calculation results of fan air volume.

Table 2 .
Air volume rate.After the air volume rate and the average inlet speed of MCM and ACM are obtained, the thermal simulation of MCM and ACM is carried out respectively.The ambient temperature is set to 40 °C.