High-precision calorimetric measurement of EV converter power losses

Article By : Maurizio Di Paolo Emilio

Calorimetric measurement is capable of achieving high precision without requiring any electrical connection to the converter.

Climate change, with the consequent need to reduce carbon dioxide emissions, is revolutionizing the entire transport sector, increasingly oriented toward electric mobility or e-mobility. Electric vehicles (EV) use high-efficiency power converters, with values close to 99 percent.

To design or evaluate a power converter, it is essential to measure its power loss with great accuracy. Normally measured by a wattmeter, the power loss is expressed as the difference between the value of the input power and that of the output power. Due to the high efficiency, this difference is very small and therefore only full-scale errors can be highlighted.

An alternative solution to the electrical measurement with the wattmeter is the one that is based on the calorimetric method, capable of achieving high precision without requiring any electrical connection to the converter.

The technique we will now describe uses a single thermostatic chamber, a Peltier cell, and a room temperature control system. The Peltier cell, operating in reverse mode, leading to the Seebeck effect, generates a current at its electrodes as an effect of the difference in heat between the cold and warm sides.

Conventional calorimeters

Since power losses in an electronic circuit are mainly due to heat dissipation, they can be determined by measuring the heat generated by the system. Calorimetric methods, in particular, use a medium to remove the heat produced by the device under test (DUT). In an ideal calorimeter, the dissipated heat is absorbed entirely by the medium, which can be air, water, or another type of coolant.

Conventional calorimeters are of three types:

• Open-type calorimeter: The DUT is placed directly in the measuring chamber, while the coolant is represented by common air. The advantages of this solution are the simplicity of construction and the speed of execution of the measurement. The main disadvantage is the difficulty of measuring the heat capacitance of the air.
• Closed-type single-cased calorimeter: It comprises a separate cooling loop for heat exchange with ambient environment. By using water for coolant, it achieves greater accuracy than the open-type calorimeter. However, since the heat capacitance of water is larger than that of air, the measurement time becomes longer.
• Closed-type double-cased calorimeter: It allows for active control of the air temperature in the gap between the two cases, thus leading to improved accuracy.

Regardless of type, the primary source of error is the loss of heat (P_wall) through the walls of the calorimeter. For the open-type and the closed-type single-cased calorimeter, P_wall is denoted as:

P_wall = (T_test – T_amb) / R_th,wall

Here, T_test is the temperature in the test chamber, T_amb is the ambient temperature, and R_th,wall is the thermal resistance of the calorimeter walls.

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For the closed-type double-cased calorimeter, P_wall can be estimated as:

P_wall = (T_test – T_gap) / R_th,wall

Whereas T_gap is the air temperature in the gap between cases.

The proposed solution

The proposed solution uses a single chamber, a Peltier cell on the faces of which (inside and outside the chamber) there are two heat sinks, temperature sensors, and finally fan motors to cool the heat sinks (Figure 1).

Figure 1 The calorimeter scheme uses a Peltier cell.

The disadvantage of a single-room solution is represented by the error introduced by P_wall, or the heat leakage across the walls. In order to improve the accuracy of the measurement, the temperature in the chamber is kept equal to T_amb thanks to the action carried out by the Peltier cell.

The total amount of heat developed is shown in the following formula:

u_c = S_pT_cI_p – (T_h – T_c)/R_p – 0.5 R_pI_p²

Here, S_p is the Seebeck coefficient, T_c is the cold side temperature, T_h is the hot side temperature, R_p is the thermal resistance of the Peltier cell, and I_p is the input current to the Peltier cell.

When the temperatures inside and outside the chamber are the same, the cooling capacity of the Peltier cell is equal to the power loss dissipated as heat. The power loss of the DUT (P_loss) can be calculated as follows:

P_loss = S_pT_cI_p – (T_h – T_c)/R_p – 0.5 R_pI_p² – Q_Fc

Whereas Q_Fc is the power dissipation of the cold side fan motor.

Figure 2 shows the proposed calorimeter control system. P₁ is the plant of the calorimeter, P₂ is the buck converter for current control, C₁ is the PI controller for temperature tracking, and C₂ is the PI controller for current tracking.

Figure 2 Here is the calorimeter feedback control system.

C₁ and C₂ are noted as follows:

Here, KP_i and KP_T are proportional gains, and KI_i and KI_T are integral gains.

Experimental results

Initially, a simulation of the thermal equivalent circuit model was developed in the Matlab and Simulink environments. Through this simulation, it was possible to derive the trend of T_in as a function of time, observing how, after a transient lasting about 600 seconds, the temperature in the chamber follows the trend of T_amb.

By operating in the same way, it was possible to derive the temperature trends on the hot and cold sides of the Peltier cell, the input current to the Peltier cell and, finally, the estimated power loss. The estimated power loss coincides with the power dissipation of the converter under test. The results obtained experimentally are aligned with the data produced by the simulation, confirming the validity of the proposed calorimetric method.

Maurizio Di Paolo Emilio is Editor of Power Electronics News and EEWeb and European correspondent for EE Times. He holds a Ph.D. in Physics and is a telecommunication engineer.

Reference

1. K. Mitsugi, Y. Noge and M. Deng, “Simple Calorimetric Power Loss Measurement System Using Single Chamber and Peltier Device with Ambient Temperature Tracking Control,” IEEE 2020.

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