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.
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:
Regardless of type, the primary source of error is the loss of heat (Pwall) through the walls of the calorimeter. For the open-type and the closed-type single-cased calorimeter, Pwall is denoted as:
Pwall = (Ttest – Tamb) / Rth,wall
Here, Ttest is the temperature in the test chamber, Tamb is the ambient temperature, and Rth,wall is the thermal resistance of the calorimeter walls.
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For the closed-type double-cased calorimeter, Pwall can be estimated as:
Pwall = (Ttest – Tgap) / Rth,wall
Whereas Tgap 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).
The disadvantage of a single-room solution is represented by the error introduced by Pwall, 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 Tamb thanks to the action carried out by the Peltier cell.
The total amount of heat developed is shown in the following formula:
uc = SpTcIp – (Th – Tc)/Rp – 0.5 RpIp2
Here, Sp is the Seebeck coefficient, Tc is the cold side temperature, Th is the hot side temperature, Rp is the thermal resistance of the Peltier cell, and Ip 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 (Ploss) can be calculated as follows:
Ploss = SpTcIp – (Th – Tc)/Rp – 0.5 RpIp2 – QFc
Whereas QFc 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, KPi and KPT are proportional gains, and KIi and KIT are integral gains.
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 Tin as a function of time, observing how, after a transient lasting about 600 seconds, the temperature in the chamber follows the trend of Tamb.
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.