# Thermal Mapping of Power Semiconductors in H-Bridge Circuit +
## 2 Operation Principle of H-Bridge Circuit  
For the grid-tied converter used in renewable energy systems, the loading of the power devices is dominated by the grid requirements (e.g., active and reactive power). In respect to the power converter applied in generator/motor control, the generated current is tightly dependent upon the machine characteristic. Thus, regardless of the converter applications, its loading can simply be considered as a controlled current source.  
A single-phase H-bridge circuit is shown in Figure 1a, which consists of a test leg and a load leg. An inductor in between is needed for the circuit operation. The test leg provides the reference voltage,  
Figure 1: Configuration of H-bridge circuit and phasor diagram between test leg and load leg. (a) Single-phase H-bridge circuit. (b) Phasor diagram in the case of inverter mode. (c) Phasor diagram in the case of rectifier mode.  
while the load leg imitates the loading feature, supplying resistive, inductive or capacitive loading conditions. Consequently, the current flowing through the test leg can be regulated freely. According to the Kirchhoff voltage law, the relationship between the output voltage of the test leg \(\overline{U_{te}}\) and the load leg \(\overline{U_{lo}}\) can be found as:  
\[\overline{U_{te}}=\overline{U_{lo}}+\overline{U_{Lf}} \tag{1}\]  
where \(\overline{U_{Lf}}\) is the voltage drop across the filter inductor \(L_{f}\).  
For typical operation conditions, the converter under the inverter mode and the rectifier mode are shown in Figure 1b,\(c\). In the case of the inverter mode, the loading current \(l_{lo}\) is in the opposite direction to the reference voltage \(U_{te}\). The amplitude of the load voltage \(U_{lo}\) becomes slightly higher and lags the reference voltage with \(\varphi_{u}\). Similarly, in the case of the rectifier mode, the loading current is in phase with the reference voltage, which results in the leading load voltage.  
According to the modulation scheme of the test and load legs in [18], the switching period can be divided into five stages. As shown in Figure 2, the activation status of power switches is described in detail, where both the positive and negative currents are considered. For the inverter mode, it is obvious that \(T_{1}\) and \(T_{4}\) conduct depending on the direction of loading current. Besides, the inductor current increases in stages 2 and 4, but it maintains the same in the rest of the period. Similarly, the loading current flowing through the power devices during rectifier mode is presented in Figure 2b. At different directions of the loading current, the conducting devices change to the complementary switches at all stages.  
Regardless of the current direction, it is evident that the turn-on and turn-off loss of the IGBT, and reverse-recovery loss of the diode happen once during a switching period. Moreover, in the case of  
Figure 2: Activated power switches at various operation modes. (**a**) Inverter mode. (**b**) Rectifier mode.  
the inverter mode, the IGBTs \(T_{1}\) and \(T_{4}\) are primarily conducting. However, in the case of the rectifier mode, the diodes \(D_{1}\) and \(D_{4}\) conduct mainly. Consequently, the conduction loss is dominated by the IGBT in the inverter mode but by the diode in the rectifier mode.