2 When Should You Cascode Two TPSI3050 Devices

In the automotive industry, electric vehicles (EV) and hybrid electric vehicles (HEV) are using up to 400-V or 800-V batteries. In industrial applications within grid infrastructure and power delivery, 120-Vac or 240-Vac control is commonly used. These are high voltage (HV) systems that require high voltage switching technologies. The most common solutions in this space are Si MOSFETs, IGBTs, and SiC MOSFETs Figure 2-1. The TPSI3050-Q1 offers 5 kV reinforced isolation and AEC-Q100 qualification for automotive applications while the TPSI3050 offers 3 kV basic isolation for industrial applications.

Figure 2-2 shows I_D vs V_DS (or V_CE) curves for the three different power transistors. The R_DS(ON) for MOSFETs can be extracted from these curves; lower R_DS(ON) results in lower power dissipation in the transistor. The lowest R_DS(ON) can be estimated using the relation between I_D, V_DS, and V_GS. For the same I_D current for different V_GS curves, the higher the V_DS drop, the higher the resulting R_DS(ON). In the case of the IGBTs, the curves represent Collector-Emitter voltage drop (V_CE). The higher the V_CE , the higher the power dissipation in the IGBT. Therefore, for a given collector current operation, it is helpful to operate the IGBT at a higher V_GE to achieve a lower V_CE, resulting in lower power dissipation.

For the Si MOSFET (left plot of Figure 2-2) when V_GS is greater than 7-V and the MOSFET is in the ohmic region, R_DS(ON) is at the lowest possible value. The ohmic region shows overlapping curves for V_GS greater than 7-V representing relatively constant R_DS(ON). TPSI3050 generates a 10-V gate drive which is suitable for many Si MOSFETs. For IGBTs and SiC MOSFETs, the optimal operating condition might require higher V_GS voltage to obtain full enhancement.

IGBTs and SiC MOSFETs can a have higher dependence on the drive voltage as can be observed in the linear region (IGBTs) or ohmic region (MOSFETs) with different slopes in Figure 2-2 . If insufficient V_GE (IGBTs)/V_GS (SiC MOSFETs) is used for IGBTs or SiC MOSFETs power switches, then high conduction losses can occur. High conduction losses can lead to potential damage of the power switch. Care must be taken to ensure thermal considerations are understood as well as ensuring the device's safe operating area is not violated. For this IGBT (center plot of Figure 2-2), V_GE higher than 15-V is desired to obtain lower V_CE for lower power dissipation. For the case of the SiC MOSFET (right plot of Figure 2-2), R_DS(ON) is highly dependent on V_GS and 20-V would provide the lowest R_DS(ON).

These are examples where cascoding two TPSI3050 devices would provide optimal gate drive to reduce conduction losses. TPSI3050 generates a floating secondary supply of 10-V and when two devices are cascoded, these can be combined to obtain higher voltage levels. Two TPSI3050 devices allow for gate drive voltages up to 20-V. In addition, it can be configured to provide other options such as -5-V to 15-V. Figure 3-1 shows how to configure two TPSI3050 devices to obtain voltages from -5-V to 15-V and 0-V to 20-V.