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4 \ VISHAY An Automotive Grade Above A s dual board net systems–vehicles with both a 12V bus and a 48V bus–are becoming more popular, a high- power bi-directional 48V to 12V DC/DC converter is a key building block in the vehicle's architecture. To optimize the overall efficiency of the vehicle, energy must be transferred in either direction between the 12V battery and the 48V battery, depending on the vehicle's electrical demands and the state of the batteries' health. Therefore, a complete 3kW 48V/12V buck-boost converter design (Figure 1) utilizes an insulated metal substrate (IMS) with a heat sink for the power stage with a standard FR4 controller board mounted on top. In general, these types of converter designs cannot operate at maximum efficiency over a wide power range. However, this design features six modular power stages capable of 500W each. By switching the protection MOSFETs (TR1/TR2 and TR5/TR6 in Figure 2) on and off, it is possible to activate or deactivate the power stages separately. Therefore, this topology allows for efficiency to be maximized under any operating condition. Also, a total breakdown can be prevented in the event of an individual power stage failure (such as a defect of TR4 in Figure 2). Another important schematic detail is the half-bridge design (TR3/TR4) with different MOSFETs. The high-side MOSFET usually operates at a fourth of the output current, therefore, the R DS(ON) is not critical. The gate-source charge (Q gs ) and the gate-drain charge (Q gd ) parameters are more important. These MOSFETs are driven through gate resistors R3 and R4 in Figure 2 (Vishay MMU0102). Unlike low power thick film resistors, thin film MELFs are designed to handle large pulses and will not drift over time and temperature. These values influence the switching losses, which are the dominating power-loss factor at a frequency of 100kHz to 150kHz. The low-side MOSFET consists of two Vishay SQJQ184E connected in parallel to minimize the resistance. In this case, the R DS(ON) is the dominating power loss. The primary storage inductor (L2 in Figure 2) needs to support both the DC output current along with the ripple current. The switching frequency and inductance value determine the ripple current amplitude. Increasing the switching frequency or the inductance value will reduce the ripple current, but tradeoffs in size and performance must be considered. The designer must Free Up Your Design With 6 Independent Phases VISHAY INTERTECHNOLOGY Figure 1: 3kW 48V/12V buck-boost converter (Source: Vishay) Learn More WSLP3921 Power Metal Strip ® Current Sense Resistors