Providing low loss and high power MOSFET

The PN junction of a silicon power diode typically has a voltage drop of about 1.2V. This voltage drop causes a considerable amount of energy to be dissipated in the power diode, resulting in a loss of power efficiency. For a solar panel with a 120W power supply and a nominal voltage of 24V, a diode to prevent backflow may generate a power loss of 6W, or 5% of the controlled energy. In addition, the cost of developing a cooling system to dissipate the heat of the diode may also be an issue. This example shows a more economical solution, which replaces the traditional power diode with a MOSFET transistor operating in switch mode. Figure 1 shows a rectifier circuit using a MOSFET transistor Q1, which has a low drain-source resistance in the on state. In the circuit, V2 represents a 36V AC power supply. The load includes a 9Ω resistor RL and a 25mH coil L1. When the positive terminal of the power supply is higher than the negative terminal of the drain voltage, the comparator IC1 generates the gate voltage of Q1. Therefore, the source acts as the positive terminal of the rectifier, while the drain acts as the negative terminal. The circuit takes advantage of the ability of the transistor to conduct current in the source-drain direction. The conduction of Q1 actually reduces the parasitic diode between the substrate and the drain, thus minimizing the power dissipation. When the gate-source voltage is low, Q1 and its parasitic diode are both off. Diode D1 and resistor R1 are used to limit the voltage at the two inputs of the comparator. Figure 2 shows the load voltage and the voltage drop across the rectifier Q1. Figure 3 shows the normal operation of the rectifier, where the voltage drop is 33 mV at the maximum load current of 2.65A; Q1 operates in the ohmic region. On the other hand, if a MOSFET is used, the voltage drop becomes 629 mV, and the maximum continuous power is 1.66W (Figure 4). This scheme can be used for any type of rectifier with any number of diodes. In addition, this circuit can be used in DC/DC and DC/DC converters because when power MOSFETs are used in bridge circuits, they can conduct both active and reactive currents. This scheme avoids the requirement of using a substrate-drain parasitic diode in the MOSFET.