Low-cost circuit preventing MOSFET reverse diode conduction
Publication Date: 2002-Dec-13
The IP.com Prior Art Database
Low-loss circuit preventing MOSFET reverse diode conduction
MOSFETs are used as power switches in many applications. In the device structure of the MOSFET is a diode between source and drain of the device, generally referred to as body diode (Fig. 1a). Although there are applications in which this inherent body-diode can be used to advantage, some of the characteristics of this diode compare less favourably with discrete diodes, particularly with respect to reverse recovery behaviour. Depending on topology and mode of operation, activation of the MOSFET's body-diode can result in unacceptably high losses or even in device failure. For those applications that benefit from or require a reverse diode function across the switch, this function thus needs to be provided by means of an external diode, anti-parallel to the MOSFET with deactivated body-diode. In Fig. 1B the most common way to address the problem is shown. Activation of the body diode is prevented by placing a diode in series with the MOSFET that prevents current from flowing in the reverse direction. An additional external diode is used to provide the required reverse diode function. A Shottky-diode may be used instead of a Si-PN diode to reduce the voltage drop over the diode and thereby the power loss in it.
A disadvantage of this solution is that substantial power losses take place in the diode that is in series with the MOSFET.
This problem can be prevented by making use of the solutions shown in Fig. 2 and Fig. 3.
The first solution (Fig. 2) is to make use of a source switched high-voltage MOSFET with external reverse diode. Loss reduction is achieved by driving the low voltage MOSFET into the on-state, thereby creating a much lower voltage drop across that devices on-resistance than would otherwise occur across the external series diode.
The gate of the low-voltage MOSFET is connected to a gate drive signal generator. The gate of the high voltage MOSFET can also be connected to the same gate drive signal generator but is preferably connected to a fixed voltage, e.g. 12V. The diode D-LV is not essential but will generally be used to prevent the source of the high-voltage MOSFET from becoming sub...