It is often desirable to utilize a power MOSFET as a switch. However, in conventional techniques, such as biasing the power MOSFET with a voltage doubler con- figuration or through the use of a flyback drive circuit, problems are encountered. These problems manifest them- selves in a number of forms: first, there is a requirement to bias the gate electrode of the MOSFET at a higher potential than the source electrode in order to achieve a low drain-source resistance (rDSon) and, therefore, a reduction in the power consumption of the switch; and second, present circuit implementation is complicated. Furthermore, there is a demand within a power MOSFET switch circuit for a fast operational switching time. This requirement is particularly ...
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MOTOROLA /NC. Technical Developments Volume 14 December 1991
IMPROVED BIASING FOR A POWER MOSFET ACTING AS A SWITCH
by Yitzak Cohen, Rafael Brody, Ofer Rosenzwieg and Tzvika Magril
It is often desirable to utilize a power MOSFET as a switch. However, in conventional techniques, such as biasing the power MOSFET with a voltage doubler con- figuration or through the use of a flyback drive circuit, problems are encountered. These problems manifest them- selves in a number of forms: first, there is a requirement to bias the gate electrode of the MOSFET at a higher potential than the source electrode in order to achieve a low drain-source resistance (rDSon) and, therefore, a reduction in the power consumption of the switch; and second, present circuit implementation is complicated. Furthermore, there is a demand within a power MOSFET switch circuit for a fast operational switching time. This requirement is particularly prominent when switching a MOSFET in a high frequency switched power supply.
Figure 1 illustrates a power MOSFET switching cir- cuit with improved biasing characteristics.
An increased gate bias for power MOSFET Q2 is obtained through the use of additional low cost com- ponents D,, R,, R2, C, and Q,. When MOSFET Q, is at saturation, its drain terminal can be consid- ered to be at ground potential. In this state, capaci- tor C,, is charged to a voltage potential given by:
v (VDD - VDI]
source an& hence, a high gate voltage with respect to the source i...
