A simple switch control circuit that connects Vsupply to the load

The circuit shown in Figure 1 is a simple switch control circuit that connects Vsupply to the load. Vsupply can be a positive
voltage, a negative voltage, or an AC voltage. The input voltage amplitude is limited only by the maximum rating of the MOSFET's Vds. The VDS limit of the MOSFET Q1 and Q2 shown in the figure
is 50V. The circuit uses the MAX845 chip .

The working voltage of the primary winding of the transformer and the driver IC is 5V. They generate an isolated
alternating voltage in the secondary winding of the transformer, which is rectified by D1 and D2 to generate a 10V VGS voltage for the N-channel MOSFET. The VGS generated in this way
is a constant isolation voltage and will not be affected by the change of the VDS voltage to ground. Since the negative VGS voltage applied to a single MOSFET
will still have current flowing through the MOSFET in the off state (caused by the internal parasitic diode in the forward biased state). Therefore,
the two MOSFET sources are connected to the source, so that their internal parasitic diodes are reversely connected, and no
current will flow under any circumstances.
After turning off the IC, the VGS of the MOSFET is 0V and the switch is disconnected (SD=5V, switch is disconnected, SD=0V, switch is closed).
The speed of the switch depends on the size of R1: a small R1 value can reduce the switching delay, but the corresponding power consumption is larger (when R1=1K, the load current
is 24 mA). If switching speed is not critical, a larger R1 value can be selected to reduce the power consumption current to 5 mA. Figure 2
shows the performance of the circuit under 40V, 1.2A load.
There are certain disadvantages to using other switching technologies, such as relays, whose switch contacts will vibrate and power consumption is also higher.
The maximum VGS rating of power MOSFET switches (about 20 volts for standard devices and about 15 volts for logic-level devices) makes it difficult
to withstand voltages exceeding 15 volts. Of course, it is also possible to level shift the gate voltage, but this will waste power, and
the larger resistance required for higher voltages will reduce switching speed.