Switching DC currents can become a challenging task, but simulation can be of help. When it comes to low power and a human operated push-button, the following combination of a pMos and nMos is easy to implement. Take care that the maximum VGS(th) has to be limited to be below 20volts maximum. The human operated push button is replaced by the control section within the red dashed lines. Here an On-pulse and an Off-pulse are generated by two signal generators and logically added using an OR-gate to control the push-button. The simplicity of the circuit is directly visible and can be created froma small amount of components.
Using an isolated gate driver allows driving a nMosfet that has a lower on-state resistance compared to the pMosfet used in the above example. The gate-driver model includes many details, such as the delay between turning on and off. Dimensioning of the external isolated power supply for powering the gate driver is based on the amount of gate charge and pulse width modulation and can easily be verified in simulation.
The simulation shows clearly the voltage spike that exist when using longer cables between the switch and the load. The isolated gate driver can directly drive the nMosfet. However, an isolated power supply is required to power the gate driver. More info at Isolated Gate Driver
Numerous simulation examples are available for automotive and switched mode power supplies in the simulation package Caspoc.
A free demo can be downloaded from the Caspoc web site.
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