What Is a High-Side Switch?
A high-side switch is a semiconductor device that turns the power supply on and off for loads such as motors, solenoids, inductors, and LEDs.
In most cases, P-channel MOSFETs are used as semiconductors. In the circuit wiring, the switch is placed on the power supply side of the load, so it is called a high-side switch.
When the high-side switch is ON, power is supplied to the load, and when the high-side switch is OFF, power is not supplied to the load. When the switch is placed on the output side of the load (opposite side of the power supply) in the circuit wiring, it is called a low-side switch.
Uses of High-Side Switches
High-side switches are used to supply or disconnect power to a variety of loads. Specifically, they are often used in inverters, power on/off circuits, LED drivers, and inductance load drivers such as motors and solenoids.
Since large currents are often applied to the load, it is necessary to design the inrush current countermeasures and reverse current prevention circuits into consideration. Because the power supply to the load is turned on and off by a semiconductor device, rather than a mechanical switch such as a relay, the ON/OFF speed can be increased.
If you want to control the output of the load ON/OFF while the power supply to the load remains ON, use a low-side switch.
Principle of High-Side Switches
There are two types of FETs: P-channel type and N-channel type. When FETs are used as low-side switches, N-channel type FETs are often used. When used as a high-side switch, P-channel FETs are generally used.
When a negative gate-source voltage is applied to a P-channel FET, the resistance between the drain-source decreases and current flows from the source to the drain. The power supply and load must be connected to the FET according to the direction of the current flow, so connect the power supply to the source of the P-channel FET and the load to the drain.
A P-channel FET functions as a high-side switch because current flows from the source to the drain when the gate voltage is lower than the supply voltage connected to the source.
Other Information on High-Side Switches
1. Principle of Fet
While transistors are called “base,” “emitter,” and “collector,” FETs are called “gate,” “source,” and “drain. Base” and “gate,” “emitter” and “source,” and “collector” and “drain” are similar terminals.
A transistor has a characteristic that the current flowing in the base multiplied by a certain multiple flows in the collector, while a FET has a characteristic that the resistance between the drain and the source varies depending on the voltage between the gate and the source. With a higher voltage between gate and source, there will be a smaller the resistance between drain and source.
In contrast to controlling the pace current and the collector current when controlling a transistor, controlling the voltage between gate and source and controlling the resistance between drain and source when controlling a FET is more effective.
2. Points to Consider When Selecting a High-Side Switch
When using an N-channel FET as a high-side switch, the power supply should be connected to the drain and the load to the source, and the gate voltage should be higher than the supply voltage to the load. To make the gate voltage higher than the power supply voltage to the load, a gate voltage booster circuit or a similar device should be provided.
However, the resistance between the drain and the source is larger than that of an N-channel FET, so care must be taken when selecting a FET.