How to successfully connect and disconnect a line voltage supply

Members box download this article in PDF format.

What you will learn:

  • N-channel or P-channel MOSFET to build an electronic switch?
  • Single box or two-chip solution for power switch and driver?
  • Selecting the correct MOSFET driver.

In some electrical systems, it is necessary to disconnect a supply line. This can take the form of cutting off a battery voltage to conserve battery charge, for example, or separating a load from a live line. Ideally, this involves a mechanical switch.

However, if switching is to occur via an electrical signal, an electronic switch is generally more suitable. Such an electronic switch can be constructed with a MOSFET as the switching element. In addition to a purely discrete solution with a MOSFET, numerous semiconductor integrated circuits are available for an easy implementation of an electronic switch.

Step by step method

First, a decision must be made as to whether the switching element should be an N-channel or a P-channel MOSFET. Both can be fine. Compared to a P-channel MOSFET, an N-channel MOSFET has lower resistance and therefore lower on-state losses.

The disadvantage is in the driving of the N-channel MOSFET. Here, a higher voltage than the available supply line voltage is required for the gate (Fig.1). Consequently, the driver integrated circuit must contain any charge pump. A P-channel MOSFET does not require this kind of voltage boost. Nevertheless, there is a large selection of N-channel MOSFETs.

The next step is to choose between having the power switch and driver in a single package, or using a two-chip solution with the driver circuit in a separate IC and the corresponding MOSFET in a second package.

The already optimized selection of the switch for the control circuit argues in favor of integration into a single housing. The switch is usually well protected with this configuration and is therefore not overloaded during operation. Disadvantages of such a fully integrated solution include smaller market offering and higher cost.

In the third step, a decision has to be made whether a single MOSFET is sufficient for a mechanical switch. A MOSFET always includes a body diode. Thus, it can only switch currents in one direction. If an application requires a line to be completely interrupted to prevent current from flowing in both directions, a solution with two MOSFETs connected in series in opposite directions is needed. Figure 2 shows such an arrangement of switching stages.

Choose the right driver

Finally, the appropriate IC, i.e. the driver for the MOSFET, or the package with the MOSFET and the driver, must be selected. This step seems trivial, but it is quite tedious. Load switches are often sought after. Generally, however, there are no great offers here.

Hot-swap controllers, electrical fuses, surge suppressors, ideal diodes, and power path controllers can be used in many cases, depending on the application and additional monitoring functions required. In most of these devices there is an on/off pin to interrupt the flow of current if necessary. With the LTspice simulation tool of Analog Devicesit is possible to check if the precise behavior of the solution meets the specifications (Fig.3).

Disconnecting a supply voltage can become complex depending on the application. A special MOS driver IC with an on/off control pin simplifies this design.

Comments are closed.