How many steps does it take to switch the power on and off in a complex electronic system like putting an elephant in a refrigerator?

The option for circuit designers to turn power lines on and off in electronic systems may sound like a small thing, but there are many aspects to consider for successful implementation.

In some electronic systems, it is necessary to disconnect the power lines. For example, it might be cutting off battery voltage to preserve battery charge, or disconnecting a load from a live line. Ideally, a mechanical switch would be used for this purpose. However, if switching via an electronic signal is required, then it is usually more appropriate to use an electronic switch. This type of electronic switch may use MOSFETs as switching elements. In addition to purely discrete solutions with MOSFETs, electronic switching can be easily implemented using a variety of semiconductor ICs.

Figure 1. Switching power lines using N-channel MOSFETs and a stand-alone driver circuit, the LTC7003

First, it must be decided whether the switching element is an N-channel or P-channel MOSFET. Both may be appropriate. However, compared with P-channel MOSFET, N-channel MOSFET has lower resistance, so the loss in the on-state is also lower. The disadvantage lies in the driving aspect of N-channel MOSFET. where the voltage required by the gate is higher than the available power line voltage. Therefore, some form of charge pump must be included in the driver IC. P-channel MOSFETs do not require this type of boosting. However, there is a wide selection of N-channel MOSFETs.

Next, you need to choose whether to integrate the power switch and driver in a single package or use a two-chip solution that puts the driver circuit in a separate IC and the corresponding MOSFET in another package. A selection of switches that have been optimized for the driver circuit further supports integration in the same package. In this case, the switch is generally well protected and does not experience overloading during use. This fully integrated solution also has disadvantages, including fewer related products on the market and higher costs.

In the third step, it must be determined whether a single MOSFET is sufficient to constitute a mechanical switch . MOSFETs have a body diode and, therefore, can only switch current in one direction. If the application requires a complete disconnection of the line, ensuring that current cannot flow in either direction, then a solution is required in which two MOSFETs are connected in anti-series with each other. Figure 2 shows this switch-level configuration.

Figure 2. Circuit in which two N-channel MOSFETs are connected back-to-back to block current in both directions of the circuit.

Finally, the appropriate integrated circuit must be selected, either the driver for the MOSFET or the package containing the MOSFET and driver. This step sounds simple, but is actually quite tedious. A load switch is usually required. However, the range of options is not large. In many cases, hot-swappable controllers, electronic fuses, surge protectors, ideal diodes, and power path controllers can be used, depending on the application and the additional monitoring capabilities required. Most of these devices contain switch pins that can cut off the current flow when needed. With Analog Devices' LTspice® simulation tools, you can confirm that the solution's precise behavior meets specifications.

Figure 3. LTspice simulation using the LTC4414 low-loss PowerPath™ controller as load switch

Depending on the specific application, disconnecting the supply voltage can be very complex. However, a special MOS driver IC with switch control pins can be used to reduce design difficulty.

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About the author

Frederik Dostal studied microelectronics at the University of Erlangen in Germany. He started in the power management business in 2001 and held various applications engineer positions, including 4 years in Phoenix, Arizona working on switch mode power supplies. He joined Analog Devices in 2009 and worked as a power management field applications engineer at ADI in Munich.