In many applications, safely protecting circuits from overvoltage is very important. This article explains how to neutralize, or cancel out, overvoltage using protection circuits.

■ Surge protection is necessary to protect other loads connected to the same power supply.

Overvoltage can occur in situations such as when a large load is rapidly cut off in a power distribution system.

Surge protection is necessary to protect other loads connected to the same power supply.

Figure 1 shows a protection circuit configuration using the LT4363 in front of an electronic circuit that requires protection. This example is taken from an industrial application with a rated power supply voltage of 24V.


To optimize the size of a MOSFET, attempts must be made to safely operate the smallest possible MOSFET.

In other words, avoid using excessively large components, and ensure that most of the MOSFET's SOA can be utilized in the application.

To achieve this, the control IC must be able to precisely determine the current operating state and ascertain whether the MOSFET is operating within the SOA's safety range.

However, many control ICs only measure the current flowing through the MOSFET. If the voltage drop across the MOSFET can also be determined, much more accurate judgment is possible.

The LT4363 surge protection device considers not only the current flowing through the MOSFET but also the voltage applied between the source and drain.

This means that MOSFETs can be operated much more safely in the linear region, and consequently, system costs can be reduced by selecting smaller MOSFETs.

The protection mechanism operates by charging the timer capacitor connected to the TMR pin in Figure 1, and the charging rate is determined by the measured current and dropout voltage. If the capacitor voltage exceeds 1.275V, a warning is generated, and if it exceeds 1.375V, the MOSFET is completely shut off to protect the component.


Figure 3 shows how the timer capacitor voltage of the LT4363 rises due to the VDS voltage applied to MOSFET Q1 in Figure 1.

A similar charging graph exists for the current flowing through MOSFET Q1.

These parameters ensure that the SOA curve of the MOSFET is not exceeded, and realize safe operation and overvoltage protection at the same time.

■ Small characteristics make a big difference in overvoltage protection operation mode and suitable MOSFET selection

Although overvoltage protection modules may appear simple and unremarkable on the surface, small characteristics can make a significant difference in how overvoltage protection operates and in the selection of a suitable MOSFET.

※ Author Introduction
Frederik Dostal has been working in this industry for over 20 years as a power management expert. He majored in Microelectronics at the University of Erlangen in Germany and joined National Semiconductor in 2001, where he gained extensive experience implementing power management solutions for customer projects as an FAE. During his four years at NS, he worked in Phoenix, Arizona, as an Application Engineer responsible for Switch Mode Power Supplies (SMPS). He joined Analog Devices in 2009 and subsequently held various positions related to product lines and European technical support. Currently, leveraging his extensive knowledge of design and applications, he works as a power management expert at the ADI Munich office.