In electronic circuit design, MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) are widely used in power control and signal switching circuits due to their advantages such as low on-resistance and high switching speed. When MOSFETs are used to control the shutdown of resistor voltage divider circuits, voltage overshoot phenomena may occasionally occur. This not only risks damaging other components in the circuit but also affects the stability and reliability of the entire circuit system. Understanding the causes of overshoot during MOSFET-controlled resistor voltage divider shutdown is crucial for optimizing circuit design and ensuring normal circuit operation.

The influence of parasitic parameters

The role of parasitic capacitance

MOSFETs contain various parasitic capacitances, among which the gate-source capacitance (CGS), gate-drain capacitance (CGD), and drain-source capacitance (CDS) have the most significant impact on shutdown overshoot. When the MOSFET turns off, the gate voltage rapidly decreases. Due to the presence of CGS, the gate charge does not release instantly but discharges slowly through the connected circuit. During this process, CGD induces the Miller effect. The Miller effect increases the equivalent input capacitance, slowing down the rate at which the gate voltage decreases, thereby prolonging the MOSFET's turn-off time. During the transition phase of the MOSFET turning off, the drain voltage begins to rise. At this point, the CDS forms an LC resonant circuit with the inductors in the circuit (such as line inductors, load inductors, etc.). Due to the presence of the CDS, the drain voltage experiences overshoot during the resonance process. In a resistor voltage divider-based buck converter circuit controlled by a MOSFET, when the MOSFET turns off, the resonance caused by the CDS and line inductance causes the drain voltage to momentarily exceed twice the power supply voltage, posing a serious threat to other components in the circuit.

The impact of parasitic inductance

In addition to parasitic capacitance, parasitic inductance in the circuit is also a significant factor causing overshoot. Line inductance is unavoidable, especially in high-frequency circuits or high-current circuits, where longer wires or improper routing can result in significant line inductance. When the MOSFET turns off, the current in the circuit changes rapidly. According to the law of electromagnetic induction, the inductance generates an induced emf whose direction is opposite to that of the current change. In a resistor voltage divider circuit, the induced electromotive force generated by the inductance is superimposed on the power supply voltage, causing the MOSFET drain voltage to rise sharply and form overshoot. In a high-power MOSFET drive circuit, due to unreasonable wiring, the line inductance is large. At the instant the MOSFET turns off, the drain voltage overshoot can reach up to three times the power supply voltage, causing the MOSFET to be damaged by breakdown.

Impact of the drive circuit

Rise/fall time of the drive signal

The drive signal directly affects the turn-off characteristics of the MOSFET. If the fall time of the drive signal is too long, it slows down the turn-off speed of the MOSFET. During turn-off, the drain current cannot quickly drop to zero, preventing the energy stored in the inductor from being released in time, thereby causing overshoot when the MOSFET turns off. Conversely, if the falling time of the drive signal is too short, although the MOSFET can turn off quickly, the excessive rate of current change will generate a higher induced electromotive force in the inductor, further exacerbating the overshoot. In a circuit where a conventional transistor is used to drive the MOSFET, the limited switching speed of the transistor results in a drive signal fall time of tens of microseconds, causing noticeable voltage overshoot during MOSFET turn-off and affecting the normal operation of the circuit.

Stability of the drive power supply

The stability of the drive power supply also affects the MOSFET's turn-off process. If the drive power supply has ripple or voltage fluctuations, the unstable drive power supply can cause fluctuations in the gate voltage during MOSFET turn-off, thereby affecting the MOSFET's turn-off characteristics. When the drive power supply voltage drops suddenly, it may cause the MOSFET to turn off prematurely, resulting in the sudden release of energy in the inductor and causing overshoot. In a system where a switching power supply is used to power the MOSFET drive circuit, due to the relatively high ripple of the switching power supply, voltage overshoot frequently occurs during MOSFET shutdown. After replacing it with a linear power supply with lower ripple, the overshoot issue was significantly improved.

Impact of Load Characteristics

Energy Storage and Release in Inductive Loads

When the load in a resistive voltage divider circuit is inductive, such as a motor or transformer, the inductive load stores energy during operation. When the MOSFET turns off, the current in the inductive load cannot instantly drop to zero but continues to flow through a freewheeling diode or other path. Due to the inductance's energy storage characteristics, during current reversal, the inductance generates an induced electromotive force, causing the MOSFET's drain voltage to rise. If the freewheeling diode has poor performance or improper parameter selection, it cannot effectively release the energy stored in the inductor in a timely manner, exacerbating the overshoot phenomenon. In a motor drive circuit, due to the excessive reverse recovery time of the freewheeling diode, the induced electromotive force generated by the motor inductor cannot be released through the freewheeling diode in time when the MOSFET is turned off, causing significant overshoot in the MOSFET drain voltage, which damages both the motor and the MOSFET.

Size of the load resistor

The size of the load resistor also affects the overshoot  during MOSFET turn-off. In a resistor voltage divider circuit, the load resistor and the MOSFET's on-resistance jointly determine the circuit's operating current. When the MOSFET turns off, the smaller the load resistor, the greater the rate of change in circuit current, resulting in a larger induced electromotive force from the inductor and more pronounced overshoot. In a resistor voltage divider constant current source circuit controlled by a MOSFET, when the load resistance is reduced to a certain extent, the overshoot voltage during MOSFET turn-off increases sharply. By increasing the load resistance or adjusting the circuit parameters, the overshoot issue is mitigated.

The overshoot during MOSFET-controlled resistor voltage divider turn-off is the result of multiple factors, including parasitic parameters, the drive circuit, and load characteristics. During circuit design, these factors must be fully considered. By optimizing circuit layout, selecting appropriate drive circuits, and adjusting component parameters, overshoot can be effectively suppressed to ensure stable and reliable circuit operation. As electronic technology continues to advance, the requirements for MOSFET applications are becoming increasingly stringent. In-depth research into the mechanisms causing overshoot and the implementation of effective solutions are of significant importance for enhancing circuit performance and driving the progress of electronic circuit technology.