PNP transistor plus PNP transistor overvoltage protection circuit
The PNP transistor plus PNP transistor overvoltage protection circuit is commonly used in high-voltage protection designs, especially at power input ports or in battery-powered devices, to provide automatic cutoff and voltage-limiting protection. The core idea of this architecture is to use the conduction and cutoff characteristics of transistors to detect and cut off overvoltage, making it an active overvoltage protection circuit.
This circuit is suitable for low-power DC circuits, supports automatic overvoltage cutoff, and can quickly react to overvoltage conditions. Once the supply voltage exceeds the set threshold, the output is cut off. It also features delayed recovery (if not latched), meaning the output can restart once the input voltage returns to normal (depending on the design). It operates independently without a microcontroller, relying solely on analog components for rapid response, and can be implemented at low cost with just a few transistors and resistors.
Example schematic of PNP transistor plus PNP transistor overvoltage protection circuit
In the example above, the core control components are Q2 and D2, with Q1 serving as the main switch. When the input voltage is between 4.5V and 5.1V DC, D2 is reverse-biased and cut off, Q2 is cut off, and Q1’s base voltage is 0, allowing Q1 to conduct and the circuit to operate normally. When the input voltage exceeds 5.1V, D2’s Zener diode breaks down, clamping Q2’s base voltage to 5.1V. When the input voltage exceeds 5.7V, Q2 conducts, causing Q1’s base voltage to equal VCC, which cuts off Q1 and disconnects the input. When the input voltage returns to the normal range, the circuit automatically resumes power supply.
When designing this circuit, pay attention to the selection of PNP transistors. Their voltage tolerance should be higher than the maximum input voltage (recommended with a 20–30% safety margin), and their current capacity should exceed the load’s maximum current. Choose models with low saturation voltage VCE(sat) to reduce power consumption. For threshold design, use voltage divider resistors combined with Zener diodes to set Q2’s conduction conditions. For more precise control, consider using adjustable voltage references like the TI’s TL431.
For thermal and power considerations, Q1, as the main current path, will generate power dissipation, so heat dissipation and PCB copper design must be considered. A low-value resistor (e.g., 0.1Ω) can be connected in series with Q1 to detect current and protect the base. To avoid false triggering due to transient voltage fluctuations during startup, add a capacitor for delay to improve stability.
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