How to Design a Series Voltage Regulator with Foldback Current Limiting and Its Working Operation

A reliable voltage regulator is essential for any electric power supply, ensuring steady output voltage even during load fluctuations. Current limiting, as detailed in our earlier tutorial on How to Design a Series Voltage Regulator with Current Limiting, is a crucial enhancement to protect components like the pass transistor and rectifier diodes from damage in case of a short circuit. However, traditional current limiting has a downside: it causes high power dissipation in the pass transistor during short-circuit conditions, as the full input voltage drops across it.

To address this issue, foldback current limiting can be incorporated. This advanced feature minimizes power dissipation during a short circuit, safeguarding the power supply unit (PSU). Below, we explore the design and operation of a linear power supply with foldback current limiting.

Understanding Foldback Current Limiting

In a foldback current-limiting design, the load current decreases significantly under short-circuit conditions, protecting both the pass transistor and the entire power adapter system.

Circuit Overview:
In the Series Voltage Regulator with Foldback Current Limiting circuit diagram below, the voltage across the current-sensing resistor ($R_{\text{sense}}$) is fed into a voltage divider network consisting of $R_5$ and $R_6$.This voltage divider drives the base of a transistor ($Q_1$).

The overall power supply circuit diagram is shown below.

Normal Operation of the Voltage Regulator

During normal operation, the base voltage of $Q_1$ is lower than its emitter voltage, resulting in a negative base-emitter voltage ($V_{BE}$). This keeps $Q_1$ in a cutoff state, ensuring the DC power supply operates without interruptions.

When the load current reaches a high value, the base voltage of $Q_1$ rises. As soon as $V_{BE}$ crosses approximately 0.6–0.7 V, $Q_1$starts conducting, activating the current-limiting mechanism.

Foldback Limiting under Short-Circuit Conditions

If the load resistance decreases further, simulating a short circuit, the foldback current limiting feature comes into play. This mechanism decreases the load current significantly, reducing the stress on the pass transistor. This makes foldback limiting a superior choice compared to conventional limiting for applications like uninterruptible power supply (UPS) systems.

Key Advantages of Foldback Current Limiting

1. Reduced Power Dissipation:
   By limiting the current under short-circuit conditions, the power pack reduces the thermal stress on the pass transistor.

2. Enhanced System Protection:
   Components like rectifier diodes and the pass transistor in the modular power supply are safeguarded from overheating, ensuring reliable operation.

3. Improved Efficiency:
   Foldback limiting prevents unnecessary energy wastage, making it ideal for programmable power supplies and other advanced systems.