The Importance Understanding Current Limiting in Linear Power Supplies
Current limiting circuits are crucial for protecting linear power supplies from damage caused by short circuits and overloads. These circuits prevent excessive current flow, safeguarding both the power supply and the connected load. This guide delves into the intricacies of current limiting, offering insights into different types and their practical applications.
Linear power supplies rely on current limiters to ensure reliable operation. This tutorial explores constant current and foldback current limiting, highlighting their characteristics, advantages, and disadvantages. We'll also examine circuit examples and design considerations to help you implement effective current limiting in your power supply designs.
Two Main Types Exploring Constant Current and Foldback Limiting
There are two primary approaches to current limiting in linear power supplies: constant current limiting and foldback current limiting. Each method has its unique characteristics and trade-offs.
Constant Current Limiting: Maintains a fixed output current until a maximum threshold is reached. While simple to implement, it may not fully protect the circuit during a short circuit, as the maximum current is sustained, potentially leading to increased power dissipation. However, this is the most basic and simple form.
Foldback Current Limiting: Reduces the output current as the overload increases, providing superior protection. This approach minimizes power consumption and heat generation, making it a preferred choice in many applications. Foldback current limiting is generally considered superior to constant current limiting.
Circuit Design Basic Constant Current Limiting Circuit
A simple constant current limiting circuit typically uses a sense resistor in series with the output pass transistor's emitter and two diodes connected to the base. The voltage across the sense resistor, combined with the base-emitter voltage of the transistor, determines the current limiting action.
As the output current increases, the voltage across the sense resistor rises. When this voltage plus the base-emitter voltage equals the diode forward voltage drops, the diodes conduct, pulling down the transistor's base voltage and thus limiting the output current. This design is useful for simple power supplies, but is less efficient and can generate more heat.
“Current limiters are essential to protect your power supply and the circuits it powers.
Power Supply Design Expert
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Advanced Design Transistor Foldback Current Limited Circuit
Foldback current limiting circuits, employing a few more components, including a transistor and a few resistors, provide enhanced protection compared to constant current limiting. As the load increases and the output voltage is dropped, the current is folded back, reducing the risk of damage.
As the load resistance decreases and the load current increases, the transistor will start to turn on and reduce the current further, folding back the level of current provided. With a few equations that can be used to find the maximum current for a linear voltage regulator, as well as the short circuit current of the linear voltage regulator.