UC3832/3 improves the performance characteristics of linear regulated power supply

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Basic Types and Problems of Overcurrent Protection

Overcurrent protection is a crucial feature in power supply design to prevent damage from excessive currents. The most common methods include current limiting, current reduction, and cutoff types. Each has its own advantages and disadvantages depending on the application.

The current-limiting type works by restricting the output current when it reaches a certain threshold, maintaining a constant current level. This method is simple to implement and allows for full load startup. However, during overcurrent conditions, the power supply transistor experiences significant losses, which may require larger components and increased thermal management, leading to higher costs and reduced reliability.

Cutoff-type protection, on the other hand, completely shuts down the power supply once an overcurrent is detected. This reduces power consumption to zero, but it can be prone to false triggering due to impact loads. Once triggered, the system must be manually reset, which can be inconvenient in some applications.

The current-reduction type gradually lowers the output current as the voltage drops, preventing excessive power loss in the regulator. It is commonly used in integrated circuits with built-in voltage regulators. However, this method may lead to latch-up issues under full load, where the output voltage gets stuck below normal levels, causing startup problems. To avoid this, a large margin must be set between the normal operating current and the overcurrent threshold, which can sometimes leave the load unprotected.

Combined Overcurrent Protection

A combined overcurrent protection approach offers the best of both worlds: it provides smooth startup like current limiting and low power loss like cutoff protection. Initially, the system starts in current-limit mode to handle impact loads. If an overcurrent is detected, the system enters current-limiting mode to manage transient surges. If the overcurrent persists, it transitions to cutoff mode for full protection. After a set period, it rechecks the load condition and restores normal operation if safe. This method significantly reduces the power dissipation in the regulator compared to traditional current-limiting schemes.

Implementation of Combined Overcurrent Protection

One of the easiest ways to implement this combined protection is using the ASIC UC3832/3. This IC integrates essential functions such as error amplification, driver stages, and overcurrent protection, making it ideal for linear regulated power supplies.

UC3832/3 Circuit Principle

The UC3832/3 includes a current-limiting amplifier (CSAMP) and a current-limit comparator (CSCOMP). The CSAMP is biased at 130mV, and when the input voltage exceeds this, the system enters current-limit mode. The CSCOMP, biased at 100mV, ensures that the current-limiting function activates before the main regulation circuit, allowing for fast response to overcurrent events.

The timer within the UC3832/3 charges a timing capacitor (CT), and when the voltage reaches 1.8V, the system switches to cutoff mode. Once the overcurrent is cleared, the system returns to normal operation. This cycle repeats until the fault is resolved, ensuring reliable protection without unnecessary shutdowns.

High Current Low Input/Output Differential Voltage Implementation

Minimizing the input-output voltage difference is critical for improving efficiency, especially in high-current applications. The UC3832/3 supports very low dropout voltages, making it suitable for applications requiring minimal voltage drop. Bipolar transistors can achieve less than 1V, while power FETs can go below 0.3V, offering excellent performance for high-power systems.

Practical Application of UC3832/3

When using the UC3832/3, selecting the right components—such as the power transistor, output capacitor, timing capacitor, and resistor—is essential. The power transistor must handle the maximum input voltage and current, while the output capacitor should be chosen to balance ripple and startup performance. Timing capacitors and resistors control the duration of the overcurrent protection cycles, ensuring stable and safe operation.

Conclusion

The UC3832/3 is a powerful solution for implementing combined overcurrent protection in linear power supplies. Its advanced features provide superior performance, low power loss, and efficient regulation, making it an excellent choice for a wide range of applications.