Protection for Voltage Regulators Schematic Circuit Diagram
Understanding Voltage Regulator Limits
Voltage regulators ICs often come with an overlooked upper voltage limit, typically around 35 V, especially for those with fixed output voltages. Even adjustable voltage regulators have a maximum voltage specification, typically around 40 V, between the input and output terminals. In cases where the output is shorted, the input voltage must be restricted to prevent issues. This circuit presents a solution that enables the use of these regulators in scenarios with higher input voltages. Although the remedy involves just three additional components, it is straightforward and can be constructed using readily available parts.
Voltage Limitation and Regulation
The voltage regulator’s output is controlled by a combination of T1 and zener diode D1, ensuring the regulator functions properly with loads up to its maximum rating. R1 provides the necessary current for D1 and biases T1. Using a Darlington transistor for T1 helps maintain a reasonably high value for R1. D1 carries only 10 mA of current with a 60 V input voltage, effectively limiting the voltage across the regulator. Surprisingly, even with no load, the output voltage only increases slightly, reaching 5.10 V with a 60 V input.
Transistor Selection and Circuit Variations
In our experiments, T1 was represented by BDV65B and R1 had a value of 4.7 kΩ. To guarantee short-circuit protection with a 60 V input, the chosen transistor must operate safely at the maximum input voltage under the regulator’s short-circuit current, which can exceed 2 A. BDV65B and TIP142 do not meet these criteria, with maximum voltages of 40 V and 50 V, respectively. If the transistor fails, the regulator will also malfunction. Adding SOA protection for T1 is one option, although it seems like overprotecting the protection mechanism. Alternatively, relaxing the requirements might be considered.
Optimizing Protection and Cooling
To ensure T1 receives enough current during a short circuit, R1 must provide adequate current. However, this leads to a minor difference in practice and increases the minimum load. Adequate cooling for both T1 and IC1 should be implemented according to the load requirements. Ripple suppression is minimally affected by the protection circuit, as T1 stabilizes the input. Still, it’s crucial to consider the presence of C2. When using an adjustable regulator like LM317 with an output above 40 V, C2 might momentarily increase the voltage beyond 40 V during a short circuit, potentially damaging the IC. In such cases, an alternative solution or a different regulator type might be necessary.