Voltage regulators are a cornerstone of electronic circuits, providing a steady output voltage regardless of variations in input voltage or load conditions. Among them, the basic operational amplifier (op-amp) based series voltage regulator is widely used for its simplicity and reliability. However, unlike shunt regulators, the basic op-amp series voltage regulator lacks inherent short-circuit protection, making it vulnerable to catastrophic damage in the event of a short circuit across the load terminals.
Why Short-Circuit Protection is Essential
When the load terminals are accidentally shorted, the load current can surge towards infinity. This can irreparably damage the pass transistor and potentially destroy other components in the unregulated electric power supply feeding the series regulator. To address this issue, designers incorporate some form of current limiting into the series regulator, which safeguards the circuit from accidental shorts.
Circuit Diagram and Working Principle
Below is the circuit diagram of a series voltage regulator with short-circuit protection:
The overall power supply circuit diagram is shown below.
Current-Sensing Resistor (Rsense):
Rsense is a small resistor that monitors the load current. In this example, we use an Rsense of 1 Ω.
The load current passes through Rsense, generating a voltage across it that serves as the base-emitter voltage for Q1.
Normal Operation:
When the load current is below 600 mA, the voltage across Rsense remains less than 0.6 V, keeping Q1 in the cutoff state. In this mode, the regulator operates as a standard linear power supply.
Current Limiting:
As the load current increases beyond 600 mA but stays below 700 mA, the voltage across Rsense rises to between 0.6 V and 0.7 V, gradually turning on Q1.
The collector current of Q1 flows through resistor R4, reducing the base voltage of Q2. This, in turn, decreases the load voltage and limits the load current.
Short-Circuit Scenario:
When the load is shorted, Q1 conducts heavily, dropping the base voltage of Q2 to approximately 1.4 V (two VBE drops above ground). This limits the current through the pass transistor to approximately 700 mA, depending on the characteristics of the transistors.
Role of Resistor R4
Resistor R4 plays a critical role in ensuring sensitive current limiting. Without R4, the current-sensing transistor (Q1) lacks sufficient voltage gain to respond effectively. However, the value of R4 must strike a balance:
It should be high enough to amplify the voltage across Rsense and enable current limiting.
It should not be so high that it hampers the op-amp’s ability to drive the pass transistor (Q2).
Typical values for R4 range from a few hundred to a few thousand ohms.
Design Example
Output Voltage Calculation
To illustrate the operation of a series voltage regulator, consider the following equation for the output voltage:
Vo = ((R1 + R2) / R1) * Vz
Given:
R1 = 10 kΩ
R2 = 10 kΩ
Vo = 4.8 V
Substitute these values into the equation to determine Vz:
Vz = Vo / ((R1 + R2) / R1)
Vz = 5 / ((10k + 10k) / 10k) Vz = 5 / 2 Vz = 2.4 V
Thus, the Zener diode voltage (Vz) required for this design is 2.4 V.
Current Limiting Calculation
Using the equation:
\(I_{SL}= V_{BE} / R_{sense}\)
Where:
Rsense = 3 Ω
VBE ≈ 0.7 V (typical value for a silicon transistor)
Substitute the values:
\(I_{SL}\) = 0.7 / 3 = 0.333 A
The short-circuit current limit (ISL) is approximately 0.7 A.
Conclusion
The inclusion of current limiting in a series voltage regulator significantly enhances its reliability and robustness. By incorporating a simple current-sensing resistor and a current-limiting transistor, designers can protect the regulator and associated circuitry from accidental short circuits. Understanding and implementing such protective measures are crucial for designing durable and efficient energy supply systems.
For more advanced designs, such as programmable power supplies or high-output shunt voltage regulators, these protective measures can be further optimized to ensure maximum performance and safety in power supply units (PSU).
