How to Design an Op-Amp Series Voltage Regulator

A series voltage regulator is a linear voltage regulator that uses a pass transistor in series with the load. The regulator adjusts the transistor's base current through an op-amp to maintain a constant output voltage. This design is often used in a low-voltage power supply or as part of a modular power supply unit (PSU) where stable and precise voltage regulation is needed.

In this power supply tutorial, it is shown how to design an op-amp series voltage regulator using LM358, 1N4733A Zener diode, and TIP3055 with precise voltage regulation for linear power supplies.

Components and Circuit Overview

Op-Amp:
- Acts as the error amplifier.
- Example: LM358 is a cost-effective choice for low-power designs.
Pass Transistor:
- Handles the load current.
- Example: TIP3055 for higher currents or 2N2222A for lighter loads.
Zener Diode:
- Provides the reference voltage.
- Example: 1N4733A with a 5.1V breakdown voltage.
Resistors $R_1$ and $R_{2}$ :
- Form a voltage divider to set the output voltage.
Power Source:
- Use a suitable AC power supply or DC power supply with sufficient voltage and current ratings.

Circuit Diagram and Operation

Shown below is the Circuit Diagram of a Op-Amp Series Voltage Regulator.

Circuit Diagram of Op-Amp Series Voltage Regulator

Operation of Op-Amp Series Voltage Regulator

An op-amp series voltage regulator operates using components like the LM358 op-amp, a 1N4733A Zener diode, and a pass transistor such as the TIP3055 or 2N2222A. The Zener diode generates a stable 5.1V reference voltage, which is applied to the non-inverting input of the op-amp. The inverting input receives a portion of the output voltage through a feedback network of resistors ( $R_1 = R_2 = 10k$ ). The operational amplifier is powered with the unregulated power supply voltage. The LM358 amplifies the difference between the reference voltage and the feedback voltage, driving the base of the pass transistor. This transistor regulates the current from the unregulated supply to the load, ensuring a stable output voltage. The circuit dynamically adjusts to variations in the input supply or load demand, making it ideal for applications requiring precise voltage regulation in linear power supplies or DC power supplies.

Step-by-Step Design

1. Set the Output Voltage

The output voltage is determined by the Zener diode and the feedback resistors $R_1$ and $R_{2}$ . The formula is:

V_{\text{out}} = V_{\text{ref}} \times \left(1 + \frac{R_1}{R_2}\right)

For example, with $V_{\text{ref}} = 5.1V$ and $R_1 = R_2 = 10k$ :

V_{\text{out}} = 5.1 \times \left(1 + \frac{10}{10}\right) = 10.2V

2. Ensure Adequate Power Supply Headroom

The op-amp's positive supply voltage ( $V_{c c}$ ) must be higher than the required output voltage. Insufficient headroom can cause the op-amp to fail in driving the pass transistor.

Example: If $V_{\text{out}} = 10.2V$ , the $V_{c c}$ should be at least: $V_{cc} \geq V_{\text{out}} + V_{\text{BE}} + \text{op-amp swing margin}$ For a TIP3055 with $V_{\text{BE}} = 0.7V$ and an op-amp swing margin of 2V, $V_{c c} \geq 13 V$

3. Pass Transistor Selection

Ensure the transistor can handle the maximum output current ( $I_{out}$ ) and power dissipation:

P = (V_{\text{in}} - V_{\text{out}}) \times I_{\text{out

Example: For $V_{in} = 20 V$ , $V_{out} = 10.2 V$ , and $I_{\text{out}} = 1A$ :

P = (20 - 10.2) \times 1 = 9.8W

A heatsink may be required for power dissipation above 1W.

4. Zener Current Sizing

The Zener diode must operate within its recommended current range. Calculate the Zener current as:

I_Z = \frac{V_{\text{in}} - V_{\text{ref}}}{R_Z}

Ensure $I_Z$ is sufficient (e.g., 5–20mA for 1N4733A).

Additional Low Pass Filter

If the unregulated voltage has ripple higher than the tolerance, a low poss filter(LPF) should be added after the input capacitor of the power supply. For the filter design we need to find out first what cutoff frequency to use and the component R and C values.

If the mains frequency is 50Hz, and you're using a full-wave rectifier (which doubles the mains frequency), the ripple frequency would be 100Hz. To design an RC filter to attenuate the ripple effectively, we want to select a cutoff frequency ( $f_c$ ) that is lower than the ripple frequency.

Step 1: Determine the Cutoff Frequency

We want the cutoff frequency $f_c$ of the RC filter to be about 10 times lower than the ripple frequency, which would be:

f_c \approx \frac{100\ \text{Hz}}{10} = 10\ \text{Hz}

Step 2: Use the RC Filter Formula

The cutoff frequency of an RC filter is given by the formula:

f_c = \frac{1}{2\pi RC

Rearranging the formula to solve for $R$ gives:

R = \frac{1}{2\pi f_c C

Step 3: Choose a Capacitor Value

A typical capacitor value for power supply filters is in the range of 100µF to 470µF. Let's assume you are using 100µF for your filter.

Step 4: Calculate the Resistor Value

Substitute $f_{c} = 10 Hz$ and $C = 100 μ F$ (which is $100 \times 10^{-6}$ F) into the formula:

R = \frac{1}{2\pi \times 10 \times 100 \times 10^{-6}} \approx 159.2\ \Omega

So, you would use a resistor of approximately 160Ω.

Step 5: Adjusting for Different Capacitors

If you decide to use a different capacitor value, the resistor will need to be adjusted accordingly:

For 220µF: $R = \frac{1}{2\pi \times 10 \times 220 \times 10^{-6}} \approx 72.3\ \Omega$
For 470µF: $R = \frac{1}{2\pi \times 10 \times 470 \times 10^{-6}} \approx 33.9\ \Omega$

Summary of Values:

For 100µF capacitor: R = 160Ω
For 220µF capacitor: R = 72Ω
For 470µF capacitor: R = 34Ω

These values will provide a low-pass filter that will effectively reduce the ripple at the 100Hz ripple frequency, giving you a smoother output.

Common Challenges

Insufficient Power Supply Voltage

If the op-amp's power supply voltage is too low, it cannot properly drive the pass transistor. Connect the op-amp's power pins to an unregulated power supply or high-voltage power supply to provide adequate headroom.

Thermal Management

For high-current designs, ensure the pass transistor can dissipate the heat generated. Using a linear power supply often requires adding a heatsink.

Applications of Series Voltage Regulators

Battery Chargers
- See: How to Build Lithium Polymer Battery Charger
DIY Power Supplies
- See: DIY 5V Regulated Power Supply for Breadboards
Reverse Polarity Protection
- See: How to Build Reverse Polarity Protection Circuit

Tips for Component Selection

Zener Diode: Choose a Zener voltage near the desired reference voltage.
Op-Amp: Use an op-amp with low input offset and rail-to-rail output for precise regulation.
Pass Transistor: Ensure adequate current gain and power dissipation ratings.

Conclusion

Designing an op-amp-based series voltage regulator requires careful selection of components and attention to circuit details. Ensuring sufficient headroom for the op-amp and proper thermal management for the pass transistor are key to achieving stable and efficient regulation.

Explore more about designing power circuits:

By following these principles, you can design a versatile and reliable linear power supply for a variety of applications.