Boost Converter with 555 Timer How to

Boost converters are widely used in applications requiring a higher output voltage than the input voltage. In this post, we will discuss a practical design for a 5V to up to 25V boost converter that can deliver 1A output current. The circuit uses the versatile 555 timer IC configured as an astable multivibrator and an IRFZ44N MOSFET as the power switch.

Circuit Overview

The boost converter with 555 timer circuit diagram is shown below.

This boost converter circuit consists of the following key components:

1. 555 Timer IC: Configured as an astable multivibrator to generate a square wave at a frequency of 25 kHz with a duty cycle of approximately 58.66%.
2. IRFZ44N MOSFET: Acts as the power switch.
3. Inductor (L): A 90 µH inductor stores and transfers energy.
4. Diode (1N5819): A Schottky diode directs energy from the inductor to the output.
5. Output Capacitor: A 470 µF capacitor smooths the output voltage.

555 Timer Configuration

The 555 timer is the heart of this circuit, generating the PWM signal that controls the MOSFET. The timer is configured as an astable multivibrator with a frequency of 25 kHz and a duty cycle ($D$) of 58.66%.

• Resistors:

  • $R_1=1\text{k}\mathrm{\Omega }$
  • $R_2 = 50 \, \text{k}\Omega$ potentiometer for fine-tuning the duty cycle.

• Capacitor (C): A 1 nF capacitor determines the timing.

• Diodes: Two 1N4148 diodes are connected to the potentiometer to adjust the charge and discharge paths for setting the duty cycle.

• Pin Connections:

  • Pins 2 and 6 are connected together.
  • Pin 5 is grounded through a 10 nF capacitor (C2) to stabilize operation.
  • Pins 4 and 8 are connected to the power supply, and pin 1 is grounded.

For more information on configuring a 555 timer, check out this guide on astable 555 timer circuits.

Power Switch and Output Stage

The timer’s output (pin 3) is connected to the gate of the IRFZ44N MOSFET via a 1 kΩ resistor. The MOSFET's drain is connected to the inductor and the Schottky diode (1N5819). The inductor and diode form the boost converter's energy storage and transfer path.

The output capacitor (470 µF) smooths the boosted voltage, ensuring a stable DC output.

For optimizing snubber circuits for MOSFETs, refer to this MOSFET snubber circuit design tutorial.

Inductor and Capacitor Selection

The values of the inductor and capacitor are critical for efficient operation.

1. Inductor (L): A 90 µH inductor was chosen to ensure energy transfer during each switching cycle without saturation.
2. Output Capacitor (C): A 470 µF capacitor provides sufficient smoothing to minimize ripple at the output.

Calculation of L and C values

Key Variables:

• Inductor $L = 90 \mu H$
• Frequency $f = 25 \, \text{kHz}$
• Input Voltage $V_i = 5 \, \text{V}$
• Output Voltage $V_o = 12 \, \text{V}$
• Duty Cycle $D = 58.66 \%$(0.5866)
• Inductor Current Ripple $\mathrm{\Delta }{I}_L$ (not provided, but can be derived)
• Output Ripple Voltage ΔVo​ = 50 mV

Energy Stored in Inductor and Capacitor:

In a boost converter, the energy stored in the inductor is transferred to the capacitor, and we can use the formula for energy stored in an inductor and capacitor:

Where:

• $E_L$ is the energy stored in the inductor
• $E_C$ is the energy stored in the capacitor
• $L$ is the inductance
• $\Delta I_L$ is the current ripple in the inductor
• $C$ is the capacitance
• $\Delta V_o$ is the voltage ripple across the output capacitor

Step 1: Inductor Ripple Current $\Delta I_L$

The ripple current in the inductor $\Delta I_L$ is given by:

Substitute the known values:

• $V_i = 5 \, \text{V}$
• $D = 0.5866$
• $L=90\mu H=90\times 10^{-6}\text{H}$
• $f=25\text{kHz}=25\times 10^3\text{Hz}$

Step 2: Energy Stored in Inductor $E_L$

Now we can calculate the energy stored in the inductor:

Substitute the known values:

Step 3: Energy Stored in Capacitor $E_C$

We assume that the energy stored in the inductor is equal to the energy stored in the capacitor:

Solving for $C$:

Step 4:Calculate Capacitor Value(C)

 If the output ripple voltage $\Delta V_o$ is 10 mV (0.01 V), we can use the equation for ripple voltage to calculate the required output capacitance $C$.

Formula for Output Ripple Voltage:

Where:

• $\Delta V_o$ = Ripple voltage (10 mV = 0.01 V)
• $\Delta I_L$ = Inductor current ripple (calculated earlier as 0.92 A)
• $f$ = Switching frequency (25 kHz = 25,000 Hz)
• $C$ = Output capacitance (to be calculated)

Rearranging the formula to solve for $C$:

Substituting the known values:

• $\mathrm{\Delta }{I}_L=0.92\text{A}$
• $f=25\times 10^3\text{Hz}$
• $\Delta V_o = 0.01 \, \text{V}$

To calculate the appropriate LC values for your converter design, explore this LC value calculator for switching converters.

Fine-Tuning and Performance Optimization

1. Duty Cycle Adjustment: The duty cycle can be adjusted via the potentiometer to regulate the output voltage between 5V to 25V.
2. Frequency Tuning: The 25 kHz switching frequency is a good balance between efficiency and component stress.

For detailed insights into designing RC snubber circuits for further optimization, visit this RC snubber circuit calculator.

Conclusion

This 555 timer-based boost converter provides a cost-effective and reliable solution for boosting a 5V input to up to 25V output. Its modular design allows for easy adjustments in duty cycle, frequency, and output voltage, making it suitable for various applications.

For additional information on designing effective output filters, check out this output LC filter calculator for switching circuits.