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Design of an Isolated Power Supply for Cell Phone Charging

Justin Alam, Saif Elsaady

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Arizona State University

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Abstract & Project Scope

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  • Objective: Design a 5V, 30W isolated power supply with universal AC input (85V–265V RMS).
  • Topology: Flyback converter for simplicity, cost, and performance.
  • Key Metrics: 75.65% efficiency, <50mV ripple, stable operation across input ranges.
  • Tools Used: Texas Instruments Power Stage Designer, MATLAB.

Key Requirements:

  • Input: Universal AC (85V–265V RMS, 60Hz).
  • Output: 5V DC, 30W with galvanic isolation.
  • Ripple Voltage: Less than 50 mV.

Applications:

  • Consumer electronics and portable chargers.

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Topology Selection

Flyback Converter

  • Why?
    • Efficiency for low power (<100W).
    • Galvanic isolation via transformer.
    • Compact and cost-effective.
  • Alternatives: Buck, Forward, Push-Pull, and Bridge converters (with limitations).

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Circuit Design

  1. AC to DC Conversion:
    • A bridge rectifier (four diodes) converts the universal AC input (85V–265V RMS) into pulsating DC.
    • An input capacitor (470 μF, 400V) smooths the rectified voltage and reduces ripple.
    • This provides a stable DC input for the primary side of the transformer.
  2. Transformer and Storage:
    • Custom transformer with a 40:1 turns ratio and 34,000 μH inductance.
    • Provides galvanic isolation between the input and output for user safety.
    • Stores energy in the magnetic core during the "on" phase of the MOSFET.
  3. Switching and Transfer:
    • MOSFET (650V, 10A, RDS(on)=37mΩ) controlled via pulse-width modulation (PWM).
    • A snubber circuit (100Ω resistor, 10nF capacitor) dampens voltage spikes caused by transformer leakage inductance.
  4. Output Rectification and Filtering:
    • A Schottky diode (200V, 6A) rectifies the transformer’s secondary voltage.
    • The output capacitor (1000 μF, 10V, low ESR) reduces ripple voltage to <50mV.
  5. Feedback Loop:
    • Monitors the output voltage and adjusts the PWM signal driving the MOSFET.
    • Ensures a steady 5V output, even under input or load variations.
    • Feedback is transmitted through an optocoupler for isolation.

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AC Input -> Diode Bridge -> IGBT Switch -> Transformer -> Diode -> RC -> Output

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Component Selection

Component

Specification

Part Number

Link

Transformer

Turns ratio: 40:1, Inductance: 34,000 μH.

Custom Designed

N/A

MOSFET

650V, 10A, RDS(on)=37 mΩ.

IPW60R037P7

Diode

Reverse voltage: 200V, Forward current: 6A.

GI752-E3/54

Output Capacitor

1,000 μF, 10V, Low ESR

EEU-FR1A102LB

Input Capacitor

470 μF, 400V

MAL219366471E3

Snubber Resistor

100 Ω, 2W

PR02000201000JR500

Snubber Capacitor

10 nF, 600V

716P10396JA1

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Simulation Setup

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Waveforms

Input Voltage AC

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Waveforms

After diode bridge… Conversion of AC -> DC

  • The smoothing capacitor reduces the ripple, creating a steadier DC signal with minor variations. This provides a stable input for the transformer in the next stage.

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Waveforms

Transformer Secondary Voltage

  • During the MOSFET "on" phase, the transformer stores energy, and during the "off" phase, it transfers this energy to the secondary winding, creating the alternating waveform.
  • The DC offset arises due to the transformer's operation in Continuous Conduction Mode (CCM), ensuring steady energy transfer and minimizing ripple.

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Waveforms

Final Output Voltage

The output waveform is a steady DC signal at 4.79V with a light ripple.

  • This regulated waveform ensures a consistent output suitable for powering sensitive electronic devices.

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Simulation Results

Waveform Analysis:

  • Input Voltage: Smooth transition from AC to pulsating DC.
  • Transformer Secondary Voltage: Accurate step-down based on turns ratio.
  • Final Output: Regulated almost 5V DC with minimal ripple.

Performance Metrics:

  • Efficiency: 75.65%.
  • Effective power conversion according to calculation!

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Loss Analysis and Efficiency

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  • Diode Losses (4.2W): Largest contributor due to forward voltage drop.
  • Copper Losses (3.61W): Significant heat from transformer winding resistance.
  • MOSFET Losses (1.63W): Moderate due to switching inefficiencies.
  • Core Losses (0.225W): Minimal, indicating efficient core material.
  • Total Losses (9.665W): Room for optimization to improve 75.65% efficiency.

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Challenges and Improvements

Challenges:

  • High switching losses.
  • Noise and EMI issues.
  • Tuning feedback control.

Proposed Improvements:

  1. Replace MOSFET with GaN transistors for lower losses and faster switching.
  2. Introduce soft-switching to minimize switching losses and improve efficiency.
  3. Optimize transformer design and implement EMI shielding for better performance.

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Conclusion

Summary:

  • Successfully designed and simulated a 5V, 30W isolated power supply.
  • Achieved galvanic isolation, regulated output, and stable operation.

Next Steps:

  • Build a hardware prototype to validate design in real-world conditions.
  • Investigate advanced topologies and methods to further enhance efficiency and reduce losses.

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Contributions

Saif: Circuit Design, Graphics, Calculations, Report, Presentation, Research, Tool usage, & Simulation.

Justin: Circuit Design, Calculations, Report, Presentation, Research, Further Improvements, & Simulation

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References

[1] Erickson, R. W., & Maksimovic, D. (2007). Fundamentals of power electronics. Springer.

[2] Infineon Technologies. (n.d.). IPW60R037P7 MOSFET datasheet. Retrieved from https://www.mouser.com

[3] KEMET. (n.d.). C330C103KDR5TA capacitor datasheet. Retrieved from https://www.mouser.com

[4] LTspice. (n.d.). LTspice user guide. Analog Devices. Retrieved from https://www.analog.com

[5] Mohan, N., Undeland, T. M., & Robbins, W. P. (2002). Power electronics: Converters, applications, and design. John Wiley & Sons.

[6] Nichicon Corporation. (n.d.). UPW1A102MPD capacitor datasheet. Retrieved from https://www.digikey.com

[7] Panasonic. (n.d.). EEUEB2G471 capacitor datasheet. Retrieved from https://www.mouser.com

[8] Singh, M. D., & Khanchandani, K. B. (2007). Power electronics. Tata McGraw-Hill Education.

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[9] Texas Instruments. (n.d.). Flyback converter design guide. Retrieved from https://www.ti.com

[10] Vishay Semiconductor Diodes Division. (n.d.). VS-20ETX06-M3 diode datasheet. Retrieved from https://www.digikey.com

[11] Wurth Elektronik. (n.d.). Transformer design for flyback converters. Retrieved from https://www.we-online.com

[12] Wu, X., & Chen, W. (2016). Resonant and soft-switching converters: Principles, designs, and applications. Wiley-IEEE Press.

[13] Yageo. (n.d.). RS-1002 resistor datasheet. Retrieved from https://www.digikey.com

[14] Digikey Electronics. (n.d.). Component selection and design resources. Retrieved from https://www.digikey.com

[15] Mouser Electronics. (n.d.). Power supply design resources. Retrieved from https://www.mouser.com

[16] MATLAB. (n.d.). Simulink control design. MathWorks. Retrieved from https://www.mathworks.com