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12V 5A SMPS: Switched Mode Power Supply Circuit Diagram IC TOP227YN

Abhishek Singh

In this article, you will learn how to design a 12V 5A SMPS circuit that converts 230VAC to regulated 12V DC using the TOP227YN in a flyback configuration.

Switched Mode Power Supplies (SMPS) are widely used for high-efficiency AC-to-DC conversion. The flyback topology is especially suitable for low- to medium-power isolated supplies. The TOP227YN is a highly integrated offline switcher IC with an internal high-voltage MOSFET and protection features, making it ideal for compact 60W designs.

We will also cover transformer design, turns ratio calculations, and practical implementation details.

SMPS Specifications

Before starting the design, let’s summarize the specifications of this 12V power supply:

This design targets moderate power applications such as LED systems, embedded electronics, and communication devices.

Overview of TOP227YN Controller

The core of the circuit is TOP227YN, an integrated off-line PWM switcher from the TOPSwitch-II family. Key features include:

It simplifies the flyback converter design by integrating the control circuitry and power switch inside one TO-220 package.

Related Articles:

What is Flyback Topology?

IC TOP227YN 12V 5A SMPS Circuit Diagram

The circuit can be divided into five major sections:

12V 5A SMPS Circuit

Input Section (AC 220V to DC Bus)

Components Involved

Working

VDC ≈ 220 × 1.414 ≈ 310V DC

This 310V DC becomes the input for the flyback converter.

Primary Switching Stage

Components

Working Explanation

This circuit operates in discontinuous or boundary conduction flyback mode under typical load conditions. TOP227YN operates in current-mode flyback topology. Let’s See Switching Cycle Operation:

Stage 1 – MOSFET ON (Energy Storage Mode)

When the internal MOSFET of TOP227YN turns ON:

Primary current slope:

di/dt = Vin / Lp

Where: Vin ≈ 310V, Lp = Primary inductance

Stage 2 – MOSFET OFF (Energy Transfer Mode)

When MOSFET turns OFF:

Peak drain voltage:

VDS(max) = Vin + (Np/Ns)(Vout + Vd) + Vspike

Snubber Network

This RCD snubber absorbs voltage spikes caused by transformer leakage inductance and protects the internal MOSFET.

Flyback Transformer (TR1) Design Calculations

The transformer is the most critical component in a flyback SMPS. For this
12V / 5A (60W) power supply using the TOP227YN and ER34/17/11 ferrite core,
the transformer must be designed considering low-line operation, maximum
duty cycle, and safe flux density.

Design specifications:

DIY Simple Switching Power Supply – Easy SMPS Circuit Build JLCPCB

Circuit Design PCB Design

Turns Ratio Calculation (Flyback Topology)

For a flyback converter, the turns ratio is determined using reflected voltage
rather than the simple Vin/Vout ratio used in forward converters.

We choose a reflected voltage of approximately 100V for good efficiency and
safe MOSFET stress.

Reflected voltage formula:

Np/Ns = Vreflected / (Vout + Vd)

Where: Vout = 12V, Vd ≈ 0.5V (Schottky diode drop)

Np/Ns = 100 / 12.5 = 8

Therefore, the primary-to-secondary turns ratio is approximately:

Np:Ns = 8:1

Primary Turns Calculation

Primary turns are calculated using the volt-second balance equation:

Np = (Vin(min) × Dmax) / (Bmax × Ae × f)

Where: Vin(min) = 255V, Dmax = 0.45, Bmax = 0.20T, Ae = 125 × 10-6 m², f = 100kHz

Substituting:

Np = (255 × 0.45) / (0.20 × 125 × 10-6 × 100000)
Np ≈ 46 turns

For design margin and ease of winding, we round up:

Primary Turns (Np) = 48 turns

Secondary Turns Calculation

Using the turns ratio of 8:1:

Ns = Np / 8
Ns = 48 / 8 = 6
Secondary Turns (Ns) = 6 turns

Auxiliary Winding Calculation

The auxiliary winding provides Vcc (~15V) for the TOP227YN.

Naux = (Vaux / (Vout + Vd)) × Ns

Where: Vaux ≈ 15V, Vout + Vd = 12.5V

Naux = (15.7 / 12.5) × 6
Naux ≈ 7.5
Auxiliary Turns = 7 turns

Final Transformer Winding Data

Voltage Stress Verification

Reflected voltage:

Vreflected = 12.5 × 8 = 100V

Maximum MOSFET voltage stress at high line (325V DC):

VDS(max) = 325 + 100 = 425V

This is well within the 700V rating of the TOP227YN internal MOSFET,
ensuring safe and efficient operation.

This transformer configuration provides:

The above calculations ensure the ER34/17/11 core is optimally designed
for a stable 12V, 5A flyback SMPS.

Secondary Rectification, Output Filter and Auxiliary Supply

Components

Operation

Auxiliary winding supplies power to the control circuitry after startup, improving efficiency by eliminating continuous startup resistor dissipation.

Feedback and Voltage Regulation

Components

Working

Output Indicator

Feedback Loop Compensation Analysis

Control architecture:

TL431 Reference

Internal reference = 2.5V

Vout = 2.5 × (1 + R9/R8)

Given: R8 = 3kΩ, R9 = 12kΩ

Vout = 12.5V

Compensation Network

R10 + C9 form Type-II compensation to:

fp = 1/(2π Rload Cout)
For Rload = 2.4Ω and Cout ≈ 2000µF: fp ≈ 33Hz

PCB Layout Guidelines for SMPS

Efficiency Optimization Tips

Thermal Management

Although 60W is moderate power, heat dissipation must be considered.

Efficiency typically ranges between 80–85% for a properly designed flyback.

Protection Features

This design includes:

Simulation and Testing

Advantages of Flyback Design

Disadvantages of Flyback SMPS

Applications of 12V 5A SMPS Circuit

A 12V 5A SMPS (60W) is widely used in:

Its compact size and high efficiency make it suitable for continuous operation devices.

Conclusion

The 12V 5A SMPS using TOP227YN is a robust and efficient flyback converter suitable for medium power applications around 60W. The integration of high-voltage MOSFET and PWM controller inside TOP227YN significantly reduces component count and design complexity.

By carefully selecting the transformer turns ratio, core material, snubber network, and feedback components, you can build a reliable and robust 12V 5A isolated power supply.

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