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⚡ A prototype 60W isolated flyback SMPS PCB design created in KiCad, converting 220V AC mains input to a regulated 12V DC 5A output using TOP227YN, TL431 voltage regulation, PC817 optocoupler feedback isolation, and an ER34 flyback transformer topology.
This project focuses on offline switch mode power supply design, isolated flyback converter architecture, 4-layer PCB layout, high-voltage routing, high-current output routing, creepage/clearance planning, and KiCad-based hardware documentation.
⚠️ Important:
This is a prototype and educational PCB design project.
It is not a certified commercial power supply.
⚡ Prototype 60W isolated flyback SMPS PCB design in KiCad, converting 220VAC to regulated 12VDC 5A using TOP227YN, TL431, PC817 feedback isolation, and ER34 transformer topology.
| Parameter | Value |
|---|---|
| Input Voltage | 220V AC |
| Rectified DC Bus | Approximately 310V DC |
| Output Voltage | 12V DC |
| Output Current | 5A |
| Output Power | 60W |
| Topology | Isolated Flyback |
| Switching IC | TOP227YN |
| Feedback Method | TL431 + PC817 Optocoupler |
| Transformer Core | ER34 / 17 / 11 |
| PCB Design Software | KiCad |
| PCB Layers | 4 Layer PCB |
| Project Status | Prototype / Educational Design |
The SMPS is divided into the following functional sections:
AC Input
↓
Fuse Protection
↓
NTC Inrush Limiter
↓
EMI Filter
↓
Bridge Rectifier
↓
High Voltage DC Bus
↓
TOP227YN Flyback Switching Stage
↓
ER34 Flyback Transformer
↓
Schottky Rectification
↓
LC Output Filter
↓
12V DC Output
↓
TL431 + PC817 Feedback Regulation# ⚡ 12V 5A Isolated Flyback SMPS Power Supply
The AC input stage includes the mains input terminals, fuse protection, NTC thermistor, EMI common mode choke, and X2 safety capacitors. This section protects the power supply from overcurrent conditions, limits inrush current, and reduces conducted EMI.
Main components:
| Reference | Component |
|---|---|
| H1 / H2 | AC input terminals |
| F1 | 3A Fuse |
| TH1 | NTC thermistor |
| L1 | Common mode choke |
| C1 / C2 | 0.1µF X2 capacitors |
The bridge rectifier converts the 220V AC input into high-voltage DC.
220V AC × 1.414 ≈ 310V DC
Main component:
| Reference | Component |
|---|---|
| D1 | KBL408 Bridge Rectifier |
The high-voltage DC bus is filtered using a large electrolytic capacitor.
| Reference | Value | Function |
|---|---|---|
| C3 | 150µF / 400V | Primary high-voltage DC filtering |
This capacitor stores energy for the switching stage. Since the rectified voltage is around 310V DC, a 400V-rated capacitor is required.
The main switching section uses the TOP227YN offline flyback switch IC.
| Reference | Component |
|---|---|
| U1 | TOP227YN |
The switching stage drives the primary winding of the flyback transformer.
Critical primary loop:
C3+ → Transformer Primary → TOP227YN → C3-
This loop must be short and compact to reduce EMI, switching noise, voltage spikes, and power loss.
The snubber circuit suppresses voltage spikes caused by transformer leakage inductance.
Main components:
| Reference | Component |
|---|---|
| D3 | UF5207 / FR207 fast recovery diode |
| R5 | 47Ω / 3W resistor |
| C5 | 4.7nF high-voltage capacitor |
Snubber loop:
Transformer Primary → Fast Recovery Diode → RC Snubber Network → Primary Bus
The snubber parts should be placed close to the transformer primary and TOP227YN switching node.
The transformer provides galvanic isolation and energy transfer between the primary and secondary sides.
| Reference | Component |
|---|---|
| T1 / TR1 | ER34/17/11 Flyback Transformer |
Transformer details:
Core Type: ER34 / 17 / 11
Topology: Isolated Flyback
Primary Winding: High-voltage switching side
Secondary Winding: 12V output side
Auxiliary Winding: Controller / feedback support
Example winding reference:
Primary winding: Pins 1 - 4
Secondary winding: Pins 2 - 6
Auxiliary winding: Pins 3 - 7
Always verify transformer winding direction, polarity dots, inductance, air gap, insulation, and pin mapping before real power testing.
The transformer secondary output is rectified using a high-current Schottky diode.
| Reference | Component |
|---|---|
| D5 / VD4 | MBR20100 Dual Schottky Diode |
Critical secondary loop:
Transformer Secondary → Schottky Rectifier → C6 → Transformer Secondary
This loop carries high-frequency pulsed current, so it should be short and wide.
The output filter smooths the rectified secondary waveform into stable 12V DC.
| Reference | Value | Function |
|---|---|---|
| C6 | 1000µF / 25V | First secondary output capacitor |
| L2 | 3.3µH | Output filter inductor |
| C7 | 1000µF / 25V | Final output capacitor |
Output filter path:
Schottky Rectifier → C6 → L2 → C7 → 12V DC Output
For 5A output current, this path should use wide copper zones or very wide traces.
The output voltage is regulated using a TL431 reference and PC817 optocoupler.
| Reference | Component |
|---|---|
| U2 | PC817 Optocoupler |
| U3 | TL431 Programmable Reference |
Feedback operation:
12V Output → Resistor Divider → TL431 → PC817 LED → TOP227YN Control Pin
The PC817 transfers the feedback signal from the secondary side to the primary side while maintaining galvanic isolation.
| Part | Description |
|---|---|
| TOP227YN | Integrated offline flyback switch IC used as the main primary-side switching controller |
| PC817 | Optocoupler used to transfer feedback signal from secondary side to primary side while maintaining isolation |
| TL431 | Programmable shunt regulator used for accurate 12V feedback regulation |
| KBL408 | Bridge rectifier used to convert 220V AC input into high-voltage DC |
| MBR20100 | Dual Schottky diode used for high-current secondary rectification |
| UF5407 / FR207 / UF5207 | Fast recovery diodes used in snubber and auxiliary sections |
| Ref | Value | Voltage Rating | Purpose |
|---|---|---|---|
| C1 | 0.1µF | X2 safety rated | AC input EMI filtering |
| C2 | 0.1µF | X2 safety rated | AC input EMI filtering |
| C3 | 150µF | 400V | Primary high-voltage DC bus filtering |
| C4 | 47µF | 63V | TOP227YN control / auxiliary filtering |
| C5 | 4.7nF | High voltage | Primary snubber capacitor |
| C6 | 1000µF | 25V | First secondary output capacitor |
| C7 | 1000µF | 25V | Final secondary output capacitor |
| C8 | 0.1µF | 50V or higher | Auxiliary / filter capacitor |
| C9 | 100nF | 50V or higher | TL431 feedback compensation capacitor |
For C1 and C2, use real X2 safety-rated capacitors, not normal ceramic or random film capacitors.
| Ref | Value | Function |
|---|---|---|
| R2 | 6.8Ω | TOP227YN control/source network |
| R3 | 330kΩ | Input bleed / discharge resistor |
| R4 | 680kΩ | Startup / bleed resistor |
| R5 | 47Ω / 3W | Primary snubber resistor |
| R6 | 22kΩ | Optocoupler feedback resistor |
| R7 | 560Ω | TL431 feedback network |
| R8 | 3kΩ | TL431 divider resistor |
| R9 | 12kΩ | TL431 divider resistor |
| R10 | 330Ω | LED / feedback resistor |
| R11 | 3.3kΩ | Output sensing / load resistor |
| R12 | 4.7kΩ | LED current limiting resistor |
| Ref | Component | Function |
|---|---|---|
| L1 | Common Mode Choke | Reduces conducted EMI from the AC input side |
| L2 | 3.3µH High-Current Inductor | Filters the 12V DC output |
| T1 | ER34/17/11 Flyback Transformer | Provides energy transfer and galvanic isolation |
Transformer reference:
Core Type: ER34 / 17 / 11
Topology: Isolated Flyback
Primary Winding: High-voltage switching side
Secondary Winding: 12V output side
Auxiliary Winding: Controller / feedback support
| Ref | Name | Description |
|---|---|---|
| H1 | IN AC + | AC input terminal marking |
| H2 | IN AC - | AC input terminal marking |
| H3 | OUT DC + | 12V positive output terminal marking |
| H4 | OUT DC - | 12V negative output terminal marking |
For real hardware, use properly rated connectors:
AC Input Connector: 250VAC rated terminal block
DC Output Connector: 5A or higher rated terminal block
The PCB is arranged in a clear left-to-right power flow:
Left Side : AC input and primary high-voltage section
Center : Transformer and isolation boundary
Right Side : Isolated 12V DC output section
Placement strategy:
AC Input → Fuse → EMI Filter → Bridge Rectifier → Bulk Capacitor
→ TOP227YN → Transformer → Schottky Diode → LC Filter → 12V Output
The PCB includes silkscreen labels for AC input, DC output, KiCad branding, open hardware marking, and project identification.
This design separates the PCB into two electrical regions:
Primary Side:
220V AC input, bridge rectifier, high-voltage DC bus, TOP227YN, transformer primary
Secondary Side:
Schottky rectifier, output capacitors, TL431, LED, 12V output
The grounds are separate:
Primary Ground: PGND / HV_GND
Secondary Ground: SGND / OUT_GND
Important isolation rules:
| Category | Parts |
|---|---|
| Protection | Fuse, NTC thermistor |
| EMI Filter | Common mode choke, X2 capacitors |
| Rectification | KBL408 bridge rectifier |
| Primary Storage | 150µF 400V capacitor |
| Switching | TOP227YN |
| Snubber | Fast diode, 47Ω resistor, 4.7nF HV capacitor |
| Isolation | ER34 transformer, PC817 optocoupler |
| Secondary Rectification | MBR20100 Schottky diode |
| Output Filtering | 1000µF capacitors, 3.3µH inductor |
| Feedback | TL431, PC817, resistor divider, compensation capacitor |
| Indicator | LED and current-limiting resistor |
| Mechanical | Mounting holes and PCB terminals |
⚠️ Danger: This project operates directly from 220V AC mains voltage.
This PCB contains lethal voltage levels. Incorrect design, assembly, routing, or testing can cause:
Safety requirements before testing:
This design is provided for educational and prototype purposes only.
This project helps understand:
This project is an educational hardware design and PCB layout exercise. It should not be used in commercial, industrial, or safety-critical applications without proper electrical safety testing, regulatory compliance, and professional validation.
The author is not responsible for damage, injury, or failure caused by incorrect construction, testing, or use of this design.
Designed by Fernando S.M.H.G.
Created using KiCad.
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