Open-source a wide input voltage, 24V 3A output flyback switching power supply (Flyback Power Supply), 72 W power, maximum efficiency 87.4%, based on UC3842 chip, synchronous rectification (UCC24612‑1DB), includes circuit and transformer parameter calculation process, schematic, PCB, PSIM simulation model, transformer manufacturing specification, etc.
Introduction
This is my first flyback power supply. Please point out any improvements or mistakes, experts.
- Flyback power supply parameter calculation and component selection, flyback transformer calculation and winding tutorial: https://blog.zeruns.com/archives/909.html
- Magnetic component / core material / core structure analysis, comparison and selection guide: https://blog.zeruns.com/archives/897.html
Project video demonstration: https://www.bilibili.com/video/BV1ES4GzQE19/
LCSC open-source platform link: https://oshwhub.com/zeruns/24v3a-Flyback-Power-Supply-uc384x
Electronics / MCU technical exchange QQ group: 2169025065
Download links are at the end of the article!
Warning: Switch‑mode power supply construction is hazardous. I do not guarantee the correctness of any circuit, parameter, or formula in this design. Replicating or referencing this work is at your own risk.
Estimated small‑batch (20 sets) average unit price is about ¥25 (excluding PCB and transformer, based on component prices from the LCSC Mall). Including PCB and transformer, it should stay under ¥50.
Design Parameters
| Parameter | Value |
|---|---|
| Rated input voltage V_{acnom} | 220 VAC |
| Minimum input voltage V_{acmin} | 85 VAC |
| Maximum input voltage V_{acmax} | 265 VAC |
| Mains frequency f_L | 50 Hz |
| Output voltage V_{out} | 24 V |
| Output current I_{out} | 3 A |
| Switching frequency f_s | 150 kHz |
| Design efficiency η | 85 % |
PCB size: 100 × 55 mm
PCB specification: double‑layer, top‑side components, bottom‑side SMD
Physical Photos
The following is the second version.
The first version had some issues and tended to blow the tube; the second version fixes them.
High‑frequency transformer:
Operation Test and Performance Measurement
First Power‑On Test
When powering on for the first time, connect a lamp in series to avoid a short‑circuit explosion. The test worked normally, with an output voltage of 24.1 V (the test used a 0.9 A load).
The purpose of the series lamp: using the lamp’s current‑limiting protection. Under normal operation the lamp’s resistance is low, voltage drop is small, and it is barely lit or not lit, which does not affect the power‑supply test. If the power supply shorts internally, the circuit current spikes; the lamp’s fixed resistance then drops most of the voltage, limiting the current and protecting the supply components from damage.
Switch‑mode power‑supply protection socket (AFF): https://s.click.taobao.com/OiMyz3q
You can also test with a DC input. I tried 60 V DC and the supply started and output 24 V, but the 200 kΩ start‑up resistor (R24 + R16) had to be changed to 100 kΩ (short one of them). The original resistance was too high for low‑voltage start‑up.
Conversion Efficiency Test
Test equipment:
- Torch‑power meter (AFF): https://u.jd.com/0gXabte
- Torch electronic load (AFF): https://s.click.taobao.com/DvbzQ4q
Measured data:
| Input Voltage (V) | Input Current (A) | Input Apparent Power (W) | Input Real Power (W) | Output Voltage (V) | Output Current (A) | Output Power (W) | Conversion Efficiency (%) | Power Factor |
|---|---|---|---|---|---|---|---|---|
| 219.85 | 0.029 | 6.38 | 2.10 | 24.13 | 0.00 | — | — | 0.33 |
| 219.83 | 0.251 | 55.18 | 28.69 | 24.10 | 1.00 | 24.10 | 83.99 | 0.52 |
| 219.59 | 0.438 | 96.18 | 55.78 | 24.07 | 2.00 | 48.14 | 86.30 | 0.58 |
| 219.65 | 0.637 | 139.92 | 82.55 | 24.05 | 3.00 | 72.15 | 87.40 | 0.59 |
| 111.55 | 0.036 | 4.02 | 1.81 | 24.13 | 0.00 | — | — | 0.45 |
| 111.13 | 0.406 | 45.12 | 28.88 | 24.10 | 1.00 | 24.10 | 83.46 | 0.64 |
| 110.89 | 0.753 | 83.50 | 56.78 | 24.06 | 2.00 | 48.12 | 84.75 | 0.68 |
| 110.58 | 1.097 | 121.31 | 84.91 | 24.00 | 3.00 | 72.00 | 84.79 | 0.70 |
Highest measured conversion efficiency: 87.4 %. No‑load power consumption is as low as 1.81 W, a bit higher than ideal.
The data above were obtained with diode rectification, not synchronous rectification, because the MOSFET I used for synchronous rectification had a relatively high on‑resistance, which reduced efficiency. You can replace it with a better MOSFET (rated ≥200 V). After soldering a 20 Ω resistor and a 2.2 nF capacitor across R9 and C8, a 150 V rated part can be considered.
Output Voltage Ripple Test
Oscilloscope: Puyuan DHO914S: https://blog.zeruns.com/archives/764.html
The probe was clamped to a ~15 cm long output lead; no ground loop or direct connection to the output capacitor, so the measured ripple may be higher than actual.
No‑load ripple: peak‑to‑peak about 730 mV, ripple frequency 138.96 kHz, close to the switching frequency.
With 3 A load: peak‑to‑peak about 562.08 mV.
MOSFET Waveforms
With 220 VAC input and 24 V / 1 A output, the primary‑side switch MOSFET’s gate‑source (GS) and drain‑source (DS) voltage waveforms are shown. Yellow is gate‑source voltage, blue is drain‑source voltage.
The drain‑source voltage spike during turn‑off reaches up to ~440 V (the series lamp was still in circuit, and the supply input voltage was only a little over 100 V, so the measured spike is lower).
Zoomed gate voltage waveform:
Output Rectifier Diode Waveforms
With 60 V DC input, no‑load output (24 V) diode voltage waveform shows a peak spike of ~56 V. After adding a 20 Ω resistor and a 2.2 nF capacitor across R9 and C8, the spike drops to ~42 V.
With 60 V DC input and 1 A load, the diode spike reaches ~190 V; after the same RC network it drops to ~81 V.
No‑load Start‑up Output Voltage Waveform
With 60 V DC input, the output rises from 0 V to 24 V in about 7 ms.
Thermal Situation
No‑load thermal image of the power‑supply bottom: hottest spot is the start‑up resistor, about 60 °C (ambient ~25 °C); primary‑side MOSFET ~48 °C.
With 3 A load: hottest spot is the primary MOSFET or the resistor in the RCD absorption circuit, >88 °C (ambient ~26 °C); secondary rectifier diode likely >60 °C as well.
Full‑load temperature is a bit high; for continuous full‑load operation the primary switch should have a heatsink or be encapsulated to conduct heat to the chassis.
Component Purchase Links (AFF)
- SMD resistor & capacitor samples: https://s.click.taobao.com/ngH2RGq
- PQ transformer core & frame: https://s.click.taobao.com/EJtb04q
- UC3842 chip: https://s.click.taobao.com/7pfvQGq
- PC817A optocoupler: https://s.click.taobao.com/Le1X04q
- MSB40M bridge rectifier: https://s.click.taobao.com/7HqQQGq
- SBDD10200CT diode: https://s.click.taobao.com/izw104q
- UCC24612‑1 synchronous rectifier controller: https://s.click.taobao.com/K1gEQGq
- TL431 voltage reference: https://s.click.taobao.com/DPxnz3q
- UU10.5 common‑mode choke: https://s.click.taobao.com/D1t1z3q
- 5D‑11 thermistor: https://s.click.taobao.com/IsOry3q
- Safety capacitor X2: https://s.click.taobao.com/vBeCPGq
- Safety Y capacitor: https://s.click.taobao.com/sas5PGq
It is recommended to purchase components from the LCSC Mall: https://activity.szlcsc.com/invite/D03E5B9CEAAE70A4.html
In the LCSC open‑source link, the BOM table can be clicked to import all required parts directly into the shopping cart.
Schematic
PCB
Top layer
Bottom layer
Download Links
The following links contain: LCSC EDA project files, schematic PDF, PCB Gerber files, SMPS design tool SMPSKit, flyback transformer calculation workbook (Mathcad), silkscreen artwork, transformer manufacturing specification, datasheets for each chip, PSIM simulation models, BODE‑plot Matlab code, and other reference documents (some collected from the Internet).
- 123 Cloud Drive: https://www.123684.com/s/2Y9Djv-2hTdH
- Baidu Netdisk: https://pan.baidu.com/s/1767xJthTFWQbxgeZQXM_Hg?pwd=9527 Extraction code: 9527
If you find this useful, feel free to tip via the 123 Cloud Drive link, or if you read the WeChat article (public account: zeruns‑gzh) you can like the article and tip the author. Thank you.
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English Version of the Article: https://blog.zeruns.top/archives/74.html
