S-360-12 (12V 30A) Cheap China PSU that works Differently! Schematic, Design Details

Summary

In this detailed electronics teardown and reverse-engineering video, the creator examines a low-cost 12V 30A switching power supply and uncovers an unexpected self-oscillating startup design commonly used in older power supplies from the late 1980s and early 2000s. He compares the circuit to newer designs that use dedicated flyback converters, explaining the unusual transformer-driven startup mechanism, the use of a TL494 controller, and the safety concerns tied to the design. Through schematic analysis, live voltage measurements, and comparisons with vintage ATX power supplies, the video provides valuable insights into repair techniques, circuit behavior, and the evolution of switch-mode power supply engineering.

Takeaways

• 🔌 The video analyzes a low-cost S-360-12 12V 30A switching power supply and compares it with a previously reverse-engineered model.
• ⚡ The power supply uses a half-bridge topology without active PFC and is controlled by a TL494 PWM controller.
• 🛠️ The creator always visually inspects and sockets controller ICs before powering up unknown power supplies for safety and easier troubleshooting.
• 💡 The power supply surprisingly self-oscillates and partially works even when the TL494 controller chip is removed.
• 📚 The design uses an old self-oscillating startup method common in late 1980s and early 2000s power supplies.
• 🔄 Instead of a dedicated auxiliary flyback converter, the power supply derives startup power through a special driver transformer arrangement.
• 🧲 The driver transformer is critical because the full primary current passes through it, requiring strong insulation and robust construction.
• ⚠️ The creator criticizes the design because a failure in the driver transformer isolation could expose dangerous mains voltage on the secondary side.
• 🔍 A full reverse engineering and schematic of the power supply were completed to better understand and document the design.
• 📈 The self-oscillating circuit generates around 7.5V auxiliary voltage, enough to start the TL494 controller.
• 🔋 Once the TL494 is installed and operating normally, the auxiliary voltage rises to around 13V and the output stabilizes at 12V.
• 🏭 Similar self-oscillating designs were commonly found in old ATX computer power supplies from the 1990s.
• 🧠 The creator understands the design well enough for repair purposes but considers it outdated and unnecessarily complicated compared to modern flyback startup circuits.
• 💸 The video argues that using a modern low-cost flyback converter would be simpler, safer, and not significantly more expensive.
• 📖 The creator plans to make a future repair guide and PDF manual explaining step-by-step troubleshooting and repair methods for these power supplies.
• 🙏 Viewers are encouraged to subscribe because reverse engineering and documenting these circuits requires many hours of detailed work.
• 🌡️ The video closes with casual discussion about changing weather conditions and appreciation for viewers watching the channel.

Q & A

What is the model of the power supply analyzed in the video?

The power supply analyzed is the S360-12, which is a 12V, 30A unit.

What prompted the creator to study this specific power supply?

The creator purchased it cheaply from a local auction to examine its internal construction and to obtain a 12V, 30A supply for use.

What initial observation caused surprise when powering the supply for the first time?

The creator noticed that there was no controller chip inside during startup, revealing a self-oscillating design instead of a conventional auxiliary-controlled circuit.

What type of switching configuration does the S360-12 use?

It uses a half-bridge configuration with high-voltage BJTs instead of MOSFETs.

How does the S360-12 generate auxiliary voltage for the TL494 controller?

It uses a self-oscillating design where one of the primary-side BJTs starts oscillation, inducing a voltage on the secondary auxiliary winding sufficient to power the TL494.

What are the advantages and disadvantages of the self-oscillating design compared to modern designs?

Advantages include eliminating an extra auxiliary IC, potentially reducing component count. Disadvantages include higher complexity, high current through the driver transformer, safety risks, and difficulty in repair.

What did measurements show about the output voltage during self-oscillation?

During self-oscillation without the TL494 controller, the output voltage was about 6V, and the auxiliary voltage was around 7.4V, enough to start the TL494.

How does the output voltage behave once the TL494 controller is active?

Once controlled by the TL494, the output voltage stabilizes at approximately 12V, with the auxiliary voltage increasing to around 13V.

What are the key components on the secondary side of the power supply?

The secondary side includes a full-wave rectification with snubber circuits, a current shunt, filter capacitors, and a small trimmer to adjust the output voltage slightly.

Why does the creator consider the self-oscillating design unnecessary?

Because a modern cheap flyback converter could provide the auxiliary voltage more safely, cheaply, and with simpler construction than the complex self-oscillating method used here.

What future resource does the creator plan to provide for viewers?

The creator plans to make a step-by-step manual and a PDF guide to help repair and understand these types of power supplies, including self-oscillating and newer flyback-based designs.

What precautions are recommended when inspecting or powering the S360-12?

It is recommended to perform an optical inspection first, use a current-limited isolation transformer when powering, and socket the controller IC to prevent accidental damage.