Simplest SMPS Circuit with TNY267: Build Your Own 19V Power Supply

Hey, if you’re looking to build a compact switched-mode power supply that converts mains AC to a stable 19V DC, this schematic using the TNY267 IC is a great starting point. The image labels it as the “Simplest SMPS With TNY267,” featuring a minimal design with a custom transformer on an EE19 core, basic rectification, and feedback via an optocoupler. As an electronics engineer, I’ve pored over this circuit, and it’s a clever, low-cost way to get around 5-10W output—perfect for charging laptops, powering LEDs, or small devices. We’ll cover the analysis, components, how it operates, and a build guide. Just remember, working with mains voltage means safety first; if you’re new, double-check everything. Let’s dive in.

What Is an SMPS and Why Use TNY267?

A switched-mode power supply, or SMPS, converts AC to DC efficiently by switching a transistor at high frequency, unlike linear supplies that waste power as heat. This makes SMPS smaller, lighter, and more efficient (up to 80-90%), ideal for modern gadgets. The downside? They can generate noise (EMI), so good design is key.

The TNY267 from Power Integrations’ TinySwitch-II family simplifies things. It integrates a 700V MOSFET, oscillator, current limit, and protection into one chip, reducing parts count. Operating at 132 kHz with jitter for EMI reduction, it supports universal input (85-265V AC) and outputs up to 13W in adapters. Why this IC? It’s cheap (under $2), reliable, and handles faults like shorts automatically. Similar designs appear in phone chargers or LED drivers, but this one’s stripped down for hobbyists.

Analyzing the Circuit Schematic

The schematic is basic but effective for a flyback topology SMPS. Input is 100-240V AC at 50-60Hz, output 19V DC. Key elements:

• Input Rectification and Filtering: AC enters through two BA159 fast-recovery diodes in series for half-wave rectification, handling up to 800V each. A 10µF 400V electrolytic capacitor smooths the DC, creating a high-voltage bus (around 140-340V DC). A 10Ω 1W resistor limits inrush current, and a 1N4007 diode might be for reverse protection or clamping.
• TNY267 IC: The heart, in a DIP-8 package. Pins: Likely pin 8 (DRAIN) connects to the transformer primary, pin 1 (BYPASS) to a 2.2nF 25V capacitor for internal supply, pin 2 (EN/UV) for feedback and undervoltage sense, pins 3-7 (SOURCE) to ground. The IC switches the primary at 132 kHz, storing energy in the transformer during on-time and releasing it to the secondary during off-time.
• Transformer: Custom on EE19 ferrite core with 0.4mm air gap to prevent saturation. Primary: 140 turns of 0.15mm wire, secondary: 27 turns of 0.4mm wire. This ratio (140:27 ≈ 5.2:1) steps down the voltage, considering the flyback’s duty cycle. No bias winding needed, thanks to TNY267’s internal current source.
• Output Stage: Secondary connects to a BA159 diode for rectification, then a 220µF 35V capacitor for smoothing. Output is 19V, with a 100Ω resistor, possibly for discharge or sensing.
• Feedback Loop: A PC817 optocoupler provides isolation. From output, a 470Ω resistor drives the LED side, with a Zener diode (18V) for reference—ensuring regulation by pulling EN/UV low when output exceeds setpoint. A 100R resistor and 2.2nF cap filter the feedback.

This flyback design is isolated, safe for touchable outputs. Power around 5-10W, based on core size. Potential issues: Half-wave rectification causes ripple; full-bridge would be better for efficiency.

Components You’ll Need

Gather these for the build. Most are available online or at electronics stores.

• IC: TNY267PN (DIP-8)
• Diodes: 3 x BA159 (fast recovery, 800V 1A), 1 x 1N4007 (general, 1000V 1A), 1 x 18V Zener (1W)
• Capacitors: 10µF 400V (electrolytic input), 2.2nF 25V (bypass, ceramic or film), 220µF 35V (output electrolytic)
• Resistors: 10Ω 1W (inrush), 100Ω (0.25W), 470Ω (0.25W feedback)
• Transformer: EE19 core, ferrite, 0.4mm gap; wind primary 140 turns 0.15mm enameled wire, secondary 27 turns 0.4mm
• Optocoupler: PC817
• Other: PCB or protoboard, fuse (0.5A slow-blow on input), heatsink if needed (TNY267 has thermal protection), output connector

Cost: $5-15, plus transformer winding tools. Use varnish on the transformer for insulation.

How the Circuit Works: A Quick Explainer

This is a flyback converter. AC is rectified to HV DC, filtered by the 10µF cap. The TNY267’s internal oscillator runs at 132 kHz, switching the MOSFET on/off.

• On Cycle: MOSFET conducts, current ramps in primary, storing energy in the core’s magnetic field. No secondary current due to the diode reverse bias.
• Off Cycle: MOSFET off, field collapses, inducing voltage in secondary. Diode conducts, charging output cap to 19V.

Regulation: Optocoupler senses output via Zener. If output >19V, the LED lights, the transistor pulls EN/UV low, skipping cycles to reduce power. Undervoltage on EN/UV (via resistor from HV) prevents startup glitches.

Auto-restart handles faults: If shorted, limits duty to 5.6%, protecting components. Thermal shutdown at 135°C adds safety.

Efficiency: Around 70-80%, low no-load power <250mW. Jitter reduces EMI peaks by 10dB.

Step-by-Step Building Guide

Build on a PCB for safety—mains traces need spacing (6mm creepage).

1. Wind Transformer: Gap EE19 core 0.4mm (use tape). Wind primary 140 turns 0.15mm wire evenly, insulate, then secondary 27 turns 0.4mm in the opposite direction for flyback polarity.
2. Input Section: Solder BA159 diodes in series for rectification, parallel 10µF cap. Add a 10Ω resistor and a 1N4007 as shown.
3. IC and Primary: Mount TNY267. Connect DRAIN (pin 8) to the primary end, other primary to HV+. SOURCE pins to ground. BYPASS (pin 1) to 2.2nF cap to ground.
4. Output: Secondary to BA159 anode, cathode to +19V with 220µF cap. Ground secondary.
5. Feedback: Output + to 470Ω to PC817 LED anode, cathode to Zener cathode, Zener anode to ground. PC817 transistor: collector to EN/UV (pin 2) via 100R? , emitter to ground.
6. Test: Use a variac to ramp the voltage. Check no-load output ~19V, add load (e.g., 20Ω resistor for 1A). Measure ripple <100mV with scope.

Isolate primary/secondary during test.

Safety Considerations

Mains voltage kills—use a fused input, insulated tools. Ensure isolation: Optocoupler provides it, but test with a megger. Transformer insulation prevents arcs. TNY267’s protections help, but add MOV for surges. For production, meet UL/IEC standards.

Performance and Optimization

At 19V, expect 5-10W clean output. Regulation ±5% with load changes. To optimize: Add an input filter (pi network) for EMI, a larger output cap for less ripple. For higher power, use a bigger core or TNY268.

Compared to linear supplies, this is compact but noisier—use for non-sensitive loads.

Troubleshooting Common Problems

No output? Check transformer polarity (reverse secondary if needed). IC hot? Faulty clamp—add RCD snubber (100Ω, 0.1µF, diode across primary).

Oscillating? Feedback loop issue—adjust Zener or add a cap across the opto.

Low voltage? Too many skipped cycles—check load or EN/UV resistor.

Use datasheet for limits: Max drain voltage 700V, but clamp to <600V.

Conclusion

This simplest SMPS with TNY267 is a rewarding build for efficient power conversion. With its integrated features and minimal parts, you’ll get a reliable 19V supply for various projects. Experiment safely, and you might customize it for 12V or 5V. If you add features like better filtering, it rivals commercial units. Let me know how yours turns out!