Complete Tny268pn Switching Power Supply Circuit Diagram and Analysis

tny268pn circuit diagram

Start with a 25W flyback transformer rated at 13V output to match the load requirements. Ensure the primary side includes a clamp circuit with a 1N4007 diode and a 22Ω resistor in series to dissipate excess energy during switching transitions, preventing voltage spikes from damaging the controller.

Select a 47μF/25V electrolytic capacitor for input smoothing, placed within 10mm of the primary winding to minimize high-frequency noise. For output regulation, pair a 4.7μH inductor with a 1000μF/16V low-ESR capacitor–this combination reduces ripple to under 50mV at full load (1.9A). Avoid cheaper capacitors with high ESR, as they increase power loss by 15-20% under transient conditions.

Use a TL431 shunt regulator coupled with a 10kΩ potentiometer for precise voltage trimming. The feedback path must include a 2kΩ resistor in series with a 10nF ceramic capacitor to stabilize the loop response and suppress oscillations above 100kHz. Failure to optimize this network can lead to audible noise or thermal runaway in the secondary rectifier.

Grounding demands a star topology: route all high-current returns (input, output, and controller grounds) to a single point near the transformer’s core. Separate signal grounds from power grounds using 0Ω resistors or ferrite beads to prevent noise coupling into sensitive components like the feedback divider. For layouts, keep trace lengths under 50mm for switching nodes to limit parasitic inductance–longer traces introduce voltage overshoot of up to 30%.

Test the design under minimum input voltage (85VAC) with a 1.5x nominal load to verify efficiency drops remain below 5%. Monitor the primary MOSFET’s drain voltage with an oscilloscope: spike amplitudes should not exceed 650V for a 400V-rated device. If spikes exceed this limit, reduce the clamp resistor value or add a 100pF snubber capacitor across the MOSFET’s drain-source to absorb transients.

Building a Flyback Converter: Step-by-Step PCB Assembly Guide

Select a 47μF input capacitor with a voltage rating at least 1.5× the maximum input voltage to prevent ripple-induced stress on the primary components. For a 24V input, a 50V-rated capacitor ensures reliable operation without premature failure under transient loads.

Position the switching regulator’s drain pin trace with a minimum width of 3mm to handle peak currents up to 2A. Use 2oz copper thickness on the PCB to reduce thermal resistance, keeping the junction temperature below 110°C during continuous operation. Ensure vias near the thermal pad are plated-through with a diameter of 0.5mm to improve heat dissipation.

Critical placement rules for auxiliary components:

  • Keep the feedback resistor divider (200kΩ and 10kΩ) within 5mm of the control IC’s feedback pin to minimize noise pickup.
  • Locate the output diode (1N5822) no more than 10mm from the transformer’s secondary winding to reduce parasitic inductance.
  • Use a 1μF ceramic capacitor for the bypass pin, positioned directly on the IC’s underside to filter high-frequency transients.

Wind the flyback transformer with a 1:0.1 primary-to-secondary turns ratio for a 5V output. Use 0.4mm enameled wire for the primary and 0.3mm for the secondary to handle the 60kHz switching frequency without skin-effect losses. Apply a 2mm air gap in the core to prevent saturation at nominal load currents (up to 1.2A). Verify the transformer’s leakage inductance stays below 5μH; higher values require a snubber network (100Ω resistor + 1nF capacitor in series) across the primary winding.

Test the assembled board with a 12Ω load resistor before connecting sensitive devices. Measure the output ripple with an oscilloscope; it should not exceed 50mVpp under full load. If ripple exceeds this, increase the output capacitor to 100μF (low-ESR type) and verify the feedback loop stability by checking for overshoot during load transients (0.5A to 1A step change). Adjust the compensation network (47kΩ resistor + 220pF capacitor in parallel) if ringing persists beyond 10% of the steady-state output voltage.

Critical Elements and Pin Layout of the TNY-Series Flyback Regulator

Connect the drain (pin 5) directly to the primary winding of the high-frequency transformer using a trace width capable of handling at least 3A peak current to prevent voltage drops during switching transitions. Use a 2oz copper pour for the drain node to dissipate heat efficiently–thermal vias should be placed under the IC’s exposed pad if present, spaced no more than 1.2mm apart with a minimum diameter of 0.3mm.

The bypass (pin 2) requires a 0.1µF ceramic capacitor mounted within 2mm of the IC; X7R dielectric is mandatory for stable Self-Bias Undervoltage (SBUV) protection. Skip electrolytic capacitors here–their ESR introduces noise that disrupts the internal oscillator’s precision. For input voltages above 100Vdc, parallel a 1nF capacitor to reduce switching noise coupling into the control circuitry.

Enable (pin 1) accepts a logic-level signal (3.3V or 5V) but must be driven through a 1kΩ resistor to limit current–direct GPIO connection risks latch-up. Floating this pin while the device is powered triggers unpredictable shutdown cycles; tie it high or low if unused, never leave it open. For remote on/off, a 10kΩ pull-down resistor ensures clean transitions.

Source (pin 4) must return to the ground plane with minimal inductance–avoid daisy-chaining ground returns from other components. A star-ground topology is non-negotiable; route source traces as short, wide paths directly to the input capacitor’s negative terminal. The internal MOSFET’s 700V rating demands this separation to prevent ground bounce from triggering false overcurrent events.

The feedback (pin 3) network consists of an optocoupler’s collector and a 24kΩ precision resistor to ground for 5V outputs. For 12V outputs, reduce the resistor to 12kΩ–errors here shift the regulation point by ±10%. Use a 1% tolerance resistor and pair it with a 2.2µF stabilization capacitor across the optocoupler’s LED to dampen overshoot during load transients. Omit this capacitor, and the loop will oscillate at 200kHz.

Undervoltage lockout (pin 6) is optional but critical for noisy environments; a resistive divider from Vin (typically 2MΩ + 1MΩ) sets the threshold at 200V–adjust ratios to match your minimum input voltage. Skip this, and brownout conditions will cause erratic startups. For transient immunity, add a 100nF capacitor from this pin to source, forming a 1ms delay.

Output diode selection dictates efficiency: for 65kHz operation, use a 200V ultrafast diode (e.g., STTH1L06); at 132kHz, switch to a 150V part with

Step-by-Step Assembly for a 12V Flyback Power Supply

Begin by mounting the isolated switching regulator IC on a heat-resistant PCB with a minimum copper pad area of 20mm² per pin to prevent thermal throttling at loads exceeding 5W. Verify the input voltage range (90–265VAC) matches the application before proceeding.

  • Solder the primary-side fuse rated for 1.5× the maximum expected current (e.g., 1A for a 15W design) directly to the AC input pads.
  • Connect a 0.1µF X2-class capacitor in parallel with the rectifier bridge to suppress high-frequency noise above 1MHz.
  • Use a 10Ω, 1W resistor between the bridge output and the bulk storage capacitor to limit inrush current to 30A peak.

Select a high-voltage MOSFET with a breakdown voltage of at least 700V and a drain current rating 2× the calculated peak (e.g., 2A for a 12W output). Gate drive traces must be kept under 15mm in length to minimize parasitic inductance.

  1. Wind the transformer primary with 40 turns of AWG 28 enameled wire for a 12V/1A output, ensuring a leakage inductance below 15µH measured at 100kHz.
  2. Add a secondary winding of 8 turns using AWG 24 bifilar wire for improved coupling, with a center-tap for post-regulation feedback.
  3. Incorporate a 0.47µF snubber capacitor across the MOSFET drain-source to clamp flyback voltage spikes to 600V maximum.

For output regulation, use a TL431 shunt regulator with a feedback divider set to 2.5V reference. Place the divider resistors (e.g., 10kΩ and 2.2kΩ) within 5mm of the optocoupler’s LED to avoid noise pickup on the feedback loop.

Finalize the build by testing on a variac, incrementing input voltage in 10VAC steps while monitoring output voltage ripple (target <50mVpp) and efficiency (target >80% at full load). Isolate the primary and secondary grounds with a 1mm PCB clearance for reinforced insulation per IEC 60950.

Common Mistakes in Assembling Switching Regulator Layouts

Incorrect placement of the input capacitor is the most frequent error, leading to voltage spikes exceeding 15% of the nominal value. The bypass capacitor (typically 0.1μF–1μF X7R ceramic) must be positioned within 2mm of the controller’s VDD and EN/UV pins–any deviation increases ESR and degrades transient response. For 12V input designs, omitting a series resistor (10Ω–100Ω) on the BYPASS pin causes erratic startup, especially at temperatures below 0°C. Always verify capacitor voltage ratings: a 16V cap fails in a 12V system with >4V spikes, despite nominal calculations.

Component Error Impact Corrective Action
Feedback resistor divider Resistor values >100kΩ Output voltage drift >5% Use 20kΩ–50kΩ, place traces away from switching nodes
Drain clamp circuit Zener voltage >1.5× max input MOSFET failure in 50ms Select 1N4937 diode + 18V Zener for 12V input
Output inductor Inductance below datasheet minimum Core saturation at 80% load Use 47µH for 5V/0.5A output; verify saturation current >1.5× max load

Ensure the feedback path is routed on the same layer as the controller’s pin, avoiding vias–each via adds ~0.5nH inductance, distorting regulation. For multi-layer boards, stitch the ground plane with vias spaced 200mV.