TNY275PN-TL - TinySwitch-III Offline AC-DC Converter, 11.5W, by Power Integrations

What Is the TNY275PN-TL and How Does It Work?

The TinySwitch-III takes the complexity of an AC-DC flyback converter and stuffs it into one chip. Inside: a 700V power MOSFET, an oscillator running at ~132 kHz with frequency jitter for EMI reduction, a cycle-by-cycle current limiter, a thermal shutdown circuit, and a simple ON/OFF control scheme. There's no error amplifier and no compensation network. Instead, the feedback comes through the EN/UV pin: when the output voltage is below regulation, the controller skips cycles. When it's above, the controller enables switching. A current on the EN/UV pin above ~25 μA starts the chip; below ~25 μA stops it. This hysteretic ON/OFF control means the loop is inherently stable - no phase margin to worry about, no compensation components to tune, no mysterious oscillations that only appear at 3am before a design review.

The flyback topology works like this: during the MOSFET on-time, current ramps up in the primary winding of the transformer, storing energy in the magnetic core. During the off-time, that energy transfers to the secondary winding and flows into the output capacitor through a rectifier diode. The transformer provides galvanic isolation between the AC mains side and your DC output. The 132 kHz switching frequency with ±8 kHz of jitter spreads the EMI spectrum, which reduces the size and cost of the input filter. It's not as effective as a proper spread-spectrum modulator, but for 11W it's more than enough to pass CISPR 22 Class B with a simple pi filter.

Startup is self-biased: the chip draws current from the DRAIN pin through an internal high-voltage current source to charge the BP/M capacitor. Once the BP/M pin reaches 5.85V, the controller starts switching. Once the output comes up, the transformer's bias winding takes over supplying the chip through the BP/M pin. The internal current source shuts off to save power. This self-biasing means you don't need a separate startup resistor or auxiliary winding - though an auxiliary winding improves efficiency by a few percent once running.

What Are the Specifications of TNY275PN-TL?

Numbers That Actually Matter in a Design

• The BP/M capacitor value sets your power budget. 0.1 μF = standard current limit (~560 mA peak drain). 1 μF = reduced limit (fewer watts but higher efficiency and lower EMI). 10 μF = increased limit (more peak power, useful for short overloads). This isn't a secret feature hidden in an app note - it's in the datasheet table on page 2. But an astonishing number of designs just copy the reference schematic with a 0.1 μF cap and never realize they could get more power or better efficiency by changing one component. Per the PI datasheet, TNY275 with 10 μF BP/M goes from 9.3W to 11.8W on universal input - a 27% bump with zero silicon change.
• The DIP-8 package dissipates heat through the source pins. Those four source pins (pins 5–8) aren't just for current - they're the primary heat path from the die to the PCB. θJA with 0.36 in² of copper is 70 °C/W; with 1 in² it drops to 60 °C/W. At 9.3W output with 80% efficiency, the chip dissipates about 2.3W - that's a 138°C rise at 60 °C/W, junction near 163°C. That's above the 150°C max and thermal shutdown at 142°C will trip. The headline power numbers assume you've got enough copper to keep Tj under 100°C. In a real design, use PI Expert's thermal estimates, not the datasheet headline.
• Frequency jitter helps EMI but doesn't eliminate the need for proper filtering. The ±8 kHz jitter at 132 kHz spreads the fundamental and harmonics, reducing peak EMI by 5–10 dB in conducted emissions testing. This typically saves one ferrite bead or one X-cap stage. But it's not a free pass - you still need a proper input filter, a snubber on the drain, and careful transformer design. E-Shield windings between primary and secondary reduce common-mode noise coupling. The datasheet has specific transformer construction recommendations - follow them.
• The EN/UV pin is more finicky than it looks. To program undervoltage lockout, you connect a resistor from the rectified DC bus to the EN/UV pin. The chip enables when current into this pin exceeds ~25 μA. With a typical 4.4 MΩ resistor, that's ~121 VDC on the bus. Board contamination, flux residue, or humidity can create leakage paths that alter this threshold. If your power supply won't start until 110 VAC input, clean the board around the EN/UV pin with isopropyl alcohol. Confirmed repeatedly on the PI community forum.
• Output power collapses under load? It's probably the transformer, not the chip. The TNY275 has robust protection that prevents it from destroying itself. If the output drops from 5V to 0.5V when you apply load, the chip is hitting its current limit because the transformer can't deliver enough energy per cycle. Wrong turns ratio, inadequate primary inductance, or core saturation. Use PI Expert to design the transformer - guessing winding ratios for flyback converters is a recipe for disappointment.

What Is TNY275PN-TL Used For?

Low-Power AC-DC Adapters and Chargers

The TNY275's sweet spot: cell phone chargers (5V/1A = 5W, well within spec), USB chargers, and small wall adapters. The EcoSmart technology keeps standby power under 140 mW with no load, which meets every energy efficiency regulation on the planet. Add a USB PD controller on the secondary side via an optocoupler to the EN/UV pin, and you've got a complete USB-C adapter that fits in a wall plug.

Auxiliary / Standby Power Supplies

Larger systems - TVs, set-top boxes, appliances, and industrial equipment - need a small always-on supply to keep the MCU alive while the main power stage is off. The TNY275 is the standard choice for this. Its no-load power is low enough that the auxiliary supply's contribution to the system standby budget is negligible. TNY275 on the always-on rail, big LLC resonant converter for the main rail - classic two-stage architecture.

Industrial Metering and IoT Power

Smart meters, sensor nodes, and IoT gateways running off AC mains need a small, reliable, isolated supply. The TNY275's 85–265 VAC universal input handles any grid voltage worldwide without a range switch. The built-in protection suite (OTP, OVP, OCP, SCP) means one less thing to worry about in a product that's going to sit in a utility closet for 10 years.

LED Driver Bias Supplies

LED lighting often uses a dedicated constant-current converter for the LEDs themselves, but needs a small auxiliary supply for the wireless module, MCU, or sensor. The TNY275 provides that bias rail from the same AC input, with isolation that keeps the low-voltage control circuitry safe. At 1–3W output, it's barely breaking a sweat.

Appliance Control Boards

Washing machines, refrigerators, and air conditioners all have control boards that need a few watts of isolated DC from the AC line. The DIP-8 package handles the vibration and temperature extremes of appliance environments better than tiny SMD packages, and the through-hole mounting is preferred for wave-soldered single-sided boards that are still common in appliance manufacturing.

TNY275PN vs TNY274PN vs TNY276PN - Which TinySwitch Do You Need?

Pick TNY274PN when: Your output is 5W or less. USB chargers at 5V/1A, simple bias supplies, sensor nodes. The lower Rds(on) of the TNY275 would be wasted silicon - you'll never hit the current limit. Save the BOM cost.

Pick TNY275PN when: You need 6–11.5W. This covers 5V/2A chargers, 12V/0.8A adapters, multi-output bias supplies with a few watts to spare. The TNY275 is the middle child that covers the most common power levels. From what we see, it's the most popular part in the TinySwitch-III family by volume.

Pick TNY276PN when: You need 12–15W. The next step up without changing package or pinout. If the TNY275 is hitting its current limit (output collapses under load), the TNY276 is the drop-in fix.

Important: These three are pin-compatible but not always a simple drop-in swap. Changing the IC changes the peak current, which changes the transformer's flux swing, which may require a different core size or gap. Always re-run the PI Expert design when swapping TinySwitch parts - the output capacitor ripple current rating in particular can change significantly between adjacent family members.

Frequently Asked Questions About TNY275PN-TL

Q1: My TNY275 power supply outputs the right voltage with no load but drops to near zero under load. Why?

A: The chip is hitting its current limit. The most common cause is a transformer that can't deliver enough energy per switching cycle - wrong primary inductance, incorrect turns ratio, or core saturation. Second possibility: the BP/M capacitor is set to the reduced current-limit mode (1 μF) but the design needs the standard or increased limit. Try 10 μF on BP/M to raise the limit. Third: the primary bulk capacitor is undersized or has high ESR, and the DC bus is drooping too far between AC half-cycles. This is extensively discussed on the Power Integrations community forum.

Q2: How do the three current-limit modes work on the TNY275?

A: The BP/M pin capacitor value selects the mode. 0.1 μF = standard limit (default for most designs). 1 μF = reduced limit - lower peak drain current, lower output power, but higher efficiency and cooler operation because RMS currents are lower. 10 μF = increased limit - 27% more peak power capability, useful for handling short overloads or peak demands. The chip reads the capacitor value during startup by measuring the charging time. It only checks once, at power-up. Changing the cap while running has no effect until the next power cycle. Per the PI datasheet.

Q3: Why does my TNY275 burn out after months of normal operation?

A: This is typically slow degradation from thermal stress or overvoltage, not a sudden defect. Three things to check. One: junction temperature. If Tj routinely exceeds 100–110°C, the MOSFET's Rds(on) drifts up over time (positive tempco), increasing dissipation further - thermal runaway. Add more source-pin copper or reduce power. Two: bulk capacitor aging. As the input electrolytic dries out, ripple voltage increases and peak drain voltage spikes get worse. Use 105°C-rated capacitors with voltage headroom. Three: sustained high mains voltage. If your grid consistently delivers 250–260 VAC, the reflected voltage and leakage spikes stress the 700V MOSFET. Confirmed on the PI forum in a case where a TNY275 "burned with a hole in the package" after years of 250 VAC operation.

Q4: How do I set the undervoltage lockout threshold?

A: Connect a resistor from the rectified DC bus (after the bridge rectifier and bulk cap) to the EN/UV pin. The chip enables switching when current into the EN/UV pin exceeds ~25 μA. For a turn-on at 100 VDC on the bus: R = (100 − 1.2V) / 25 μA ≈ 3.9 MΩ. For 120 VDC: R ≈ 4.7 MΩ. Leave EN/UV unconnected and the chip starts at its internal threshold (typically ~50 VDC). Board contamination can shift the effective threshold - a clean PCB around this pin matters. Power Integrations provides detailed UVLO design guidance in the TNY274-280 datasheet application section.

Q5: Can I use TNY275PN as a direct replacement for TNY274PN?

A: Mechanically yes - same package, same pinout. Electrically, maybe. The TNY275 has a larger MOSFET with lower Rds(on) and higher peak current. If your transformer was designed for the TNY274's lower peak current, dropping in a TNY275 may work fine at the same power level (more headroom) but may also change the transformer's operating point enough to cause saturation or instability at higher loads. Run PI Expert with the TNY275 selected and compare the transformer design to what's on your board. Output capacitor ripple current rating is the most commonly mismatched parameter after a swap.

Q6: My TNY275 supply won't start below 100 VAC. What's wrong?

A: Check three things. One: the bulk capacitor value. At low line, the rectified DC bus has deeper valleys between half-cycles. If the capacitance is too low, the bus droops below the UVLO threshold during the valleys and the chip shuts off between cycles. Increase the bulk cap or lower the UVLO resistor value. Two: the EN/UV resistor. If it's too large, the UVLO threshold is too high for low-line operation. Three: check for contamination on the EN/UV pin creating leakage. These three causes account for ~90% of low-line startup problems based on PI forum discussions.

Q7: Does the TNY275 need a heatsink?

A: At full power in an enclosed adapter: yes, plan for it. At 8.5W output with 80% efficiency, the chip dissipates ~2.1W. With θJA = 60 °C/W (1 in² Cu), Tj rise = 126 °C; at 40 °C ambient, Tj = 166 °C - above the shutdown threshold. You need either more copper area, a clip-on heatsink on the DIP package, or both. At 5W output, dissipation drops to ~1.25W, rise = 75 °C, Tj = 115 °C at 40 °C ambient. That's fine without a heatsink if you've got adequate copper. The source pins (pins 5–8) are the thermal path - maximize the copper area connected to them, on both sides of the board with thermal vias.

Pricing & Availability

Contact ICMASS for current pricing and volume quotes. We stock genuine Power Integrations TinySwitch-III parts with batch traceability. The full family (TNY274 through TNY280) available, plus the TinySwitch-IV (TNY284–TNY290) for designs that need even lower no-load power. Transformer design support available through PI Expert - ask us about reference designs for your output voltage and power level.