TNY274PN-TL - 700V Offline Switcher IC by Power Integrations (TinySwitch-III)

What Is TNY274PN-TL and How Does It Work?

It's a flyback switcher with the high-voltage MOSFET and controller on one die. The control method is simple ON/OFF regulation: when the output drops below the regulation threshold, the IC switches at 132 kHz. When the output is above it, it stops switching. No PWM, no duty cycle calculation, no loop compensation. This makes the TinySwitch-III family exceptionally simple to design with.

The BP/M (Bypass/Multi-function) pin is your configuration interface. The value of the capacitor on this pin selects the current limit mode:

• 0.1 μF - Standard current limit (default for TNY274)
• 1 μF - Full current limit (required for TNY274 to deliver its rated power)
• 10 μF - Increased current limit (TNY275–TNY280 only - do not use on TNY274)

Here's the thing we see most often on the Power Integrations forum: engineers use a 10 μF cap because they read it gives "increased current limit." But on the TNY274, the 10 μF cap puts the IC into reduced current limit mode. The output never reaches the rated power, and you spend days debugging what looks like a transformer issue. Use 1 μF for full current limit on the TNY274.

The PN suffix is the standard DIP-8 through-hole package. The -TL suffix means tape-and-reel for automated assembly. The silicon is identical.

Position in the product line: The TinySwitch-III family ranges from the TNY274 (8.5W) to the TNY280 (36.5W). All share the same pinout and basic design. The main difference is the MOSFET current rating. The TNY274 is the entry-level part, suitable for small adapters, auxiliary supplies, and standby power. If your design needs more than 10W, we typically recommend the TNY275 or TNY276 instead.

What Are the Specifications of TNY274PN-TL?

Key Numbers That Matter

• 8.5W across universal input: That's roughly 5V at 1.7A or 12V at 700 mA. Enough for chargers, auxiliary supplies, and small appliances. For enclosed designs without airflow, derate to about 5.5W. We see customers regularly push it past 8.5W in open-frame designs, but the transformer size and thermal management become critical.
• 48 Ω R_DS(on): At 75 mA switching current, conduction loss is about 270 mW. Not a problem for 8W output. But compared to modern CoolMOS parts (<1 Ω), it's an order of magnitude higher - the trade-off is integration and simplicity.
• 132 kHz with jitter: The frequency jitter spreads the switching energy across a wider spectrum, reducing peak EMI by about 10 dB. This is the difference between passing EN55022 Class B and failing it. It's enabled by default - no extra components needed.
• 1 μF BP/M cap is critical: As mentioned above, the TNY274 needs a 1 μF capacitor on BP/M to get the full current limit. A 0.1 μF cap limits output to about 50% of rated power. A 10 μF cap puts it into reduced mode. This is the single most common mistake on the Power Integrations community forum.

What Is TNY274PN-TL Used For?

Chargers and Small Adapters

Cell phone chargers, tablet chargers, and generic wall warts up to 10W. The TinySwitch-III's EcoSmart technology meets CEC and Energy Star no-load requirements (< 150 mW). At standby, the IC reduces switching frequency to a low level, and the auto-restart feature keeps fault power under 3% of full load.

PC Standby and Auxiliary Supplies

Desktop PC power supplies, TVs, and set-top boxes need a small auxiliary rail (typically 5VSB) that runs even when the main supply is off. The TNY274 is a common choice for this: it needs minimal PCB space and meets the < 1W standby power requirements of the EU ErP directive.

Appliance Power Supplies

Washing machines, microwave ovens, and air conditioners need low-power isolated supplies for the control board. The TNY274's flyback topology gives you isolation (unlike the LinkSwitch-TN non-isolated buck), which is important for safety in appliance designs that need to meet IEC 60950 or IEC 62368.

Industrial Control Power

Industrial sensors, PLCs, and metering equipment often need a small isolated supply (5V or 12V at a few hundred mA) from the 220 VAC mains. The TNY274's integrated 700V MOSFET handles the wide input range needed for industrial environments.

Common Design Issues - and How to Avoid Them

Based on the Power Integrations community forum, here are the most common TNY274 issues and their fixes:

Output Voltage Stuck at Low Level (e.g., 3V instead of 12V)

This is the most frequently reported problem. The output sits at 3–4V and won't rise. The usual suspects, in order: (1) BP/M capacitor is 10 μF instead of 1 μF - the IC is in reduced current limit mode. (2) The flyback transformer gap or inductance is wrong - use PI Expert to calculate the correct values. (3) The output rectifier diode is too slow - must be an ultrafast or Schottky type, not a 1N4007. (4) The primary clamp diode failed - replace with UF4007 or equivalent.

Hiccup Mode / Clicking Sound (Overcurrent Protection Cycling)

The output cycles on and off every 800 ms, with a clicking sound from the transformer. The root cause is usually transformer saturation (gap too small, primary inductance too low), a slow output diode causing excessive reverse recovery spikes, or noise on the EN/UV pin. Fix: check the transformer design, replace the output diode with an ultrafast type, and add a small capacitor (1 μF) between EN/UV and ground to filter interference.

IC Exploding After Removing Input Capacitor

A specific issue reported on the forum: removing the bulk capacitor after the input inductor causes the inductor to force constant current into the switching transistor. When the switch turns off, the voltage rises uncontrollably past 700V and the IC fails. Fix: never remove the bulk capacitor after the input inductor. If you need to reduce component count, remove the inductor instead and use a single larger capacitor.

No Output / Zero Voltage

First check: what's the BP pin voltage? It should be about 5.8V. If it's 0V, the IC isn't starting. Check the input bulk capacitor voltage (should equal peak of AC input), check the transformer polarity, and check that the EN/UV pin isn't being pulled low by a fault condition. If BP voltage is present but output is zero, check the feedback optocoupler and TL431 circuit.

TNY274PN-TL vs. the Competition

Layout Guidelines

The TinySwitch-III is simple to lay out, but a few rules make the difference between a design that passes EMI and one that doesn't:

1. Keep the DRAIN node physically short. The DRAIN pin switches at 132 kHz with fast edges. Minimize the copper area on this node to reduce radiated EMI, but keep it wide enough for thermal dissipation.
2. BP/M capacitor right at the pin. The 1 μF bypass capacitor must be placed within 5 mm of the BP/M pin. This capacitor provides the internal supply for the control logic. Too far away = noise injection = erratic operation.
3. Primary side layout: The input bulk capacitor, transformer primary, and DRAIN pin form the high-di/dt loop. Keep this loop as small as possible. Use a separate return path to the bulk capacitor's negative terminal.
4. Secondary side layout: The output diode, output capacitor, and transformer secondary form the secondary loop. Keep it tight. If using a snubber across the output diode, place it directly across the diode leads.
5. Feedback routing: The feedback trace from the TL431/optocoupler to the EN/UV pin is high-impedance. Keep it away from the DRAIN node and the transformer. A 1 kΩ resistor in series with the optocoupler output helps filter noise.

Frequently Asked Questions

Q1: Is the TNY274PN-TL the same as TNY274PN?

A: Electrically identical. PN is the standard DIP-8 through-hole package. The -TL suffix means tape-and-reel packaging. If you're hand-assembling prototypes, the PN version without -TL is easier to order in small quantities.

Q2: What capacitor value should I use on the BP/M pin?

A: Use 1 μF for the full current limit on the TNY274. A 0.1 μF cap gives reduced current limit (about 50% of rated power). A 10 μF cap does not increase the current limit on the TNY274 - it activates a different mode that actually reduces it. This is the single most common mistake on the Power Integrations forum.

Q3: Why is my output voltage stuck at 3V instead of 12V?

A: Four things to check, in order: (1) Is the BP/M capacitor 1 μF? If 0.1 μF or 10 μF, the current limit is reduced. (2) Is the flyback transformer gapped correctly? Use PI Expert to verify the gap and inductance. (3) Is the output rectifier an ultrafast diode? A 1N4007 is too slow and will limit output. (4) Did the primary clamp diode fail? Check the UF4007 in the clamp circuit.

Q4: Why does my TNY274 supply cycle on and off with a clicking sound?

A: The IC is in auto-restart / hiccup mode due to overcurrent protection. The most common cause is transformer saturation - the core gap is too small or the primary inductance is too low. Second most common: a slow output diode causing reverse recovery current spikes that trigger the current limit. Replace with an ultrafast diode (t_RR ≤ 75 ns).

Q5: Can I use the TNY274 for a 5V output design?

A: Yes. At 5V output, expect about 1.5–1.7A maximum in open-frame design (8.5W / 5V). The transformer turns ratio needs to be designed for 5V output - use PI Expert to calculate the primary-to-secondary ratio. For a typical 5V/1A design, you need about 8 turns on the secondary with a 100–120:1 primary-to-secondary turns ratio.

Q6: Does the TNY274 need a heatsink?

A: At 8.5W open-frame: not if you have adequate copper area on the DRAIN pin and the PCB breathes. At 11W or in an enclosed design: yes, you need a heatsink or a larger PCB copper area. The DIP-8 package's thermal resistance is about 80°C/W junction-to-ambient. At 11W with 80% efficiency, you're dissipating about 2.2W, which gives a 176°C temperature rise - too high without heatsinking.

Q7: What happens if the TNY274 overloads?

A: The IC enters auto-restart mode. It delivers less than 3% of rated power in fault conditions (short circuit, open loop), trying to restart every 800 ms. When the fault is removed, the IC recovers automatically. This is safer than latching off because it doesn't require a power cycle to recover.

Q8: Can I drop a TNY275 into a TNY274 design for more power?

A: Pin-compatible, same package. You can. But the transformer must be redesigned for the higher power - the primary inductance and gap need to change. The feedback loop components and BP/M capacitor configuration may also need adjustment. Always run PI Expert with the target IC before making the swap.

Q9: What output diode should I use?

A: An ultrafast recovery diode rated for at least 100V (for 5V output) or 200V (for 12V–15V output). The SB5100 Schottky (5A, 100V) works well for 5V outputs. For 12V outputs, the UF4007 (1A, 1000V) or MUR420 (4A, 200V) are solid choices. Do not use a standard 1N4007 - it's too slow and will overheat.

Q10: Can I use the TNY274 for a multiple-output supply?

A: Yes, with multiple secondary windings on the transformer. The cross-regulation isn't as tight as a post-regulated supply, but for applications that need a main regulated output (e.g., 5V) plus a loosely regulated auxiliary (e.g., 12V for a relay), it works well. The feedback should come from the most critical output.