TNY279GN — 18W Off-Line Switcher with 700V MOSFET (SMD-8C)

What Are the Technical Specifications of TNY279GN?

The thermal rows carry a detail that decides package questions: the datasheet specifies θ_JA and θ_JC for the P or G package, as a single figure. Rev. N (12/21) does not print a separate derating for the surface-mount body.

So the 70°C/W and 60°C/W numbers describe how much copper you soldered to, not which package you bought. That is the whole thermal argument, and it runs the other way from most families.

Build the footprint for seven pads, not eight. A library part copied from a generic SOIC-8 template will place a pad where pin 3 should be, and the high-voltage gap to the BP/M pin shrinks back to a standard lead pitch.

Note which side the drain sits on. Pins 5 through 8 are all SOURCE, and they are the thermal path — the datasheet's θ_JA figures assume those pins are soldered to the stated copper area.

How Much Output Power Does One TNY279GN Really Cover?

Three power classes, one part number — selected by a single capacitor. The BP/M pin doubles as the current-limit selector, and Rev. N gives the peak power for each setting at 230VAC ±15%.

The spread is 8.5W, or roughly 36%, from the same die in the same socket. It also lines up with the neighbours: the reduced setting lands near the TNY278's standard limit, and the increased setting near the TNY280's.

That is deliberate. Power Integrations made the three current-limit levels of adjacent family members line up, so a design can move one step up or down by changing a capacitor instead of a part number.

Read the diagram as a design lever, not a curiosity. If your enclosure runs hot, dropping to a 1µF capacitor cuts the current limit by 100mA and the peak power by 8.5W — and the part stays in place.

When Should You Use (and NOT Use) the TNY279GN?

✅ Use TNY279GN when:

• You need 12–18W from a universal input. 12W enclosed and 25W peak on 85–265VAC covers most 12V adapters without moving up the family.
• You want one part number across several power SKUs. The BP/M capacitor lets a 12W build and an 18W build share a BOM line.
• Your board is reflow-assembled. SMD-8C removes the through-hole step, and the power rating does not change for it.
• You have copper to spare under the SOURCE pins. The 60°C/W figure assumes 645mm² of 2oz copper. Give it that and the thermal budget closes.
• You're replacing a TNY277 in a design with margin. Same footprint, one step more current — but check the clamp and snubber first, because the extra current shows up there.

❌ Don't use TNY279GN when:

• Your board has no copper pour. θ_JA climbs fast below the stated areas, and the part will thermally shut down rather than fail politely.
• You need more than about 20W continuous. Move to the TNY280, or out of the family to a TinySwitch-4 eSOP-12B part.
• You're designing for no-load under 50mW. That needs a bias winding feeding the BP/M pin; budget it now rather than after the efficiency test.
• Your clamp is a plain 1N4007. The clamp diode's recovery speed shows up as heat in the clamp resistor and the chip. Use a fast or glass-passivated part.
• You need a 5V–3.3V rail directly. This is a primary-side switcher. It needs a transformer and secondary-side regulation.

What Are the Alternatives to TNY279GN?

Here's the thing that catches people upgrading within the family: the current limit is the first thing a board feels, not the last.

A designer who swapped a TNY277 for a TNY279 on an existing board put it into thermal shutdown under load. The clamp and snubber were sized for the smaller device's leakage spike.

So the extra 100mA is not free. If your transformer and RCD clamp were tuned at 350mA, budget the new spike before the new part.

Going the other way is easier. The reduced current limit of the TNY279GN equals the standard current limit of the TNY278, so an existing TNY278 design can sometimes absorb a TNY279GN at the lower setting without touching the magnetics.

What Are the Typical Applications of TNY279GN?

18W universal-input adapters. The 12W enclosed rating on 85–265VAC is the conservative number for a sealed plastic case; the 18W row assumes 230VAC ±15%. Pick your row by the worst-case line voltage you actually ship.

Standby and auxiliary rails inside larger equipment. Power Integrations lists PC standby supplies directly. The ON/OFF control keeps efficiency flat down to very light load, which matters for a rail that idles at milliwatts for most of its life.

Appliance and metering supplies. Non-ventilated enclosures and long service life are the constraints here, and the hysteretic thermal shutdown with automatic recovery means the supply recovers on its own instead of needing a power cycle.

Chargers for small battery equipment. The ON/OFF scheme allows the output to fall close to zero volts while still delivering power, so constant-current charging doesn't need a forward bias winding.

Why Buy TNY279GN from ICMASS?

We check incoming TinySwitch lots against the Rev. N rows that matter here: the 700V DRAIN rating and the current limit at each of the three BP/M capacitor values. A remarked part is usually a smaller family member, and the current limit is where that shows up first.

Family sourcing is where we add the most. We stock the TinySwitch-III SMD line and the TinySwitch-4 line together — TNY277GN, TNY279GN, TNY280GN on one hand, TNY284DG and TNY288DG on the other — so a power-class change is a stock question instead of a redesign.

Tell us your output power, worst-case line voltage and enclosure type and we'll tell you which row of the power table you should actually be designing against. That answer is frequently one step below the part you asked about.

Orders ship same day from Shenzhen, and the official Power Integrations PDF for every part we quote travels with the quote.

Frequently Asked Questions About TNY279GN

Q1: I replaced a TNY277 with a TNY279 and it now shuts down under load. Why?

A: The new part delivers more current into a clamp that was sized for the old one. This exact swap has been reported on Power Integrations' own forum, with the chip heating until thermal shutdown. Check the RCD clamp and snubber first — the leakage spike scales with the current limit. If the clamp is already tight, fit a TNY280 with a 1µF BP/M capacitor: it sets the limit back to the TNY279 level with a lower on-resistance.

Q2: What bypass capacitor does the TNY279GN need?

A: 0.1µF gives the standard current limit and is the default. Use 1µF to drop to the reduced limit or 10µF to raise it. The capacitor is also the chip's energy store and its high-frequency filter, so place it as close as possible to the BP/M and SOURCE pins. An open BP/M capacitor stops the supply starting altogether; a shorted one puts it into a fault state.

Q3: The supply won't start and clicks every few seconds. What is that?

A: Auto-restart, and the 2.5-second period is the signature. If the EN/UV pin is not pulled low for 64ms, switching is disabled for 2.5s and then retried. A genuine auto-restart cycle is about 2.5s — if you measure 2s, look elsewhere. Common causes: an output overload, a stuck optocoupler, or start-up taking longer than 50ms to reach regulation.

Q4: Does the SMD-8C version deliver less power than the DIP-8C?

A: No. Rev. N prints one output-power row and one thermal-resistance figure covering P or G. The TNY279P and TNY279GN share the 18W row and the 60°C/W figure on 645mm² copper. Only the current-limit maximum differs — 695mA on the P against 709mA on the G — with the same 650mA typical.

Q5: Why does the chip run hotter at 240VAC than at 120VAC?

A: Because the losses at high line are switching losses, not conduction losses. A user measuring an 8W design saw 58°C at 240VAC against 48°C at 100VAC, both at the same output. The drain voltage and the RCD clamp carry that difference. Lower the clamp's series resistor to trim the leakage spike, and check the snubber before adding copper.

Q6: What is pin 3 on the TNY279GN?

A: There is no pin 3. Both the DIP-8C and SMD-8C versions have seven leads: 1 is EN/UV, 2 is BP/M, 4 is DRAIN, and 5 through 8 are SOURCE. The missing lead widens the creepage distance between the high-voltage DRAIN pin and the low-voltage BP/M pin. If your footprint came from a generic SOIC-8 library part, delete that pad.

Q7: Can I use a 1N4007 as the clamp diode?

A: No — and this one has a documented failure. A builder of Power Integrations' DI177 9V/9W reference design got no output and a very hot 100Ω clamp resistor by fitting a 1N4007 where the design called for a glass-passivated part. A fast-recovery diode fixed it. The slow diode's recovery turns the clamp resistor into the load.

Q8: How much copper do the SOURCE pins need?

A: 645mm² (1 in²) of 2oz copper gets the 60°C/W figure; 232mm² gives 70°C/W. Those are the only board-level thermal numbers Rev. N publishes, and both assume the SOURCE pins are soldered to that area. Below 232mm² the power table no longer applies to your board.

Q9: What happens if I leave out the 4MΩ resistor on EN/UV?

A: You lose the line undervoltage function, and the chip detects that itself. With no external resistor the device disables undervoltage sensing and starts switching as soon as the bus comes up. Fit the 4MΩ resistor and switching is inhibited until the DC line passes about 100V, which is what you want in a universal-input design.

Q10: Can I run the TNY279GN without a bias winding?

A: Yes, and most designs do. The chip is self-biased from the DRAIN pin, so no bias winding or its components are needed. No-load consumption is under 150mW at 265VAC that way. If you need below 50mW, add a bias winding feeding the BP/M pin with a minimum of 1mA — that removes the DRAIN supply current from the no-load budget.