TN2124K1-G Sourcing Guide: Microchip's 240V Depletion MOSFET
TN2124K1-G: The Part, in Plain Terms
The TN2124K1-G is a 240V N-channel depletion-mode MOSFET in a TO-252 (DPAK) package, originally a Supertex part and now sold under the Microchip brand after the 2014 acquisition. Depletion mode means the channel is on at zero gate bias — you drive the gate negative to shut it off. That one behavior is the whole reason the part exists, and the reason engineers keep specifying it decades after its introduction.
If you're sourcing it, you're almost certainly supporting one of a handful of classic circuits: an inrush current limiter, a constant-current source for LED strings or sensor excitation, a start-up bleed path in an offline supply, or a clamp in a solid-state relay. None of these are glamorous, but they're everywhere in industrial power, telecom line interfaces, and lighting gear.
Where the TN2124K1-G Actually Gets Used
The most common application I see on BOMs is the two-component current source: the TN2124K1-G plus a single resistor between gate and source gives you a crude but very effective current limiter that needs no control circuitry and works the moment voltage appears. Designers use it to tame inrush into bulk capacitors on 85–265VAC inputs, or to set a fixed current through an LED string fed from a poorly regulated high-voltage rail.
On-resistance at zero gate bias sits in the single-digit-ohm range, and the saturation current (IDSS) is modest — this is not a power switch, it's a current-shaping device. Pull the Microchip datasheet and check IDSS and RDS(on) at your actual operating point before you assume a drop-in will work, because the spread on depletion parts is wider than most engineers expect from enhancement-mode experience.
There's a broader context here too. The industry's power-management conversation keeps drifting toward smart, autonomous control — as Power Electronics News reported on Nanopower's nPZ2100, newer ICs can manage sensors and peripherals without waking the MCU at all. That's the direction new designs go. But the installed base of industrial drives, ballasts, and telecom line cards built around simple depletion-FET current sources isn't getting redesigned. Those boards get repaired and reproduced for years, which is why a plain 240V depletion MOSFET still shows up on shortage lists.
Sourcing Pitfalls Specific to This Part
The -G suffix matters on paper, not in silicon. The "G" denotes the green/RoHS-compliant package plating. Electrically, TN2124K1 and TN2124K1-G are the same die. Where it bites you is compliance documentation: if your customer's quality system requires a RoHS declaration tied to the exact orderable part number, shipping non-G stock won't clear incoming inspection even though the parts work identically. Confirm which suffix the end customer audits against before you buy.
Marking is compact and easy to misread. On DPAK, Supertex/Microchip marks these with abbreviated codes, and date codes from the Supertex era look different from current Microchip-era marking. Neither is wrong on its own. What should raise a flag is mixed marking styles inside a single reel or lot, or laser marks that sit proud of the package surface — a tell for resurfaced parts.
Refurbished risk is real but moderate. The TN2124K1-G isn't a top-tier counterfeit target like a TI op-amp or a Xilinx FPGA, but DPAK high-voltage parts do show up as pulled devices sold as new. On pulled stock you'll often see slight solder shadow on the tab or inconsistent lead finish under a microscope. For anything going into a medical or industrial-safety product, insist on traceability or an authorized-channel paper trail.
Lead times swing hard. Microchip runs these older Supertex lines on relatively long cycle times, and when a lighting OEM or telecom customer places a blanket order, the open-market price on the TN2124K1-G can move within weeks. If the part is on your AVL for a product with a long service life, a modest buffer buy during calm periods usually beats paying a spot premium later.
A Realistic Buying Scenario
Say a contract manufacturer is mid-build on an industrial lighting driver and the TN2124K1-G line on the BOM comes back with a 20-week factory lead time. The build is due in nine. The options, roughly in order of preference: first, check authorized distributors for residual stock; second, source from a vetted independent distributor with inspection capability — XingHuan International (icxing.com), for example, handles hard-to-find discrete semiconductors with documented QC and can often turn DPAK discretes around in days rather than weeks; third, evaluate a substitute, which means requalification.
That third option deserves a caution. Higher-voltage siblings in the same family, or depletion devices from other vendors, can often do the job electrically, but IDSS, gate cutoff voltage, and thermal behavior in the SOT/DPAK footprint all differ enough that "form-fit-function equivalent" claims on a broker's line card are not a substitute for bench validation. In a current-source application, a mismatch in IDSS directly changes your output current. Budget the engineering time if you go this route.
Adjacent Components Worth Checking on the Same BOM
When the TN2124K1-G goes short, it's worth auditing the rest of the power stage at the same time, because these designs often share age-related supply risk. High-voltage electrolytics, surge-rated film caps, and NTC inrush devices on the same board frequently come from product families that have been consolidated or moved to extended lead times. The industry keeps pushing components toward harsher operating conditions — see KYOCERA AVX's recent vibration-proof capacitor launch, covered by Power Electronics News — which is a reminder that passive availability moves around as vendors chase automotive and industrial sockets. A consolidated shortage list beats three separate emergency buys.
FAQ
Q: Is the TN2124K1-G still in active production, or is it obsolete?
A: As of this writing it's an active Microchip part number, not on a last-time-buy notice. That said, factory lead times on legacy Supertex discretes commonly run 16–26 weeks, so treat it as "active but slow" and plan buffer stock if it sits on a long-lifecycle product.
Q: Can I substitute TN2124K1 (without the -G) in a RoHS product?
A: Electrically yes — same die, same package outline. The risk is documentation: older non-G stock may have lead-bearing terminal plating, which fails RoHS material declaration at incoming inspection. Ask the supplier for the plating composition and lot date code before you commit.
Q: How do I spot refurbished TN2124K1-G parts in the open market?
A: Check the tab for solder residue or reflow shadows, inspect lead finish uniformity under 10x magnification, and compare marking style against a known-good date code from the same era. Mixed marking fonts within one lot, or blacktopping texture on the mold compound, are strong indicators of reworked devices.
If you're qualifying stock for a production run, a simple curve-tracer check of IDSS and gate cutoff on a sample of five to ten parts per lot will catch most substitutions and pulls before they reach the line. It's fifteen minutes of bench time that has saved more than one build I've been involved with.