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Nie, dziękuję Przekieruj mnie tamako zmerat lkn 304gn ohmertom nemam vnutorne zapojenie
• An ordinary ohmmeter (or the resistance / diode-test range of a DMM) can only reveal catastrophic faults of the LNK304GN, e.g. a shorted high-voltage MOSFET (Drain ↔ Source) or a blown bond-wire that has gone open-circuit.
• Measure out of circuit, discharge all capacitors, and compare Drain–Source, Bypass–Source and Feedback–Source readings in both polarities.
• A healthy device shows “open” (>! 1 MΩ) or typical semiconductor forward junction readings (≈ 0.4-0.7 V in diode mode) depending on polarity; any reading < 100 Ω Drain ↔ Source or a short between pins that should be isolated means the IC is bad.
• Because the LNK304GN is a complete offline SMPS controller with an internal power MOSFET, an ohmmeter cannot prove that the part is good—only that it is definitely bad if a short is found.
What the device is
• LNK304GN belongs to Power-Integrations’ LinkSwitch-TN family – an offline switch-mode power-supply controller that contains a start-up circuit, oscillator, current-limit, thermal shutdown and a high-voltage MOSFET rated ≈ 650 V.
• Typical packages: PDIP-8 (-P), SO-8 (-G). “GN” = SO-8, RoHS.
Relevant pins (check your datasheet; packages differ, but the functional groups are fixed)
– BP/M (BYPASS / MULTI-FUNCTION): external 0.1 µF–1 µF to Source, internal 5.8 V regulator.
– FB (FEEDBACK): regulates output via opto or resistor divider.
– S (SOURCE): controller ground; multiple pins are often shorted together for thermal reasons.
– D (DRAIN): internal MOSFET drain; several pins tied together; at mains potential.
Safety note: the Drain pins sit on live mains—probe only with the supply fully isolated and caps discharged.
Why static resistance tests are of limited value
• Inside the IC you have MOSFET structures, ESD diodes, high-voltage depletion devices and a 6 V shunt regulator.
• These appear as non-linear, polarity-dependent impedances. An ohmmeter sources only a few hundred mV, so most PN junctions are reverse-biased and read “open”.
• The only common hard failure is a punch-through of the MOSFET, visible as low ohms Drain ↔ Source (typically < 10 Ω).
Recommended static measurements (device removed from the PCB)
| Test leads (DMM) | Expected healthy reading | Fault indication |
|---|---|---|
| Drain ➜ Source (both polarities) | “OL” or > 1 MΩ | < 100 Ω = MOSFET short |
| Bypass ➜ Source | “OL” (> 1 MΩ) one way, ≈ 0.4-0.7 V in diode mode the other (internal clamp) | 0 Ω or < 100 Ω either way |
| Feedback ➜ Source | “OL” both ways (may show ~0.6 V forward if meter current high enough) | Low resistance or short |
| Any pin ➜ any other unrelated pin | “OL” | Any measurable continuity |
Always swap probes because semiconductor polarity matters.
Dynamic tests (when static test passes)
• Solder the part back, power the SMPS through an isolation transformer and an incandescent series safety lamp or VARIAC/Limiter.
• Measure BP pin; it should charge to ≈ 5.8 V (the IC’s internal supply).
• Observe Drain waveform with an isolated scope; you should see ≈ 70-120 kHz pulses whose duty cycle varies with load.
• If BP never charges, or pulses do not appear, suspect the IC even if static ohm tests were OK.
• Field repair practice in 2024: because LNK30x cost < 1 € and labour dominates, professionals seldom waste time bench-testing—if Drain-Source is short or the PSU is dead, the IC is routinely replaced together with the primary electrolytic capacitors.
• Power-Integrations application notes now recommend adding an NTC or fusible resistor and an MOV to reduce surge-induced MOSFET failures—common reason for D-S shorts that your ohmmeter will detect.
• The high-voltage MOSFET occupies ≈ 75 % of the die area; once it punctures, D ↔ S resistance becomes a few ohms—easy to spot.
• The BP/M pin contains a 5.8 V zener-like clamp. In diode mode with red lead on BP and black on S you usually read 0.6-0.7 V (forward drop of the clamp transistor).
• Analogy: Testing LNK304GN with an ohmmeter is like tapping a car engine with a screwdriver—you might hear something obviously broken, but silence does not prove the engine runs.
• The Drain pin is directly connected to the mains. Any powered measurement must be made through an isolating transformer and compliant CAT II instruments.
• Always ensure the main electrolytic capacitors are discharged before ohmmeter or soldering work to avoid electric shock.
• Replacing the IC without respecting the original creepage/clearance may violate product safety norms (IEC 62368-1, IEC 61558).
Potential challenges and how to overcome them
• False shorts caused by in-circuit components → remove at least one pin of the IC or lift suspect parts.
• Meter auto-range lag → use manual range for resistances < 1 kΩ to avoid mis-reading intermittent shorts.
• Passing the ohmmeter test does not guarantee the IC is good; only dynamic measurements or substitution can prove it.
• Pin numbering differs between PDIP-8B and SO-8C versions; always refer to the exact datasheet revision.
• If the PSU has suffered surge or lightning, other primary components (bridge, start-up resistor, opto, secondary diode) may be damaged as well.
• Power-Integrations Application Note AN-32: “LinkSwitch-TN Family Design Guide” – contains full pinout and recommended test points.
• AppNote “SMPS Troubleshooting With a Mains Isolation Lamp” – various repair blogs.
• Study surge immunity techniques (IEC 61000-4-5) to understand why LNK devices fail short.
An ohmmeter can only reveal gross faults of the LNK304GN, principally a Drain-Source short. Remove the IC, ensure caps are discharged, verify all inter-pin resistances are open or show only normal diode drops. Anything below ~100 Ω is a fail. If the static test passes, confirm functionality under power (BP ≈ 5.8 V and drain switching). Because the device is inexpensive and resistance checks cannot prove it healthy, industry practice is to replace the IC whenever a switching supply built around it is suspect.
