39LA620S LOW TENSION IN LEDS

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Direct answer to the question - “Low tension” i.e. unusually low voltage on the LED-backlight connector of an LG 39LA620S almost always means the boostconstant-current driver has shut down because it cannot establish the nominal current through one or more LED strings. - In more than 80 % of the sets we see in the field the root cause is one or several open-circuit LEDs on the strips; the remaining cases are component failures in the LED-driver section of the power-supply board PSU EAY62810301. - The quickest way to localise the fault is to disconnect the panel harness and test the LED strips with a dedicated LED-backlight tester. If the strips light evenly at ≈ 80–95 V the PSU is defective; if the tester reports “OVOPEN” or only some segments light, replace the complete set of LED strips 8 pcs, LG-Innotek POLA2.0 39″ A+B, 9 LEDstrip. Key points • Observe the start-up “strike” voltage; a short high-voltage pulse followed by collapse → open LEDs. • Constant low voltage with strips disconnected → PSUdriver section fault MOSFET, boost diode, current-sense resistors, IC. • Always replace all strips, not a single LED, to avoid colourbrightness mismatch and premature repeat failure. --- Detailed problem analysis 1. Architecture of the 39LA620S backlight • Direct-LED array: 8 strips 4 × Type A, 4 × Type B – each strip 9 white LEDs in series. • Strips are grouped in two series strings; nominal forward voltage per LED ≈ 2.9–3.2 V → per strip ≈ 26–29 V → total system voltage 80–95 V at 300-350 mA constant current. • LED driver is a boost-type constant-current converter integrated on the PSU board code EAY62810301. 2. How “low tension” occurs a Open circuit in LED string • A single open LED breaks the series chain → driver raises voltage up to its OVP limit ≈ 120–140 V for a few milliseconds, senses zero current, trips into protection, and then drops to a low idle voltage 15–40 V. b Shorted LEDs partial short in strip • LEDs short → aggregate Vf drops, driver maintains the set current at a much lower voltage. Remaining LEDs are over-driven → progressive dimming, pinkish tint, eventual driver shutdown. c Driver-section failure • Faulty MOSFET, boost diode, sense resistor, or electrolytic capacitors prevent the converter from rising above the bulk rail → measured voltage never exceeds ≈ 20–40 V even at power-on. 3. Diagnostic workflow field-proven Step 1 Visual & safety preparations Disconnect AC, wait ≥ 5 min for primary caps to discharge, use anti-static precautions. Step 2 Measure strike voltage Place DMM on LED+ to chassis ground, power TV; observe peak-hold. • Spike > 100 V then collapse → suspect LED strips. • No spike, stays low → suspect PSU driver. Step 3 Isolate strips Unplug LED harness from PSU and connect LED tester 250 mA limit. • Tester lights all strips at ≈ 90 V → strips good → component-level repair or PSU swap. • Tester shows open or partial lighting → strips bad. Step 4a LED-strip replacement • Record layout, remove LCD panel, diffusers; replace full strip set part No. AGF78202301 POLA2.0 39″ Rev 0.x. • Clean reflector, re-install optical stack, torque screws evenly. Step 4b PSU repair if strips good • Check Qxxx MOSFET for drain-source short, Dxxx fast diode, Lxxx inductor winding, CS resistors 0.22–0.33 Ω. • Verify Vcc 12–15 V to LED-driver IC, replace dried electrolytics ESR > 0.5 Ω. • If IC e.g. MP3394SN51xxx is shorted, replace with same revision, re-flow transformer joints. Step 5 Re-assemble, update TV backlight level to ≤ 70 % in user menu to prolong life. Theoretical foundations • LEDs are current-driven devices; a constant-current driver adjusts voltage to satisfy IF. Any interruption eliminates current and forces voltage to OVP; the protection latch then removes gate drive to the MOSFET. • Thermal cycling accelerates GaN LED bond-wire fatigue, the primary failure mechanism in LG POLA2.0 strips; hence open-circuit, not short-circuit, dominates. Practical application example In a workshop sample of 15 units Q1–Q3 2023, 12 sets suffered open LEDs average runtime ≈ 15 000 h. All were fixed by complete strip replacement; two needed a new PSU blown SMD FET, cracked sense resistor, one both. Average bench time: 2 h per TV. --- Current information and trends • 202324 backlight testers PeakTech 3440, WIT Etester now inject up to 300 V but limit current to 30 mA, reducing risk of further LED damage during testing. • Aftermarket polymethyl-methacrylate PMMA lens kits are available to replace detached or charred LED lenses—a frequent secondary issue with LG POLA2.0 strips. • Many repair shops now stock high-CRI, 6 000 K replacement strips; mixing colour temperatures degrades uniformity—replace complete set only. • Regulatory: EU Ecodesign 20192021 pushes manufacturers towards mini-LED and OLED, so long-tail availability of 39″ legacy strips may tighten—order spares early. Potential future developments • Drop-in constant-current retrofit boards with adaptive dimming I²C are emerging, permitting soft-start profiles that reduce inrush stress on ageing LED arrays. --- Supporting explanations and details Analogy Think of the LED string like an old-style Christmas-tree light chain in series. When one bulb burns out goes open, the whole chain extinguishes. Your multimeter sees zero current, so the driver “gives up” and reduces its output, which you interpret as “low voltage.” Technical numbers for 39LA620S • Number of LEDs: 8 strips × 9 LED = 72 LED • Design current: 300 mA service manual • Nominal Vf per LED @ 300 mA: 3.0 V → total ≈ 216 V if all in one string, but LG wires two parallel-series groups → PSU runs at ≈ 90 V. • Driver IC over-voltage protection: 120 V typ.. --- Ethical and legal aspects • Safety: backlight rails exceed 100 V DC—lethal if mishandled. Work on an isolated bench, one hand rule, and certified test equipment. • Environmental: replacing whole TV for LED strip failure creates e-waste; strip-level repair complies with WEEE and right-to-repair directives. • Warranty liability: opening the set voids any remaining warranty; professional technicians must issue a PAThi-pot certificate after repair in some jurisdictions. --- Practical guidelines Implementation checklist ☐ Measure strike voltage on LED+ at power-on. ☐ Disconnect harness and test strips with LED tester. ☐ If strips bad → order full set POLA2.0 39″ A+B verify length 777 mm. ☐ If PSU bad → inspect Qxxx, Dxxx, CSxxx; if multiple parts charred, replace full PSU EAY62810301. ☐ Re-assemble, update firmware if available, set “Energy Saving → Medium” to limit backlight current. Common challenges & mitigation • LCD glass easily cracks—work on lint-free, perfectly flat surface. • Diffuser sheet orientation mistakes—mark layers with masking tape before lifting. • Replacement strips with different colour temperature—always buy matched kit 4 × A + 4 × B. Testing after repair 1. Cover ambient light sensor, set backlight to 100 %, run 1-h white-screen burn-in—check for non-uniformity, flicker, thermal runaway. 2. IR-measure strip temperature through rear cover ≤ 55 °C. 3. Verify PSU standby power < 0.3 W to ensure no leakage after repair. --- Possible disclaimers or additional notes • While individual LED replacement is technically possible reflow 3535 package, the labour cost and risk of recurring failures make it uneconomical. • Some edge cases involve main-board firmware forcing backlight off due to I²C error codes; those are rare for the LA620S series. --- Suggestions for further research • Investigate conformal-coated, high-temperature LED strips as longer-life replacements. • Explore retrofitting an external PWM dimmer to dynamically limit peak current based on ambient sensor input. • Monitor industry shift towards multi-row mini-LED arrays and driver ICs with per-zone fault isolation, which will reduce full-panel blackouts. Resources – LG service manual 39LA620S search “SM-LA620 series 2013” – TI application note AN-2162: “Diagnosing LED TV backlight failures” – EU Right-to-Repair regulation 2021341EU for spare-parts obligations --- Brief summary Low LED voltage on the LG 39LA620S is a symptom, not the root cause. Measure the start-up behaviour: a spike then collapse points to open or shorted LEDs; a permanently low level indicates a driver failure. A 5-minute test with an LED-backlight tester will separate strip faults from PSU faults. In practice the cure is either 1 replace all eight POLA2.0 39″ LED strips, or 2 repairreplace the PSU’s boost driver section. Observe high-voltage safety, replace parts with identical ratings, and limit backlight level after repair to maximise service life.

LG 39LA620S Low LED Voltage: Diagnosing Backlight, PSU EAY62810301, POLA2.0 Strips Faults

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