Grundig/Loewe/Finlux ZUV-19xR-xx TV: 2A Fuse Blows After 30s, MOSFET & PWM IC Suspected
User question
Similar problem, the very same fusible resistor blew and main 2A fuse too. the fusible resistor was replaced, and the fuse 2A too. The TV set operated properly for about 30 seconds and then the new 2A fuse blew too.
Artificial Intelligence Response
Direct answer to the question
- The 30‑second delay before the 2 A mains fuse opens means the original fault on the PSU is still present and only develops once the supply is under load/at temperature. Simply replacing the fuse and the “fusible resistor” treats symptoms, not the root cause.
- Most likely culprits: primary switch stage (MOSFET + snubber + current‑sense network), active PFC switch/diode, startup/Vcc circuitry of the PWM/PFC ICs, or a shorted secondary rectifier/backlight rail that overloads the primary after ramp‑up.
- Do not apply full mains again. Test with a series “dim‑bulb” limiter or variac + isolation transformer and systematically check the parts listed below.
Detailed problem analysis
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Why it lasts ~30 s
- A dead short would blow the fuse instantly. A 10–40 s delay strongly points to a part that fails hot (semiconductor junction leakage increasing until breakdown) or to control/Vcc that collapses after warm‑up, driving the primary switch abnormally and over‑currenting the input.
- When active PFC is present, the supply often starts on the rectified bus, then the PFC loop engages and boosts the bus. If the PFC MOSFET or boost diode is marginal, it can survive for a short period then fail catastrophically once PFC duty increases.
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Two common “fusible resistor” roles on TV SMPS:
1) Line fusible/flameproof resistor (typically 5–33 Ω, 1–3 W) in series with the bridge or mains—opens when the primary is over‑stressed.
2) Current‑sense resistor in the MOSFET source (typically 0.15–0.47 Ω, 0.5–2 W metal‑film/metal‑strip). If this blew, assume the MOSFET shorted D‑S and often the driver IC and gate network were hit too.
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Primary side suspects and mechanisms
- Primary MOSFET (or two‑transistor forward/flyback pair): partial short when hot; avalanche/over‑voltage from a failed snubber; incorrect gate drive due to a damaged PWM IC or open/shifted gate resistor/zener clamp.
- RCD/snubber network: shorted/snapped ceramic snubber cap or shorted ultrafast snubber diode will overheat the switch and blow the line fuse after a brief run.
- Startup/Vcc: dried‑out startup electrolytic (typically 10–47 µF, 35–50 V) or high‑value startup resistors drifting high. The controller browns out, misdrives the switch, current spikes rise, and the mains fuse opens.
- PFC stage (if fitted): shorted PFC MOSFET, leaky/short SiC/ultrafast boost diode, or PFC controller fault. These often fail a few seconds after the PFC comes up.
- Bridge rectifier and bulk cap: a leaky bridge or bulk capacitor can run for seconds before current rises; however, these usually trip earlier.
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Secondary side suspects
- Shorted secondary Schottky/MOSFET rectifier on the main rails or a shorted LED backlight rail can pull excessive power once the main board enables full operation, reflecting as primary over‑current and popping the mains fuse.
- Faulty optocoupler/TL431 feedback can drive the primary wide‑open.
Current information and trends
- Modern LED TV PSUs commonly integrate active PFC; the bus transitions from ~160–170 VDC (120 VAC mains) or ~310–325 VDC (230 VAC) to ~380–400 VDC once PFC engages. Failures that appear only after some seconds are frequently in the PFC switch/diode or in the startup/Vcc capacitor feeding the controller.
- Slow‑blow (time‑lag, “T”) fuses are standard on these supplies due to inrush and PFC engagement. Using a fast‑acting (“F”) 2 A will often nuisance‑blow during normal startup.
Supporting explanations and details
Ethical and legal aspects
- Mains and PFC sections contain lethal voltages and high stored energy. Use an isolation transformer, discharge the bulk capacitor safely, and follow IEC/UL 62368‑1 safety practices.
- Replace safety‑critical parts (fuse, fusible resistor, Y/X capacitors) only with flameproof, safety‑rated equivalents. Never bypass fuses or uprate beyond the design.
Practical guidelines
- Don’t re‑apply full mains. Build a dim‑bulb tester (60–100 W incandescent in series with line) or use variac + isolation transformer.
- Verify parts and types:
- Fuse: T2A (time‑lag). If you installed an F2A, it can open during normal inrush and muddy the diagnosis.
- Fusible resistor: exact resistance and wattage; use flameproof type.
- Recommended “kit” replacement when the sense/fusible resistor has blown:
- Primary MOSFET.
- PWM controller IC and any PFC controller IC if present.
- Gate resistor and gate clamp zener; check/replace the small series diode in the gate network if fitted.
- Startup/Vcc electrolytic and any high‑value startup resistors feeding Vcc.
- RCD/snubber diode and HV ceramic snubber capacitor across the primary.
- Step‑by‑step test flow
1) Cold checks (ohmmeter/diode mode) on bridge, MOSFET, PFC switch/diode, secondary rectifiers, sense and gate network.
2) Replace any suspect parts above as a set.
3) First power‑up through a 60–100 W lamp. Observe:
- Lamp flashes then goes dim: no hard short; proceed to measure rails.
- Lamp stays bright: primary short/PFC fault still present.
4) Measure bulk bus before/after PFC engagement. If it rises toward ~380–400 V and lamp flares, suspect PFC.
5) Reconnect the main board/backlight last; if adding a load turns the lamp bright, the downstream load is bad.
Possible disclaimers or additional notes
- A MOSFET that “measures OK” cold can fail under high voltage or temperature. If the current‑sense resistor opened previously, replacing the MOSFET and driver parts proactively is prudent.
- If the PCB shows carbonization around the primary switch, tracking can cause recurrent failures; clean and seal or replace the board.
- Without the exact board ID and the resistor’s marked value, the guidance remains general to TV SMPS topologies.
Suggestions for further research
- Obtain the PSU schematic or service manual for your exact PSU board; look up the controller ICs’ datasheets to verify UVLO/Vcc thresholds and typical waveforms.
- Read application notes on flyback/PFC snubber design and MOSFET gate drive integrity to understand repetitive‑stress failures.
- If available, examine the waveforms on the MOSFET drain and controller Vcc with an isolated scope during lamp‑limited power‑up.
Brief summary
- A 30‑second delay before the 2 A fuse opens points to a thermal or control‑related fault, commonly in the primary switch, snubber, PFC stage, or controller Vcc/startup network—less often a hard short at the bridge/bulk cap.
- Replace/verify the primary MOSFET, sense resistor, gate network, controller IC(s), startup cap, and snubber parts; confirm the fuse is T2A. Bring up with a dim‑bulb limiter while monitoring the bulk bus and secondary rails to isolate whether PFC engagement or a downstream load triggers the over‑current.
- Provide the PSU board code, the exact value/marking of the “fusible resistor,” and any controller/MOSFET part numbers, and I’ll give you component‑level targets and expected voltages for that specific design.
Disclaimer: The responses provided by artificial intelligence (language model) may be inaccurate and misleading. Elektroda is not responsible for the accuracy, reliability, or completeness of the presented information. All responses should be verified by the user.
User question
I can't agree that Vestel 17IPS12 or 17IPS20 PSU boards are similar in topology to ZUV-194R-06, because there are two MOSFETs on primary side as I can tell from the printed board, whereas the suggested PSUs have only one
Artificial Intelligence Response
Direct answer to the question
Detailed problem analysis
Circuit partitioning
a. Vestel 17IPS12 / 17IPS20
• AC input → Bridge rectifier → Bulk-cap (~325 Vdc)
• ONE N-MOSFET (e.g. 7N80, 10N60) driven by a PWM/Fly-back IC (ICE3B, STR-A, etc.)
• No discrete PFC switch in low-power versions; at most an integrated PFC stage inside the controller IC for higher-end variants.
b. ZUV-194R-06
• AC input → Bridge rectifier → PFC choke → MOSFET #1 (active PFC boost) → 390–400 V bus
• 390 V bus → Half-bridge MOSFET #2 + MOSFET #3 (in many boards both FETs sit on a single heatsink and are often mistaken for “two” when in fact the PFC FET makes three).
• Resonant tank (Cr + Lr integrated in the transformer) → multiple low-voltage rails and LED driver.
• Even if only two TO-220 devices are visible, PCB labelling usually reads “QPFC” and “QHS / QLS” or “Q1 / Q2” indicating a split-duty converter.
Topological consequences
• Half-bridge/LLC demands matched FETs, symmetrical gate drive, resonant timing and reliable bootstrap-cap health.
• Fly-back demands only a single fast-switching FET, snubber network and tightly-coupled auxiliary winding for bias.
Failure signatures (why they look different on the bench)
ZUV-194R-06
• Simultaneous short of both half-bridge FETs or PFC FET → mains fuse + fusible resistor open, scorch marks between drain nodes, blown driver IC (L6599, IRS2153, FAN7621, etc.).
• High-ESR bootstrap or VCC electrolytic → erratic gate drive, shoot-through after 10-40 s.
Vestel 17IPS12/20
• Single FET short; controller startup cap dry-out; opto-feedback open.
• Primary snubber RC fails; secondary Schottkys short; but never cross-conduction because only one switch exists.
Diagnostic difference
• ZUV-194R-06: check ALL high-side/low-side gate resistors, the bootstrap diode/cap, resonant capacitor, PFC boost diode, current-sense resistors.
• Vestel: focus on lone MOSFET, RCD clamp, startup resistor, opto, secondary rectifiers.
Current information and trends
• Modern TV suppliers (>80 W) have migrated to two-stage PFC + LLC to meet EU ErP Lot 5 / U.S. DOE efficiency. The ZUV-194R-06 reflects this trend.
• Vestel cost-optimised chassis (17IPSxx before ~2014) remain single-switch because the target sets are ≤60 W and exempt from full-load PFC.
• Replacement boards now often integrate the half-bridge FET pair into a single “bridge-FET” module (Infineon CoolSET-F3, Power-Integrations InnoSwitch-3CP) to further raise efficiency and reduce BOM. Expect future drop-in boards to carry SMD power stages rather than TO-220 discretes.
Supporting explanations and details
Half-bridge LLC fundamental equations
Switching frequency
[ f_s = \frac{1}{2\pi\sqrt{L_r C_r}} ]
where L_r is the resonant inductance (magnetising + external) and C_r the series resonant capacitor.
Voltage-gain curve shows conversion ratio >1 below resonance (f < f_r) and <1 above resonance. Proper design sweeps frequency 30–140 kHz to regulate output while maintaining ZVS.
Fly-back power equation (continuous conduction)
[ P = \frac{1}{2}Lp \left(\frac{V{in}}{L_p}\right)^2 D (1-D) f_s \eta ]
Limitation: peak currents scale with duty-cycle; beyond ~120 W cost explodes; reason Vestel uses it only in smaller panels.
Ethical and legal aspects
• Both designs switch 325–400 Vdc at tens of kHz; lethal voltages remain for minutes after unplugging. Discharge bulk capacitors with 10 kΩ/5 W resistor before handling.
• Substituting MOSFETs with lower SOA or avalanche energy violates CE/UL safety approval; always match V_DS, E_AS, R_DS(on), Q_G.
• Do not bypass input EMI filter or NTC thermistor—doing so breaks EMC compliance.
Practical guidelines
Component set to replace in ZUV-194R-06 after primary blow-up
• Both half-bridge FETs (or all three if PFC FET exists).
• Driver IC (IRS2153/L6599/FAN7621) and its 10-47 µF/50 V V_CC electrolytic.
• Bootstrap diode (UF4005-UF4007 class) + 2.2–4.7 µF/25 V MLCC or low-ESR electrolytic.
• Resonant capacitor (47–220 nF/630 V polypropylene).
• Current-sense and gate resistors.
• Fusible resistor (e.g. 4R7 / 2 W) + input fuse (T2A).
Bring-up sequence
• Series 60–100 W incandescent bulb or electronic CC-source <0.3 A.
• Scope across each FET drain: expect ~½ V_BUS square wave, 50 % duty, ≈80–130 kHz; no simultaneous conduction.
• Verify PFC boost to 390 V once main relay clicks.
Common pitfalls
• Replacing only the visibly shorted FET—latent driver damage kills the new silicon within seconds.
• Overlooking dried 7–12 mm electrolytics causing intermittent undervoltage lock-out.
Possible disclaimers or additional notes
• Without a full schematic, ZUV-194R-06 might implement a two-switch forward rather than LLC; visually confirm presence of resonant capacitor and series inductor. Principles above still apply—two tightly timed switches vs. one in Vestel.
• A few late-revision Vestel boards (17IPS71+) added a discrete PFC MOSFET; those have three primary FETs and ARE closer to ZUV-194R-06, but not the early 17IPS12/20 your comment refers to.
Suggestions for further research
• Trace the ZUV-194R-06 gate-drive nets—identify controller IC marking, then download its datasheet for exact timing/UVLO specs.
• Use an LCR meter to compare resonant capacitor ESR to new part; drift beyond 0.1 Ω drastically shifts f_r.
• Investigate Infineon “600 V CoolMOS CFD7” or ON-Semi “650 V FDPAK” replacements for higher avalanche ruggedness.
Brief summary
ZUV-194R-06 and Vestel 17IPS12/20 share a similar footprint but differ fundamentally: the former uses a dual-switch primary (PFC + half-bridge/LLC) whereas the latter relies on a single-switch fly-back. This architectural divergence explains the extra MOSFET you observed and dictates unique diagnostic and repair tactics. Always verify topology before applying service data from “look-alike” boards to avoid repeat failures or safety hazards.
User question
two-switch forward would require two recovery diodes but on the printed board I don't see 2, only 1 big one (high current) and a smaller one
Artificial Intelligence Response
Direct answer to the question
Key points
• Two small primary clamp diodes → magnetising energy, low average current
• One big secondary diode → full load current, therefore large package
• Modern sets often replace the second secondary diode with a synchronous MOSFET, so only one big rectifier is visible on the low-voltage side.
Detailed problem analysis
Two-switch forward topology refresher
• Two MOSFETs, QH (high-side) and QL (low-side), are in series across the bulk capacitor (≈ 390 V).
• While both MOSFETs are ON, the full Vin is applied to the transformer primary and power is transferred.
• When both switches turn OFF, the primary voltage reverses. To prevent over-voltage, each drain must be clamped to the nearest rail:
– DH (high-side clamp) from QH drain to +Vin
– DL (low-side clamp) from QL drain to GND
• These diodes conduct only the transformer’s magnetising current Imag for a few hundred nanoseconds per cycle. Peak current can be several amps, but the average current is a few milliamps, so TO-220 packages are unnecessary.
What you see on the PCB
• Large diode on heatsink ‑ rated tens of amps at 60…200 V; connected to output filter capacitors ⇒ secondary rectifier.
• Smaller through-hole or SMD diode near the MOSFET area – could be one clamp diode or a bootstrap/snubber device.
• Missing second clamp? Usually a tiny SMD part: BYV26, UF400x, ES1-x, MUR1xx, etc. Common hiding places: under the mains-side heatsink, between MOSFET pins, or on the solder side.
Why clamp diodes are small
• Energy to reset the core is ( \frac{1}{2} Lm I{mag}^2 ) and returns each cycle. Conduction time is very short, so temperature rise is minimal.
• Fast recovery time (trr < 50 ns) is more important than continuous current.
Secondary side variants
• Schottky pair → two large diodes in common-cathode or common-anode pack (e.g. MBR, STPS).
• Synchronous rectification (SR) → one diode replaced by a MOSFET driven in antiphase to reduce forward losses; board then shows only one big diode plus an SR MOSFET.
• This is typical in >80 W LED/LCD TV power supplies designed after ≈2015.
Current information and trends
Supporting explanations and details
Package comparison (typical)
Analogy: Think of the clamp diodes as the seat-belt tensioners (only fire briefly in a crash) whereas the rectifier diode is the main brake system (works continuously).
Ethical and legal aspects
Practical guidelines
Possible disclaimers or additional notes
Suggestions for further research
Brief summary
Your board almost certainly does contain the two mandatory clamp diodes; they are just small, fast, and easy to miss. The conspicuous large diode is the secondary rectifier handling the full load current. Carefully trace the high-side and low-side MOSFET drains to the rails and you will locate the two small recovery diodes essential for transformer reset in a two-switch forward converter.