BU808 power suply

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BU808 power suply

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Direct answer to the question - BU808 is a high‑voltage NPN Darlington power transistor commonly the BU808DFIDFX variants with an integrated damperfree‑wheel diode. It is used most often as the horizontal output switch in CRT TVsmonitors and in older offline flyback SMPS stages. - If your “power supply” has a BU808, you are likely repairing a CRT chassis whose low‑voltage rails are derived from the flyback. A failed BU808 or its drivesnubbers will make the set look “dead.” - For new SMPS designs, a modern 600–800 V MOSFET is a better choice than a BU808 Darlington due to lower switching losses and simpler drive. Key points - Typical ratings vary by variantpackage: Vceo ≈ 700 V; Ic ≈ 8 A continuous; Ptot ≈ 50–75 W insulated packages toward the low end; Darlington Vbeon ≈ 1.2–2.0 V; higher VCEsat than MOSFETs. - Critical to reliability: solid base‑drive waveform, proper snubbering, and good heatsinking. - In popular CRT chassis e.g., various Vestel families, dried base‑drive electrolytic capacitors and cracked yokeflyback solder joints are frequent root causes behind recurring BU808 failures. Detailed problem analysis - Devicevariants - BU808 family = monolithic NPN Darlington. DFIDFX versions include an internal damper diode andor electrically insulated package for easier heatsinking. - Engineering implications of Darlington topology: - High current gain simplifies base drive current but - Higher VCEsat and Vbeon raise conduction loss and thermal stress versus a MOSFET. - Why the TV “power supply” appears dead when BU808 fails - In many CRT sets, the horizontal stageflyback LOPT generates multiple secondary rails. If the BU808 shorts or won’t switch, the B+ regulator may shut down or never rise, so the entire set looks dead even though the mains SMPS is attempting to start. - Typical failure chain field experience - Degraded base‑drive electrolytic often 10–22 µF, 50–63 V, 105 °C, low‑ESR distorts the drive, keeping the Darlington in its linear region → overheating → C‑E short. - Opencold joints at the deflection yoke connector or flyback pins cause arc spikes → overvoltage at the collector. - Stressed or incorrect snubber RCD or RC lets leakage energy ring up collector voltage → second breakdown. - Counterfeit replacements remarked low‑voltage BJTs fail instantly. - Electrical checks bench - With transistor removed: - B–E diode test Darlington: forward drop typically ~1.2–1.4 V; if ~0.6–0.7 V, suspect a fakestandard BJT. - C–E measurement will show the internal damper conduction in one direction for DFIDFX variants; should be open the other way. - Base‑drive waveform oscilloscope: - Clean, fast edges; amplitude and current consistent with forced gain target. For Darlingtons, design for forced β in the 100–200 range under peak current to keep it hard‑on; for non‑Darlington BJTs use 10–20. Verify turn‑off is snappy adequate base discharge. - Collector waveform: - Peak VCE should remain comfortably below rating design ≤ 0.7–0.8× Vceo with snubber margin. Excessive spikes indicate snubber or flyback issues. - Thermal and SOA - Respect switching SOA RBSOA with inductive loads. Darlington devices are more prone to second‑breakdown if run with slow turn‑off or inadequate snubbing. - Heatsink to keep Tj < 125 °C in real use; re‑check thermal interface micasilicone, torque, paste coverage. Insulated packages have higher θJC; derate accordingly. Current information and trends - BU808 parts are obsoleteend‑of‑life at many distributors; availability is inconsistent and counterfeits are common. Many service technicians now: - Prefer verified originalspulls from donor boards, or - Use equivalent Darlingtons e.g., 2SC5388 after checking pinout and SOA, or - Where practical non‑deflection SMPS, redesign around a 600–650 V superjunction MOSFET for higher efficiency and easier drive. - Industry shift: modern SMPS topologies QR flyback, LLC overwhelmingly use MOSFETsIGBTs, not Darlington BJTs, for improved efficiency and EMI. Supporting explanations and details - Snubbering the collector - RCD across the primary or RC across C–E clamps leakage spikes. Typical starting values: 10–47 nF ≥1 kV and 10–47 Ω, fast diode UFFR type in RCD clamps. Optimize by measuring peak VCE and ring‑down. - Drive network - Provide a low‑impedance drive and a proper discharge path: - Series base resistor sized to limit peak base current and damp ringing. - A small B–E resistor if not integrated helps discharge stored charge at turn‑off. - A reverse B–E diode if not integrated protects against negative drive excursions. - Start‑upbring‑up safety for repairs - Use a 60–150 W series incandescent lamp or variac + isolation transformer for first power‑up after replacing BU808. This limits surgeovercurrent and saves the new part if upstream faults remain. - Identifying counterfeits - Visual: inconsistent mold marksprint, wrong package mass. - Electrical: B–E drop ~0.6–0.7 V not Darlington‑like, unusually high leakage at room temperature, or low breakdown. Ethical and legal aspects - Safety: CRTs store lethal charge; discharge the CRT anode and primary bulk capacitor properly before service. Mains isolation is mandatory when probing primarydeflection stages. - Compliance: If substituting parts or redesigning the supply e.g., MOSFET conversion, verify creepageclearance, EMC, and thermal compliance. Do not defeat original protection circuits. Practical guidelines - If you are repairing a CRT chassis that “won’t power up”: 1. Remove and test BU808; replace only with a verified Darlington BU808DFIDFX, 2SC5388, etc. of matching or better VISOA and the same pinout. 2. Replace the base‑drive electrolytics near the driver transformertransistor with 105 °C low‑ESR parts often 10–22 µF, 50–63 V. 3. Inspectresolder yoke connector and flyback pins; check snubber components and the B+ regulator electrolytics. 4. Check secondary loads e.g., vertical IC, secondary rectifiers for shorts before first power‑up. 5. First power‑up through a series lamp; scope the collector and base to confirm clean switching and acceptable VCE peaks. - If you intend to use BU808 in a new power supply: - Prefer a 600–800 V MOSFET for primary switching. If a BU808 must be used, keep switching frequency modest, provide strong base drive with fast turn‑off, and over‑spec the snubber and heatsink. Validate against the device’s switching SOA, not only DC ratings. Possible disclaimers or additional notes - Ratings and internal features vary across BU808 variants and packages; always verify the exact suffix e.g., DFIDFX and datasheet limits before substitution. - Many “cheap” marketplace parts are re‑marked; bench‑test before committing. Suggestions for further research - Obtain the service manual or chassis number of your set; locate the horizontal drive electrolytics and snubber values. - Review the exact BU808 variant datasheet for SOA and thermal derating. - If redesigning, review application notes on RCD snubbers and base‑drive design for bipolar switches in flyback converters. Brief summary - BU808 is a high‑voltage Darlington typically used as the horizontal switch; failures often trace to poor drive or snubbering, not the transistor itself. For repair, replace the BU808 with a genuine equivalent, refresh the base‑drive electrolytic, verify snubbersjoints, and soft‑start the set. For new SMPS work, select a modern MOSFET instead. If you share the devicechassis model, inputoutput specs, and symptoms dead, ticking, blowing fuse, overheating, I can provide a targeted schematic snippet, test points, and exact component values to check.

BU808 Darlington Transistor in CRT TV Power Supply: Faults, Substitutes, and Repair Tips

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