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• BP2958 is a non-isolated, high-PF, critical-conduction-mode (CRM) buck-type constant-current LED-driver IC that runs directly from rectified 230 VAC.
• A functional circuit therefore consists of:
– Input protection (fuse, MOV, NTC).
– Bridge rectifier + small “bulk” capacitor (≈1-4.7 µF/400 V) to obtain 310 VDC.
– BP2958 whose DRAIN/VIN pin is tied to the bulk rail, SW pin to an inductor, then a fast diode, LED string, and back to ground through a sense resistor RS.
– RS sets the LED current: RS = VCS(th) / IPEAK (VCS(th)≈0.2 V). ILED ≈ IPEAK / 2.
– L (470 µH…1.5 mH) and a ≥400 V Schottky or UF diode complete the buck stage.
• Optional DIMming is achieved by PWM-gating the BP2958 supply or the LED string.
• All PCB tracks around DRAIN-SW-L-D must be short; star-ground the CS node; keep creepage ≥3 mm.
• Because the circuit is NOT galvanically isolated from the mains, all parts of the board are at lethal potential. Use Class II insulation and comply with IEC / EN 61347, 60598, EMC, RoHS.
IC overview
– Internal 900 V MOSFET, PF>0.9 without PFC network (uses valley/zero-current switching).
– Typical switching freq. 50–120 kHz (variable with load).
– Key pins: DRAIN/VIN, SW, CS, GND (and DIM/VCC on “BP2958X” options).
Core schematic (single channel)
AC(L) ─ Fuse ─ MOV ─┐
AC(N) ──────────────┘
│
Bridge Rectifier (KBPC-04, etc.)
│ ≈310 VDC
+─── Cbulk 1–4.7 µF/400 V (X cap, high-frequency film or MLCC in parallel)
│
DRAIN ───┤BP2958
│ SW ─────── L 470 µH–1 mH / I_sat ≥1.2×I_PK ───► UF4007 / ES1J diode ──► LED(+)
│ ▲
│ |
CS ─ RS 0.27–1 Ω (≤0.5 W) ─────── LED(–) ── Cout 22–100 µF/100 V (optional)
│
GND ────────────────────────────────────────────────────────────────────────────Component calculation example (230 VAC, 16 × 3 V LEDs ≈ 48 V, desired ILED = 300 mA)
• Select RS:
IPEAK = 2 × ILED ≈ 0.6 A → RS = 0.2 V / 0.6 A ≈ 0.33 Ω (choose 0.33 Ω, 0.5 W).
• Inductor: use 680 µH, I_sat ≥0.8 A, DCR <1 Ω.
• Diode: ES3J (600 V, 3 A) or SS2H10; trr ≤150 ns.
• Cbulk: 2.2 µF/400 V X2 film in parallel with 0.1 µF MLCC for HF.
• Cout: 47 µF/100 V low-ESR if ripple or flicker must be minimised.
Dimming / dual-channel CCT usage
• BP2958 itself has no linear dim pin; versions tagged “BP2958D/X” expose a DIM input.
• Common practice in smart fixtures (e.g. BK7231N Wi-Fi CCT lamp) is to PWM-switch the entire LED string with a MOSFET on the low side, or PWM-gate the VCC of the BP2958 through an NPN/MOSFET.
• For CCT lamps you duplicate the circuit: one driver for warm-white, one for cool-white; the MCU supplies two independent PWM streams.
PCB layout & EMI
• Minimise loop area DRAIN–SW–L–D (hot loop).
• Place RS next to CS pin; route CS as Kelvin sense.
• Add RC snubber (e.g. 47 Ω + 1 nF) between SW and GND if conducted EMI exceeds EN 55015 limits.
• Use a solid ground copper pour as heatsink for the IC.
Protection & reliability
• Front-end: T-type fuse (250 mA–1 A slow-blow) + S14K275 MOV + 5R NTC.
• Thermal: ensure T_J < 125 °C; metal-core board or 2-oz copper for >10 W.
• Add TVS (SMBJ440A) across LED string for surge immunity.
• Many low-cost smart bulbs now replace bare BP2958 supplies with combined Wi-Fi + PSU modules (LN882H, HLK-LDxx) to improve standby efficiency and EMC, but BP2958 remains common in retrofit driver boards because of its minimal BOM and integrated HV MOSFET.
• Newer pin-compatible parts (BP5918, OB3350) claim lower THD (<10 %) and built-in OTP.
• Why very small bulk capacitor? Critical-conductance mode lets input current follow the rectified sine, achieving PF≈0.9 without a large electrolytic.
• Why high inductance? To maintain CCM/CRM at 50/60 Hz valley (worst-case VIN).
• Why Schottky not ultrafast? Forward drop and reverse recovery dominate efficiency; choose >400 V rating to survive line peaks.
• Non-isolated design means entire secondary is at mains potential – accidental LED touch is lethal. Consumer products must provide double insulation or isolated enclosure per IEC 60598-1 Class II.
• Disposal: BP2958 contains lead-free materials but whole board counts as WEEE; recycle accordingly.
• Datasheets from BPS (Bright Power Semiconductor) show VCS(th) spread 180–250 mV; design for the worst-case to stay within LED absolute max current.
• BP2958 cannot meet SELV; for user-accessible LEDs an isolated flyback driver (e.g. BP37xx or LYT168) is mandatory.
• Investigate synchronous-rectifier replacements (BP5012) for higher efficiency at >500 mA.
• Examine digital dimming protocols (0–10 V, DALI) wrapped around BP2958 by modulating VCC.
• Look into GaN FET retrofits for the inductor-based buck to shrink magnetics. Datasheet app-notes: BPS AN_BP2958_Rev2.1, ON-Semi LED-buck CRM models.
BP2958 enables an ultra-low-cost, high-PF buck LED driver that connects straight to rectified mains. The heart of the design is the sense resistor (sets current) and the buck L-D network (stores and transfers energy). Correct component sizing, tight PCB layout, and rigorous mains-safety practices are essential. For dual-channel CCT fixtures simply duplicate the stage and PWM each channel via a microcontroller. Always respect insulation rules: although the circuit is electrically simple, it operates at lethal voltage and must comply with IEC lighting safety and EMC standards.