Flyback Controller Selection: Quasi-Resonant (QR) vs Fixed-Frequency PWM — On-Bright OB2201 / OB2358 / OB2500
How to pick a flyback IC: QR vs fixed-frequency PWM, external-MOSFET controller vs integrated-MOSFET power switch, and PSR vs SSR — using On-Bright OB2201, OB2358 and OB2500 as worked examples.
Quasi-resonant (QR) vs fixed-frequency PWM
A quasi-resonant (QR) flyback turns the switch on at the valley of the drain-voltage ring (valley switching), minimizing turn-on switching loss and lowering EMI — at the cost of a switching frequency that varies with load and line (it rises at light load / high line); QR mainly cuts turn-ON loss, while turn-off peak current can be higher. A fixed-frequency PWM flyback holds a constant switching frequency: simpler and more predictable design and EMI filtering, usually lower controller cost; modern parts add frequency dithering (“shuffling”) to spread the noise and pass conducted-EMI limits. As a rule, chasing efficiency standards (DoE Level VI, EU CoC Tier 2) favours QR/multi-mode, while simplicity and cost favour fixed-frequency PWM.
Controller (external MOSFET) vs integrated-MOSFET power switch
A controller (e.g. OB2201) contains no power switch; it drives an external power MOSFET through a GATE pin. The upside is that power scales with whatever MOSFET you choose and thermals/EMI/efficiency are tunable; the cost is more BOM and a larger board. An integrated-MOSFET power switch (e.g. OB2358, OB2500) puts the high-voltage MOSFET on-chip: fewest parts, smallest board, lowest cost at low power; the cost is that power is bounded by the on-chip MOSFET rating and heat concentrates in the IC. Low-power chargers/adapters lean integrated, while higher power or thermal headroom favours a controller plus external MOSFET.
PSR vs SSR: primary- vs secondary-side regulation
Primary-side regulation (PSR, e.g. OB2500) samples the auxiliary winding as an image of the output, so it can drop the optocoupler and TL431 — better BOM and standby, ideal for single-output, cost-driven chargers/adapters; the trade-off is looser regulation (commonly ~±3–5% in the industry) and weaker fast-transient response. Secondary-side regulation (SSR) senses the output directly and feeds back through an optocoupler (with TL431): tighter voltage accuracy (commonly ~±1–2%), better transient response and support for multiple outputs; the cost is the opto-plus-TL431 parts and bias current, and optocoupler CTR degrades over temperature and age. (Accuracy figures are general industry typicals, not a guaranteed spec of any specific part.)
How the three On-Bright parts map, and a selection checklist
OB2201 is a quasi-resonant (QR) flyback controller that drives an external MOSFET via a GATE pin — the corner for designs needing power headroom and chasing efficiency standards (the datasheet lists adapter, open-frame SMPS, set-top-box and NB/DVD applications; treat exact wattage as design-dependent). OB2358 is a fixed-frequency current-mode PWM power switch with an integrated high-voltage MOSFET and frequency shuffling for EMI, positioned for cost-driven offline flyback in the sub-27W class. OB2500 is a high-precision CC/CV PSR power switch with an integrated 2A/650V MOSFET that needs no optocoupler or TL431, for low-power chargers/adapters. To choose: very low power → integrated parts (OB2500/OB2358); higher power or thermal headroom → controller plus external MOSFET (OB2201); to pass efficiency standards lean QR/multi-mode or a strong-standby PSR; if you stay fixed-frequency pick a part with frequency dithering; for tight accuracy / multiple outputs / fast transients use SSR, while single-output with cost and standby priority and ±3–5% acceptable favours PSR.
Products mentioned
OB2201 Quasi-Resonant (QR) Flyback PWM Controller
The OB2201 is an On-Bright offline flyback Quasi-Resonant (QR) PWM controller — a controller-type IC that drives an external power MOSFET. It runs at ~52kHz CCM at low line, switches to QR valley-switching at high line, and enters PFM at light load to raise efficiency across the range and lower EMI. With cycle-by-cycle over-current (OCP), output over-voltage (OVP), VCC under-voltage lockout (UVLO), and pin open/short protection, in SOP-8 / DIP-8, it suits adapters, chargers, and set-top-box supplies.
OB2358 Integrated-MOSFET Current-Mode PWM Power Switch
The OB2358 is an On-Bright current-mode PWM power switch with an integrated high-voltage power MOSFET, for low-cost off-line flyback AC/DC converters — up to ~27W at 230VAC and ~16W over a universal 85~265VAC input. It runs at a fixed 50kHz with frequency shuffling for low EMI, uses extended burst mode for low standby power, and includes cycle-by-cycle OCP, OLP, OVP, and UVLO auto-recovery protections in a DIP8 package — for adaptors, set-top boxes, and auxiliary/standby supplies (note: a PWM power switch, not an LED driver).
OB2500 PSR Power Switch with Integrated MOSFET
The OB2500 is an On-Bright offline primary-side-regulation (PSR) power switch with an integrated driver power MOSFET, achieving CC/CV constant-current/voltage output from primary-side sensing alone — eliminating the TL431 and opto-coupler and cutting BOM. It features frequency shuffling for low EMI, UVLO and over-temperature (OTP) protection, very low standby power, and DOE Level 6 average efficiency, in OB2500N (SOP7) and OB2500P (SOP8) — for 5V USB phone chargers and low-power AC/DC adapters.
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