Diodes Incorporated AP3981CS-13

Overview

AP3981CS-13 from Diodes Incorporated is a primary-side regulated (PSR) flyback power switcher integrating a 650V N-channel MOSFET and PWM/PFM controller for AC/DC adapters and battery chargers. It delivers accurate constant voltage (2.4V FB threshold ±2%) and constant current (tONS/tSW = 0.5 above SUVP, 0.75 below) without optocoupler or secondary-side feedback. Achieves 75mW no-load input power and operates in DCM with valley-on switching for high efficiency and low EMI in STB, router, and network adapter power supplies.

For engineers reviewing the AP3981CS-13 datasheet, AP3981CS-13 pinout, AP3981CS-13 application, or AP3981CS-13 equivalent, key selection considerations include its integrated 650V/3A MOSFET, SO-8 package thermal resistance (θJA = 65°C/W), multi-segment AM/PFM control for audible noise suppression, and comprehensive protection suite including SOVP (3.6V FB threshold), SUVP (1.35V), SCP (750mV), and OTP (145°C shutdown).

Technical Context

The AP3981CS-13 implements primary-side regulation via auxiliary winding sampling during tONS, using programmable sample ratio (50–80% of tONS) to maintain CV accuracy across load and line conditions. Its dual-mode control combines PFM at light loads and peak-current amplitude modulation (AM) across three segments (VCS_L = 180mV, VCS_H = 630mV) to sustain switching frequency >20kHz and suppress audible noise.

Valley-on turn-on reduces switching losses and introduces natural frequency jitter; an additional active VCS_REF dither (±2% over 12-cycle period) further improves conducted EMI. Line compensation is implemented via external RFB1 and internal 55Ω RLC to correct CC drift across universal AC input, while transformer saturation protection triggers on three consecutive VCS > 800mV events.

Key Specifications

Pinout & Package

AP3981CS-13 is housed in a standard SO-8 package with exposed thermal pad (θJC = 3°C/W), optimized for surface-mount assembly and thermal management in compact adapter PCBs.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar primary-side regulated flyback controller applications.

Compared with MP6004DJ-LF-P and LNK364PG, the AP3981CS-13 uniquely combines valley-on switching, 3-segment AM/PFM, and external line compensation-enabling both sub-100mW standby compliance and <20kHz noise-free operation across universal input, making it optimal for premium STB/router adapters.

Availability

AP3981CS-13 is available at Aetrix Electronics and suitable for router power supplies, set-top box adapters, and network adaptors requiring stable component supply, regulatory-compliant standby power, and robust primary-side regulation.

Supply support for AP3981CS-13 includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Diodes Incorporated is a global manufacturer of discrete semiconductors and analog ICs, specializing in power management, signal integrity, and protection solutions for consumer, industrial, and communications markets.

The AP3981C belongs to Diodes' PSR AC/DC controller product line, designed specifically for cost-sensitive, space-constrained adapter applications demanding high integration, low no-load power, and full protection without secondary feedback.

FAQ

What is the recommended minimum R_CS value for AP3981CS-13 current sensing?

The minimum R_CS is determined by the peak current limit: R_CS = V_{CS_H}/I_{PK_MAX}. With V_{CS_H} = 630mV (typ) and I_{PK_MAX} ≤ 3A, R_CS ≥ 0.21Ω. A 0.22Ω, 1% tolerance, 1W metal film resistor is recommended to ensure stable current limiting and thermal margin under worst-case line/load conditions.

How does the AP3981CS-13 achieve constant current without secondary-side sensing?

It uses primary-side tONS/tSW ratio detection: during secondary conduction (tONS), the FB pin voltage reflects output voltage scaled by transformer turns ratio. The controller measures tONS duration relative to switching period (tSW) and adjusts switching frequency to hold tONS/tSW constant at 0.5 (above SUVP) or 0.75 (below SUVP), directly regulating output current independent of secondary components.

Can AP3981CS-13 support quasi-resonant (QR) operation?

No-it operates exclusively in discontinuous conduction mode (DCM) with valley-on detection, not true QR. Valley-on ensures turn-on near drain voltage minima but does not synchronize to resonant tank zero-crossings. Its tVAL-ON window (26–38μs) is fixed and not adaptive to variable resonant frequency, distinguishing it from QR controllers like INN3266C.

What is the maximum allowable V_CC capacitor value for stable startup?

The V_CC capacitor must balance hold-up time and startup delay. With ICC_OPR = 550μA (max) and VTH_ST = 16V, a 10μF/50V electrolytic capacitor provides >100ms hold-up after AC removal while keeping startup time <500ms. Larger values (>22μF) risk exceeding V_CC OVP (32V) during surge events unless clamped with Zener diode.