Design Idea DI-89 ® LinkSwitch-XT Low Cost 2 W CV Power Adapter Application Device Power Output Input Voltage Output Voltage Topology Adapter LNK362P 2W 85-265 VAC 6.2 V Flyback Design Highlights • • • • • • • the ringing of the filter. Switching frequency jitter and PIʼs E-Shield transformer construction technology enable this design to meet EN55022 Class-B conducted EMI with good margin (see Figure 4). Y capacitor C4 (optional) can improve the unit-to-unit repeatability of EMI scans. Low-cost, low parts-count CV solution: 20 components Proprietary IC design and winding techniques enable a Clampless™ drain-node ± 5% over-temperature threshold – with hysteretic recovery – keeps PCB temperatures below safety limits Auto-restart: output short circuit and open loop protection IC creepage > 3.2 mm: no arcing in humid environments Easily meets all EPS energy efficiency standards Meets CISPR-22 Class B EMI with sufficient margin This supply also takes advantage of PIʼs Clampless transformer techniques, which uses T1ʼs primary winding capacitance to clamp the voltage spike that its leakage inductance causes, each time the MOSFET in U1 turns off. Therefore, this converter has no primary clamp components connected to the drain-node. Operation From no-load until maximum output power (2 W) is delivered, the LNK362P (U1) regulates the output voltage by skipping switching cycles, based on the current delivered into the feedback (FB) pin. If the output is over loaded and no feedback (< 49 µA) is received within a 40 ms period, U1 goes into autorestart mode. In auto-restart, MOSFET switching is enabled for about 40 ms approximately every 800 ms if no feedback is received within the 40 ms window of enabled switching. This LinkSwitch-XT based flyback converter (Figure 1) provides 2 W of tightly regulated constant voltage (CV) output power, while meeting the active-mode efficiency and no-load power consumption requirements of all harmonized energy efficiency (EPA, CEC) standards (see Figure 2 and Figure 3). Diodes D1– D4 rectify the AC input. The resulting DC is filtered by bulk storage capacitors C1 and C2. Components L1, L2, C1 and C2 form a conducted EMI noise filter. Resistor R1 dampens L1 1 mH C4* 100 pF 250 VAC T1 EE16 9 4 5 3 J1 RF1 8.2 k 2.5 W 8 C5 330 µF 16 V 6.2 V, 322 mA J3-1 D5 1N4934 NC NC D1 1N4005 D2 1N4005 C1 3.3 µF 400 V 85-265 VRMS LinkSwitch-XT U1 LNK362P D4 1N4005 L2 1 mH J3-2 R3 390 Ω 1/8 W C2 3.3 µF 400 V J2 D3 1N4005 VR1 BZX79B5V1 5.1 V, 2% R1 3.9 k 1/8 W D U2 PC817A FB R2 1k 1/8 W BP S *Optional component C3 100 nF 50 V PI-4205-110805 Figure 1. LNK362 Based 6.2 V, 322 mA, 2 W, Low-cost, Flyback CV Output Power Adapter. DI-89 www.powerint.com November 2005 Key Design Points 68 • 66 • • • 0.1 85 VAC 115 VAC 230 VAC 265 VAC EPA Std 58 56 54 0 0.5 1 1.5 2 2.5 Output Power (W) Figure 3. Harmonized (EPA, CEC) Active-Mode Efficiency vs. Output Power (25, 50, 75 & 100%). 80 70 0.08 60 Amplitude (dBµV) Input Power (W) 60 PI-4235-110805 0.12 62 PI-4237-110805 • 64 Efficiency (%) • The PI Xls spreadsheet calculates all of the parameters required to specify and build transformer T1. The power transformer must have a two layer primary winding to ensure that its intra-winding capacitance is sufficient for Clampless operation. The reflected output voltage (VOR) of this design was kept < 90 V (74 V) for Clampless operation. Since this supply has a Clampless drain node, it must be verified that the maximum drain voltage does not exceed 650 V when the flyback voltage spike occurs. The primary current ripple-to-peak ratio (KP) factor should be > 1 (ensures discontinuous conduction mode operation) to minimize conducted EMI. The maximum operating flux density (BM) was kept < 1500 Gauss to eliminate audible. PI-4236-110805 DI-89 0.06 0.04 0.02 50 40 30 20 -10 0 0 50 100 150 200 250 Input Voltage (VAC) Figure 2. No-load Input Power Consumption vs. Input Voltage. 300 0 -10 -20 0.15 1 10 30 Frequency (MHz) Figure 4. Conducted EMI Scan to EN55022B Limits: Full-Load, 115 VAC, 60 Hz Input, with Artificial Hand. 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