Design Idea DI-59 ® LinkSwitch Low Cost 2.5 W CV/CC Charger or Adapter Application Device Power Output Input Voltage Output Voltage Topology Charger/Adapter LNK500 2.5 W 85-265 VAC 5.5 V Flyback Design Highlights • Replaces a linear transformer based power supply at the same or lower cost, with better output regulation • No-load input power consumption is less than 300 mW, at 230 VAC input: meets worldwide conservation guidelines • Extremely simple circuit – requires only 23 components! • No alternate path of secondary-side feedback is required: output feedback is obtained through the main transformer! • Output Voltage (CV) tolerance ±10% at peak power point • Output Current (CC) tolerance ±25%, when L ≤ ±10% • Features the following auto-recovering protection functions: output short-circuit, open feedback loop, thermal shutdown • Typically about 68% efficient! • Meets EN550022 B EMI limits, without a Y-1 type Safety capacitor between the primary and the secondary • Ultra-low leakage current: < 5 µA at 265 VAC input Operation Fusible resistor RF1 gives short-circuit fault protection and limits start-up inrush current. Diodes D1–D4 provide full bridge rectification that charges capacitors C1 and C2. Inductors L1 and L2 and capacitors C1 and C2 form a low-cost pi (π) filter that attenuates conducted EMI. Transformer (T1) winding phasing and D6 orientation let no secondary winding current flow when the MOSFET in U1 is ON, so the current that flows through the primary winding stores its energy in the core of T1. When the MOSFET in U1 is turned OFF, the energy stored in L1 1.5 mH S C D1-D2 1N4007 L 85-265 VAC N C2 4.7 µF 400 V C1 4.7 µF 400 V RF1 8.2 Ω 1 W Fusible D3-D4 1N4007 L2 Ferrite Bead The RCD network of C3, D5 and R1 has two functions: 1) It clamps the reflection of the output voltage (VOR) on the primary winding, as the MOSFET turns OFF. 2) It holds VOR, for use as output feedback, eliminating the cost and complexity of a separate feedback path. Resistor R1 attenuates the switching noise from the VOR. Resistor R2 determines the amount of feedback current that flows into the CONTROL pin of U1. The output voltage can be fine-tuned by varying the value of R2. CONTROL pin capacitor C4 stores power and supplies it back to U1 during start-up, determines the restart “attempt” frequency in the auto-restart mode, shunts high frequency noise around U1 and provide U1 with the instantaneous MOSFET gate-drive current it requires. Resistor R3, capacitor C5, the 22-turn core cancellation winding and the 5-turn shield winding all reduce EMI. LinkSwitch based solutions are designed to operate only in the discontinuous conduction mode. PO ~ 0.5 L I2f, where PO = Output Power, L = transformer primary inductance, I = LinkSwitch peak current, f = Switching frequency and η = Efficiency. I2f is accurately controlled for the LinkSwitch; therefore, PO is proportional to L. T1 EE13 LP = 2.3 mH 5 1 LinkSwitch D T1 drives current out of the secondary winding, forward biasing D6, charging C6 and developing/maintaining the output voltage across C6. U1 LNK500 R2 17.4 kΩ 1% D5 1N4937 C4 0.22 µF 25 V C3 100 nF 63 V FILM 90T, #36 15T, #30, TIW 2 22T, #34 3 6 2 5T, #30 x 3 C6 330 µF 16 V D6 SB1100 R3 100 Ω C5 100 pF 500 V R1 100 Ω Figure 1. 2.5 W LinkSwitch Based Charger/Adapter DI-59 5.5 V, 0.45 A www.powerint.com R5 22 kΩ RTN C7 R4 33 Ω 150 pF 100 V PI-3692-091903 September 2003 DI-59 Typical applications are chargers for cell phones, PDAs, portable audio devices and shavers or power sources embedded within home appliances and consumer electronics, such as TV standby and bias supplies. TRANSFORMER PARAMETERS TDK PC40 EE13, AL = 284 nH/T2 Core Bobbin EE13 Horizontal 8 pin Key Design Points • Set VOR between 36 V and 60 V (50 V is optimum) • Transformer primary inductance tolerance must be ≤10%, to maintain CC limit tolerance of ±25% • If battery (load) voltage is less than 2 V, then the LinkSwitch will not come out of its auto-restart mode • To increase the time period allowed for regulation to be reached at startup (or into a full-power resistive load), C4 must be increased in value to 1 µF • To lower the ripple voltage into non-battery loads, an LC filter or LDO must be added onto the output 4 2 0 100 200 300 400 500 Primary Resonant 300 kHz (minimum) Frequency 350 6 0 2.3 mH ± 10% 600 PI-3693-090503 8 Primary Inductance Core cancellation (2-3, clockwise), tape, Primary (1-2, clockwise), tape, Shield (2-open, anti-clockwise), tape, Secondary (5T: 5-7, anti-clockwise, 10T: 7-6, anti-clockwise) Table 1. Transformer Design Parameters. PI-3694-091803 VIN = 85 VAC VIN = 265 VAC Low Limit High Limit Winding Order (pin numbers) No Load Consumption (mW) Output Voltage (V) 10 Winding Details Core Cancellation: 22T, 34 AWG Primary: 90T, 36 AWG Shield: 5T, 3 x 30 AWG Secondary: 5T, 2 x 30 AWG (TIW) + 10T, 30 AWG (TIW) 300 250 200 150 100 50 0 Output Current (A) 60 Figure 2. Load Regulation - CV/CC Characteristics. 90 120 150 180 210 240 270 Input Voltage (VAC) Figure 3. No-load Input Power Consumption. A 9/03 www.powerint.com