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Design Idea DI-59 (R) LinkSwitch Low Cost 2.5 W
CV/CC Charger or Adapter
Application Charger/Adapter Device LNK500 Power Output 2.5 W Input Voltage 85-265 VAC Output Voltage 5.5 V Topology 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 A at 265 VAC input
T1 drives current out of the secondary winding, forward biasing D6, charging C6 and developing/maintaining the output voltage across C6. 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 = Efficiency. I2f is accurately controlled for the LinkSwitch; therefore, PO is proportional to L.
T1 EE13 LP = 2.3 mH 5 1
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 () 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
LinkSwitch
D C S
5.5 V, 0.45 A
U1 LNK500
L 85-265 VAC N RF1 8.2 1 W Fusible
D1-D2 1N4007
C2 4.7 F 400 V
C4 0.22 F 25 V C3 100 nF 63 V FILM
90T, #36 2 22T, #34 3 R3 100 C5 100 pF 500 V
15T, #30, TIW 6 2 5T, #30 x 3 C6 330 F 16 V D6 SB1100
R2 17.4 k 1%
R5 22 k
C1 4.7 F 400 V L2 Ferrite Bead
D5 1N4937 R1 100
RTN C7 R4 33 150 pF 100 V
D3-D4 1N4007
Figure 1. 2.5 W LinkSwitch Based Charger/Adapter
DI-59
PI-3692-091903
www.powerint.com
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
Core Bobbin TDK PC40 EE13, AL = 284 nH/T2 EE13 Horizontal 8 pin 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) 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) 2.3 mH 10%
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 F * To lower the ripple voltage into non-battery loads, an LC filter or LDO must be added onto the output
10
VIN = 85 VAC VIN = 265 VAC Low Limit High Limit
Winding Details
Winding Order (pin numbers)
Primary Inductance
Primary Resonant 300 kHz (minimum) Frequency
PI-3694-091803
Table 1. Transformer Design Parameters.
PI-3693-090503
Output Voltage (V)
8
350
No Load Consumption (mW)
300 250 200 150 100 50 0 60 90
6
4
2
0 0 100 200 300 400 500 600
Output Current (A)
120 150 180 210 240 270
Figure 2. Load Regulation - CV/CC Characteristics.
Input Voltage (VAC)
Figure 3. No-load Input Power Consumption.
A 9/03
www.powerint.com

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