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Design Idea DI-58 LinkSwitch Low Cost 1.5 W
CV/CC Charger or Adapter
Application Charger/Adapter Device LNK500 Power Output 1.5 W Input Voltage 85-265 VAC Output Voltage 5.5 V Topology Flyback
Design Highlights
* Replaces a linear transformer based supply at the same or lower cost but with much higher performance * <0.3 W consumption at zero load meets worldwide guidelines (EC's 0.3 W, USA's 1 W for example) * Extremely simple circuit - only 15 components for production-worthy design * Primary based CV/CC output - no secondary sense components required * 10% output voltage and 25% output current tolerances at peak power point * Fully protected for thermal, short circuit and open loop faults * >62% efficiency (>70% with R1 replaced by an inductor) * Meets CISPR22B/EN55022B EMI limits with low cost resistive input filter * Ultra-low leakage current design <5 A * EE13 core for low cost and small size
LinkSwitch derives all feedback information from the primary. During output diode conduction, the output voltage reflected through the turns ratio is sampled and held by C4. The feedback voltage across C4 (VOR) is converted into feedback current by R2 and fed into the CONTROL pin. This feedback current regulates the output by PWM control during CV operation, and by reducing the internal current limit during CC operation. Below an output voltage of ~2 V, LinkSwitch enters auto-restart, limiting average output current to less than 50 mA. The nominal transition from CV to CC occurs at 5.5 V, 0.27 A. The output envelope characteristic and specification limits are shown in Figure 2. Together with D3, C4 and R2 are also part of the primary clamp, limiting the peak drain to source voltage due to leakage inductance. Resistor R3 filters the leading edge leakage inductance spike, reducing the error in the feedback voltage. The CONTROL pin capacitor C3 provides energy storage for supply start-up and sets auto-restart timing during fault conditions.
Operation
The AC input is rectified and filtered by D1, D2, C1 and C2. Conducted EMI filter is provided both by a filter (C1, R1 and C2) and a differential filter (RF1 and C1). Together with a shield in the transformer (formed from part of the primary), the design meets conducted EMI limits with no Y-capacitor between primary and secondary. Resistor RF1 also functions as a fuse.
Key Design Points
* Select transformer turns ratio to give a VOR of 40 V to 60 V. Lower values reduce power capability; higher values increase no-load consumption. * R2 provides 2.3 mA into the CONTROL pin at the peak power point at 85 VAC. The value can be adjusted to center the output voltage.
T1 1 C3 0.22 F C4 0.1 F 100 V FILM 104T 34 AWG 4 12T 2 x 30 AWG 3 EE13 D4 LP = 1.36 mH UF4002 1A 100V 5 15 T 30 AWG T.I.W. C5 220 F 16V RTN 5.5 V, 0.27 A
R1 68 Flameproof
LinkSwitch
D C S
U1 LNK500
L 85-265 VAC N
RF1 10 Fusible
D1 1N4007 R2 21 k C1, C2 2.2 F 400 V
6
D3 1N4937 R3 100
D2 1N4007
Figure 1. LinkSwitch 1.5 W Charger Power Supply: 85 VAC to 265 VAC Input, 5.5 V, 0.27 A Output
DI-58
PI-3687-091903
www.powerint.com
September 2003
DI-58
* To maintain the 25% CC tolerance the primary inductance tolerance should be tighter than 10%. * Minimize zero load consumption by reducing drain node capacitance: Use double coated/grade 2 wire for primary and do not vacuum impregnate. Avoid using an RC snubber across the output diode. * For resistive loads, increase C3 to 1 F (electrolytic) to allow adequate time for start-up at full load. * For battery loads, an output LC filter is typically not required but can be added for resistive loads to reduce switching ripple. * R1 can be replaced with an inductor for higher efficiency (~10% increase). * Adding a 1 mA to 2 mA pre-load reduces zero load voltage by ~1 V but increases power consumption by ~10 mW. * Diode D4 can be replaced with a Schottky for higher efficiency. * See AN-35 for more information.
VIN = 85 VAC VIN = 265 VAC Low Limit High Limit
PI-3686-091803
TRANSFORMER PARAMETERS
Core Bobbin TDK PC40 EE13, ALG= 101 nH/T2 EE13 Horizontal 8 pin Primary: 104T, 34 AWG Shield: 12T, 2 x 30 AWG Secondary: 15T, 30 AWG T.I.W. Flux Band: 1T, 6 mm Cu foil (T.I.W.: Triple Insulated Wire) Secondary (5-6), tape, Shield (3-4), tape, Primary (4-1), tape, Flux band (3-NC) Primary: 1.36 mH 10%, Leakage: 50 H (max.) 300 kHz (minimum)
Winding Details
Winding Order (pin numbers) Inductance Primary Resonant Frequency
Table 1. Transformer Construction Information.
No-Load Power Consumption (W)
PI-3373-091803
10
0.3 0.25 0.2 0.15 0.1 0.05 0 50
Output Voltage (V)
8
6
4
2
0 0 0.1 0.2 0.3 0.4
100
150
200
250
300
Output Current (A)
Input Voltage (VAC)
Figure 3. No-load Input Power Consumption.
Figure 2. Load Regulation - CV/CC Characteristics with Limits.
A 9/03
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