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Design Idea DI-85 (R) LinkSwitch-LP 2 W Charger:
Application Charger Device LNK564P Power Output 2W Input Voltage 90-265 VAC
Replaces Unregulated Linear Solutions
Output Voltage 6V Topology Flyback
Design Highlights
* Low-cost, low parts-count solution: 14-17 components * Proprietary IC design and winding techniques enable a ClamplessTM design with simple FilterfuseTM input stage * 5% over-temperature threshold - with hysteretic recovery - keeps PCB temperatures below safety limits * Auto-restart: output short circuit and open loop protection * IC creepage distance >3.2 mm: no arcing in high humidity * Easily meets all EPS energy efficiency standards * Meets CISPR-22 Class B EMI with good margin
Past the peak power point, cycle skipping ceases and U1 limits the supplys output current by reducing its oscillator (MOSFET switching) frequency, as the voltage on the FB pin drops. If the load demand causes the FB pin voltage to drop below an auto-restart threshold voltage VFB(AR) of 0.8 V (1 V to 1.5 V on the supply output) for more than 100 ms, the IC goes into auto-restart mode. In auto-restart, MOSFET switching is enabled for about 100 ms, approximately every 800 ms, until the FB pin voltage increases above 0.8 V. Due to the frequency jittering of the U1 internal oscillator and the E-ShieldTM winding techniques used in constructing the transformer (T1), conducted EMI is adequately attenuated by the LC filter formed by L1 and C1. Inductor L1 serves as both a differential mode choke and a fuse. This Filterfuse is sleeved with heat-shrink tubing, and its winding wire diameter was selected so that it will open like a fuse if any single component fails shorted. Thanks to the tight current limit tolerance of the LinkSwitch-LP family and construction techniques used on T1, the primary winding can be left Clampless, since the peak drain voltage does not approach the 700 V drain-to-source breakdown voltage (BVDSS) of U1. With no optocoupler and such a low parts count, this supply is cost competitive with the unregulated line frequency transformer based solution it was designed to replace.
Operation
This LinkSwitch-LP based flyback converter (Figure 1) provides an output VI curve (Figure 2) similar to that of an unregulated line frequency transformer based supply, but with output current that is limited past the maximum rated output power (peak power point). From no-load to the 2 W peak power point, the LNK564P (U1) regulates the output voltage by skipping switching cycles, based on the current delivered into the FEEDBACK (FB) pin. At the peak power point, the supply delivers >300 mA of load current at >5.7 VDC.
L J-1
D1 1N4937
L1 RF1* 8.2 3300 H 2.5 W C1 10 F 400 V
2
T1 EE16 7
D4 UF4002
C5 220 F 25 V
VR1* R3 1N5240B 6 V, 2 k 10 V 0.33 A J3-2
90-265 VAC J-2 N D2 1N4005
1
6 4 5 D3 1N4005 R1 37.4 k C3 330 nF 50 V
J3-1 RTN
D
LinkSwitch-LP U1 LNK564P
FB BP
C4* 100 pF 250 VAC
S
C2 0.1 F 50 V
R2 3 k
*Optional components
PI-4106-101105
Figure 1. LNK564 Based 6 V, 330 mA, 2 W, Low-Cost, Flyback Charger Power Supply.
DI-85
www.powerint.com
February 2006
DI-85
PI-4159-092905
Key Design Points
* The PI Xls spreadsheet calculates all of the parameters required to specify and build transformer T1. * This design uses one of two "standard" transformers (see AN-39). With this transformer, the output voltage of the supply can be set between 4 V and 7.5 V by selecting the appropriate value of R1. * Figure 1 contains three optional components: RF1, VR1 and C4. A fusible resistor must be used if a safety agency disapproves of using L1 as a fusible component. The auto-restart function limits the output power during open loop operation, so only a 1/2 W VR1 is required if the open loop output voltage is unacceptable. Y capacitor C4 improves EMI repeatability but is not required to meet EMI limits. * For the IC to limit the supplys output current past the point of peak power delivery, the voltage on the FB pin must begin to drop below 1.69 V as the load increases. Therefore, the value of R1 should be selected so the FB pin voltage is 1.69 V at the peak power point.
PI-4158-092905
160 140
Input Power (mW)
120 100 80 60 40 20 0
50
100
150
200
250
300
AC Input Voltage (VAC)
Figure 3. No-load Input Power vs. Line Voltage.
70 60
Efficiency (%)
10 9 8 7 6 5 4 3 2 1 0
90 VDC 115 VDC 230 VDC 265 VDC Min Limit Max Limit
50 40 30 20 10 0 0.00
90 VDC 115 VDC 230 VDC 265 VDC
Output Voltage (V)
0.50
1.00
1.50
2.00
2.50
3.00
Output Power (W)
Figure 4. Efficiency vs. Output Power and Line Voltage.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Average Output Current (A)
Figure 2. Output VI Characteristic Curves vs. Load & Line
For the latest updates, visit www.powerint.com Power Integrations reserves the right to make changes to its products at any time to improve reliability or manufacturability. Power Integrations does not assume any liability arising from the use of any device or circuit described herein. POWER INTEGRATIONS MAKES NO WARRANTY HEREIN AND SPECIFICALLY DISCLAIMS ALL WARRANTIES INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF THIRD PARTY RIGHTS. The products and applications illustrated herein (transformer construction and circuits external to the products) may be covered by one or more U.S. and foreign patents or potentially by pending U.S. and foreign patent applications assigned to Power Integrations. A complete list of Power Integrations' patents may be found at www.powerint.com. Power Integrations grants its customers a license under certain patent rights as set forth at http://www.powerint.com/ip.htm.
The PI logo, TOPSwitch, TinySwitch, LinkSwitch, DPA-Switch, EcoSmart, Clampless, E-Shield, Filterfuse, PI Expert and PI FACTS are trademarks of Power Integrations, Inc. Other trademarks are property of their respective companies. (c)Copyright 2005, Power Integrations, Inc.
Power Integrations
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B 2/06
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APPLICATIONS HOTLINE +1 408-414-9660
www.powerint.com
PI-4160-092905
80

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