Design Idea DI-118
®
TinySwitch-III
4.5 W CV/CC Charger with <260 mW
No-Load Consumption
Application
Device
Power Output
Input Voltage
Output Voltage
Topology
Charger
TNY276P
4.5 W
90-265 VAC
5.7 V, 800 mA
Flyback
WhentheprimarycurrentreachestheMOSFETcurrentlimit,
the controller turns it off, and the energy in T1 is transferred
to the output. Schottky diode D6 and capacitor C5 rectify
and filter the output. Inductor L3 and capacitor C7 attenuate
the switching ripple on the output.
Design Highlights
•
•
•
•
Simple, low cost, low parts count CV/CC solution
Low input power at no-load: <260 mW at 265 VAC
Efficiency >65% at 90 VAC
Meets CEC / ENERGY STAR requirements for active
mode efficiency (66.8 % vs 62.6 % requirement)
Compact design using small, low cost EE16 core size
>15 dBµV margin to EN55022B conducted EMI limits
No Y-capacitor: <10 µA line frequency leakage current
The primary clamp (D5, R2, C4 and R1) limits the maximum
peak drain voltage to less than the 700 V BVDSS rating of
the internal MOSFET. Resistor R2 reduces high-frequency
leakage inductance ringing and thereby EMI. The tightly
controlled tolerances of U1 allow this configuration to still
meet low no-load input power levels.
•
•
•
Operation
The CV/CC charger circuit shown in Figure 1 was designed
as a Flyback converter, using TNY276P (U1). The device
has its four SOURCE pins on one side of its 8-pin package,
whichsimplifiesthelayoutofthePCBcopperforheatsinking.
Additionally, the value of C3 selects the reduced current limit
ofU1.BothfactorsallowU1todeliverfullpowerfromwithin
a sealed enclosure, at an external ambient temperature of
40°C. Inlessthermallychallengingapplications, aTNY275P
operating at its standard current limit (C3 = 0.1 µF) could be
used to reduce cost, without any other changes.
ThepifilterformedbyC1,L1,L2andC2attenuatesconducted
EMI while C8 and R8 on the secondary side reduce high
frequency ringing. The integrated frequency jitter feature
of U1 along with E-ShieldTM techniques allow such simple
EMI filtering to ensure compliance with EN55022B, even
without a Y capacitor across the primary to secondary
isolation barrier.
The output is regulated using optocoupler feedback.
During CV operation, reference IC U3 senses the output
TheinternalMOSFETwithinU1conductscurrentthroughthe
primary winding of T1 during each enabled switching cycle.
C8
1 nF
50 V
R8
10 Ω
L3
R3
1.2 Ω
2 W
C5
Ferrite Bead
3.5 × 7.6 mm
5.7 V,
800 mA
L1
1 mH
470 µF
10 V
1
2
NC NC
3,4
10
7
D1
1N4005
D2
1N4005
C7
100 µF
10 V
D6
SB260
C4
1 nF
1 kV
R1
200 kΩ
RF1
8.2 Ω
2.5 W
T1
EE16
RTN
R4
100 Ω
C1
4.7 µF
400 V
C2
4.7 µF
R2
400 V
100 Ω
90-265
VAC
D5
1N4007GP
U2-B
D3
1N4005
D4
1N4005
R7
13.7 kΩ
1%
L2
Ferrite
Bead
TinySwitch-III
U1
TNY276P
C6
100 nF
50 V
R5
1 kΩ
D
EN/UV
BP/M
S
S
U2-A
PC817A
C3
1 µF
50 V
U3
TL431
R6
10 kΩ
1%
PI-4422-051006
Figure 1. 4.5 W, 5.7 V CV/CC Charger Using TNY276P.
September 2006
DI-118
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