Design Idea DI-117
TinySwitch-III
Application
Adapters / General Purpose
Device
®
15 W, 12 V Adapter with <100 mW No-Load Consumption
Power Output
15 W
Input Voltage
90-265 VAC
Output Voltage
12 V
Topology
Flyback
TNY279P
Design Highlights
•
•
•
•
•
•
•
Simple, low cost, low parts count solution
Low input power at no-load: <100 mW at 265 VAC
High efficiency >81% at 90 VAC
Meets CEC requirements for active mode efficiency
(79 % vs. 73.5 % requirement)
>10 dBµV margin to EN55022B conducted EMI limits
Simple, primary-side output overvoltage latching shut-
down protects load under fault conditions
Low line frequency leakage current <10
µA
Operation
The TNY279 (U1) in the 8-pin DIP package selected for the
flyback design in Figure 1 is ideal for adapter applications.
The arrangement of the four SOURCE pins on one side of
the package allows a small metal heatsink to be inserted.
This allows the device to operate in a sealed adapter with an
external ambient of 40
°C.
L4
1 mH
C5
1 nF
R5
R6
1 kV
240 kΩ 240 kΩ
The internal MOSFET in U1 conducts current through
the primary winding of T1 during each enabled switching
cycle. When the primary current reaches the MOSFET
current limit, the controller turns it off, and the energy
in T1 is transferred to the output. Schottky diode D7 and
capacitor C8 rectify and filter the output. Inductors L3 and
capacitor C9 attenuate the switching ripple on the output.
Capacitor C3 selects the standard current limit of U1. For
higher efficiency, the next larger (TNY280P) device may
be used. In this case, the value of C3 would be changed to
1
µF,
selecting the reduced current limit for the larger device,
but no other circuit changes would be required.
The primary clamp (D5, R1, C5, R5 and R6) limits the
maximum peak drain voltage below the 700 V breakdown
voltage of U1's internal MOSFET. Resistor R1 dampens the
high-frequency ringing of the T1 leakage inductance.
C6
68 pF
250 VAC
C11
R7
1 nF
100 V 20
Ω
1
T1
10
D7
SB580
C8
680
µF
25 V
L3
Ferrite Bead
3.5 × 7.6 mm
C9
220
µF
25 V
+12 V, 1.25 A
D1
1N4005GP
F1
3.15 A
90-265
VAC
D2
1N4005
C1
10
µF
400 V
C2
22
µF
400 V
R1
100
Ω
D5
FR107
3
NC
9
5
RTN
D6
1N4148
C7
1
µF
50 V
R8
56
Ω
4
EF20
D3
1N4005GP
D4
1N4005
L5
3.3
µH
VR3
BZX79-B6V8
6.8 V
R4
47
Ω
R3
10 kΩ
EN/UV
BP/M
U2-B
TinySwitch-III
U1
TNY279P
D
R9
1 kΩ
R11
39 kΩ
1%
R2
100
Ω
C4
33 pF
1 kV
S
S
C10
220 nF
50 V
U2-A
PC817A
U3
LM431
R10
3 kΩ
C3
100 nF
50 V
R12
10 kΩ
1%
PI-4417-051106
Figure 1. 15 W, 12 V Output Supply Using TNY279P.
DI-117
September 2006
POWERINT [ POWER INTEGRATIONS, INC. ]
