Design Idea DI-77
®
TinySwitch-II 3 W Adapter:
<200 mW No-Load Consumption
Application
Device
Power Output
Input Voltage
Output Voltage
Topology
Adapter
TNY263P
3 W
85-265 VAC
9 V / 330 mA
Flyback
Design Highlights
(a similarly rated linear transformer adapter will typically
consume approximately 1 W under no-load). This low no-load
consumption is the result of the inherent cycle skipping feature
of the TinySwitch-II and careful transformer design.
•
Replaces a linear transformer based power supply at the
same or lower cost, but with much higher performance
<200 mW no-load consumption (at 115/230 VAC)
Meets CISPR 22-B with >15 dB margin, w/o Y capacitor
Low cost, low component count solution: only 24 parts!
TinySwitch-II On/Off regulation scheme produces ±5%
output regulation over temperature with a 2% Zener
Does not require a transformer bias winding
•
•
•
•
Key Design Points
•
Minimize secondary-side power consumption. Zener diode
VR1 receives its bias current from the opto-LED in U2.
Using a low current device minimizes consumption while
providing an output voltage tolerance of ±5%. A TL431
can be used for tighter output voltage accuracy.
•
Operation
•
•
•
Minimize clamp losses: design the transformer to have a low
reflected voltage (VOR). In this case, VOR was set to ≈ 90 V.
Minimize leakage inductance: select wire gauge sizes that
completely fill each winding layer of the transformer.
Minimize intra-winding capacitance and no-load
consumption: put a layer of tape between each primary
winding layer.
Use >200 kΩ resistor in the RCD clamp to further reduce
power losses if the following two conditions are met: 1) the
EMIperformanceisnotcompromised,and2)thereisenough
drain voltage (BVDSS) margin for the internal MOSFET.
ThisTinySwitch-IIbasedflybackconverterproducesanisolated
3W, 9VDCoutputfroman85VACto265VACinput. Fusible
resistorRF1givesshortcircuitfaultprotectionandlimitsstartup
inrush current. Inductors L1 and L2 and capacitors C1 and C2
form a low-cost pi (π) filter that attenuates conducted EMI. In
addition, the internal frequency jitter of the TinySwitch-II, the
use of shield windings, an output RC snubber, and a primary-
side RCD clamp makes the circuit meet CISPR-22 Class B
conducted EMI limits without aYcapacitor, resulting in a very
low value of AC leakage current (<300 µA).
•
The key performance characteristic of this circuit is the
extremely low no-load power consumption of <200 mW
C7
R2
16 Ω
1 µF
50 V
R1
L2
L3
NC NC
200 kΩ
Bead
1/2 W
1
Bead
9 V, 330 mA
10
8
C3
1 nF
1 kV
D6
11DQ06
C5
330 µF
16 V
C6
100 µF
D1
D2
3
RF1
8.2 Ω
1.0 W
25 V
1N4007 1N4007
R3
L
T1
EE16
200 Ω
C1
4.7 µF
400 V
Lp = 1.6 mH
D5
1N4007G
RTN
85 - 265
VAC
C2
VR1
BZX79-B8V2
4.7 µF
400 V
TinySwitch-II
U1
N
D
S
TNY263P
EN
BP
U2
PC817A
D3
1N4007
D4
1N4007
C4
L1
1.0 mH
0.1 µF
50 V
PI-3877-060404
Figure 1. TinySwitch-II 3.0 W Adapter.
May 2004
DI-77
www.powerint.com
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