®
Design Idea DI-27
TinySwitch® 3 W AC Adapter:
<10 mW No-load Consumption
Topology
Flyback
Device
Power Output
Input Voltage
Output Voltage
Application
Adapter
3 W
85-265 VAC
TNY254
12 V
consume <100 mW at no-load. However, by providing
external bias, the internal high voltage current source, which
normally powers the IC from the DRAIN pin, is disabled and
a further reduction in consumption is therefore achieved.
Design Highlights
• Extremely low no-load power consumption (<10 mW at
115/230 VAC input)
• Simple RC snubber/clamp reduces EMI
• Low cost, low component count solution
The bias winding should provide enough current to fully
disable the internal current source. Figure 2 shows that the
bias winding and choice of C5 and R3 should provide
250 µA to 300 µA at no-load to minimize consumption.
Zener VR3 is required to clamp the BYPASS pin voltage.
Operation
The TinySwitch flyback converter in Figure 1 generates an
isolated 12 VDC, 3 W output from 85 VAC to 265 VAC input.
Typical applications include wall mounted AC adapters for
consumer products.
The output Zener VR2 is not biased, minimizing the
secondary circuit consumption. A low current Zener there-
fore provides the best output voltage tolerance.
The key performance characteristic of the circuit shown is
extremely low no-load consumption of <10 mW. A linear
transformer adapter of similar rating will typically consume
1 W to 4 W at no-load. At $0.12/kWh, the TinySwitch can
therefore reduce energy costs by $1 to $4 per year.
Components R2 and C3 provide a low cost clamp and also
help reduce EMI by softening switching edges. This type of
snubber increases switching losses and would therefore
normally be avoided in low consumption circuits. However,
with TinySwitch, these losses are negligible at no-load as
switching frequency is so low. This is illustrated in Figure 3
where replacing R2 and C3 with a Zener clamp across the
primary winding reduces consumption by <0.25 mW.
The no-load performance is achieved using a transformer bias
winding as a low voltage source for TinySwitch operating
current. Even without this winding, a TinySwitch circuit will
C8
2.2 nF Y1
D6 UF4003
+12 VDC
1
10
L1
1.0 mH
L2
Ferrite
Bead
C7
C6
180 µF
16 V
49T
100 µF
16 V
D1 & D2
1N4007
28 AWG
9
3
223T
36 AWG
D5 1N4148
RTN
RF1
8.2 Ω
R1
4.7 kΩ
C5
100 µF
10 V
30T
x2
37 AWG
R3
4.7 kΩ
2
4
L
VR2
BZX79B11
85 - 265
VAC
TinySwitch
R2
1.5 kΩ
1/2 W
C1 & C2
3.3 µF
400 V
U1
TNY254P
D
EN/UV
N
BP
D3 & D4
1N4007
S
VR3
BZX79-
B6V2
U2
PC817A
C3
68 pF
1 kV
U2
C4
0.1 µF
50 V
PI-3011-091802
Figure 1. 3 W AC Adapter with <10 mW No-load Consumption.
September 2002
DI-27
www.powerint.com