Design Idea DI-27
®
TinySwitch
3 W AC Adapter:
<10 mW No-load Consumption
Application
Adapter
Device
TNY254
Power Output
3W
Input Voltage
85-265 VAC
Output Voltage
12 V
®
Topology
Flyback
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
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.
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.
The output Zener VR2 is not biased, minimizing the
secondary circuit consumption. A low current Zener there-
fore provides the best output voltage tolerance.
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.
C8
2.2 nF Y1
1
10
D6 UF4003
49T
28 AWG
D5 1N4148
C5
100
µF
10 V
R3
4.7 kΩ
VR2
BZX79B11
C6
180
µF
16 V
L2
Ferrite
Bead
+12 VDC
C7
100
µF
16 V
RTN
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 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.
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
D1 & D2
1N4007
RF1
8.2
Ω
L
85 - 265
VAC
L1
1.0 mH
223T
36 AWG
R1
4.7 kΩ
9
3
2
30T
x2
4
37 AWG
C1 & C2
3.3
µF
400 V
R2
1.5 kΩ
1/2 W
D
TinySwitch
U1
TNY254P
EN/UV
BP
N
D3 & D4
1N4007
C3
68 pF
1 kV
S
C4
0.1
µF
50 V
U2
PC817A
VR3
BZX79-
B6V2
U2
Figure 1. 3 W AC Adapter with <10 mW No-load Consumption.
DI-27
PI-3011-091802
www.powerint.com
September 2002
POWERINT [ POWER INTEGRATIONS, INC. ]
