Design Idea DI-77
®
TinySwitch-II
3 W Adapter:
Application
Adapter
Device
TNY263P
Power Output
3W
<200 mW No-Load Consumption
Input Voltage
85-265 VAC
Output Voltage
9 V / 330 mA
Topology
Flyback
Design Highlights
•
•
•
•
•
•
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
(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.
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.
Minimize clamp losses: design the transformer to have a low
reflected voltage (V
OR
). In this case, V
OR
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
EMI performance is not compromised, and 2) there is enough
drain voltage (BV
DSS
) margin for the internal MOSFET.
R2
16
Ω
NC NC
C7
1
µF
50 V
L3
Bead
D6
11DQ06
9 V, 330 mA
Operation
This
TinySwitch-II
based flyback converter produces an isolated
3 W, 9 VDC output from an 85 VAC to 265 VAC input. Fusible
resistor RF1 gives short circuit fault protection and limits startup
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 a Y capacitor, 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
L2
Bead
R1
200 kΩ
1/2 W
C3
1 nF
1 kV
RF1
8.2
Ω
1.0 W
D1
1N4007
D2
1N4007
C1
4.7
µF
400 V
R3
200
Ω
D5
1N4007G
C2
4.7
µF
400 V
D
•
•
•
•
1
10
3
T1
EE16
Lp = 1.6 mH
8
C5
330
µF
16 V
C6
100
µF
25 V
L
85 - 265
VAC
RTN
VR1
BZX79-B8V2
N
TinySwitch-II
U1
TNY263P
EN
BP
D3
1N4007
D4
1N4007
U2
PC817A
C4
0.1
µF
50 V
PI-3877-060404
S
L1
1.0 mH
Figure 1. TinySwitch-II 3.0 W Adapter.
DI-77
www.powerint.com
May 2004
POWERINT [ POWER INTEGRATIONS, INC. ]
