Design Idea DI-34
®
TinySwitch-II
5 W Universal Adapter
Application
Adapter
Device
TNY266P
Power Output
5W
Input Voltage
85-265 VAC
Output Voltage
5V
®
Topology
Flyback
Design Highlights
• Simple design: low parts count (23)
• <300 mW no-load without transformer bias winding
• Frequency jittering reduces EMI - meets EN55022 Class B
with no Y capacitor: very low primary-to-secondary leakage
current
•
±7%
output tolerance with simple Zener reference
• High Frequency (132 kHz) operation allows small, low cost
EE16 transformer
• Built-in thermal shutdown protection
• ON/OFF digital regulation: No analog control loop to
compensate
The
TinySwitch-II
frequency jitter feature and transformer
construction allow use of a simple filter (L1, L2, C1 and C2)
to meet EMI requirements with L2 used to attenuate high
frequency EMI in the radiated spectrum (>30 MHz). The
circuit also uses a low cost RCD clamp (C7, D5, R3, and R4).
Diode D5 is a Glass Passivated (GP) general purpose diode,
allowing partial recovery of the leakage energy by
recirculating clamp current during the D5 reverse recovery
time, thus improving efficiency. Resistor R3 reduces EMI by
limiting the peak D5 reverse current and thus softening its
recovery characteristic.
The combined voltage drops of Zener diode (VR7) and
optocoupler diode (U2) set the output voltage. Resistor R7
biases VR7 according to its specification. The
TinySwitch-II
feedback current is independent of load allowing
±7%
output
voltage tolerance with this simple Zener reference.
Due to the digital nature of the
TinySwitch-II
control scheme,
no loop compensation is required. In addition, the current
transfer ratio (CTR) of the optocoupler is not critical and no
DC gain setting resistor in series with VR7 is required.
Operation
Figure 1 is a simple flyback converter utilizing several
TinySwitch-II
built-in features such as switching frequency
jittering, thermal shutdown and auto-restart fault protection.
Typical applications are wall mount adapters and other AC/DC
applications requiring very low cost and small size. The ability
to meet EMI requirements with no Y capacitor makes this
circuit ideal for applications requiring low primary-to-secondary
leakage currents.
T1
EE-16
C7
1.0 nF
1 kV
R3
100
Ω
D5
1N4005 GP
C1
6.8
µF
400 V
C2
6.8
µF
400 V
R4
200 kΩ
1/2 W
NC
L1
1.0 mH
D1
1N4005
D2
1N4005
RF1
8.2
Ω
2W
L
85 - 265
VAC
1
5
D6
1N5822
C4
1000
µF
10 V
L3
3.3
µH
1.0 A
C5
470
µF
10 V
5 V, 1.0 A
4
3
8
TinySwitch
U1
TNY266P
D
EN
BP
RTN
2
VR7 BZX79C3V9, 2%
N
D3
1N4005
D4
1N4005
S
L2
Bead
C3
0.1
µF
50 V
U2
PC817A
R7
150
Ω
PI-3326-051303
Figure 1. TinySwitch-II – TNY266P, 5 V, 1 A, 5 W Charger-Adapter Power Supply.
DI-34
www.powerint.com
May 2003
DI-34
PI-3325-090602
Key Design Points
• Use K
RP
(ripple-to-peak current ratio) in the range of 0.4 to
0.6 and V
OR
(output reflected voltage) of 90 V to
110 V for best efficiency.
• Use low cost optocoupler - gain is non-critical.
• PCB traces which carry high switching voltages and current
should be short and wide to reduce EMI.
• Reduce leakage inductance by filling each winding layer
across the entire width of the bobbin.
• R4 should be large enough to limit dissipation, to meet
<300 mW no-load target, while still limiting peak DRAIN
voltage to a safe value – 200 kΩ is a good start value for
most designs.
• A layer of insulation tape between each layer of primary
winding will further reduce inter-winding capacitance and
therefore switching losses.
• Ferrite bead L2 reduces radiated EMI.
I
O
= 0 A
PI-3324-090602
80
70
60
QP
AV
Amplitude (dBµV)
50
40
30
20
10
0
-10
-20
0.15
1.0
10.0
-100.0
Average
Quasi Peak
Frequency (MHz)
300
250
Figure 3. Conducted Emissions–EN55022 Class B (QP and AVG),
5 V, 1 A, 230 VAC, with Artificial Hand Connected to
Secondary Return.
TRANSFORMER PARAMETERS
Core
Bobbin
EE16, Nippon Ceramic NC-2H or
equivalent, A
L
= 135 nH/T
2
EEL16 Vertical
Shield (1-NC), Primary (4-1), Shield
(3-2), and Secondary (10-8).
1660
µH ±10%
400 kHz (minimum)
Input Power (mW)
200
150
100
50
0
50
100
150
200
250
300
Winding Order
(pin numbers)
Primary Inductance
(Pins 1-4 all others open)
Primary Resonant
Frequency (Pins 1-4, all
other open)
Input Voltage (VAC)
Figure 2. No-load Input Power Consumption.
Leakage Inductance
(Pins 1-4, with Pins 8-14
shorted)
70
µH
(maximum)
Table 1. Transformer Design Parameters.
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Power Integrations reserves the right to make changes to its products at any time to improve reliability or manufacturability.
Power Integrations does not assume any liability arising from the use of any device or circuit described herein, nor does it convey any
license under its patent rights or the rights of others. The products and applications illustrated herein (including circuits external to the
products and transformer construction) may be covered by one or more U.S. and foreign patents or potentially by pending U.S. and foreign
patent applications assigned to Power Integrations. A complete list of Power Integrations’ patents may be found at www.powerint.com.
The PI Logo,
TOPSwitch
,
TinySwitch
,
LinkSwitch
and
EcoSmart
are registered trademarks of Power Integrations, Inc.
PI Expert
is a trademark of Power Integrations, Inc. ©Copyright 2003, Power Integrations, Inc.
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