Design Idea DI-91
TinySwitch®-III
12 W Universal Input CV Adapter
Application
Device
Power Output
Input Voltage
Output Voltage
Topology
General Purpose
TNY278P
12 W
85-265 VAC
12 V
Flyback
The use of an E-shield™ in the transformer, a low-noise drain-
node clamp (R2, C4 and D5), a single Y capacitor (C5) and the
deviceʼs switching frequency jitter function enable a low-cost
pi (π) filter (C1, L1 and C2) to attenuate conducted EMI enough
toprovidemorethan10dBofmargintoEN550022requirements
(see Figure 4). The clamp circuit Zener (VR1) only conducts
briefly during startup and at worst-case overload, just before
the auto-restart function is activated.
Design Highlights
•
•
•
No-load consumption <50 mW at 265 VAC (R8 fitted)
Active mode efficiency >75% (CEC standard: ≥71%)
BP/M pin capacitor value selects MOSFET current
limit (ILIMIT–1, ILIMIT, or ILIMIT+1): for design flexibility
Output OVP function senses overvoltage on the output
through the primary bias winding
Accurate (±5%), auto-recovering, hysteretic thermal
shutdown function maintains safe PCB temperatures
Auto-restart function protects against output short
circuits and open feedback loops
•
•
•
•
Although TinySwitch-III family members do not need a bias
windingtooperate,onewasusedinthisdesigntotakeadvantage
of the ICʼs output OVPdetection function. If an open loop fault
causes an overvoltage on the output, VR2 will conduct. When
the current into the BP/M pin exceeds 5.5 mA, an internal latch
triggers and disables MOSFET switching (see Figure 3), until
ACpowerisremovedandreapplied(toresetthelatch). Resistor
R3 limits the current through VR2, while R3 and R7 attenuate
the portion of the leakage inductance spike that appears in the
bias winding.
> 3.2 mm package pin creepage: reliable operation in
high humidity and high pollution environments
Operation
This universal input, 12 V, 1 A, flyback converter was
designed around a member of the TinySwitch-III IC family,
the TNY278P. Because of the many features integrated into
the device family, the supply only requires 31 through-hole
components (no SMT), which enables a simple, single-sided
PCB layout to be used.
C5
2.2 nF
250 VAC
L2
D7
Ferrite Bead
3.5 × 7.6 mm
VR1
P6KE150A
BYV28-200
T1
+12 V, 1 A
NC
1
8
6
J3
J4
C11
100 µF
25 V
C10
1000 µF
25 V
D1
1N4007
D2
1N4007
R2
100 Ω
F1
RTN
C4
3.15 A
J1
R1
1 kΩ
C1
C2
10 nF
1 kV
6.8 µF
400 V
22 µF
400 V
R7
3
20 Ω
4
85-265
VAC
RV1
275 VAC
D5
1N4007GP
2
5
D6
UF4003
R5*
3.6 MΩ
J2
D3
1N4007
D4
1N4007
VR2
1N5255B
28 V
L1
1 mH
C6
VR3
BZX79-C11
11 V
1 µF
60 V
R3
47 Ω
*R5 and R8 are optional
components
1/8 W
R6
390 Ω
1/8 W
R8*
21 kΩ
1%
†C7 is configurable to adjust
U1 current limit, see circuit
description
U2
PC817A
D
EN/UV
BP/M
S
S
R4
2 kΩ
1/8 W
C7 †
100 nF
50 V
TinySwitch-III
U1
TNY278P
PI-4244-021406
Figure 1. TinySwitch-III 12 W, Universal Input, CV Adapter.
January 2006
DI-91
www.powerint.com
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