Design Idea DI-42
®
TinySwitch
Non-isolated Flyback
(Buck/Boost) Converter
Application
Appliances
Device
TNY254P
Power Output
1.2 W
Input Voltage
85-265 VAC
Output Voltage
12 V
®
Topology
Buck/Boost
Design Highlights
•
•
•
•
•
Lowest cost non-isolated flyback converter
Inherently short circuit & thermal overload protected
Excellent line and load regulation characteristics
Operates at 85 ˚C, without forced cooling
Meets EN61000-4-5 (2 kV line surge – simple
modification provides a 6 kV line surge withstand)
• Input AC line is fed directly to the output
• Available for prototyping in DAK-7
there is no analog control loop to compensate. The low
switching frequency under light and no-load conditions
provides very low power consumption under these
conditions – see Figures 2 and 3.
Providing power for driving TRIACs normally requires the
output of the supply to be referenced to the input line as
shown in Figure 1. Therefore, only half-wave rectification
is used and the switch must be in the return or neutral side of
the primary switching circuit. In these applications, EMI
filtering is normally done at the system level and is therefore
not shown here.
The circuitry shown allows this converter to meet 2 kV surge
requirements according to IEC standards. To extend surge
withstand to 6 kV, an MOV must be added between the Line
input and the node where D3 connects to R2. Additionally,
D3 must be replaced with 100
Ω,
5 W surge-rated resistor
(See DAK-7 for details).
The high efficiency of this converter (see Figure 4) allows
the circuit to operate to very high ambient temperatures of up
to 85 ˚C, as sometimes specified in household appliances and
industrial applications.
+12 VDC
D2
UF4005
U2
VR1
1N5342B
C3
220 uF
25 V
Circuit Features
This non-isolated Flyback (Buck-Boost) converter has been
implemented using a
TinySwitch
TNY254P, which minimizes
cost, PCB space and design time.
The most typical application for the circuit of Figure 1 is
powering a TRIAC drive and associated circuitry, in a consumer
or industrial appliance, having TRIAC controlled motors.
The combined voltage drops of Zener diode (VR1) and
optocoupler diode (U2) set the output voltage. Due to the digital
nature of the
TinySwitch
output voltage regulation scheme, the
current transfer ratio (CTR) of the optocoupler is not critical and
+
L
C1
6.8 uF
400 V
D3
1N4007
N
R2
180
Ω,
3 W
Fusible
D1
1N4007
C2
0.1 uF
50 V
85 - 265
VAC
L1
1.5 mH
TinySwitch
TNY254P
S
D
BP EN
U2
SFH615-2
PI-3280-090502
Figure 1. TinySwitch – TNY254P, 1.2 W, 12 V 100 mA.
.
DI-42
www.powerint.com
September 2002
POWERINT [ POWER INTEGRATIONS, INC. ]
