®
Design Idea DI-42
TinySwitch® Non-isolated Flyback
(Buck/Boost) Converter
Topology
Buck/Boost
Device
Power Output
Input Voltage
Output Voltage
Application
Appliances
1.2 W
85-265 VAC
TNY254P
12 V
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.
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
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.
Circuit Features
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).
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
poweringaTRIAC driveandassociatedcircuitry,inaconsumer
or industrial appliance, having TRIAC controlled motors.
The high efficiency of this converter (see Figure 4) allows
thecircuittooperatetoveryhighambienttemperaturesofup
to85˚C,assometimesspecifiedinhouseholdappliancesand
industrial applications.
The combined voltage drops of Zener diode (VR1) and
optocouplerdiode(U2)settheoutputvoltage.Duetothe digital
nature of the TinySwitch output voltage regulation scheme, the
currenttransferratio(CTR)oftheoptocouplerisnotcriticaland
+12 VDC
+
D2
UF4005
C3
U2
220 uF
25 V
VR1
1N5342B
L
C1
6.8 uF
400 V
L1
1.5 mH
85 - 265
VAC
TinySwitch
R2
180 Ω, 3 W
Fusible
D3
1N4007
D1
1N4007
TNY254P
S
D
C2
0.1 uF
50 V
N
BP EN
U2
SFH615-2
PI-3280-090502
Figure 1. TinySwitch – TNY254P, 1.2 W, 12 V 100 mA.
.
DI-42
September 2002
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