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
DI-42
Key Design Points
• Use low cost optocoupler - CTR is unimportant for output
regulation.
• PCB traces that carry switching currents should be short
and wide to minimize EMI.
• Select input and output capacitors to meet ripple current
requirements.
• Diode D2 must be an ultra-fast type, with reverse recovery
time (t
rr
)
≤
50 ns.
• Minimum recommended output voltage in this topology is
5 V.
Lower voltages may cause continuous mode operation,
forcing high reverse recovery currents in D2 and therefore,
lowering efficiency.
PI-3379-091602
PI-3380-091602
100
80
Input Power (mW)
60
40
20
0
50
100
150
200
250
300
Input Voltage (VAC)
Figure 3. No-load Power Consumption.
PI-3381-091602
1.2
1
80
Input Power (W)
0.8
70
0.6
0.4
0.2
0
0
.200
.400
.600
.800
40
0
20
40
60
80
100
VIN = 115 VAC
VIN = 230 VAC
Efficiency (%)
60
50
VIN = 115 VAC
VIN = 230 VAC
Output Power (W)
Figure 2. Output Power vs. Input Power.
120
Output Power (mA)
Figure 4. Efficiency vs. Output Current.
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www.powerint.com
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 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
and
EcoSmart
are registered trademarks of Power Integrations, Inc.
PI Expert
is a trademark of Power Integrations Inc. ©Copyright 2002, Power Integrations, Inc.
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