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Design Idea DI-42 (R) 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
(R)
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 , 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 , 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 (trr) 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.
For the latest updates, visit our Web site: 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. (c)Copyright 2002, Power Integrations, Inc.
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A 9/02
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