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Design Idea DI-34 (R) TinySwitch-II
5 W Universal Adapter
Application Adapter Device TNY266P Power Output 5W Input Voltage 85-265 VAC Output Voltage 5V
(R)
Topology Flyback
Design Highlights
* Simple design: low parts count (23) * <300 mW no-load without transformer bias winding * Frequency jittering reduces EMI - meets EN55022 Class B with no Y capacitor: very low primary-to-secondary leakage current * 7% output tolerance with simple Zener reference * High Frequency (132 kHz) operation allows small, low cost EE16 transformer * Built-in thermal shutdown protection * ON/OFF digital regulation: No analog control loop to compensate
The TinySwitch-II frequency jitter feature and transformer construction allow use of a simple filter (L1, L2, C1 and C2) to meet EMI requirements with L2 used to attenuate high frequency EMI in the radiated spectrum (>30 MHz). The circuit also uses a low cost RCD clamp (C7, D5, R3, and R4). Diode D5 is a Glass Passivated (GP) general purpose diode, allowing partial recovery of the leakage energy by recirculating clamp current during the D5 reverse recovery time, thus improving efficiency. Resistor R3 reduces EMI by limiting the peak D5 reverse current and thus softening its recovery characteristic. The combined voltage drops of Zener diode (VR7) and optocoupler diode (U2) set the output voltage. Resistor R7 biases VR7 according to its specification. The TinySwitch-II feedback current is independent of load allowing 7% output voltage tolerance with this simple Zener reference. Due to the digital nature of the TinySwitch-II control scheme, no loop compensation is required. In addition, the current transfer ratio (CTR) of the optocoupler is not critical and no DC gain setting resistor in series with VR7 is required.
Operation
Figure 1 is a simple flyback converter utilizing several TinySwitch-II built-in features such as switching frequency jittering, thermal shutdown and auto-restart fault protection. Typical applications are wall mount adapters and other AC/DC applications requiring very low cost and small size. The ability to meet EMI requirements with no Y capacitor makes this circuit ideal for applications requiring low primary-to-secondary leakage currents.
T1 EE-16 C7 1.0 nF 1 kV R3 100 D5 1N4005 GP C1 6.8 F 400 V C2 6.8 F 400 V R4 200 k 1/2 W NC
L1 1.0 mH D1 1N4005 D2 1N4005
RF1 8.2 2W L 85 - 265 VAC
1
5
D6 1N5822 C4 1000 F 10 V
L3 3.3 H 1.0 A C5 470 F 10 V
5 V, 1.0 A
4 3
8
TinySwitch
U1 TNY266P
D EN BP
RTN 2 VR7 BZX79C3V9, 2%
N
D3 1N4005
D4 1N4005
S
L2 Bead
C3 0.1 F 50 V
U2 PC817A
R7 150
PI-3326-051303
Figure 1. TinySwitch-II - TNY266P, 5 V, 1 A, 5 W Charger-Adapter Power Supply.
DI-34
www.powerint.com
May 2003
DI-34
PI-3325-090602
Key Design Points
* Use KRP (ripple-to-peak current ratio) in the range of 0.4 to 0.6 and VOR (output reflected voltage) of 90 V to 110 V for best efficiency. * Use low cost optocoupler - gain is non-critical. * PCB traces which carry high switching voltages and current should be short and wide to reduce EMI. * Reduce leakage inductance by filling each winding layer across the entire width of the bobbin. * R4 should be large enough to limit dissipation, to meet <300 mW no-load target, while still limiting peak DRAIN voltage to a safe value - 200 k is a good start value for most designs. * A layer of insulation tape between each layer of primary winding will further reduce inter-winding capacitance and therefore switching losses. * Ferrite bead L2 reduces radiated EMI.
IO = 0 A
PI-3324-090602
80 70 60
QP AV
Amplitude (dBV)
50 40 30 20 10 0 -10 -20 0.15 1.0 10.0 -100.0
Average Quasi Peak
Frequency (MHz)
300 250
Figure 3. Conducted Emissions-EN55022 Class B (QP and AVG), 5 V, 1 A, 230 VAC, with Artificial Hand Connected to Secondary Return.
TRANSFORMER PARAMETERS
Core Bobbin EE16, Nippon Ceramic NC-2H or equivalent, AL = 135 nH/T2 EEL16 Vertical Shield (1-NC), Primary (4-1), Shield (3-2), and Secondary (10-8). 1660 H 10% 400 kHz (minimum)
Input Power (mW)
200 150 100 50 0 50 100 150 200 250 300
Winding Order (pin numbers) Primary Inductance (Pins 1-4 all others open) Primary Resonant Frequency (Pins 1-4, all other open)
Input Voltage (VAC)
Figure 2. No-load Input Power Consumption.
Leakage Inductance (Pins 1-4, with Pins 8-14 shorted)
70 H (maximum)
Table 1. Transformer Design Parameters. 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 (including circuits external to the products and transformer construction) 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, LinkSwitch and EcoSmart are registered trademarks of Power Integrations, Inc. PI Expert is a trademark of Power Integrations, Inc. (c)Copyright 2003, Power Integrations, Inc.
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B 5/03
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