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Design Idea DI-91 (R) TinySwitch-III
Application General Purpose Device TNY278P
12 W Universal Input CV Adapter
Power Output 12 W Input Voltage 85-265 VAC Output Voltage 12 V Topology Flyback
Design Highlights
* * * * * * * No-load consumption <50 mW at 265 VAC (R8 fitted) Active mode efficiency >75% (CEC standard: 71%) BP/M pin capacitor value selects MOSFET current limit (ILIMIT-1, ILIMIT, or ILIMIT+1): for design flexibility Output OVP function senses overvoltage on the output through the primary bias winding Accurate (5%), auto-recovering, hysteretic thermal shutdown function maintains safe PCB temperatures Auto-restart function protects against output short circuits and open feedback loops > 3.2 mm package pin creepage: reliable operation in high humidity and high pollution environments
The use of an E-shieldTM in the transformer, a low-noise drainnode clamp (R2, C4 and D5), a single Y capacitor (C5) and the devices switching frequency jitter function enable a low-cost pi () filter (C1, L1 and C2) to attenuate conducted EMI enough to provide more than 10 dB of margin to EN550022 requirements (see Figure 4). The clamp circuit Zener (VR1) only conducts briefly during startup and at worst-case overload, just before the auto-restart function is activated. Although TinySwitch-III family members do not need a bias winding to operate, one was used in this design to take advantage of the ICs output OVP detection function. If an open loop fault causes an overvoltage on the output, VR2 will conduct. When the current into the BP/M pin exceeds 5.5 mA, an internal latch triggers and disables MOSFET switching (see Figure 3), until AC power is removed and reapplied (to reset the latch). Resistor R3 limits the current through VR2, while R3 and R7 attenuate the portion of the leakage inductance spike that appears in the bias winding.
C5 2.2 nF 250 VAC D7 BYV28-200 C10 1000 F 25 V
L2 Ferrite Bead 3.5 x 7.6 mm
Operation
This universal input, 12 V, 1 A, flyback converter was designed around a member of the TinySwitch-III IC family, the TNY278P. Because of the many features integrated into the device family, the supply only requires 31 through-hole components (no SMT), which enables a simple, single-sided PCB layout to be used.
VR1 P6KE150A D1 1N4007 D2 1N4007 C1 6.8 F 400 V C2 22 F 400 V R1 1 k
NC R2 100 C4 10 nF 1 kV
T1
8
+12 V, 1 A J3 C11 100 F 25 V J4 RTN
1
J1 85-265 VAC J2
F1 3.15 A RV1 275 VAC
6 R7 4 20 2 5 D6 UF4003
3
D3 1N4007
D4 1N4007
R5* 3.6 M L1 1 mH
D5 1N4007GP
VR2 1N5255B 28 V R3 47 1/8 W R8* 21 k 1%
EN/UV
C6 1 F 60 V
VR3 BZX79-C11 11 V
*R5 and R8 are optional components
R6 390 1/8 W U2 PC817A
C7 is configurable to adjust U1 current limit, see circuit description
D S
BP/M
TinySwitch-III U1 TNY278P
S
C7 100 nF 50 V
R4 2 k 1/8 W
PI-4244-021406
Figure 1. TinySwitch-III 12 W, Universal Input, CV Adapter.
DI-91
www.powerint.com
January 2006
DI-91
When R8 (optional) is installed, it supplies current to the BP/M pin, which reduces the no-load consumption of the supply. When R5 (also optional) is installed, the current it feeds into the EN/UV pin of the IC enables the input under-voltage lockout function (UVLO), and sets the start-up threshold at 65 VAC. The output voltage is determined by the series sum of the voltages across VR3, R6, and the LED in U2. The values of R4 and R6 can be adjusted to fine tune the output voltage of the supply, and a TL431 can be used in place of VR3, for tighter output voltage regulation.
86 84 82 80 78 76 74 72 70 68 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5
TNY279 (with ILIMIT-1) TNY278 (with std ILIMIT) TNY277 (with ILIMIT+1) CEC/ENERGY STAR Minimum TNY278 (with ILIMIT+1)
Efficiency (%)
Output Current (A)
Key Design Points
* In the TinySwitch-III family of devices, the MOSFET current limit can be selected from 3 levels by the value of capacitance on the BP/M pin. This gives the designer a number of options, which are shown in Figure 2. First, a TNY279 could be used, with the current limit of a TNY278, to obtain higher efficiency. Second, a TNY277 could be used, with the current limit of a TNY278, to deliver the same power at a lower efficiency. Lastly, a TNY278 could be used, with the current limit of a TNY279, to extend the power capability of the supply*. The UVLO function integrated into U1 only enables MOSFET switching to occur once the current flowing into the EN/UV pin exceeds 25 A. Changing the UVLO threshold voltage requires changing the value of R5 so that 25 A flows through it at the desired threshold voltage. If output OVP is not required and no-load power consumption of 150 mW (max, at 265 VAC input) is acceptable, then R7, D6, C6, R3 and VR2 can be eliminated and the bias winding removed from T1, for an even lower parts count.
Figure 2. Operating Efficiency vs. ILIMIT+, ILIMIT- and Device Size.
PI-4288-012506
Voltage (V)
15
0
*
0
250
500
Time (ms)
Figure 3. BP/M Pin OVP Function Shuts Down Power Supply.
PI-4289-012506
80 70 60
*
Amplitude (dBV)
50 40 30 20 -10 0 -10 -20 0.15 1 10 30
*Note: This supply was not thermally designed to continuously deliver the extended power range that could be obtained by operating a TNY278 with its MOSFET current limit set at that of a TNY279.
Frequency (MHz)
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Figure 4. Conducted EMI Scan (Line): 115 VAC, Full Load.
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. POWER INTEGRATIONS MAKES NO WARRANTY HEREIN AND SPECIFICALLY DISCLAIMS ALL WARRANTIES INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF THIRD PARTY RIGHTS. The products and applications illustrated herein (transformer construction and circuits external to the products) 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. Power Integrations grants its customers a license under certain patent rights as set forth at http://www.powerint.com/ip.htm.
The PI logo, TOPSwitch, TinySwitch, LinkSwitch, DPA-Switch, EcoSmart, Clampless, E-Shield, Filterfuse, PI Expert and PI FACTS are trademarks of Power Integrations, Inc. Other trademarks are property of their respective companies. (c)Copyright 2006, Power Integrations, Inc.
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PI-4274-013106
88

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