TNY274-280
C5
2.2 nF
250 VAC
L2
D7
Ferrite Bead
3.5 × 7.6 mm
VR1
BYV28-200
T1
12 V, 1 A
P6KE150A
NC
8
6
C11
C10
1000 μF
25 V
100 μF
D1
1N4007
D2
1N4007
R2
25 V
1
3
100 Ω
F1
RTN
C4
10 nF
1 kV
3.15 A
R1
C1
6.8 μF
400 V
C2
22 μF
400 V
1 kΩ
R7
20 Ω
4
85 - 265
VAC
RV1
275 VAC
D5
1N4007GP
2
5
D6
UF4003
R5*
3.6 MΩ
D3
1N4007
D4
1N4007
VR2
1N5255B
28 V
L1
1 mH
C6
VR3
BZX79-C11
11 V
1 μF
60 V
R3
47 Ω
1/8 W
*R5 and R8 are optional
components
R6
390 Ω
1/8 W
R8*
21 kΩ
1%
†C7 is used to adjust U1
current limit. See circuit
description
U2
PC817A
D
EN/UV
BP/M
S
S
R4
2 kΩ
1/8 W
C7 †
100 nF
50 V
TinySwitch-III
U1
TNY278PN
PI-4244-111708
Figure 14. TNY278P, 12 V, 1 A Universal Input Power Supply.
voltage falls below the feedback threshold, a conduction cycle
is allowed to occur and, by adjusting the number of enabled
cycles, output regulation is maintained. As the load reduces,
the number of enabled cycles decreases, lowering the effective
switching frequency and scaling switching losses with load.
This provides almost constant efficiency down to very light
loads, ideal for meeting energy efficiency requirements.
Applications Example
The circuit shown in Figure ꢀ4 is a low cost, high efficiency,
flyback power supply designed for ꢀ2 V, ꢀ A output from
universal input using the TNY278.
The supply features undervoltage lockout, primary sensed
output overvoltage latching shutdown protection, high
efficiency (>805), and very low no-load consumption (<10 mW
at 261 VAC). Output regulation is accomplished using a simple
zener reference and optocoupler feedback.
As the TinySwitch-III devices are completely self-powered, there
is no requirement for an auxiliary or bias winding on the
transformer. However by adding a bias winding, the output
overvoltage protection feature can be configured, protecting the
load against open feedback loop faults.
The rectified and filtered input voltage is applied to the primary
winding of Tꢀ. The other side of the transformer primary is
driven by the integrated MOSFET in Uꢀ. Diode D1, C2, Rꢀ, R2,
and VRꢀ comprise the clamp circuit, limiting the leakage
inductance turn-off voltage spike on the DRAIN pin to a safe
value. The use of a combination a Zener clamp and parallel RC
optimizes both EMI and energy efficiency. Resistor R2 allows
the use of a slow recovery, low cost, rectifier diode by limiting
the reverse current through D1. The selection of a slow diode
also improves efficiency and conducted EMI but should be a
glass passivated type, with a specified recovery time of ≤2 μs.
When an overvoltage condition occurs, such that bias voltage
exceeds the sum of VR2 and the BYPASS/MULTIFUNCTION
(BP/M) pin voltage (28 V+1.81 V), current begins to flow into the
BP/M pin. When this current exceeds ISD the internal latching
shutdown circuit in TinySwitch-III is activated. This condition is
reset when the BP/M pin voltage drops below 2.6 V after
removal of the AC input. In the example shown, on opening the
loop, the OVP trips at an output of ꢀ7 V.
For lower no-load input power consumption, the bias winding
may also be used to supply the TinySwitch-III device. Resistor
R8 feeds current into the BP/M pin, inhibiting the internal high
voltage current source that normally maintains the BP/M pin
capacitor voltage (C7) during the internal MOSFET off time.
This reduces the no-load consumption of this design from
ꢀ40 mW to 40 mW at 261 VAC.
The output voltage is regulated by the Zener diode VR3. When
the output voltage exceeds the sum of the Zener and opto-
coupler LED forward drop, current will flow in the optocoupler
LED. This will cause the transistor of the optocoupler to sink
current.When this current exceeds the ENABLE pin threshold
current the next switching cycle is inhibited. When the output
8
Rev. I 01/09
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