TNY263-268
2.5 W CV/CC Cell-Phone Charger
line sense resistors R2 and R3 sense the DC input voltage
for line under-voltage. When the AC is turned off, the under-
voltage detect feature of the TinySwitch-II prevents auto-restart
glitches at the output caused by the slow discharge of large
storage capacitance in the main converter. This is achieved by
preventing the TinySwitch-II from switching when the input
voltagegoesbelowalevelneededtomaintainoutputregulation,
and keeping it off until the input voltage goes above the under-
voltage threshold, when the AC is turned on again. With R2
and R3, giving a combined value of 2 MΩ, the power up under-
voltage threshold is set at 200 VDC, slightly below the lowest
required operating DC input voltage, for start-up at 170 VAC,
with doubler. This feature saves several components needed to
implement the glitch-free turn-off compared with discrete or
TOPSwitch-II based designs. During turn-on the rectified DC
input voltage needs to exceed 200 V under-voltage threshold
for the power supply to start operation. But, once the power
supply is on it will continue to operate down to 140 V rectified
DC input voltage to provide the required hold up time for the
standby output.
As an example, Figure 14 shows a TNY264 based 5 V,
0.5 A, cellular phone charger operating over a universal input
range (85 VAC to 265 VAC). The inductor (L1) forms a
π-filter in conjunction with C1 and C2. The resistor R1 damps
resonances in the inductor L1. Frequency jittering operation
of TinySwitch-II allows the use of a simple π-filter described
aboveincombinationwithasinglelowvalueY1-capacitor(C8)
to meet worldwide conducted EMI standards. The addition
of a shield winding in the transformer allows conducted EMI
to be met even with the output capacitively earthed (which is
the worst case condition for EMI). The diode D6, capacitor
C3 and resistor R2 comprise the clamp circuit, limiting the
leakage inductance turn-off voltage spike on the TinySwitch-II
DRAIN pin to a safe value. The output voltage is determined
by the sum of the optocoupler U2 LED forward drop (~1 V),
and Zener diode VR1 voltage. Resistor R8 maintains a bias
current through the Zener diode to ensure it is operated close
to the Zener test current.
Asimple constant current circuit is implemented using the VBE
of transistor Q1 to sense the voltage across the current sense
resistor R4. When the drop across R4 exceeds the VBE of
transistor Q1, it turns on and takes over control of the loop by
driving the optocoupler LED. Resistor R6 assures sufficient
voltage to keep the control loop in operation down to zero volts
at the output. With the output shorted, the drop across R4 and
R6 (~1.2 V) is sufficient to keep the Q1 and LED circuit active.
Resistors R7 and R9 limit the forward current that could be
drawnthroughVR1byQ1underoutputshortcircuitconditions,
due to the voltage drop across R4 and R6.
The auxiliary primary side winding is rectified and filtered by
D2 and C2 to create a 12 V primary bias output voltage for the
mainpowersupplyprimarycontroller.Inaddition,thisvoltageis
usedtopowerthe TinySwitch-II via R4.Althoughnot necessary
for operation, supplying the TinySwitch-II externally reduces
the device quiescent dissipation by disabling the internal drain
derived current source normally used to keep the BYPASS pin
capacitor (C3) charged. An R4 value of 10 kΩ provides 600 µA
into the BYPASS pin, which is slightly in excess of the current
consumption of TinySwitch-II. The excess current is safely
clamped by an on-chip active Zener diode to 6.3 V.
10 and 15 W Standby Circuits
Figures 15 and 16 show examples of circuits for standby
applications.Theybothprovidetwooutputs:anisolated5 Vand
a 12 V primary referenced output. The first, using TNY266P,
provides 10 W, and the second, using TNY267P, 15 W of
output power. Both operate from an input range of 140 VDC to
375 VDC, corresponding to a 230 VAC or 100/115 VAC with
doubler input. The designs take advantage of the line under-
voltage detect, auto-restart and higher switching frequency of
TinySwitch-II. Operation at 132 kHz allows the use of a smaller
and lower cost transformer core, EE16 for 10 W and EE22 for
15 W. The removal of pin 6 from the 8 pin DIP TinySwitch-II
packages provides a large creepage distance which improves
reliability in high pollution environments such as fan cooled
power supplies.
The secondary winding is rectified and filtered by D3 and C6.
Fora15 Wdesignanadditionaloutputcapacitor,C7,isrequired
duetothelargersecondaryripplecurrentscomparedtothe10 W
standby design. The auto-restart function limits output current
during short circuit conditions, removing the need to over rate
D3. Switching noise filtering is provided by L1 and C8. The
5 V output is sensed by U2 and VR1. R5 is used to ensure that
the Zener diode is biased at its test current and R6 centers the
output voltage at 5 V.
In many cases the Zener regulation method provides sufficient
accuracy (typically ± 6% over a 0 °C to 50 °C temperature
range). This is possible because TinySwitch-II limits the
dynamic range of the optocoupler LED current, allowing the
Zener diode to operate at near constant bias current. However,
if higher accuracy is required, a TL431 precision reference IC
may be used to replace VR1.
Capacitor C1 provides high frequency decoupling of the high
voltage DC supply, only necessary if there is a long trace
length from the DC bulk capacitors of the main supply. The
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POWERINT [ Power Integrations ]
