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TNY263-268
A simple constant current circuit is implemented using the V BE
of transistor Q1 to sense the voltage across the current sense
resistor R4. When the drop across R4 exceeds the V BE of
transistor Q1, it turns on and takes over control of the loop by
driving the optocoupler LED. Resistor R6 assures suf ?cient
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 suf ?cient to keep the Q1 and LED circuit
active. Resistors R7 and R9 limit the forward current that
could be drawn through VR1 by Q1 under output short circuit
up undervoltage 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 TinySwitch-II based designs. During turn-on the
recti?ed DC input voltage needs to exceed 200 V undervoltage
threshold for the power supply to start operation. But, once
the power supply is on it will continue to operate down to
140 V recti?ed DC input voltage to provide the required hold
up time for the standby output.
conditions, due to the voltage drop across R4 and R6.
The auxiliary primary side winding is recti?ed and ?ltered by
10 and 15 W Standby Circuits
Figures 15 and 16 show examples of circuits for standby
applications. They both provide two outputs: an isolated 5 V
and a 12 V primary referenced output. The ?rst, 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
undervoltage 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.
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
line sense resistors R2 and R3 sense the DC input voltage for
line undervoltage. When the AC is turned off, the undervoltage
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
voltage goes below a level needed to maintain output
regulation, and keeping it off until the input voltage goes above
the undervoltage threshold, when the AC is turned on again.
With R2 and R3, giving a combined value of 2 M Ω , the power
8
Rev. H 02/09
D2 and C2 to create a 12 V primary bias output voltage for the
main power supply primary controller. In addition, this voltage
is used to power the TinySwitch-II via R4. Although not
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 k Ω
provides 600 μ A 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.
The secondary winding is recti?ed and ?ltered by D3 and C6.
For a 15 W design an additional output capacitor, C7, is
required due to the larger secondary ripple currents compared
to the 10 W standby design. The auto-restart function limits
output current during short circuit conditions, removing the
need to over rate D3. Switching noise ?ltering 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 suf ?cient
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.
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