TNY253/254/255
is provided by L2 and C6. The output voltage is determined by
the sum of the optocoupler U2 LED forward drop (~ 1V) and
Zener diode VR1 voltage. The resistor R8, maintains a bias
current through the Zener to improve its voltage tolerance.
Key Application Considerations
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A simple constant current circuit is implemented using the VBE
of transistor Q1 to sense the voltage across the current sense
resistor R4, which can be made up of one or more resistors to
achieve the appropriate value. R3 is a base current limiting
resistor. When the drop across R4 exceeds the VBE of transistor
Q1, it turns on and takes over the control of the loop by driving
theoptocouplerLED. R6dropsanadditionalvoltagetokeepthe
control loop in operation down to zero volts on the output. With
the output shorted, the drop across R4 and R6 (~ 1.5V) is
sufficient 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 conditions, due to the
voltage drop across R6 and R4.
Design
Output Power Range
The power levels shown in the TinySwitch Selection Guide
(Table 1) are approximate, recommended output power ranges
that will provide a cost optimum design and are based on
following assumptions:
1. The minimum DC input voltage is 90 V or higher for
85VAC input or 240V or higher for 230 VAC input or
115VAC input with a voltage doubler.
2. The TinySwitch is not thermally limited-the source pins are
solderedtosufficientcopperareatokeepthedietemperature
at or below 100 °C. This limitation does not usually apply
to TNY253 and TNY254.
AC Adapter
Manyconsumerelectronicproductsutilizelowpower50/60Hz
transformer based AC adapters. The TinySwitch can cost
effectively replace these linear adapters with a solution that is
lighter, smaller and more energy efficient . Figure12 shows a
9V, 0.5W AC adapter circuit using the TNY253. This circuit
operates from a 115VAC input. To save cost, this circuit runs
without any feedback, in discontinuous conduction mode to
deliver constant power output relatively independent of input
voltage. The output voltage is determined by the voltage drop
across Zener diode VR1. The primary inductance of the
transformer is chosen to deliver a power that is in excess of the
required output power by at least 50% to allow for component
tolerances and to maintain some current through the Zener VR1
at full load. At no load, all of the power is delivered to the Zener
which should be rated and heat sinked accordingly. In spite of
aconstantpowerconsumptionfromthemainsinput,thissolution
is still significantly more efficient than linear adapters up to
output power levels of approximately 1W.
ThemaximumpowercapabilityofaTinySwitchdependsonthe
thermal environment, transformer core size and design
(continuous or discontinuous), efficiency required, minimum
specified input voltage, input storage capacitance, output
voltage, output diode forward drop, etc., and can be different
from the values shown in the selection guide.
Audible Noise
At loads other than maximum load, the cycle skipping mode
operation used in TinySwitch can generate audio frequency
components in the transformer. This can cause the transformer
to produce audio noise. Transformer audible noise can be
reduced by utilizing appropriate transformer construction
techniques and decreasing the peak flux density. For more
information on audio suppression techniques, please check
the Application Notes section on our Web site at
www.powerint.com.
The AC input is rectified by diodes D1 and D2. D2 is used to
reduce conducted EMI by only allowing noise onto the neutral
line during diode conduction. The rectified AC is then filtered
by capacitors C1 and C2 to generate a high voltage DC bus,
which is applied to the series combination of the primary
windingofT1andthehighvoltageMOSFETinsidetheTNY253.
The resistor R2 along with capacitors C1 and C2 form a π-filter
which is sufficient for meeting EMI conducted emissions at
these power levels. C5 is a Y-capacitor which is used to reduce
common mode EMI. Due to the 700V rating of the TinySwitch
MOSFET, a simple capacitive snubber (C4) is adequate to limit
the leakage inductance spike in 115VAC applications, at low
powerlevels. Thesecondarywindingisrectifiedandfilteredby
D3 and C6.
CeramiccapacitorsthatusedielectricssuchasZ5U, whenused
inclampandsnubbercircuits,canalsogenerateaudionoisedue
to electrostriction and piezo-electric effects. If this is the case,
replacing them with a capacitor having a different type of
dielectric is the simplest solution. Polyester film capacitor is a
good alternative.
Short Circuit Current
The TinySwitch does not have an auto-restart feature. As a
result, TinySwitch will continue to deliver power to the load
during output short circuit conditions. In the worst case, peak
short circuit current is equal to the primary current limit (ILIMIT
)
multiplied by the turns ratio of the transformer (Np/Ns). In a
typical design the average current is 25 to 50% lower than this
peak value. At the power levels of TinySwitch this is easily
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