Rev. F 08/16

8

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TNY253/254/255

TNY253/254/255

achieve the appropriate value. R3 is a base current limiting

resistor. When the drop across R4 exceeds the V

BE of transistor

Q1, it turns on and takes over the control of the loop by driving

the optocoupler LED. R6 drops an additional voltage to keep

the control loop in operation down to zero volts on the output.

With the output shorted, the drop across R4 and R6 (~ 1.5 V) 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.

AC Adapter

Many consumer electronic products utilize low power 50/60 Hz

transformer based AC adapters. The TinySwitch can cost ef-

fectively replace these linear adapters with a solution that is

lighter, smaller and more energy efficient . Figure 12 shows a

9 V, 0.5 W AC adapter circuit using the TNY253. This circuit

operates from a 115 VAC input. To save cost, this circuit runs

without any feedback, in discontinuous conduction mode to

deliver constant power output relatively independent of in-

put 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

a constant power consumption from the mains input, this solu-

tion is still significantly more efficient than linear adapters up

to output power levels of approximately 1 W.

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 wind-

ing of T1 and the high-voltage MOSFET inside the TNY253.

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 700 V rating of the TinySwitch

MOSFET, a simple capacitive snubber (C4) is adequate to limit

the leakage inductance spike in 115 VAC applications, at low

power levels. The secondary winding is rectified and filtered

by D3 and C6.

Key Application Considerations

For the most up to date information visit our Web site

at: www.powerint.com

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. TheminimumDCinputvoltageis90Vorhigherfor85VAC

input or 240 V or higher for 230 VAC input or 115 VAC

input with a voltage doubler.

2. The TinySwitch is not thermally limited - the source

pins are soldered to sufficient copper area to keep the die

temperature at or below 100 °C. This limitation does not

usually apply to TNY253 and TNY254.

The maximum power capability of a TinySwitch depends on

the thermal environment, transformer core size and design

(continuous or discontinuous), efficiency required, minimum

specified input voltage, input storage capacitance, output volt-

age, 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.

Ceramic capacitors that use dielectrics such as Z5U, when used

in clamp and snubber circuits, can also generate audio noise

due 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 (I

LIMIT)

multiplied by the turns ratio of the transformer (N

p/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