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TNY254 Datasheet(PDF) 8 Page - Power Integrations, Inc. |
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TNY254 Datasheet(HTML) 8 Page - Power Integrations, Inc. |
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8 / 20 page  C 7/01 TNY253/254/255 8 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. A simple constant current circuit is implemented using the V BE 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 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 (~ 1.5V) 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. AC Adapter Many consumer electronic products utilize low power 50/60Hz 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 a constant power consumption from the mains input, this solution is still significantly more efficient than linear adapters up to output power levels of approximately 1W. 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 winding 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 700V rating of the TinySwitch MOSFET, a simple capacitive snubber (C4) is adequate to limit the leakage inductance spike in 115VAC 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. 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 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 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. 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 easily |
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