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TNY278PN Datasheet(PDF) 9 Page - Power Integrations, Inc. |
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TNY278PN Datasheet(HTML) 9 Page - Power Integrations, Inc. |
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9 / 24 page  9 TNY274-280 E 2/06 For lower no-load input power consumption, the bias winding may also be used to supply the TinySwitch-III device. Resistor R8 feeds current into the BP/M pin, inhibiting the internal high voltage current source that normally maintains the BP/M pin capacitor voltage (C7) during the internal MOSFET off time. This reduces the no-load consumption of this design from 140 mW to 40 mW at 265 VAC. Under-voltage lockout is configured by R5 connected between the DC bus and EN/UV pin of U1. When present, switching is inhibited until the current in the EN/UV pin exceeds 25 µA. This allows the startup voltage to be programmed within the normal operating input voltage range, preventing glitching of the output under abnormal low voltage conditions and also on removal of the AC input. In addition to the simple input pi filter (C1, L1, C2) for differential mode EMI, this design makes use of E-Shield™ shielding techniques in the transformer to reduce common mode EMI displacement currents, and R2 and C4 as a damping network to reduce high frequency transformer ringing. These techniques, combined with the frequency jitter of TNY278, give excellent conducted and radiated EMI performance with this design achieving >12 dBµV of margin to EN55022 Class B conducted EMI limits. For design flexibility the value of C7 can be selected to pick one of the 3 current limits options in U1. This allows the designer to select the current limit appropriate for the application. • Standard current limit (I LIMIT) is selected with a 0.1 µF BP/M pin capacitor and is the normal choice for typical enclosed adapter applications. • When a 1 µF BP/M pin capacitor is used, the current limit is reduced (I LIMITred or ILIMIT-1) offering reduced RMS device currents and therefore improved efficiency, but at the expense of maximum power capability. This is ideal for thermally challenging designs where dissipation must be minimized. • When a 10 µF BP/M pin capacitor is used, the current limit is increased (I LIMITinc or ILIMIT+1), extending the power capability for applications requiring higher peak power or continuous power where the thermal conditions allow. Furtherflexibilitycomesfromthecurrentlimitsbetweenadjacent TinySwitch-III family members being compatible. The reduced current limit of a given device is equal to the standard current limit of the next smaller device and the increased current limit is equal to the standard current limit of the next larger device. Key Application Considerations TinySwitch-lll Design Considerations Output Power Table The data sheet output power table (Table 1) represents the minimum practical continuous output power level that can be obtained under the following assumed conditions: 1. The minimum DC input voltage is 100 V or higher for 85 VAC input, or 220 V or higher for 230 VAC input or 115 VAC with a voltage doubler. The value of the input capacitance should be sized to meet these criteria for AC input designs. 2. Efficiency of 75%. 3. Minimum data sheet value of I2f. 4. Transformer primary inductance tolerance of ±10%. 5. Reflected output voltage (V OR) of 135 V. 6. Voltage only output of 12 V with a fast PN rectifier diode. 7. Continuous conduction mode operation with transient K P* value of 0.25. 8. Increased current limit is selected for peak and open frame power columns and standard current limit for adapter columns. 9. The part is board mounted with SOURCE pins soldered to a sufficient area of copper and/or a heatsink is used to keep the SOURCE pin temperature at or below 110 °C. 10. Ambient temperature of 50 °C for open frame designs and 40 °C for sealed adapters. *Below a value of 1, K P is the ratio of ripple to peak primary current. To prevent reduced power capability due to premature termination of switching cycles a transient K P limit of ≥0.25 is recommended. This prevents the initial current limit (I INIT) from being exceeded at MOSFET turn on. For reference, Table 2 provides the minimum practical power deliveredfromeachfamilymemberatthethreeselectablecurrent limit values. This assumes open frame operation (not thermally limited) and otherwise the same conditions as listed above. These numbers are useful to identify the correct current limit to select for a given device and output power requirement. Overvoltage Protection The output overvoltage protection provided by TinySwitch-III uses an internal latch that is triggered by a threshold current of approximately 5.5 mA into the BP/M pin. In addition to an internal filter, the BP/M pin capacitor forms an external filter providing noise immunity from inadvertent triggering. For the bypass capacitor to be effective as a high frequency filter, the capacitor should be located as close as possible to the SOURCE and BP/M pins of the device. |
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