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AN10829 Datasheet(PDF) 7 Page - NXP Semiconductors |
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AN10829 Datasheet(HTML) 7 Page - NXP Semiconductors |
7 / 22 page AN10829 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2011. All rights reserved. Application note Rev. 3 — 22 June 2011 7 of 22 NXP Semiconductors AN10829 SSL2101 dimmable high efficiency flyback design • the convertor frequency must remain above the audible range even when the frequency reduces while the circuit is being dimmed. The convertor frequency also effects the efficiency of the converter and the required transformer size. The inductance and peak current are determined to achieve a frequency that provides a good balance of converter efficiency and transformer size. 2.1.3 Primary peak current and primary inductance The SMPS primary switch is located inside the SSL2101. This selection is made to eliminate the cost of an expensive high voltage low RDS(on) MOSFET. The switch is optimized for the maximum output power of the IC: 12 W for the SSL2101. The switch RDS(on) is typically 6.5 at 25 C junction temperature (Tj) and 9.0 at 100 C (Tj). The peak current through the inductor is very important. A high current causes high switching losses inside of the IC. As a result Tj rises, the RDS(on) increases and the switch losses increase further. The peak current through the inductor depends on: • the switch on-time • the primary inductance • the primary buffer voltage The SSL2101 detects this peak current by measuring the voltage drop over a shunt resistor on the SOURCE pin. The switch stops conducting 160 ns (typical) after the voltage level at the SOURCE pin crosses the 0.5 V threshold. This delay causes a higher actual peak current, especially with a high buffer voltage. A source resistor of 2.4 is used in the circuit which together with the delay and input voltage, leads to a maximum peak current of 0.26 A through the inductor. The primary inductance can be calculated with Equation 2, resulting from Equation 1. (2) Using the values mentioned in this section will result in a required primary inductance of 1.6 mH. 2.1.4 Turns ratio A relatively high percentage of the power can be lost in the flyback diode, due to a low output voltage in relation to the diode forward voltage. A Schottky diode has a lower forward bias voltage that would provide higher efficiency. However, high voltage Schottky diodes have a relatively large forward bias voltage. Increasing the transformer ratio reduces the secondary reverse voltage. The maximum turns ratio is limited by the minimum buffer voltage, the maximum switch voltage and the required output voltage. For this application, the output voltage is determined by the number of LEDs in series, the current through the LEDs, and the forward voltage over the flyback diode. The estimated required output voltage is 13.2 V. It is estimated that the buffer voltage on the primary side can go as low as 80 V as a result of the buffer capacitance, the load and the sinusoidal mains voltage. Consequently, the winding ratio must be smaller than 6.06 : 1 to be able to generate a constant output voltage, and a turns ratio of 6 : 1 is selected. This results in a maximum reverse voltage of 67 V on the flyback diode. L p 2P in I P 2 f = |
Similar Part No. - AN10829_15 |
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Similar Description - AN10829_15 |
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