AN10829

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Application note

Rev. 3 — 22 June 2011

7 of 22

NXP Semiconductors

AN10829

SSL2101 dimmable high efficiency flyback design

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

optimized for the maximum output power of the IC: 12 W for the SSL2101. The switch

The peak current through the inductor is very important. A high current causes high

losses increase further. The peak current through the inductor depends on:

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







=