User manual

Design Information

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The AC mains voltage, at the line frequency f

, is assumed to be perfectly sinusoidal and the diode bridge

ideal. This yields a perfect rectified sinusoid at the input to the flyback. The input voltage V

(t) is defined in

terms of the peak input voltage:

(1)

The controller and the transformer are also assumed to be ideal. These assumptions yield a sinusoidal

peak primary current envelope I

P-pk

(t) and peak secondary current envelope I

S-pk

(t) as shown in Figure 12.

Both are defined in terms of the peak primary current:

(2)

The output voltage reflected to the primary is defined:

(3)

CRM control yields a variable duty cycle over a single line cycle with a minimum occurring at the peak

input voltage:

(4)

The resulting sinusoidal average input current I

(t), shown in Figure 12, is approximated as the average of

each triangular current pulse during a switching period. The peak input current occurs at the peak primary

current:

(5)

Turns Ratio

The first thing to decide with an isolated design is the desired transformer turns ratio. This should be

based on the specified output voltage and the maximum peak input voltage. Frequently the MosFET is

already chosen for a design, given its cost and availability. With a desired MosFET voltage, the maximum

reflected voltage at the primary is calculated:

(6)

Generally, an integer turns ratio is selected to achieve a reflected voltage at or below the defined

maximum:

(7)

Switching MosFET

The main switching MosFET (Q3) can be sized as desired; to block the maximum drain-to-source voltage,

operate at the maximum RMS current, and dissipate the maximum power:

(8)

The peak current limit should be at least 25% higher than the maximum peak input current:

AN-2150 LM3450A Evaluation Board SNVA485B–June 2011–Revised May 2013

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