AN-6067(2010_01) Ver la hoja de datos (PDF) - Fairchild Semiconductor

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AN-6067
(Rev.:2010_01)

Design and Application of Primary-Side Regulation (PSR) PWM Controller

Fairchild Semiconductor 

AN-6067

voltage reaches the turn-off threshold voltage of the PSR

controller. Therefore, if na was calculated, the Vo,“B” can be

obtained as:

VO,"B"

=

⎛

⎜

V

fa

⎝

+ 6.75 -Vf

na

⋅ na

⎞

⎟

⎠

(12)

where:

Vfa is forward-voltage of rectifier diode of auxiliary winding.

Vf is forward-voltage of output diode.

6.75V is typically the turn-off threshold voltage of the PSR

controller.

The maximum duty ratio can be calculated by using a point

“B” output condition:

d on. max," B "

=

np ⋅ (Vo,"B" + V f )

Vin.min,"B" + np ⋅ (Vo,"B" + V f

)

(13)

The transformer inductance (Lp) is designed specifically for

DCM operation and a CC tolerance of +/-10% should be

considered. The transformer inductance can be obtained as:

Lp

η2

2

= ⋅V ⋅ d ," B" in.min," B" max," B"

2 ⋅Vo,"B" ⋅ Io ⋅ fs

(14)

where:

η,”B” is the estimated system efficiency of point “B.”

If no values are available, use 0.45~0.5 as an initial value.

fs is the PWM frequency.

After the primary inductance is calculated, the maximum

duty ratio of point “A” can be expressed as:

d = on. max," A"

2 ⋅VO," A" ⋅ IO ⋅ LP

η ⋅V ⋅ T 2

," A" in. min," A" s

(15)

where Ts is the switching period.

The primary peak inductor current (IPK) of point “A” at full

load and low line input voltage condition is:

ipk ,"A"

=

Vin.min," A"

Lp

⋅ don.max," A"

⋅ TS

(16)

APPLICATION NOTE

Determine Primary Inductance Turn Number

Based on Faraday’s law and the peak inductor current, the

minimum turns for the primary inductance is calculated as:

N pri

=

Lp ⋅ ipk ," A"

Bmax ⋅ Ae

⋅106

(17)

where:

Bmax is the saturation magnetic flux density,

Ae is the effective area of the core-section.

The number of turns for the secondary winding is defined as:

Nsec

=

N pri

np

(18)

Once the secondary winding has been calculated, the

number of turns for the auxiliary winding is defined as:

Naux = na ⋅ Nsec

(19)

Determine the Divider Resistor (R1) and Current-

Sense Resistor (RS)

Once the output voltage VO and auxiliary winding have been

defined, the feedback signal divider resistor, R1, can be

calculated as:

( ) R1

=

R2

⋅

⎡

⎢

na

⎢⎣Vref

⋅

VO + Vf

⎤

−1⎥

⎥⎦

(20)

where Vref=2.5V, R2 is typically set to 15~20KΩ.

As discussed in the Constant Current Output Regulation

section, the region of constant current output operation can be

adjusted by the current-sense resistor. After the turn ratio (np)

has been determined, the relationship between the output

current IO and current sense resistor Rs is expressed as:

RS

=

0.111875⋅ np

IO

(21)

As Figure 27 shows, a design spreadsheet can be used to

calculate the transformer design and select the power system

components for a first prototype. A 5V/1A design example

is shown in Figure 27.

© 2008 Fairchild Semiconductor Corporation

Rev. 1.0.1 • 1/26/10

11

www.fairchildsemi.com

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