AND8099/D

http://onsemi.com

3

In order for the regulator to operate in discontinuous mode

under worse case conditions and to maximize power, the

maximum on time is 48% of the full period, therefore the the

maximum primary inductance is calculated based on a

maximum duty cycle of 48%. Using a larger inductance than

calculated will cause the power supply output to fall out of

regulation.

Lpri +

Vin(low) · max

Ipeak ·fop

max is the maximum

duty cycle

Lpri +

80.2 · 0.48

0.667 * 100 · 103

+ 0.577 mH

Primary flyback voltage:

VFB +

Vin(low) ·ton

toff

+ 80.2 · 4.8 · 10

-6

5.2 · 10-6

+ 74.03 V, where

ton is 4.8 ms

Primary to secondary turns ratio:

Npri

N sec

+

VFB

Vout ) VF

+ 74.03

5

) 0.525 +

13.4

[ 13 turns

yields

≈1 turn.

Energy entering the core during on-time (when the power

switch is conducting):

Estored +

Lpri ·Ipeak2

2

,

where the stored energy is

measured in Joules.

Estored +

0.577 · 10-3 · 0.6672

2

+ 1.28

10-4 Joules

One can double check if the power capability of the

transformer is large enough to supply enough power to the

output with the following equation:

Pin(core) +

Lpri ·Ipeak2

2

·fop u Pout

Pin(core) + 1.28

10-4 · 100 kHz

+ 12.8 W u Pout

+ 10 W

Input Snubber

Because of the high dv/dt characteristic of the power

transistor drain voltage and of the transformer leakage

inductance, voltage spikes and ringing occur at the drain

when the power switch is turned off. Resistor R1, C3, D5

compromise an RCD snubber. In parallel to the primary

winding are R2 and C4 which compromise an RC ringing

damper which slows down the dv/dt and reduces the

peak voltage therefore decreasing the ringing due

+ C· dv

dt

, increasing the

capacitance will also reduce the magnitude of the voltage

ripple. The snubber and ringing damper act together to

protect the IC from voltage transients greater than 700 V and

reduce radiated noise.

Output Block

The output consists of a diode rectifier, a pi-filter, and a

voltage regulator. For a flyback converter with an output

voltage less than 7.5 V, a Schottky rectifier provides the

maximum efficiency and is therefore the best choice. The

Schottky rectifier used is the ON Semiconductor 1N5822, in

purpose of this rectifier is to take the secondary voltage and

convert it to a DC voltage. The following equations are used

in selecting the Schottky rectifier:

Maximum reverse peak voltage (calculated at high line):

VRout u Vout ) Vpeak(max) ·

N sec

Npri

VRout u 5 ) 375 ·

1

13

+ 33.85 V

For discontinuous mode, the maximum forward peak

current can be approximated using:

IFout + 4·Iout + 8A

Diode D6 along with C7, C8, C9, L2, and C11 rectify the

transformer secondary and filter the output in order to

provide a tightly regulated DC output . Capacitor C7, C8 and

C9 are placed in parallel in order to reduce ESR. In addition,

the voltage rating of C7, C8 and C9 should be high enough

for them to withstand the voltage spikes and the output

voltage. L2 and C11 form a low pass filter that attenuates

high frequency noise.

Output filter capacitor:

Cout +

Iout(max) ·Toff(max)

Vripple(desired)

,

+ min · 1

fop

where Toff(max)

and Vripple(desired) + 40 mV

Cout +

8 · 0.52 ·

1

0.040

+ 1040 mF

Output filter choke (designed for a break frequency of

4.0 kHz):

L

+

1

2·

p ·f· C

2

,

where C

+ C11 + 330 mF and f

is the corner frequency.

L

+

1

2·

p · 4k · 330 mF

2

+ 4.8 mH