AN1300 APPLICATION NOTE

6/9

Design of the Input Circuit

The input filter capacitor’s value is determined by the amount of ripple one desires on the DC input voltage of

the converter. The ripple voltage of 5-10V is typical. The input capacitor can then be estimated by:

[Eq. 7]

The approximate value of the input capacitor is then 100-150

µF

The 60Hz input rectifiers for the typical capacitor input filter would be determined by the current that is higher

than the average input current but lower than the estimated peak current. The input current rating is then ap-

proximated by Eq. 8.

[Eq. 8}

The input current rating of the input rectifier should then be equal to or greater than 0.8A. The reverse voltage

rating would then be 2 times the peak input voltage or 400V. The input rectifiers should then br 1N4004.

Selection of the Semiconductors

The power Switches:

Since this is a half-bridge 155 VAC application, the power MOS FETs need only have a breakdown voltage

The output Rectifiers:

The minimum reverse voltage that the output rectifiers will see will be about 25 volts. The RMS current that

each output rectifier will see will be about 1.5 amperes. The diode current waveform will be a half-rectified sine,

which will have an approximate peak value of 6 amperes. Since its peak current is much greater than its average

current, it is a good idea to select a schottky rectifier that is about the value of the peak value, to keep the voltage

drop during the conduction time to a minimum. Therefore a rectifier of 5 or more amperes would wise. A good

choice would be STPS8L30B. This rectifier is an 8 A schottky diode with a reverse voltage of 30 V.

Design of the Control Circuitry:

The functions that need setting-up are the excursion limits of the frequency control. These are controlled by the

frequency of the tank circuit. Some adjustment will be necessary after the converter is built, to set the control

loop to the actual resulting resonance frequency of the physical tank circuit. In a production setting, these control

limits should be checked with the production transformer and resonant capacitor.

There is some interaction between the frequency-setting pins. This is necessary for consistent operation of the

VCO.

The soft-start capacitor is determined by:

[Eq. 9]

For a 0.5 second soft-start period the soft-start capacitor is approximately 0.3

µF.

[Eq. 10]

Rfmin is then approximately 36k

Ω.

C

in

()

0.3P

in

()

f

()

() V

rip ple p

p

–

()

()

----------------------------------------------------------------------

≈

I

in rec t

()

P

in rated

()

0.4 V

in m in

()

------------------------------

≈

C

SS

0.15

/

µF

sec

()

T

SS

-------------------------------------

=

R

fmin

1.41

f

()

---------------------

=