MC33364

http://onsemi.com

10

The primary inductance value is given by:

Lp =

∂max Vin(min)

=

0.5

127 V

0.472 A70 kHz =

1.92 mH

The manufacturer recommends for that magnetic core a

maximum operating flux density of:

Bmax = 0.2 T

Ac = 33.5 mm2

The operating flux density is given by:

Bmax =

LpIppk

NpAc

From this equation the number of turns of the primary

winding can be derived:

np =

LpIppk

BmaxAc

AL =

Lp

n2p

=

2

ppk



2



=

2

2 =

105 nH

From the manufacturer‘s catalogue recommendation the

of turns of the primary winding is:

np =

AL



1∕2

=

100 nH



1∕2

= 139 turns

The number of turns needed by the 6.0 V secondary is

(assuming a Schottky rectifier is used):

ns =

=

= 7 turns

The auxiliary winding to power the control IC is 16 V and

its number of turns is given by:

naux =

(Vaux + Vfwd)(1 −∂max)np

=

(16 V + 0.9 V)(1 − 0.5)139

[0.5(127 V)]

= 19 turns

The approximate value of rectifier capacitance needed is:

C1 =

toff(Iin)

Vripple

=

(5 m sec)(0.118 A)

50 V

= 11.8 mF

where the minimum ripple frequency is 2 times the 50 Hz

haversine cycle, is assumed to be half the cycle period.

Because we have a variable frequency system, all the

calculations for the value of the output filter capacitors will

be done at the lowest frequency, since the ripple voltage will

be greatest at this frequency. When selecting the output

capacitor select a capacitor with low ESR to minimize ripple

from the current ripple. The approximate equation for the

output capacitance value is given by:

C5 =

Iout

(fmin)(Vrip)

=

2A

(70 kHz)(0.1 V) =

286 mF

Determining the value of the current sense resistor (R7),

one uses the peak current in the predesign consideration.

Since within the IC there is a limitation of the voltage for the

current sensing, which is set to 1.2 V, the design of the

current sense resistor is simply given by:

R7 =

Vcs

Ippk

=

1.2 V

0.472 A =

2.54 Ω ≈ 2.2 Ω

The error amplifier function is provided by a TL431 on the

secondary, connected to the primary side via an optoisolator,

the MOC8102.

The voltage of the optoisolator collector node sets the

peak current flowing through the power switch during each

cycle. This pin will be connected to the feedback pin of the

MC33364, which will directly set the peak current.

Starting on the secondary side of the power supply, assign

the sense current through the voltage--sensing resistor

divider to be approximately 0.25 mA. One can immediately

calculate the value of the lower and upper resistor:

Rlower = R11 =

Vref (TL431)

Idiv

=

2.5 V

0.25 mA =

10 k

Rupper = R10 =

Vout − Vref(TL431)

Idiv

=

6.0 V − 2.5V

0.25 mA

= 14 k

The value of the resistor that would provide the bias

current through the optoisolator and the TL431 is set by the

minimum operating current requirements of the TL431.

This current is minimum 1.0 mA. Assign the maximum

current through the branch to be 5 mA. That makes the bias

resistor value equal to:

Rbias = RS =

Vout − [Vref(TL431) + VLED]

ILED

=

6.0 V − [2.5V + 1.4V]

5.0 mA

= 420 Ω ≈ 430 Ω