AND8031/D

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3

A snubber is required across the primary winding because

of the leakage inductance. An RCD snubber is implemented.

The diode will be an MURS160 ultrafast diode. The resistor

and capacitor are dependent upon the leakage inductance of

the primary windings of the transformer. Preliminary values

of the resistor and capacitor are derived from the following

equations. The final values will be determined on the bench

with an examination of the trade–offs between system

efficiency and peak voltage.

Vreflect

+ (Vout ) Vdiode) ·

Np

Ns

(eq. 10)

R

+

2 · Vmax · (Vmax

* (Vout ) Vdiode) ·

Np

Lleak · Ip2 ·freq

(eq. 11)

C

+

Vmax

Vcr · R · freq

(eq. 12)

Vmax is the desired peak voltage on the power switch

minus the bulk voltage, Vcr is the clamping ripple usually

20 V, and freq is the switching frequency of the MC33363B.

Most of the values necessary to calculate the secondary

components are complete. To select the diode, the secondary

current peak, the blocking voltage and the forward voltage

drop are the main criteria. Ripple current from the

transformer, the output voltage and the output ripple current

are the criteria to select the output capacitors.

Vblock

+ Vout ) Vdc · Ns

Np

(eq. 13)

C

+

1

Vripple · freq

(eq. 14)

Utilizing the above equations and previous calculations,

the diode needs to be rated at a minimum of 13 volts and 4.44

amperes. The MBRD835L is selected as the Schottky diode.

The typical forward diode drop is 0.40 volts at 4.0 amperes.

Rubycon capacitor YXG 6.3 volts 820 uF is chosen for the

output filter. The voltage on the output capacitor needs to be

rated twice the output voltage for safety. The rated

maximum allowable ripple current of the capacitors is 0.865

amperes, so 2 capacitors in parallel will be used. A 1.0 uF

ceramic capacitor will also be used in parallel with the

aluminum electrolytics for fast transient response.

Equations 13 and 14 are used to calculate the auxiliary

winding values. The auxiliary winding diode is an ultrafast

recovery surface mount, MURS120. The diode is rated for

1 ampere and 200 volts blocking. The typical forward

voltage drop is 0.6 volts. The higher forward voltage drop is

acceptable due to the lower power requirements of the

auxiliary versus the secondary output. The auxiliary

Feedback Network

With the low voltage output requirement, a programmable

precision reference with a low operating voltage is essential.

Isolated feedback networks provide an additional challenge

since optoisolator diode forward voltages remain at 1.25 volts.

With a conventional circuit using the TL431 and TLV431, the

lowest ideal power supply output voltages are 3.75 and 2.5

volts respectively. If the reference operating voltage range is

reduced to 0.9 volts, the minimum output voltage is lowered to

2.15 volts. See Figure 4 for the circuit schematic.

This reduction is significant but will not satisfy a 1.8 volt

output. By adding a resistor and PNP transistor, the

reference voltage is reduced to the emitter–base voltage

drop plus the programmable reference operating voltage.

With the conventional TLV431 the minimum operating

voltage is reduced to 1.95 volts. With the reduced reference

voltage available in the NCP100, the minimum operating

voltage is 1.6 volts. See Figure 5 for the circuit schematic.

Figure 6 illustrates the output voltage ranges between the

TL431, TLV431 and NCP100.

R1

U2

U1

R2

C2

GND

R3

Figure 4. Traditional Isolated Feedback Network

R1

C3

Q1

U2

U1

NCP100

R2

C2

GND

R3

R4

Figure 5. 1.8 Volt Isolated Feedback Network