AND8031 Datasheet, PDF (3/12 Pages) ON Semiconductor – Isolated Precision Regulation of a Single 1.8 Volt Output from a Universal Line Input

AND8031/D
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)
Lleak · Ip2 · freq
·
Np
Ns
)
(eq. 11)
C
+
Vmax
Vcr · R · freq
(eq. 12)
See AND8023/D page 7 for further information on equations 11 and 12.⁽²⁾
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.
ǒ Ǔ 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
winding filter is the capacitor on the V_CC pin.
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.
V_out
R3
R1
U2
C2
U1
R2
GND
Figure 4. Traditional Isolated Feedback Network
V_out
R3
R1
Q1
C2
R4
C3
U2
U1
NCP100 R2
GND
Figure 5. 1.8 Volt Isolated Feedback Network
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