AND8031 Datasheet, PDF (2/12 Pages) ON Semiconductor – Isolated Precision Regulation of a Single 1.8 Volt Output from a Universal Line Input

AND8031/D
Since the power level is low, an RC EMC filter is utilized.
The RC filter is not as efficient as an LC, but it uses less
board area and will cost less. 1N4005 600 volt fast diodes
provide the full wave rectification of the AC input. The bulk
capacitor is the filter. The selection of the capacitor is
determined from three factors: input voltage, ripple current
and maximum output ripple. The equations for each are as
follows.
Vdc + Vac · ^Ǹ2
(eq. 1)
Ǹ Irms +
Ipk2 ·
D
3
(Discontinuous
mode only)
(eq. 2)
Cin
+
f
·
k · Pin (AV)
(Vripple (p–p))2
(eq. 3)
Pin
+
Pout
h
(eq. 4)
Where k is 1 for AC inputs the peak to peak ripple is 6.0
volts and η is estimated to be 70% with the low power level
of the board and the RC input filter. Solving for the equations
above and using 50% for the maximum duty cycle, Vdc
maximum is 375 volts, Vdc minimum is 120 volts, Ipeak and
Irms are solved for in the transformer calculations, Pin is 3.2
watts, and Cbulk is 0.9 uF.⁽¹⁾
The transformer converts the rectified line voltage to the
output voltage and is controlled by the MC33363B. To
design the transformer, the following data is necessary. Vdc
minimum is 100 volts (allowing for bulk voltage ripple and
power switch voltage drop), the frequency is 100 kHz, the
maximum duty cycle of the power switch and the reset time
for the secondary are both 45% to maintain discontinuous
conduction, and the secondary diode forward voltage drop
is 0.45 volts (estimated).
Psec + (Vout ) Vdiode) · Iout
(eq. 5)
1
Pin + ²
(Vdc · Ton)2
Lpri · T
, or
(eq. 6)
Lpri
+
1
2
(Vdc · Ton)2
Pin · T
Ǹ Ipeak +
2·P·
T
L
(eq. 7)
(eq. 8)
ǒ Ǔ ^Lpri
Lsec
+
Np ²
Ns
(eq. 9)
For the primary side of the transformer, Lpri is 3.16 mH,
Ipeak is 142 milliamperes, ton is 4.5 usec and Irms is 55
milliamperes–rms. For the secondary, Psec is 2.25 watts, L
is 2.28 uH, Ipeak is 4.44 amperes and Irms is 1.72
amperes–rms. The turns ratio is 37.2. The auxiliary winding
is set to 12 volts and 10 milliamperes. Using the secondary
turns ratio, the auxiliary turns ratio is calculated as 6.6 in
reference to the primary.
With the peak primary current and the frequency
determined, the components surrounding the MC33363B
can be selected. Referring to Figures 2 and 3, RT will be set
to 30 k Ohms and CT will be set to 390 pF. The Over Voltage
Protection, pin 11, will not be used. Its main function is for
loss of optocoupler protection. Pin 8, the reference voltage,
requires a 1.0 uF ceramic capacitor for stability. The voltage
compensation and voltage feedback, pins 9 and 10, will be
covered in the feedback section of this document. The Vcc
pin requires a 10 uF capacitor for stability.
1.0 M
C_T = 100 pF
500 k _{CT = 200 pF}
200 k ^CT ^{= 500 pF}
100 k ^{CT = 1.0 nF}
50 k ^{CT = 2.0 nF}
V_CC = 20 V
T_A = 25°C
20 k ^{CT = 5.0 nF}
10 k ^{CT = 10 nF}
7.0
10
15 20
30
50 70
R_T, TIMING RESISTOR (kΩ)
Figure 2. Oscillator Frequency
versus Timing Resistor
1.0
0.8
V_CC = 20 V
C_T = 1.0 mF
0.6
T_A = 25°C
0.4
0.3
0.2 ^{Inductor supply voltage and}
inductance value are
0.15 adjusted so that I_pk turn–off
is achieved at 5.0 ms.
0.1
7.0
10
15 20
30 40 50 70
R_T, TIMING RESISTOR (kΩ)
Figure 3. Power Switch Peak Drain Current
versus Timing Resistor
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