MC33364 Datasheet, PDF (8/14 Pages) ON Semiconductor – Critical Conduction GreenLine SMPS Controller

MC33364
The AL factor is determined by:
2
ǒ Ǔ _{+ + AL}
Lp
n2p
Lp BmaxAc
2
ƪ ^{ǒ Ǔ} ƫ^{Lp Ippk}
ǒ Ǔǒ Ǔ _{0.2 T 33.5 E–6 m2} ²
+ _{ǒ Ǔ} + _{.00192 H (0.472 A)2}
105 nH
From the manufacturer‘s catalogue recommendation the
core with an AL of 100 nH is selected. The desired number of
turns of the primary winding is:
+ǒ Ǔ ^ń +ƪ ƫ ^ń + np
12
Lp
AL
12
(0.00192 H)
(100 nH)
139 turns
The number of turns needed by the 6.0 V secondary is
(assuming a Schottky rectifier is used):
+ ^{ǒ ) Ǔǒ ē Ǔ} ns ^{Vs Vfwd 1– max np}
ƪē ǒ Ǔƫ max Vin(min)
ǒ Ǔ ₊ ) ǒ * Ǔ ₊ 6.0 V 0.3 V 1 0.5 139
7 turns
ƪ ǒ Ǔƫ 0.5 127 V
The auxiliary winding to power the control IC is 16 V and its
number of turns is given by:
+ _ƪ^{)ē * ē} _ƫ naux
(Vaux Vfwd)(1 max)np
max(Vin(min))
+ ) * + (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 line
frequency and toff, the discharge time of C1 during the
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. 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:
+ + + [ W W R7
Vcs
Ippk
1.2 V
0.472 A
2.54
2.2
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
currernt 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)
ILED
VLED]
+ * ) + [ W W 6.0 V [2.5V 1.4V]
5.0 mA
420
430
The MOC8102 has a typical current transfer ratio (CTR) of
100% with 25% tolerance. When the TL431 is full–on, 5 mA
will be drawn from the transistor within the MOC8102. The
transistor should be in saturated state at that time, so its
collector resistor must be
+ ^* + ^* + W Rcollector
Vref Vsat
ILED
5.0 V 0.3 V
5.0 mA
940
Since a resistor of 5.0 k is internally connected from the
reference voltage to the feedback pin of the MC33364, the
external resistor can have a higher value
+ + _* + _* Rext
R3
(Rint)(Rcollector)
(Rint) (Rcollector)
(5.0 k)(940)
5.0 k 940
+ [ 1157 W 1200 W
This completes the design of the voltage feedback circuit.
In no load condition there is only a current flowing through
the optoisolator diode and the voltage sense divider on the
secondary side.
The load at that condition is given by:
+ ₎ Rnoload
Vout
(ILED Idiv)
+ ) + W 6.0 V
(5.0 mA 0.25 mA)
1143
The output filter pole at no load is:
+ _p fpn
1
( 2 Rnoload Cout )
+ + p m 1
(2 )(1143)(300 F)
0.46 Hz
8
MOTOROLA ANALOG IC DEVICE DATA