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SEMICONDUCTOR APPLICATION NOTE
by Joël Turchi
Power Management Products Operation
Application Laboratory, Motorola, Toulouse, France
The purpose of this application note is to present a way of
designing the MC44603 in a fly–back dedicated to a 110 W
output power supply in two distinct cases: 110 Vrms mains
and 220 Vrms mains.
For this range of power, the discontinuous mode must be
chosen as it limits the stress on the power switch and the out-
put diodes. This kind of working can be guaranteed, thanks to
the demagnetization arrangement of the MC44603.
This application note considers both high and low mains
voltages.
The outputs:
120 V
28 V
15 V
8 V
0.5 A
1.0 A
1.0 A
1.0 A
Consequences for the Transformer
This output voltage values are obtained using four second-
ary windings of the transformer. Now, as a diode is located
between the output capacitor and the winding, this diode volt-
age must be taken into account in the choice of the number of
turns of each of the windings.
So, if the diode voltage is considered equal to 1 V, the
desired voltages on the windings are nearly:
121 V
— In the high voltage a.c. line case, it deals with both
MOSFET and BIPOLAR transistor use.
— In the low mains voltage case, only the MOSFET solu-
tion is considered as the inductor peak current is high.
The MC44603 is a high flexibility SMPS controller. It offers
a wide variety of protection (accurate maximum current limita-
tion, Vcc overvoltage detection, fold–back, demagnetization,
soft start).
29 V
16 V
9 V
In addition, distinct working modes can be used with this
circuit:
To obtain the output voltage values of the specification with
a good accuracy, we need at least 3 turns for the 9 V and then:
9 V
3 turns
— a fixed frequency working mode
16 V
5 turns
Here, the frequency is given by the circuit oscillator.
29 V
10 turns
40 turns
— a variable frequency working mode
This kind of working can be obtained when the chosen
oscillator period is shorter than the cycle time for trans-
former magnetization and complete demagnetization.
121 V
This solution seems to be correct, since with three turns, the
9 V should be well coupled; also, 40 turns (for the 120 V) is not
too large a value (no ferrite saturation).
— a stand–by mode
In our application, in order to obtain a very well regulated
high voltage output, an opto coupler is used. By this means,
the regulation uses feedback drawn from the 120 V output.
This reduces the working frequency during this phase.
This method reduces the stand–by losses that are
mainly proportional to the switching frequency, and
avoids the unstable very high frequency working that
is generally associated with traditional free frequency
systems.
II — GENERAL DESIGN EQUATIONS
Calculation of the Main SMPS Parameters
II–1 — Fixed frequency working mode:
In the MC44603 design proposed here, the variable
frequency mode is not used. Indeed, as shown later, the fixed
frequency mode enables a more accurate control of the power
that is drawn from the mains.
To obtain a fixed frequency discontinuous working mode
(the discontinuous mode is guaranteed by the demagnetiza-
tion section), the transformer primary inductor magnetization
and demagnetization cycle must be shorter than the oscillator
period (that is the chosen working period) for any working
point.
I — THE APPLICATIONS
OUTPUT: 110 W
In the following calculations, the maximum input power will
This condition can be expressed by the following inequality:
be considered equal to 135 W (η
80%).
(ineq 1)
Ton Toff
Tosc
Application 1: 110 V mains 80 Vrms ↔ 140 Vrms
Application 2: European mains 180 Vrms ↔ 280 Vrms
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