Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-12-26
2002-09-10
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S049000, C363S056070
Reexamination Certificate
active
06449172
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a switching power supply apparatus. More particularly, it relates to a switching power supply apparatus in which a rectifying current that flows in a primary side of a transformer connected to a load side is formed to provide nonlinear characteristics when a power supply is started up, thereby preventing an excessive current from flowing in a switching transistor or the like provided at the primary side of the transformer.
As a switching power supply apparatus, there is known an apparatus based on a current resonance system.
FIG. 1
shows a conventional example of switching power apparatus of this current resonance system, the apparatus having a SEPP (Single Ended Push Pull) arrangement.
A switching power supply apparatus
10
shown in
FIG. 1
comprises switching signal generating means
12
including a variable frequency oscillating circuit
14
and a drive circuit
16
. The oscillating circuit's oscillation signal is supplied to the drive circuit, thereby generating a pair of switching signals, for example, having a reverse phase relationship therebetween. In the case where the switching generating means
12
is composed of IC circuits, an oscillating element (capacitor
18
and resistor
20
) that determines an oscillation frequency is externally provided at any of external terminals
12
a
and
12
b
of this IC circuit.
A pair of switching signals Sp, Sp bar are supplied to a pair of switching elements
22
and
24
having SEPP arrangement. A MOS type electric field effect transistor or the like may be utilized as the switching elements
22
and
24
. An resonating capacitor
28
is connected to both a ground and a connection neutral point ‘p’ between a pair of these switching elements
22
and
24
via a primary coil
26
a
of an insulation transformer
26
.
Respective diodes
30
a
and
30
b
rectify a secondary current that flows in a pair of secondary coils
26
b
and
26
c
of the insulation transformer
26
as full-wave rectifier. The full-wave rectified current allows a smoothing capacitor
32
to be charged. Therefore, voltage ‘vb’ obtained at both ends
34
of the smoothing capacitor
32
is supplied to a load (not shown) as an output voltage.
The output voltage is supplied to an amplifier
36
, as voltage comparison means, wherein the voltage is compared with a reference voltage, Vref Its comparison output is supplied to a photo-coupler
38
that configures inductance control means
37
provided in order to insulate the primary and secondary sides of the transformer
26
. The photo-coupler
38
comprises a photodiode
40
and a phototransistor
42
that functions as a variable inductance element. A current based on the comparison output flows in this phototransistor
42
.
The phototransistor
42
is connected to the external terminal
12
b
through a stationary resistor
44
. Therefore, when the phototransistor
42
is ON, this resistor
44
and serial impedance caused by the phototransistor
42
are connected in parallel to a resistor
20
, which is an oscillation element.
In this arrangement, it is known that a relationship between a resonation frequency ‘f’ and a resonation impedance Z of the resonating circuit on the primary side of the transformer
26
formed of its primary coil
26
a
and the capacitor
28
is based on upper side operation as indicated by a curve ‘Lo’ in FIG.
2
.
In this resonating circuit, when switching frequencies of the switching signals Sp and Sp bar supplied to a pair of switching elements
22
and
24
are increased, the resonance impedance Z increases. The resonance impedance Z is lowered as the switching impedance Z is lowered. Such change in the resonance impedance Z causes a resonance current i
1
that flows in the primary coil
26
a
to be changed. Thus, controlling this resonance current i
1
allows an output voltage Vb induced at the secondary side of the transformer
26
to be controlled.
When the output voltage Vb obtained at an output terminal
34
is illustratively higher than the reference voltage Vref, the phototransistor
42
has its impedance according to the comparison output. Thus, the composite resistance of the external terminal
12
b
becomes smaller than a case of a simplex of the resistor
20
, whereby an oscillation frequency ‘fsw’ increases.
When the oscillation frequency ‘fsw’ is increased, the resonance impedance Z determined depending on the primary coil
26
a
and the capacitor
28
increases. Thus, a current that flows in this primary coil
26
a
is limited, and its value decreases. With this decrease in current, the currents induced at the secondary coils
26
b
and
26
c
are reduced as well. As a result, a charge voltage with the capacitor
32
decreases. Namely, the output voltage Vb is controlled in the direction of the reference voltage Vref.
Conversely, when the output voltage Vb is lower than the reference voltage Vref, the impedance of the phototransistor
42
increases, and the composite resistance value at the external terminal
12
b
increases. Then, the variable frequency oscillating circuit
14
is controlled so that its oscillation frequency ‘fsw’ may be lowered. As a result, the switching frequency is lowered relevant to the switching elements
22
and
24
, and the primary resonance impedance Z of the transformer
26
is lowered accordingly. This causes the resonance current to increase. When the resonance current increases, the secondary current increases as well. Thus, the charge voltage Vb with the capacitor
32
rises, and a closed loop control is performed so as to be close to the reference voltage Vref.
In the meantime, in this switching power supply apparatus
10
, a large amount of resonance current flows from a time when a power supply is turned ON to a time when the capacitor
32
rises to a voltage in its constant state. Thus, this current may damage the switching elements
22
and
24
.
In order to reduce such damage, there has been conventionally provided a soft start circuit
50
, which functions as frequency control means
60
, for limiting a resonance current during startup. This soft start circuit
50
is provided in the switching signal generating means
12
. An external charging capacitor
52
is connected to an external terminal
12
c
arranged at this soft start circuit
50
, so that charging for this capacitor
52
is started in synchronism with turning ON the power. Then, a change in charge voltage Va at this time causes a charge current of the oscillating capacitor
18
, which is an oscillating element, connected to the external terminal
12
a
to be changed.
When a charge current with the oscillating capacitor
18
changes with an elapse of time, the oscillation frequency ‘fsw’ changes accordingly. This fact will be described with reference to
FIGS. 3A
to
3
E.
FIG. 3A
shows a change in charge voltage Va when and after the power is turned ON, wherein the charge characteristics are linear as indicated by line La. The variable frequency oscillating circuit
14
is changed in the oscillation frequency ‘fsw’ by the charge voltage Va of the capacitor
52
associated with the soft start circuit
50
connected to the oscillating circuit
14
. The oscillation frequency fsw changes almost linearly as indicated by characteristic line Lb in FIG.
3
B. When a charge voltage Va is zero volt, oscillation occurs at a high frequency, and the oscillation frequency ‘fsw’ is lowered as the charge voltage Va increases.
On the other hand, the primary resonance impedance Z is characterized by characteristic curve Lo such that the resonance impedance Z increases as a frequency increases from the resonance frequency ‘fo’ as shown in
FIG. 2. A
relationship between the impedance Z and a time is illustrated as shown in FIG.
3
C. Namely, there are nonlinear characteristics that the resonance impedance Z is initially high, and then, lowers rapidly; the impedance gently changes as the charge voltage Va is close to a full charge.
As a result, there is provided nonlinear characteristics such that, although not so much pr
Maioli Jay H.
Riley Shawn
Sony Corporation
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