Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2002-05-21
2003-07-01
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
Reexamination Certificate
active
06587358
ABSTRACT:
TECHNICAL FIELD
This invention relates to a switching power supply circuit that can be incorporated as a power supply in various electronic devices.
BACKGROUND ART
A switching power supply circuit may adopt a switching converter such as a flyback converter or a forward converter. Since these switching converters use a rectangular waveform signal for a switching operation, a switching power supply circuit adopting such a converter is also called a hard switching power supply.
FIG. 7
depicts a hard switching power supply circuit
700
adopting a Ringing Choke Converter (“RCC”) system. Power supply circuit
700
is used as a standby power supply provided separately from a main power supply and is constructed so as to satisfy, for example, a low load condition where the load power (Po) is 50 W or less or another condition where the load power Po is 0.5 W or less.
As shown in
FIG. 7
, power supply circuit
700
includes a converter transformer CVT having a driving winding NB, a primary winding N
1
on a primary side, and a secondary winding N
2
on a secondary side. Thus, power supply circuit
700
is divided into a primary side
710
and a secondary side
715
.
Power supply circuit
700
includes a rectifier smoothing circuit
705
for receiving a commercial Alternating Current (“AC”) power supply with an input AC voltage VAC and producing a Direct Current (“DC”) input voltage Ei. Rectifier smoothing circuit
705
is a full-wave voltage-mulitplying rectifier circuit composed of a bridge rectifier circuit Di and a smoothing capacitor Ci. Rectifier smoothing circuit
705
produces rectified smoothed DC input voltage Ei that is substantially equal to AC input voltage VAC. Further, an inrush current limitation resistor Ri is interposed in a rectifier current path of rectifier smoothing circuit
705
in order to suppress any initial inrush current spike from flowing into smoothing capacitor Ci, for example, when power supply AC is initially provided to circuit
705
.
A switching element Q
1
receives and switches DC input voltage Ei to produce a switching output. Illustratively, a bipolar transistor is used for switching element Q
1
.
The collector of switching element Q
1
is connected to a positive electrode terminal of smoothing capacitor Ci through a series connection to primary winding N
1
of converter transformer CVT.
The base of switching element Q
1
is connected to receive rectified smoothed voltage Ei through a starting resistor RS so that a base current may be supplied thereto upon starting. Further, a series circuit connection of a base current limiting resistor RB, a diode D
4
, and driving winding NB is connected to the base of switching element Q
1
. An end of driving winding NB is grounded. A capacitor CB is connected in parallel to diode D
4
. Base current limiting resistor RB, diode D
4
, driving winding NB, and capacitor CB cooperatively form a self-excited oscillation drive circuit which oscillates and drives switching element Q
1
in a self-excited manner.
The emitter of switching element Q
1
is connected to ground through a resistor R
7
.
Converter transformer CVT is provided to transmit a switching output obtained by primary side
710
of power supply circuit
700
to secondary side
715
and has primary winding N
1
and secondary winding N
2
wound thereon. Also, driving winding NB for self-excited oscillation described above is wound on the primary side of converter transformer CVT.
A half-wave rectifier circuit formed by a rectifier diode D
01
and a smoothing capacitor C
01
is connected to secondary winding N
2
of converter transformer CVT and produces and outputs a secondary side DC output voltage E
01
. Secondary side DC output voltage E
01
is supplied to a load (not shown) and further inputted as a detection voltage to a control circuit
7
for constant voltage control.
Control circuit
7
includes a photo-coupler PC to isolate, in DC, parts thereof on secondary side
715
from parts on primary side
710
. On secondary side
715
, control circuit
7
comprises a pair of resistors R
3
and R
4
that divide secondary side DC output voltage E
01
, and the divided voltage is inputted to a detection input of a detection element Q
3
. An end of detection element Q
3
is connected to receive secondary side DC output voltage E
01
through a series connection of a resistor R
1
and a photo-diode PD of photo-coupler PC. The other end of detection element Q
3
is grounded.
A series circuit connection of a capacitor C
11
and a resistor R
2
is connected in parallel to resistor R
4
. Another series circuit connection of a capacitor C
12
and a resistor R
5
is connected across a junction between resistors R
4
and R
3
and a junction between detection element Q
3
and photo-diode PD.
On primary side
710
, control circuit
7
includes a phototransistor PT of photo-coupler PC. A half-wave rectifier circuit formed by a diode D
3
and a capacitor C
3
for rectifying and smoothing an alternating voltage excited in driving winding NB is connected to the collector of phototransistor PT so that a low DC voltage obtained by the half-wave rectifier circuit (D
3
and C
3
) may be supplied as operating power supply to phototransistor PT.
The emitter of phototransistor PT is connected to the base of a transistor Q
4
, which serves as an amplifier. A series circuit connection of a resistor R
8
and a Zener diode ZD is inserted between the emitter of phototransistor PT and a junction between driving winding NB and diode D
4
.
The collector of transistor Q
4
is connected to the base of switching element Q
1
, and the emitter of transistor Q
4
is grounded.
The base of transistor Q
4
is connected to the emitter of switching element Q
1
through a parallel circuit connection of a resistor R
6
and a capacitor C
13
, and is, thus, grounded through resistor R
7
.
A reset circuit
10
is formed by serially connecting diode DRS to a parallel circuit connection of a resistor RRS and a capacitor CRS. Reset circuit
10
is connected in parallel to primary winding N
1
.
A snubber circuit
11
includes a capacitor Csn serially connected to a resistor Rsn. The collector of switching element Q
1
is grounded through snubber circuit
11
.
Reset circuit
10
and snubber circuit
11
are required to suppress a spike voltage that appears upon turning off switching element Q
1
.
A switching operation is started by applying current to switching element Q
1
through starting resistor RS, thereby turning on switching element Q
1
. When switching element Q
1
is on, magnetic energy is stored into primary winding N
1
of converter transformer CVT. When switching element Q
1
is off, the magnetic energy stored in primary winding N
1
is discharged to the secondary side of converter transformer CVT. This operation is repeated to produce an output voltage on the secondary side of converter transformer CVT.
Control circuit
7
varies the amount of current passing through detection element Q
3
in response to secondary side DC output voltage E
01
. Photo coupler PC controllably varies the base current supplied to transistor Q
4
in response to the amount of current flowing through detection element Q
3
, thereby varying the collector current of transistor Q
4
. Since the collector of transistor Q
4
is connected to the base of switching element Q
1
, the base current (drive current amount) to flow from the self-excited oscillation drive circuit (resistor RB, diode D
4
, driving NB, and capacitor CB) to the base of switching element Q
1
varies in accordance with the collector current of transistor Q
4
. Consequently, the on time of switching element Q
1
is varied, and as a result, the switching frequency is controllably varied, thereby performing constant voltage control.
In power supply circuit
700
having the construction shown in
FIG. 7
, a constant voltage effect may be obtained by controlling a switching frequency (fs) to increase in response to a rise in AC input voltage VAC or a decrease in the load power Po. The range of control of switching frequency fs is set to a wide range of 25 KHz to 2
Berhane Adolf Deneke
Frommer WIilliam S.
Frommer & Lawrence & Haug LLP
Sony Corporation
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