Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-01-11
2001-09-25
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S097000, C363S131000
Reexamination Certificate
active
06295211
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a switching power supply unit, and more particularly, it relates to a switching power supply unit of a ringing choke converter (hereinafter referred to as an RCC) system.
2. Description of the Related Art
In general, electronic equipment such as an electronic calculator or a communication device needs a stable DC voltage. Thus, in order to supply the stable DC voltage from a commercial AC power supply, a switching power supply unit of the RCC system, in which relatively easy formation can be conducted to obtain high efficiency, has been widely used. The structure of such a switching power supply unit will be illustrated referring to FIG.
8
.
In this figure, reference numeral
10
indicates a switching power supply unit, which has an input circuit
2
, a DC-DC converter circuit
3
, a voltage detection circuit
4
and a control circuit
5
.
Among these components, the input circuit
2
has a fuse F connected to an AC power supply AC, a filter circuit LF, and a rectifying diode bridge DB.
Additionally, the DC-DC converter circuit
3
has a smoothing capacitor C
1
disposed between the output ends of the diode bridge DB of the input circuit
2
, a transformer having a primary winding N
1
, a secondary winding N
2
having the polarity opposite to that of the primary winding N
1
, and a feedback winding Nb with the same polarity as that of the primary winding N
1
, an FET Q
1
as a main switching element connected in series to an end of the primary winding N
1
of the transformer T, a starting resistor R
1
connected between the other end of the primary winding N
1
and the gate as the control terminal of the FET Q
1
, a resistor R
10
connected between the gate and the source of the FET Q
1
, a rectifying diode D
1
connected in series to an end of the secondary winding N
2
of the transformer T, and a smoothing capacitor C
4
connected between both ends of the secondary winding N
2
.
The voltage detection circuit
4
disposed on the output side of the DC-DC converter circuit
3
includes a resistor R
5
, a light-emitting diode PD on the light-emitting side of a photo coupler PC, a shunt regulator Sr, and resistors R
6
and R
7
. Among these components, the resistor R
5
, the light-emitting diode PD, and the shunt regulator Sr are mutually connected in series and disposed in parallel to the capacitor C
4
of the DC-DC converter circuit
3
. In addition, the resistors R
6
and R
7
are also mutually connected in series and similarly disposed in parallel to the capacitor C
4
. The node of the resistors R
6
and R
7
is connected to the shunt regulator Sr.
The control circuit
5
includes a resistor R
13
and a capacitor C
3
connected in series between one end of the feedback winding Nb and the gate of the FET Q
1
, a transistor Q
2
connected between the gate and the source of the FET Q
1
, a resistor R
2
connected between one end of the feedback winding Nb and the base of the transistor Q
2
, a resistor R
3
and a capacitor C
2
connected in parallel between the base and the emitter of the transistor Q
2
, a resistor R
4
, a diode D
2
, and, a photo transistor PT on the light-receiving side of the photo coupler PC, which are mutually connected in series between one end of the feedback winding Nb and the base of the transistor Q
2
.
Next, a description will be given of the operation of a switching power supply unit
10
having such a structure.
First, on startup, voltage is applied to the gate of the FET Q
1
via the resistor R
1
to turn on the FET Q
1
. When the FET Q
1
is turned on, a power supply voltage is applied to the primary winding N
1
of the transformer T, and voltage is generated in the feedback winding Nb in the same direction as that of the voltage generated in the primary winding N
1
, whereby the FET Q
1
is rapidly turned on by a positive feedback. Under this situation, excitation energy is charged in the primary winding N
1
.
When the base potential of the transistor Q
2
reaches a threshold, the transistor Q
2
is turned on, whereas the FET Q
1
is turned off. This permits the excitation energy charged in the primary winding N
1
of the transformer T during the ON time of the FET Q
1
to be discharged as electric energy via the secondary winding N
2
. The energy is supplied to a load after being rectified by the diode D
1
and smoothed by the capacitor C
4
.
In this way, when the excitation energy charged in the primary winding N
1
of the transformer T is all discharged via the secondary winding N
2
, voltage is again generated in the feedback winding Nb, and the FET Q
1
is thereby turned on. When the FET Q
1
is turned on, voltage is again applied to the primary winding N
1
of the transformer T to charge excitation energy in the primary winding N
1
.
In the switching power supply unit
10
, the above-described oscillating operation is repeated.
Under normal conditions, an output voltage on the load side is divided by the resistors R
6
and R
7
, and the divided detection voltage is compared with a reference voltage of the shunt regulator Sr. After this comparison, the amount of fluctuations in the output voltage is amplified by the shunt regulator Sr, and current flowing through the light-emitting diode PD of the photo coupler PC changes, so that the impedance of the photo transistor PT changes according to the light-emitting amount of the light-emitting diode PD. This operation permits the time for charging/discharging the capacitor C
2
to be changed, so that the output voltage is controlled to be fixed.
In the conventional switching power supply unit
10
, however, as a characteristic of the RCC system, as shown in FIG.
4
(
d
), the switching frequency of the FET Q
1
varies approximately inversely with a load power. Therefore, under light load, the switching frequency is increased, and accordingly, switching losses are increased, whereas under heavy load, the switching frequency is decreased, and accordingly, conduction losses are increased. As a result, these losses lead to reduction in circuit efficiency.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a switching power supply unit with high circuit efficiency, in which the switching frequency is reduced under light load, whereas a switching frequency equivalent to that of a conventional switching power supply unit can be obtained under heavy load.
To this end, the present invention provides a switching power supply unit including a DC power supply, a transformer having a primary winding, a secondary winding and a feedback winding, a main switching element connected in series to the primary winding, and a control circuit connected between the feedback winding and the control terminal of the main switching element, so that a DC output can be obtained. In this switching power supply unit, there are provided a voltage generating unit disposed on the primary side of the transformer to output a voltage according to a load power, and a delay circuit for reducing the switching frequency by delaying the turn-on of the main switching element according to a voltage output from the voltage generating unit thereby prolonging the OFF time of the main switching element.
In addition, the voltage generating unit includes a rectifying and smoothing circuit connected between both ends of the feedback winding.
In addition, an auxiliary feedback winding connected between the control circuit and the control terminal of the main switching element is disposed in the transformer, and the voltage generating unit includes a rectifying and smoothing circuit connected between both ends of the auxiliary feedback winding.
Furthermore, the delay circuit includes a switching unit disposed between the feedback winding and the control terminal of the main switching element, and a time-constant circuit.
Furthermore, the time-constant circuit includes a capacitor and a resistor connected to the control terminal of the switching unit.
Furthermore, a Zener diode is connected in series to the resistor of the
Nakahira Koji
Nishida Akio
Tani Ryota
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
Vu Bao Q.
Wong Peter S.
LandOfFree
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