Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
1999-04-26
2001-03-27
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
Reexamination Certificate
active
06208530
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a switching power supply device and, in particular, to a switching power supply device of a self-oscillation type ringing choke converter (hereinafter referred to as RCC) system.
2. Description of the Related Art
Generally speaking, electronic apparatus such as computers and communication apparatus require a stable DC voltage. To supply a stable DC voltage to such apparatus from a commercial AC power supply, a switching power supply device comprising an RCC circuit having a relatively simple structure and high efficiency is widely used. The construction of such a switching power supply device will be described with reference to FIG.
4
.
In
FIG. 4
, numeral
1
indicates a switching power supply device, which comprises an input circuit
2
, a DC-DC converter circuit
3
, a voltage detection circuit
4
and a control circuit
5
.
The input circuit
2
comprises a rectifying diode bridge DB, a fuse F provided between an AC power source and the input terminal of the diode bridge DB, and a filter circuit LF.
The DC-DC converter circuit
3
comprises a smoothing capacitor C
1
provided between the output terminals of the diode bridge DB of the input circuit
2
, a transformer T having a primary coil N
1
, a secondary coil N
2
of a polarity opposite to that of the primary
466
coil N
1
and a feedback coil Nb of the same polarity as the primary coil N
1
, an FET Q
1
as a main switching element connected in series to one end of the primary coil N
1
of the transformer T, a starting resistor R
1
connected between the other end of the primary coil N
1
and the gate serving as the control terminal of the FET Q
1
, a resistor R
10
connected between the gate and source of the FET Q
1
, a rectifying diode D
1
connected in series to one end of the secondary coil N
2
of the transformer T and a smoothing capacitor C
4
connected between the ends of the secondary coil N
2
.
The voltage detection circuit
4
, which is provided on the output side of the DC-DC converter circuit
3
, comprises a resistor R
5
, a light-emitting diode PD on the light emission side of a photocoupler PC, a shunt regulator Sr and resistors R
6
and R
7
. The resistor R
5
, the light-emitting diode PD and the shunt regulator Sr are connected in series to each other and arranged in parallel with the capacitor C
4
of the DC-DC converter circuit
3
. The resistors R
6
and R
7
are also connected in series to each other and arranged in parallel with the capacitor C
4
. The point of connection of the resistor R
6
and R
7
are connected to the shunt regulators Sr.
The control circuit
5
comprises a resistor R
13
and a capacitor C
3
connected in series between one end of the feedback coil Nb and the gate of the FET Q
1
, a transistor Q
2
connected between the gate and source of the FET Q
1
, a resistor R
2
connected between one end of the feedback coil 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 emitter of the transistor Q
2
, a resistor R
4
connected in series between one end of the feedback coil Nb and the base of the transistor Q
2
, a diode D
2
and a phototransistor PT on the light reception side of the photocoupler PC.
Next, the operation of the switching power supply device
1
, constructed as described above, will be explained.
When starting the device, voltage is applied to the gate of the FET Q
1
through the resistor R
1
to turn on the FET Q
1
. When the FET Q
1
is turned on, power source voltage is applied to the primary coil N
1
of the transformer T and a voltage is generated in the feedback coil Nb in the same direction as that of the voltage generated in the primary coil N
1
, the FET Q
1
being rapidly turned on by positive feedback. At this time, excitation energy is accumulated in the primary coil N
1
.
When the base electric potential of the transistor Q
2
has reached a threshold value, the transistor Q
2
is turned on and the FET Q
1
is turned off. As a result, the excitation energy accumulated in the primary coil N
1
of the transformer T during the ON-period of the FET Q
1
is discharged as electrical energy through the secondary coil N
2
, rectified by the diode D
1
, and smoothed by the capacitor C
4
before it is supplied to the load.
When the excitation energy accumulated in the primary coil N
1
of the transformer T has been entirely discharged, a voltage is generated in the feedback coil Nb and the FET Q
1
is turned on. When the FET Q
1
is turned on, a voltage is again applied to the primary coil N
1
of the transformer T, and excitation energy is accumulated in the primary coil N
1
.
This oscillating operation is repeated in the switching power supply device
1
.
In the normal state, the output voltage on the load side is divided by the resistors R
6
and R
7
, and the detection voltage obtained through this division is compared with a reference voltage of the shunt regulator Sr. Then, the variation in the output voltage is amplified by the shunt regulator Sr and the current flowing through the light-emitting diode PD of the photocoupler PC varies. The impedance of the phototransistor PT varies according to the light emission amount of the light-emitting diode PD, whereby it is possible to vary the charge/discharge time of the capacitor C
2
, thereby effecting control such that the output voltage is constant.
In the conventional switching power supply device
1
, however, the switching frequency of the FET Q
1
varies substantially inversely with the load power due to the characteristics of the RCC. In particular, the switching frequency increases when the load is light. As a result, the switching loss increases, and the circuit efficiency deteriorates. That is, as shown in
FIG. 5
, the lengths of the ON period and the OFF period determining the switching frequency of the FET Q
1
are in proportion to the load.
FIG. 5
shows the case in which the load is relatively heavy (a), the case in which the load is a medium one (b) and the case in which the load is light (c). Numerals t
1
, t
11
and t
21
indicate OFF-periods of the FET Q
1
, and numerals t
2
, t
12
and t
22
indicate ON-periods of the FET Q
1
. Under the condition in which the input/output voltage is constant, the ratio of the ON-period to the OFF-period is always constant regardless of whether the load is heavy or light. The value of t
1
:t
2
in (a), the value of t
11
::t
12
in (b) and the value of t
21
:t
22
in (c) are equal to each other. Thus, the switching frequency fluctuates to a large degree as a result of variation in the load. When the load is light, the switching frequency increases and the switching loss increases, with the result that the circuit efficiency deteriorates.
Further, when the switching frequency increases, the control circuit
5
cannot respond thereto, and a so-called intermittent operation is generated. Due to this intermittent operation, the output ripple noise voltage, for example, increases. Further, when the switching frequency increases, the EMI noise of the switching power supply device
1
is more difficult to cope with than in the case in which the switching frequency is low.
Further, at the time of short-circuiting, the FET Q
1
performs an oscillating operation, in which starting and stopping are repeated, so that, when, the starting time is short, the oscillation frequency is high, which means there is a fear of the FET Q
1
generating excessive heat and being damaged.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a switching power supply device in which the increase in the switching frequency of the main switching element is restrained, whereby it is possible to reduce the switching loss, prevent the increase in the output ripple noise voltage due to intermittent operation, and restrain the heat generation of the main switching element at the time of short-circuiting.
To achieve the above object, there is provided, in accordance with the present invention, a switchin
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
Riley Shawn
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