Electrical transmission or interconnection systems – Plural load circuit systems – Control of current or power
Reexamination Certificate
2001-01-09
2002-08-13
Tso, Edward H. (Department: 2838)
Electrical transmission or interconnection systems
Plural load circuit systems
Control of current or power
C363S021150, C363S021160, C363S097000
Reexamination Certificate
active
06433443
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a switching power supply and more particularly to a switching power supply of a self-excited ringing choke converter (hereinafter referred to as RCC) type.
2. Description of the Related Art
Generally, electronic equipment such as electronic computers and communication devices require stable DC voltages. In order to supply stable DC voltages for such electronic equipment using the commercially available power source, switching power supplies of RCC type which has a relatively simple construction and shows high degrees of efficiency are widely used.
FIG. 7
is the circuit diagram of such a conventional switching power supply of RCC type. In
FIG. 7
, a switching power supply
1
is provided with an input circuit
2
, a main operating circuit
3
, a voltage detecting circuit
4
, a voltage output terminal OUT, and a ground terminal GND. The input circuit
2
comprises a diode bridge circuit DB for rectification, and a fuse F and a filter circuit LF both of which are provided between an AC power supply and the input terminal of the diode bridge circuit DB.
Furthermore, the main operating circuit
3
comprises a capacitor C
1
for smoothing provided between the output terminals a and b of the diode bridge DB in the input circuit
2
; a transformer T containing a primary winding N
1
, a secondary winding N
2
having the opposite polarity to that of the primary winding N
1
, and a feedback winding having the same polarity as the primary winding N
1
; a FET Q
1
as a switching element connected in series to one end of the primary winding N
1
of the transformer T; a resistor R
1
for starting connected between the other end of the primary winding N
1
and the gate as a control terminal of the FET Q
1
, a resistor R
8
connected between the gate and source of the FET Q
1
, a diode D
1
for rectification connected in series to one end of the secondary winding N
2
of the transformer T, and a capacitor C
4
for smoothing connected between the other end of the secondary winding N
2
and the voltage output terminal OUT.
Furthermore, the voltage detecting circuit
4
is provided on the output side of the main operating circuit
3
and contains a resistor R
5
, a light-emitting diode PD on the emission side of a photo coupler PC, a shunt regulator Sr, and resistors R
6
and R
7
. The resistor R
5
, the light-emitting diode PD, and the anode and cathode of the shunt regulator are connected in series and are provided so as to be parallel to the capacitor C
4
of the main operating circuit
3
. Furthermore, the resistors R
6
and R
7
are connected in series and are also provided to be parallel to the capacitor C
4
. The connection point of the resistors R
6
and R
7
is connected to the control terminal of the shunt regulator Sr.
Furthermore, a control circuit
5
comprises a resistor R
9
and a capacitor C
3
connected 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 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 emitter of the transistor Q
2
, and a resistor R
4
, a diode D
2
, and a phototransistor PT on the light-receiving side of a photo coupler PC connected in series between one end of the feedback winding NB and the base of the transistor Q
2
.
Next, the operation of the switching power supply
1
thus constructed is described. First of all, at start, a voltage is applied to the gate of the FET Q
1
through the resistor R
1
and the FET Q
1
is turned on. When the FET Q
1
is turned on, the voltage of the power supply is applied to the primary winding N of the transformer T, a voltage in the same direction as the voltage generated in the primary winding N
1
is generated in the feedback winding NB, and then the FET Q
1
is rapidly turned on because of positive feedback. At this time, excitation energy is stored in the primary winding N
1
.
When the capacitor C
2
is charged through the resistor R
2
and the potential of the base of the transistor Q
2
reaches the threshold, the transistor Q
2
is turned on and the FET Q
1
is tuned off. Because of this, the excitation energy stored in the primary winding N
1
of the transformer T, while the FET Q
1
is turned on, is discharged as electric energy through the secondary winding N
2
, and the electric energy is rectified by the diode D
1
, smoothed by the capacitor C
4
, and supplied to a load through the voltage output terminal OUT.
Furthermore, when the excitation energy stored in the primary winding N
1
of the transformer T is discharged through the secondary winding N
2
, a flyback voltage VNB is generated in the feedback winding NB. The change of this flyback voltage VNB is described with reference to FIG.
8
. In
FIG. 8
, at the time t
11
, the FET Q
1
is turned off and the flyback voltage VNB is kept at a nearly constant value Vb, that is, it goes into a so-called off-state period. Then, at the time t
12
, the voltage of the diode D
1
becomes zero and the flyback voltage starts to oscillate, and when the flyback voltage VNB rises and the gate voltage reaches the threshold Vth at t
13
, the FET Q
1
is turned on. Moreover, part of the flyback voltage VNB shown by the chain line shows the case where the flyback voltage VNB is assumed to continue to oscillate. In this way, when the FET Q
1
is turned off, a voltage is applied to the primary winding N
1
again and the excitation energy is stored.
In the switching power supply
1
, such an oscillation is repeated. In a steady state, the output voltage on the load side is divided by the resistors R
6
and R
7
, and this divided detection voltage and the reference voltage of the shunt regulator Sr are compared. Then, the change of the output voltage is amplified at the shunt regulator Sr, the current flowing in the light-emitting diode PD of the photo coupler PC is changed, and in accordance with the quantity of light emission of the light-emitting diode PD, the impedance of the phototransistor PT changes. Thus, the charge and discharge of the capacitor C
2
can be changed and the output voltage can be controlled so as to be constant.
In the conventional switching power supply
1
shown in
FIG. 7
, at light load the oscillation frequency increases and the switching loss is large, which is a factor lowering the circuit efficiency. In order to solve this problem, a method can be considered whereby on the output side of the switching power supply a circuit lowering the output voltage can be provided so that by changing the impedance on the output side the output voltage Vo is lowered.
In this case, by making use of a fact that the voltage VNB generated in the feedback winding NB of the transformer T decreases in proportion to the output voltage Vo, the degree of decrease in the output voltage Vo is adjusted. By lowering the voltage VNB, the gate voltage of the FET Q
1
is made to oscillate in the range where the gate voltage does not reach the threshold and the turn-on of the FET Q
1
is delayed, and by making the off-state period of the FET Q
1
extended the oscillation frequency is lowered, and thus the switching loss is reduced.
However, such a switching power supply of RCC type is characterized in that at light load, the frequency increases because the output current is small, and when a circuit lowering the output voltage is provided as described above, all the output voltages are decreased, and accordingly when a constant output voltage is required, there is a problem that a constant-voltage control circuit is required.
SUMMARY OF THE INVENTION
Therefore, it is an object of this invention to provide a switching power supply where the increase of switching loss is suppressed and a constant output voltage can be obtained.
According to the invention, a switching power supply is provided having two or more DC outputs, the power supply comprising a DC power supply; a transform
Nakahira Koji
Nishida Akio
Tani Ryota
Yamada Tomohiro
Laxton Gary L.
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
Tso Edward H.
LandOfFree
Switching power supply having two or more DC outputs with... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Switching power supply having two or more DC outputs with..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Switching power supply having two or more DC outputs with... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2897721