Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
2001-10-22
2003-04-22
Mis, David C. (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C363S019000
Reexamination Certificate
active
06552623
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a self-oscillation type switching power supply unit.
2. Description of the Related Art
Until now, ringing choke converters have been often used as self-oscillation type switching power supply units.
FIG. 1
is a circuit diagram of a conventional ringing choke converter (hereinafter referred to as RCC). As shown in the drawing, a switching transistor Q
1
is connected in series to a primary winding N
1
of a transformer T, and a control circuit including a phototransistor PT as a light receiving element of a photo coupler is connected to a feedback winding NB of the transformer. Furthermore, a controlling transistor Q
2
is connected between the gate and source of the switching transistor Q
1
.
A rectifying and smoothing circuit made of a rectifying diode D
3
and a smoothing capacitor C
5
is provided between the two terminals of a secondary winding N
2
of the transformer T. Furthermore, a voltage divider circuit comprising resistors R
9
and R
10
, and a voltage detector circuit comprising a shunt regulator SR, a light-emitting diode PD of the photo coupler, and a resistor R
8
are provided in the output portion of the rectifying and smoothing circuit.
The operation of the circuit shown in
FIG. 1
is as follows. First of all, at the start when the power is turned on, a voltage is applied to the gate of the switching transistor Q
1
through a starting resistor R
1
, and the switching transistor Q
1
is turned on. Thus, an input power supply voltage is applied to the primary winding N
1
of the transformer T, and a voltage having the same polarity as that of the primary winding N is generated in the feedback winding NB. This voltage signal is supplied as a positive feedback signal to the gate of the switching transistor Q
1
through a resistor R
2
and a capacitor C
2
. Because of the voltage of the feedback winding NB, a charging current flows into a capacitor C
3
through a diode D
1
, resistors R
3
and R
5
, and a phototransistor PT of the photocoupler. When the charged voltage of the capacitor C
3
exceeds the forward voltage between the base and emitter of the controlling transistor Q
2
, the controlling transistor Q
2
is turned on. Because of this, the voltage between the gate and source of the switching transistor Q
1
becomes substantially zero and the switching transistor Q
1
is forced off. At this time, a forward-bias voltage to the rectifying diode D
3
is generated in the secondary winding of the transformer T, and thus the energy stored in the transformer T while the switching transistor Q
1
is turned on is discharged through the secondary winding N
2
. Furthermore, at this time, the capacitor C
3
is reverse charged by the flyback voltage of the feedback winding NB through resistors R
6
and R
7
and a diode D
2
.
When the voltage of the capacitor C
3
reaches the forward-bias voltage between the base and emitter of the controlling transistor Q
2
or less, the controlling transistor Q
2
is turned off. When the energy stored in the transformer T is discharged through the secondary winding and the current flowing the rectifying diode D
3
reaches zero, the switching transistor Q
1
is turned on again by the kickback voltage generated in the feedback winding NB. Then, the above operation is repeated.
Here, the output voltage on the load side is detected by the voltage divider circuit of the resistors R
9
and R
10
, the detected voltage is applied as a controlling voltage to the shunt regulator SR, and the amount of current flowing in the light-emitting diode PD of the photocoupler is charged in accordance with the detected voltage. Thus, the amount of light received by the phototransistor PT as a light receiving element of the photocoupler changes, and, through the change of the impedance, the charging time-constant of the capacitor C
3
is changed. The more the output voltage decreases, the larger the above charging time-constant becomes, and accordingly the more the output voltage decreases, the longer the period from the turn-on of the switching transistor Q
1
to the turn-off of the switching transistor Q
1
by the controlling transistor Q
2
, that is, the ON period of the switching transistor Q
1
increases, and, as a result, the output voltage is increased. In this way, constant-voltage control is performed so that the output may be held constant.
In the conventional self-oscillation type switching power supply unit of an RCC operation mode as shown in
FIG. 1
, it is known that the oscillation frequency f of the switching transistor Q
1
changes substantially in inverse proportion to the input power or output power. This can be shown by the relationship of oscillation frequency f to output power P
0
as shown in FIG.
2
.
Generally, the lighter the load becomes, the more the switching loss per unit switching time decreases, but, as in
FIG. 2
, the lower the output power P
0
, that is, the lighter the load, the higher the oscillation frequency becomes, and the higher the oscillation frequency f becomes, the more the number of occurrence of switching losses per unit time increases, and accordingly even if the load becomes lighter, the percentage of reduction in the switching losses is very little. Therefore, the lighter the load, the more the efficiency of the power supply unit decreases.
In order to reduce the switching losses at such light load, the circuit constants may be designed to decrease the oscillation frequency at the rated load, but, when it is required to cope with a very wide range from a light load to a heavy load, the oscillation frequency at light loading inevitably relatively increases. That is, generally, the oscillation frequency at the rated load is determined by factors such as the flux density of the transformer, ripple, noise, etc., and when the oscillation frequency is set to be too low, saturation of the transformer, etc., are caused.
In the past, to solve the above-described problems of the self-oscillation-type switching power supply unit, in the switching power supply unit disclosed in Japanese Unexamined Patent Application Publication No. 11-235036, the loss during waiting time is improved by inputting a switching signal during waiting time thus forcing the oscillation frequency to become lower. Furthermore, in the switching power supply unit disclosed in Japanese Unexamined Patent Application Publication No. 2000-278945, the loss during waiting time is improved by continuously lowering the oscillation frequencies over the range from the frequency at the rated load to the frequency during waiting time.
FIG. 3
is the frequency characteristic of the switching power supply unit disclosed in the above Japanese Unexamined Patent Application Publication No. 11-235036, and
FIG. 4
is the frequency characteristic of the switching power supply unit in the above Japanese Unexamined Patent Application Publication No. 2000-278945.
However, each of the above switching power supply units has problems described below.
Case 1: Switching power supply unit in Japanese Unexamined Patent Application Publication No. 11-235036
In this switching power supply unit, the losses are improved during waiting time, but when switched to the normal operation mode, the RCC operation is performed. Because of this, the loss at light loading cannot be improved during the RCC operation, and accordingly the increased input power and the heat-production problem of the switching transistor cannot be solved. Alternatively, it is possible to provide for intermittent oscillation, and, in this case, a problem occurs in that the ripple in the output is increased. An example where the power supply unit becomes loaded lightly is the standby mode of a printer.
Case 2: Switching power supply unit in Japanese Unexamined Patent Application Publication No. 2000-278945
In this switching power supply unit, when the switching power supply unit is lightly loaded, the oscillation frequency automatically decreases, but, in this case, when the oscillation frequency is lowered too much, the r
Nakahira Koji
Nishida Akio
Tani Ryota
Keating & Bennett LLP
Mis David C.
Murata Manufacturing Co. Ltd.
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