Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-08-14
2001-09-18
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S021010
Reexamination Certificate
active
06292376
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to an improved switching power supply which saves power under a small load to a no load condition.
2. Description of the Prior Art
A conventional switching power supply is shown in FIG.
1
and comprises a transformer TR including a primary winding L
1
and a secondary winding L
2
; a magnetic flux detector
10
; a current loop circuit
40
for controlling a DC current applied to the primary winding L
1
; a feedback circuit
30
for detecting and feeding an output voltage supplied to a load Z, on the secondary winding L
2
side, back to the current loop circuit
40
; and a secondary circuit
20
for supplying a voltage from the secondary winding L
2
to the load Z.
The magnetic flux detector
10
includes a first comparator TRCMP, wherein a magnetic flux energy accumulated in the transformer TR is detected as a voltage signal by means of a current I
2
flowing through the secondary winding L
2
and a resistor R
1
, and to which the voltage signal and a reference voltage Vt
1
are applied; and a first flip flop circuit FF
1
, wherein the output signal V
6
from the first comparator TRCMP is inputted to the set terminal S; a gate signal V
2
for a switching device SW is inputted to the reset input terminal R; and an output signal V
7
from the output terminal Q of the first flip flop circuit FFL is connected to the set input terminal S of the second flip flop circuit FF
2
, discussed hereinafter.
The secondary circuit
20
includes the secondary winding L
2
of transformer TR; a rectifying diode D connected in series to the secondary winding L
2
; a capacitor C connected in parallel to the secondary winding L
2
; and a load Z connected in parallel to the capacitor C. The feedback circuit
30
is located on the output load side, where a voltage applied to the load Z and a reference voltage Vt
2
are inputted to the feedback circuit
30
in order to negatively feed back a current control signal V
4
from an error amplifier EA, which outputs the current control signal V
4
, to a second comparator CSCMP so that a given output voltage is maintained.
The current loop circuit
40
includes the second comparator CSCMP wherein a current L
1
flowing through the primary winding L
1
is detected by means of a resistor R
2
and the resulting voltage signal V
3
is inputted to the non-inverting input terminal and the current control signal V
4
from the feedback circuit
30
is inputted to the inverting input terminal; the second flip flop circuit FF
2
wherein the output signal V
5
of the second comparator CSCMP is inputted to the reset input terminal R; the output signal V
7
of the first flip flop circuit FF
1
is inputted to the set input terminal S and the output signal V
2
of the output terminal Q is inputted to the gate of the switching device SW; and the switching device SW is turned ON and OFF by means of the output signal V
2
from the second flip flop circuit FF
2
. The switching device SW is connected in series to the primary winding L
1
of the transformer TR to control the current L
1
flowing through the primary winding L
1
.
The switching power supply of
FIG. 1
is operated as follows, with reference to the timing chart of
FIG. 2
, wherein the current L
1
(voltage signal V
3
) flowing through the switching device SW, for applying a voltage to the primary winding L
1
, reaches the current control signal V
4
; the output signal V
5
of the second comparator CSCMP changes to a high state and the output V
2
of the second flip flop circuit FF
2
is changed from a high state to a low state to turn OFF the switching device SW. That is, the switching device SW remains turned ON until the current flowing through the switching device SW reaches the current control signal V
4
, so that magnetic flux energy is accumulated in transformer TR.
When the switching device SW is turned OFF, the magnetic flux energy accumulated in transformer TR is supplied to the load Z through the secondary winding L
2
, rectifying diode D, etc, as a load current.
When the magnetic flux energy in the transformer TR becomes depleted, as time lapses, the voltage of the secondary winding L
2
drops rapidly to fall below the reference voltage Vt
1
. Thus, the output signal V
6
of the first comparator TRCMP goes low and the output signal V
7
of the first flip flop circuit FFL goes high, thereby causing the output signal V
2
of the second flip flop circuit FF
2
to be changed to a higher state. This in turn causes the switching device SW to be turned ON. If the switching device SW is turned ON, the current I
1
flowing through the switching device SW (i.e. voltage signal V
3
) continues to rise until the current again reaches the current control signal V
4
level. In this manner, the switching power supply repeats the above operation to sustain self excited oscillation.
This means that the self excited oscillation in the conventional switching power supply is based on the mechanism wherein energy accumulated in the transformer TR is controlled by switching ON and OFF, the switching device SW to the difference between the voltage detected on the secondary winding L
2
side and the reference voltage Vt
1
In the described switching power supply, the current supplied to the load is, in principle, inversely proportional to the oscillation frequency. This is because the energy exchanged with the transformer TR at each cycle is also reduced when the amount of current supplied to the load is decreased, thereby resulting in a shorter time interval at which the switching device SW is turned ON and OFF. When the ON-OFF time interval of the switching device SW is shortened, the frequency of self excited oscillation is increased. This could cause such problems as power loss in the switching device SW, core loss in the transformer TR, increase in noise, and failure in oscillation. An excess increase in the oscillation frequency thus must be avoided. For this purpose, the minimum load is fixed using a bleeder resistor in some cases. This could also cause a problem, namely, that the power consumption then is increased even when the load is small or there is no load at all, since increases in the above discussed losses and other losses result from the frequency increases. Consequently, in the convention apparatus, it is difficult to reduce power consumption.
Moreover, in the art, the unresolved problem is how to prevent the ON-OFF time interval of the switching device SW from becoming shortened more or less under a small load to no load condition, and thereby avoid any excess increase in the frequency of self-excited oscillation.
An unsatisfactory attempt to resolve the above problem is shown in
FIG. 3
, wherein attempt was made to prevent an increase in switching frequency by providing the switching power supply with an oscillator for outputting a fixed frequency pulse signal, that is high state and low state pulses, as ON-OFF signals, rather than using a magnetic flux detector
10
of FIG.
1
. Further power saving is required, however, even when the switching power supply is under a small load condition or a no load condition.
SUMMARY OF THE INVENTION
An object of the invention is to overcome the aforementioned and other deficiencies and problems of the prior art.
The foregoing and other objects are attained in the switching power supply of the invention, wherein delay circuit means are provide for prolonging the time at which the switching device makes the OFF to ON transition. In an illustrative embodiment, the delay circuit comprises a comparator having a hysteresis characteristic and a gate circuit for determining priority of signals applied to a set input terminal and a reset input terminal of a flip flop so as to suitably control the switching device. By such control, the switching power supply of the invention saves power when the load is small or no load condition exists.
REFERENCES:
patent: 5440473 (1995-08-01), Ishii et al.
patent: 6028776 (2000-02-01), Ji et al.
Moonray Kojima
Patel Rajnikant B.
Yokogawa Electric Corporation
LandOfFree
Switching power supply 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, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Switching power supply will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2496533