Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2001-08-30
2002-09-10
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
Reexamination Certificate
active
06449176
ABSTRACT:
DESCRIPTION OF THE INVENTION
1. Field of the Invention
The invention relates to a switching power supply. More particularly, the invention relates to a switching power supply having a novel magnetic amplifier control circuit.
2. Description of the Related Art
In a conventional switching power supply, an output voltage generated at a secondary side of a transformer in the switching power supply is fed back to a primary side of the transformer in order to stabilize the output voltage of a pulse width-modulated (PWM) controller.
Typically a conventional switching power supply having multiple-outputs, e.g., 5 V, 10 V and so on, uses magnetic amplifiers for control to stabilize each of the output voltages. The magnetic amplifiers are suitable for independently stabilizing the output voltages of the multiple outputs, since each of the output voltages is controlled at the secondary side of the transformer.
FIG. 5
illustrates a conventional switching power supply controlled by a forward magnetic amplifier in which a transformer T
2
having a primary side winding N
0
and first and second secondary side windings N
1
and N
2
is provided.
An induced AC voltage at the first winding N
1
passes though a rectifying diode D
11
, a fly-wheel diode D
12
and a smoothing circuit comprised of a smoothing coil L
11
and a smoothing condenser C
11
in order to output a first DC output voltage V
01
. The output first voltage V
01
is fed back to a transistor Q
11
serially coupled to the winding N
0
at the primary side of the transformer T
1
for stabilizing the first output voltage V
01
by using a PWM controller
16
.
An induced AC voltage at the second winding N
2
passes through a rectifying diode D
13
, a fly-wheel diode D
14
, a smoothing coil L
12
and a smoothing condenser C
12
in order to provide a second DC output voltage V
02
. In order to stabilize the second output voltage V
02
independently of the stabilization control for the first output voltage V
01
, PWM control is performed by using a magnetic amplifier control circuit that includes a saturable inductor LS
3
.
The magnetic amplifier control circuit includes an operational amplifier OP
4
. The second output voltage V
02
is supplied to a plus (+) input terminal of the operational amplifier OP
4
through a variable resistor R
22
. A reference voltage V
ref
generated at a Zener diode ZD
1
is supplied to a minus (−) input terminal of the operational amplifier OP
4
for comparison with the second output voltage V
02
. By comparing these voltages, the operational amplifier OP
4
supplies a control current Im to a conning point between the saturable inductor LS
3
and the rectifying diode D
13
through a resistor R
23
and a diode D
15
. In
FIG. 5
, R
21
is a current supplying resistance for the diode ZD
1
.
When the connecting point between the saturable inductor LS
3
and the diode D
13
has a negative potential, the control current Im flows toward the saturable inductor LS
3
from the operational amplifier OP
4
through the diode D
15
. In accordance with the magnitude of the control current Im, the magnetic fluxes of the saturable inductor LS
3
are reset. When the magnetic fluxes have reset, the saturable inductor LS
3
changes from a saturation state to a non-saturation state.
When the saturable inductor LS
3
changes to the non-saturation state, the inductance of the saturable inductor LS
3
becomes a large value. Consequently, even when a voltage E is supplied to the saturable inductor LS
3
from the winding N
2
in a forward direction at a next time after the saturable inductor LS
3
has changed to a non-saturation state, the flow of control current Im is delayed by a time &Dgr;T that corresponds to a reset amount &Dgr;&PHgr; of the magnetic flux of the saturable inductor LS
3
. Here, the magnetic flux &Dgr;&PHgr; is represented as a product of the voltage and the time, i.e., &Dgr;T×voltage E. Consequently, the delayed time &Dgr;T is obtained by the following equation:
&Dgr;
T
=&Dgr;&PHgr;/
E.
Thus, it becomes possible to perform PWM control of the output voltage at the secondary side of the transformer T
2
with the changing pulse width of the pulse current flowing through the second winding N
2
of the transformer T
2
by controlling the amount of the reset of the magnetic flux &Dgr;&PHgr;.
FIG. 6
illustrates a conventional push-pull magnetic amplifier type switching power supply. As illustrated in
FIG. 6
, a winding N
0
is provided at the primary side of a transformer T
1
and a plurality of windings N
1
-N
4
are provided at the secondary side of the transformer T
1
. The winding N
0
is coupled to two pairs of transistors F
2
and F
3
, and F
1
and F
4
. By alternately turning ON and OFF the pairs of the transistors F
2
and F
3
, and F
1
and F
4
, it becomes possible to alternately switch the positive and negative current flow through the winding N
0
of the transformer T
1
. C
0
is a condenser for preventing saturation of the transformer T
1
.
Induced AC voltages at the windings N
3
, N
4
are full-wave rectified by a pair of rectifying diodes D
21
and D
22
, and smoothed at a smoothing coil L
21
and a smoothing condenser C
21
. Then a first DC voltage, e.g., 5 V, is outputted at a resistor R
31
. Further, the induced AC voltages at the windings N
1
, N
2
are full-wave rectified by a pair of rectifying diodes D
1
and D
2
and smoothed by a smoothing coil L
1
and a smoothing condenser C
1
. Then a second DC voltage, e.g., 10 V, is outputted at a resistor R
42
. Here, in order to stabilize the output voltage at the resistor R
42
, a magnetic amplifier control circuit performs PWM control by using saturable inductors LS
1
and LS
2
.
In the magnetic amplifier control circuit, a transistor Q
12
is coupled to the resistor R
42
through a resistor R
41
connected in parallel with the resistor R
42
. A control current Im outputted from the transistor Q
12
is supplied to respective connecting points d
1
, d
2
between each of the saturable inductors LS
1
, LS
2
and each of the rectifying diodes D
1
, D
2
, through the respective diodes D
3
, D
4
. In
FIG. 6
, an ON/OFF control circuit for the transistor Q
12
, such as an operational amplifier, is not illustrated. Thus, stabilization of the output voltage is achieved by resetting the magnetic fluxes of the saturable inductors LS
1
, LS
2
during a period when each of the connecting points d
1
, d
2
has a negative potential, and the pulse current flows in the windings N
1
and N
2
for the duration of the pulse width.
In the push-pull magnetic amplifier type switching power supply, it is possible to achieve low noise of the switching power supply by using a phase shift full-bridge circuit comprised of the plurality of the transistors F
1
-F
4
on the primary side of the transformer T
1
.
However, voltages VS
1
and VS
2
of the respective saturable inductors LS
1
, LS
2
do not only depend upon the magnitude of the control current Im but also depend upon coercive forces Hc
1
and Hc
2
of the respective saturable inductors LS
1
, LS
2
. Consequently, if there are differences between the coercive forces Hc
1
and Hc
2
of the saturable inductors LS
1
, LS
2
, the ON widths of the respective currents that flow to the rectifying diodes D
1
and D
2
differ from each other even if the same current magnitude flows in both of the saturable inductors LS
1
, LS
2
.
Consequently, in a push-pull type switching power supply, the current balance during the push time and the pull time is destroyed, and the condenser CO is DC biased by the current difference. This causes a problem of unbalance of the product of voltage by time for the transformer between the push-time period and the pull-time period, and the transformer T
1
is saturated.
Further, in a conventional magnetic amplifier control circuit, it is desirable to insert a common mode choke in order to reduce noise in the output voltages. However, in order to insert the common mode choke in output lines, it is necessary to separate the ground reference from the reference voltage Vref supp
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Kabushiki Kaisha Toshiba
Riley Shawn
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