Electricity: power supply or regulation systems – In shunt with source or load – Using a three or more terminal semiconductive device
Reexamination Certificate
2003-07-08
2004-11-02
Riley, Shawn (Department: 2838)
Electricity: power supply or regulation systems
In shunt with source or load
Using a three or more terminal semiconductive device
C323S283000
Reexamination Certificate
active
06812676
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to a DC—DC converter that supplies a prescribed voltage to a load circuit corresponding to the supplied power source voltage.
BACKGROUND OF THE INVENTION
A DC—DC converter is usually used in order to convert a DC voltage supplied from a power source to a desired voltage that is different from the power source voltage. The DC—DC converter is composed of switching elements and an inductive element. When the switching elements are turned on/off, current flows in the inductive element, and, as a result, the stored energy is supplied to the load side. By controlling the on/off timing of the switching elements, it is possible to supply the desired voltage that is different from the power source voltage to the load.
FIG. 8
is a diagram illustrating the constitution of an example of a conventional DC—DC converter. The DC—DC converter shown in this figure is composed of switching elements (hereinafter referred to as switches) S
1
, S
2
, S
3
, S
4
, inductive element L
1
, and load capacitor C
out
. For example, inductive element L
1
may be a coil or the like. In the following, it will simply be referred to as inductor L
1
.
By means of a controller not shown in the figure, switches S
1
-S
4
are controlled to turn on/off. For the DC—DC converter shown in
FIG. 8
, there are two operating states, state 1 and state 2. These operating states will be explained below.
In state 1, switches S
1
and S
3
are kept on, and switches S
2
and S
4
are kept off. In this case, as power source voltage V
in
is applied across inductor L
1
, current flows from the power source voltage supply terminal in the path through switch S
1
, inductor L
1
, and switch S
3
. Thus, energy is stored in inductor L
1
.
Then, in state 2, switches S
1
and S
3
are kept off, and switches S
2
and S
4
are kept on. As a result, the energy stored in inductor L
1
in state 1 is released through switch S
4
to the load circuit.
By means of the controller, for example, switches S
1
-S
4
are turned on/off at a prescribed timing corresponding to a prescribed clock signal, and said state 1 and state 2 are entered repeatedly. By controlling the time ratio of state 1 and state 2 by means of a clock signal, one can supply a voltage higher or lower than power source voltage V
in
to the load circuit. Thus, the DC—DC converter shown in
FIG. 8
is also known as up/down converter. Because switches S
2
and S
4
are only required to have a rectifying effect, switches S
2
and S
4
may be made up of diodes. However, in this case, electric power losses occur due to the forward voltage drop of the diode. When high efficiency is required, all of switches S
1
, S
2
, S
3
, S
4
are all MOSFETs or other transistor elements, and the system is known as synchronized rectifying system.
Said DC—DC converter can be either a boost converter or a buck inverter, which supplies the desired stable voltage to the load circuit. In addition, since the circuit can be formed in a small size, it is now widely used.
However, when the DC—DC converter of the aforementioned synchronized rectifying system is operated at low loads, the inductor draws current from the output side, that is, the current flows backwards in the inductor. This will be explained below with reference to the inductor current I
L
waveform.
FIG. 9
illustrates the waveforms in an example of the current flowing through a coil. FIG.
9
(
a
) shows the waveform of the clock signal for controlling the on/off timing of switches S
1
-S
4
. FIG.
9
(
b
) shows current I
L
through inductor L
1
.
In this case, for example, it is assumed that the controller sets the DC—DC converter in state 1 when the clock signal is at the high level, and sets the DC—DC converter in state 2 when the clock signal is at the lower level. Consequently, in state 1, power source voltage V
in
applied across inductor L
1
, so that current I
L
of inductor L
1
rises at a rate of V
in
/L. Here, L represents the inductance of inductor L
1
. As shown in FIG.
9
(
b
), in state 1, current I
L1
through inductor L
1
rises at a rate of V
in
/L.
Then, in state 2, the energy stored in inductor L
1
is released to the load circuit. In this case, because output voltage V
out
is applied across inductor L
1
, current I
L2
in inductor L
1
falls at a rate of V
out
/L.
For inductor L
1
, as current is supplied to the load side in state 2, as shown in FIG.
9
(
b
), by taking average for current I
L2
of state 2 in one period of the clock signal, one can determine current I
out
supplied by the DC—DC converter to the load circuit.
In the following, the low-load state will be explained. In the low-load state, in state 2, current I
L2
output from inductor L
1
to the load circuit decreases. When the current drops below zero, reverse current flows from the load circuit to inductor L
1
. That is, the DC—DC converter sinks current from the load.
FIG. 10
is a waveform illustrating the current through inductor L
1
in the low-load state.
As can be seen from this figure, in state 2, the current through inductor L
1
falls gradually, and finally becomes negative.
In the low-load state, as reverse current flows through inductor L
1
, ringing occurs. As a result, an undesirable energy transfer takes place between the input and output, and the conversion efficiency of the DC—DC converter drops, which is problematic.
The purpose of the present invention is to solve the aforementioned problems of the prior art by presenting a DC—DC converter characterized by the fact that in the low-load state, it can prevent reverse current in the inductor, reduce undesirable electric power losses, and increase the conversion efficiency.
SUMMARY OF THE INVENTION
In order to realize the aforementioned purpose, the present invention provides a DC—DC converter characterized by the fact that it comprises the following parts: a first switching element connected between one terminal of a voltage source and one terminal of an inductive element; a second switching element connected between the aforementioned terminal of the aforementioned inductive element and reference potential; a third switching element connected between the other terminal of said inductive element and said reference potential; a fourth switching element connected between said other terminal of said inductive element and the output terminal; and a control means which, when said first through fourth switching elements are turned on/off at a prescribed timing, outputs a voltage corresponding to said source voltage to said output terminal, and which turns on said second and third switching elements in the standby mode.
Also, according to the present invention, the following scheme is preferred: said control means has a current detecting means that detects the current through said inductive element, and turns on said second and third switching elements corresponding to the detection result of said current detecting means. When the current in said inductive element is nearly zero, said control means turns off said fourth switching element and turns on said second and third switching elements. In this way, it is possible to keep each end of the inductive element at the same potential, to eliminate changes in the inductor current, and to reduce the undesirable electric power losses. Also, in this case, it is possible to prevent ringing caused by the inductive element and the parasitic capacitance, and to lower the noise level.
Also, in the present invention, the following scheme is preferred: said control means enters first, second and third operating states repeatedly; in said first operating state, said first and third switching elements are turned on, and said second and fourth switching elements are turned off, in said second operating state, said first and third switching elements are turned off, and said second and fourth switching elements are turned on; and, in said third operating state, said first and fourth switching element are turned off, and said second and third switching elements are turned on.
Also, in the present invention, the
Brady III W. James
Riley Shawn
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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