DC-DC converter

Electricity: power supply or regulation systems – Output level responsive – Using a three or more terminal semiconductive device as the...

Reexamination Certificate

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Reexamination Certificate

active

06621257

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a DC-DC converter which uses a &Dgr;&Sgr; modulator to limit excessive peak current flowing to an inductor and power switching element by monitoring the current flowing to said inductor with an integrator composed with a &Dgr;&Sgr; modulator.
2. Description of the Related Art
&Dgr;&Sgr; modulation is a modulation method in which modulator input signal us integrated, quantization is carried out by comparing the integrated value with a reference voltage, and modulator output signal is fed back to the modulator input.
FIG. 3
is a block drawing of a primary &Dgr;&Sgr; modulator. A DC-DC converter that switches power switching element
8
can be produced using this modulation system.
FIG. 4
shows a step-down chopper DC-DC converter as an example of a method of the prior art that uses &Dgr;&Sgr; modulation. This DC-DC converter is equipped with smoothing circuit
18
, &Dgr;&Sgr; modulator
6
is connected to the output side of this smoothing circuit
18
via an error amplification circuit
13
, and output side of this &Dgr;&Sgr; modulator
6
is connected to the input of power switching element
8
via a gate driver circuit
7
.
A constant voltage output can be obtained by comparing the converter output voltage with a reference voltage, performing &Dgr;&Sgr; modulation by using the error amplification signal voltage of the compared voltage as the modulator input signal of &Dgr;&Sgr; modulator
6
, switching power switching element
8
according to the output signal of modulator
6
, and inputting the above switching output to smoothing circuit
18
.
In contrast to DC-DC converters using a pulse width modulation system (to be referred to as PWM), which has typically been used in the past, being characterized by the switching frequency being constant, DC-DC converters using &Dgr;&Sgr; modulation are synchronized with the sampling frequency given by the &Dgr;&Sgr; modulator, and the output
1
bit signal is changed so that the error between the modulator input signal voltage and modulator output signal voltage becomes minimal. Consequently, the &Dgr;&Sgr; modulator input signal voltage changes according to the DC-DC converter output voltage and as a result, this modulation system has the characteristic of the switching frequency changing.
Although attention has been drawn to the advantage of being able to improve the power conversion efficiency of the converter as compared with the PWM system as a result of holding switching loss, which is power loss due to switching, to a low level since the number of switching cycles is reduced in &Dgr;&Sgr; modulation particularly when the DC-DC converter output current is small, this has the problems described below.
DC-DC converters have the characteristic that, during the time power switching element
8
is off, current flows through rectifier
14
while capacitor
15
discharges, and due to the presence of inductor
16
, current decreases gradually instead of sharply, while during the time power switching element
8
is on, current flows to inductor
16
through power switching element
8
, capacitor
15
discharges, and due to the presence of inductor
16
, current rises gradually, and the current that flows rises the longer the period during which power switching element
8
is on.
In the PWM system, since the switching frequency is constant, although the period during which power switching element
8
is on is nearly constant provided the DC-DC converter output current is constant, in the case of &Dgr;&Sgr; modulation, since the switching frequency is not constant, there are cases in which a state occurs in which power switching element
8
is on for a long period of time even if DC-DC converter output current is constant, and this state occurs easily particularly when the DC-DC converter output current is large.
Due to the above characteristics, since cases can occur in which the period during which power switching element
8
is on becomes long in &Dgr;&Sgr; modulation as compared with PWM modulation, there was the problem of the peak current flowing inside the converter becoming large.
Since power loss of power switching element
8
is determined by the product of the on resistance of power switching element
8
and the square of the current flowing to power switching element
8
, if the current flowing to power switching element
8
increases, the power loss in power switching element
8
becomes extremely large. Moreover, since the power loss in rectifier
14
is determined by the forward voltage and current that is flowing, and power loss also becomes large when the current flowing to rectifier
14
increases, particularly in cases in which the DC-DC converter output current is large, converters using &Dgr;&Sgr; modulation had the problem of poor power conversion efficiency as compared with those using PWM modulation due to the large current flowing inside the converter.
That is, in the case of &Dgr;&Sgr; modulation of the method of the prior art, although there is the advantage of high power conversion efficiency as compared with PWM modulation when the DC-DC converter output current is small, when the DC-DC converter output current is large, cases occur in which the peak current flowing inside the converter becomes large, thereby resulting in the problem of a decrease in power conversion efficiency.
SUMMARY OF THE INVENTION
In consideration of the above problems of the prior art, the object of the present invention is to provide a highly efficient DC-DC converter that uses &Dgr;&Sgr; modulation to limit excessive peak current flowing inside the converter by monitoring the current flowing inside the converter with an integrator located inside a &Dgr;&Sgr; modulator.
The present invention for achieving the above object is a DC-DC converter that switches a power switching element by inputting an analog input signal or multi-bit digital signal into a &Dgr;&Sgr; modulator, and conveying that modulated signal to the power switching element via a gate driver circuit, wherein excessive peak current flowing to an inductor is limited by monitoring the current flowing inside the converter with an integrator located inside the &Dgr;&Sgr; modulator.
The &Dgr;&Sgr; modulator is composed of at least one adder, integrator and quantizer, has a feedback pathway that feeds back from the output of the quantizer to the input of at least one adder, at least one integrator is connected to the output of the adder, and at least one output of the integrator is connected to the quantizer.
The DC-DC converter is also provided with an adder serving as the input of the &Dgr;&Sgr; modulator and an integrator connected to the output of that adder, and an analog adder and analog integrator are used in the case of inputting a continuous time signal in the manner of an analog signal to a modulator, while a digital adder and digital integrator are used in the case of inputting a discrete time signal in the manner of a multi-bit digital signal to a modulator.
The quantizer performs sampling on the discrete time signal, while the output of this quantizer and the output of a current control circuit, to which is connected the output of the above integrator and the output of an error amplification circuit, are connected to the input of an AND circuit that determines the final pulse width, and the output signal of this AND circuit is connected to the input of a gate driver circuit that supplies current and voltage sufficient for driving the power switching element.
The output of the above gate driver circuit is connected to the input of a power switching element that switches the input voltage of the DC-DC converter, while the output of the power switching element is connected to a smoothing circuit, and the output of this smoothing circuit becomes the output of the DC-DC converter.
The above feedback pathway in the &Dgr;&Sgr; modulator is equipped with an attenuator that conforms to output signal of the quantizer to the input signal level of the &Dgr;&Sgr; modulator. However, an attenu

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