JPH08126312A - Dc-dc converter - Google Patents

Abstract

PURPOSE: To control the electric current of a DC-DC converter over a wide range by setting the voltage resonance frequency of the converter in accordance with the output voltage of the converter. CONSTITUTION: When the electric current through a load resistor 10 is sufficiently large and almost egual to the rated currenct of a DC-DC converter, the voltage of a current detecting resistor 15 becomes higher than that of a reference voltage generator 14 and a comparator 13 does not output any signal, and then, a transistor 17 is turned off. Therefore, a voltage resonance circuit is constituted only of the primary winding of a switching transformer 1 and a capacitor 4 for voltage resonance. When the current through the resistor 10 becomes smaller, only the voltage of the resistor 15 drops and the comparator 13 outputs a signal, and then, the transistor 17 is turned on. As a result, a parallel capacitor 16 is added to the voltage resonance circuit. Therefore, the voltage resonance frequency of the voltage resonance circuit becomes lower. When such a constitution is used, the electric current can be controlled over a wide range by means of a PWM control system in the same state as that obtained when a PFM system is used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC-DC converter device suitable as a power source for a load requiring a wide range of current control, and more particularly to an on-board DC-DC converter device used in personal computers and the like.

[0002]

2. Description of the Related Art On-board DC-DC converter devices used in personal computers and the like are required to be smaller, more efficient, and less noisy in response to the demands for smaller size, lighter weight and portability of personal computers and the like. . As an inexpensive DC-DC converter device used for these devices, a conventional pulse width modulation (PWM) control method D is used.
There is a C-DC converter device. FIG. 3 is a circuit diagram showing a conventional converter device used as a DC-DC converter device of a pulse width modulation (PWM) control system. In FIG. 3, reference numeral 1 denotes a switching transformer, which is a two-winding coupling reactor. Reference numeral 2 is a power switching transistor, which is a voltage resonance type DC-D including a primary winding of the switching transformer 1 and a voltage resonance capacitor 4.
Configure a C converter. 3 is a flywheel diode. The secondary winding output of the switching transformer 1 is rectified and smoothed by the diode 7 and the capacitor 6, and becomes a DC output. Reference numeral 10 is a load resistor, which is not a part of this circuit but is described for the sake of explanation. 5 is pulse width modulation (PWM)
Although not shown, the IC is an oscillator for controlling, an oscillator for generating a triangular wave, an operational amplifier, a comparator for comparing output voltages of the oscillator and the operational amplifier, and a power switching transistor 2 for amplifying an output of the comparator. And a drive circuit for driving the. A voltage obtained by dividing the DC output voltage by the resistors 8 and 9 is input to the pulse width modulation (PWM) control IC 5, and is input to the operational amplifier. In this operational amplifier, the difference voltage between the reference power supply inside the IC and the input voltage is amplified and input to one input terminal of the comparator. That is, the voltage (Vs) proportional to the DC output voltage of the DC-DC converter is input. The triangular wave (Vw) generated by the oscillator is input to the other input terminal of the comparator. FIG. 4 shows the relationship between Vs and Vw inside the pulse width modulation (PWM) control IC. In FIG. 4, A and B show Vs, C shows a triangular wave Vw, and a and b show output waveforms of the comparator.
Now, when Vs is in the state of A, the output of the comparator is an output like a. However, if the DC output voltage of the DC-DC converter drops for some reason and Vs becomes B, the output of the comparator is b. And power switching transistor 2
The ON time of is increased and the output voltage of the DC-DC converter is increased and kept constant. The resistor 11 is a resistor for driving the power switching transistor 2, and the diode 12 is a diode for discharging the gate accumulated charge of the power switching transistor 2. In the above-mentioned pulse width modulation (PWM) control type DC-DC converter device, since the switching frequency is constant and the voltage resonance frequency is also constant, when the load current decreases, the power switching transistor causes non-zero voltage switching. There is a problem that power loss at turn-on increases and efficiency deteriorates, and noise at turn-on increases, which adversely affects other elements of the personal computer. Therefore, in the on-board DC-DC converter device used in a personal computer or the like, the P
In order to solve the problem of the DC-DC converter of the WM control system, the DC-DC of the pulse frequency modulation (PFM) control system
Using a converter, decrease in efficiency when the load current decreases,
I try to eliminate the generation of noise.

[0003]

As described above, in the pulse width modulation (PWM) control type DC-DC converter, the zero voltage switching of the power switching element cannot be performed due to the decrease of the load current, so that the power loss at turn-on occurs. This increases the efficiency, deteriorates the efficiency, and increases the noise at turn-on, which adversely affects other elements of the personal computer. Further, in the DC-DC converter of the pulse frequency modulation (PFM) control system, the control circuit is complicated and the oscillation frequency is not constant, so it is necessary to design the frequency range of the transformer and the power switching element to be wide, and further, the voltage resonance There is a problem that it becomes an expensive device because a mold control IC is required.

Therefore, the present invention is intended to solve the above problems, and an object thereof is to enable current control in a wide range equivalent to that of a pulse frequency modulation (PFM) control type DC-DC converter. Another object of the present invention is to provide a DC-DC converter device of a pulse width modulation (PWM) control system with a simple control circuit.

[0005]

In order to achieve the above-mentioned object of the present invention, the present invention comprises a voltage resonance circuit with a primary winding of a switching transformer and a first capacitor for voltage resonance. DC-D with pulse width modulation control method
In the C converter device, a DC output current detecting means,
A second capacitor connected in parallel with the first capacitor for voltage resonance of the voltage resonance circuit, and a switch for switching whether to enable the second capacitor or not by the output of the DC output current detecting means. And a pulse width modulation control type DC-DC converter device.

[0006]

Operation: Conventional pulse width modulation (PWM) control DC-DC
It was impossible to arbitrarily set the voltage resonance frequency by the output current in the converter device, but it can be arbitrarily set by the present invention, and the control IC used in the DC-DC converter device has a pulse width. Modulation (PW
M) Since it is an IC for control, the switching operation of the power switching transistor can be switched to zero voltage even if the load current fluctuates, the switching loss of the power switching transistor can be reduced, and high efficiency and low noise can be achieved. You can

[0007]

An embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 is a circuit diagram of a pulse width modulation (PWM) control type DC-DC converter device according to an embodiment of the present invention. In FIG. 1, the same parts as those shown in FIG. 4 are designated by the same reference numerals, and detailed description thereof will be omitted.

First, the configuration of this embodiment will be described.
In the conventional example shown in FIG. 3, the method of stably controlling the output voltage of the DC-DC converter is such that a switching pulse having a constant frequency and a pulse width proportional to the output voltage is generated by the IC 5 for pulse width modulation (PWM) control. However, the power switching transistor 2 is switched, and since the capacitance of the voltage resonance capacitor 4 is constant, the resonance frequency is also constant. In FIG. 4, which is an embodiment of the present invention, a capacitor 16 is provided in parallel with the voltage resonance capacitor 4,
It has a transistor 17 for switching whether to enable the capacitor 16. Transistor 17 is comparator 1
On / off control is performed by the output of 3. Comparator 13 is D
Since the terminal voltage of the current detection resistor 15 provided in the DC output circuit of the C-DC converter and the current detection resistor 15 have very small resistance values, a voltage signal proportional to the output current of the DC-DC converter and a reference The voltage generator 14 is used as an input. The resistor 18 is a resistor for driving the transistor 17.

Next, the operation of this embodiment will be described.
When the current flowing through the load resistor 10 is close to the rated current of the DC-DC converter and is sufficiently large, the current detection resistor 15
Since the voltage due to is set to be sufficiently higher than the voltage of the reference voltage generator 14, the comparator 13 does not output a signal, so the transistor 17 is off. Therefore, the voltage resonance circuit is composed of only the primary winding of the switching transformer 1 and the voltage resonance capacitor 4. The drain-source voltage waveform of the power switching transistor 2 in this case is shown in FIG. As shown in FIG. 2A, when the output current is close to the rated current of the DC-DC converter and is sufficiently large, the power switching transistor 2 performs zero voltage switching so that the pulse width modulation (PWM) control IC5. The oscillation frequency of the oscillator and the constant of the voltage resonance circuit are set. Since the voltage of the current detection resistor 15 is set to be lower than the voltage of the reference voltage generator 14 when the current of the load resistor 10 decreases, the comparator 13 outputs a signal and the transistor 17 It turns on. Therefore, the voltage resonance circuit is composed of the primary winding of the switching transformer 1, the voltage resonance capacitor 4, and the parallel capacitor 16.
Therefore, the voltage resonance frequency is represented by the following formula 1, which is lower than the frequency of the voltage resonance circuit of the conventional DC-DC converter represented by formula 2.

[Equation 1] Here, Lp is the inductance of the primary winding of the switching transformer 1, C1 is the capacitance of the voltage resonance capacitor 4, and C2 is the capacitance of the capacitor 16. Power switching transistor 2 in this case
The waveform of the drain-source voltage of is shown in FIG.
The curve indicated by the broken line in FIG. 2B is a conventional example in which the capacitor 16 is not connected in parallel. The drain-source voltage waveform of the power switching transistor 2 in this embodiment is shown by a solid line. As can be seen from FIG. 2B, when the output current is smaller than the rated current of the DC-DC converter, the power switching transistor 2 does not perform zero voltage switching in the conventional example, but in the present embodiment, the capacitor 16 is Since the voltage resonance frequency is lowered by being connected in parallel, the switching operation of the power switching transistor 2 is set to zero voltage switching by setting the set voltage of the reference voltage generator and the capacitance of the capacitor 16 to appropriate values. Can be set to

[0010]

As described in detail above, the DC-DC converter device according to the present invention has a voltage which cannot be made variable by the conventional pulse width modulation (PWM) control system DC-DC converter device. The resonance frequency can be set arbitrarily by the output current, and DC-D
Since the control IC used in the C converter device is a pulse width modulation (PWM) control IC, the switching operation of the power switching transistor can be switched to zero voltage even if the load current fluctuates. A switching loss of the switching transistor can be reduced, and a high efficiency, low noise, and low cost DC-DC converter device can be provided.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a DC-DC converter showing an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a drain-source voltage waveform of a power switching transistor according to an embodiment of the present invention.

FIG. 3 is a circuit diagram of a conventional DC-DC converter.

FIG. 4 is an operation explanatory diagram of a conventional DC-DC converter.

[Explanation of symbols]

 1 ... Switching transformer 2 ... Power switching transistor 3 ... Flywheel diode 4 ... Capacitor for voltage resonance 5 ... IC for pulse width modulation (PWM) control 6 ... Capacitor 7 ... -Diode 8 ... Resistor 9 ... Resistor 10 ... Load resistance 11 ... Resistor 12 ... Diode 13 ... Comparator 14 ... Reference voltage generator 15 ... Current Detection resistor 16 ・ ・ ・ Capacitor 17 ・ ・ ・ Transistor 18 ・ ・ ・ Resistor

Claims (1)

[Claims] 1. A DC-DC converter device of a pulse width modulation control system, comprising a voltage resonance circuit comprising a primary winding of a switching transformer and a first capacitor for voltage resonance, and a DC output current detecting means, and the voltage. A second capacitor connected in parallel to the first capacitor for voltage resonance of the resonance circuit
And a switching means for switching whether or not to enable the second capacitor by the output of the DC output current detection means.