JP3483167B2 - DC-DC converter - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC-DC converter suitable as a power supply for a load requiring a wide range of current control, and more particularly, to an on-board device used for a personal computer or the like. The present invention relates to a DC-DC converter device. 2. Description of the Related Art An on-board DC-DC converter device used for a personal computer or the like has a smaller size, a higher efficiency, and a lower noise in accordance with a demand for a smaller, lighter, and more portable device such as a personal computer. There is a request. Inexpensive DC-DC converter devices used in these devices include conventional pulse width modulation (PWM) control type D-DC converters.
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 method. In FIG. 3, reference numeral 1 denotes a switching transformer, which is a coupling reactor having two windings. Reference numeral 2 denotes a power switching transistor, which includes a voltage resonance type DC-D
Construct a C converter. 3 is a flywheel diode. The output of the secondary winding of the switching transformer 1 is rectified and smoothed by the diode 7 and the capacitor 6 to become a DC output. Reference numeral 10 denotes a load resistor, which is not a component of the present circuit, but is described for explanation. 5 is pulse width modulation (PWM)
Although not shown, the control IC includes an oscillator (not shown) that generates a triangular wave, an operational amplifier, a comparator that compares output voltages of the oscillator and the operational amplifier, and a power switching transistor 2 that amplifies the output of the comparator. And a driving circuit for driving the same. A voltage obtained by dividing the DC output voltage by the resistor 8 and the resistor 9 is input to the pulse width modulation (PWM) control IC 5 and input to the operational amplifier. In this operational amplifier, a difference voltage between a reference power supply inside the IC and the input voltage is amplified and input to one input terminal of the comparator. Namely, voltage proportional to the DC-DC converter <br/> DC output voltage (Vs) is inputted. The other input terminal of the comparator receives a triangular wave (Vw) generated by the oscillator. FIG. 4 shows the relationship between Vs and Vw inside the IC for pulse width modulation (PWM) control. In FIG. 4, A and B represent Vs, C
Represents a triangular wave Vw, and a and b represent 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 decreases for some reason and Vs becomes B, the output of the comparator becomes b. And the power switching transistor 2
Becomes longer, the output voltage of the DC-DC converter increases and is 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 charge stored in the gate of the power switching transistor 2. In the above-described DC-DC converter device of the pulse width modulation (PWM) control method, since the switching frequency is constant and the voltage resonance frequency is also constant, when the load current decreases, the power switching transistor performs non-zero voltage switching. Thus, there is a problem that power loss at the time of turn-on increases and efficiency is deteriorated, and noise at the time of turn-on increases, which adversely affects other elements of the personal computer. Therefore, the on-board DC-DC converter device used in a personal computer or the like includes the P
DC-DC of pulse frequency modulation (PFM) control method to solve the problem of DC-DC converter of WM control method
Using a converter, lowering the efficiency when the load current decreases,
I try to eliminate the generation of noise. As described above, in the DC-DC converter of the pulse width modulation (PWM) control method, the zero voltage switching of the power switching element cannot be performed due to the decrease of the load current, so that the power supply at the time of turn-on is not possible. The power loss increases, the efficiency deteriorates, and the noise at the time of turn-on increases, adversely affecting other elements of the personal computer. Further, in the DC-DC converter of the pulse frequency modulation (PFM) control method, since the control circuit is complicated and the oscillation frequency is not constant, it is necessary to design a wide frequency range of the transformer and the power switching element, and furthermore, the voltage resonance is required. There is a problem that an expensive device is required because a control IC of a mold type is required. SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to control current over a wide range equivalent to a pulse frequency modulation (PFM) control type DC-DC converter. It is an object of the present invention to provide a simple DC-DC converter device using a DC-DC converter of a pulse width modulation (PWM) control system with a simple control circuit. [0005] The object of the present invention is achieved.
Therefore, the invention of the present application
Voltage resonance circuit by the line and the first capacitor for voltage resonance
Pulse width modulation control DC-DC converter having a path
A DC output current detecting means;
Connected in parallel to the first capacitor for voltage resonance of the resonance circuit
Of the second capacitor and the switching transistor
A column circuit, and comparing the output of the DC output current detection means with a predetermined value.
And the output of the DC output current detecting means are predetermined.
When the value falls below the first comparator value, the first comparator
Switching to connect a second capacitor in parallel with the capacitor
Switching means for switching on a transistor; and a DC-DC converter device of a pulse width modulation control method, comprising: The conventional pulse width modulation (PWM) control DC-DC
In the converter device, the voltage resonance frequency cannot be set arbitrarily by the output current. However, according to the present invention, the voltage resonance frequency can be set arbitrarily, and the control IC used in the DC-DC converter device has a pulse width. Modulation (PW
M) Since it is a control IC, 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. Can be. Next, an embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 is a circuit diagram of a DC-DC converter device of a pulse width modulation (PWM) control system according to one embodiment of the present invention. In FIG. 1, the same portions as those shown in FIG. 3 are denoted by the same reference numerals, and detailed description thereof will be omitted. First, the configuration of the present embodiment will be described. In the conventional example shown in FIG. 3, a method for stably controlling the output voltage of the DC-DC converter generates a switching pulse having a constant frequency and a pulse width proportional to the output voltage by an IC 5 for pulse width modulation (PWM) control. To switch the power switching transistor 2,
Since the capacitance of the voltage resonance capacitor 4 is constant, the resonance frequency is also constant. In FIG. 1 , which is an embodiment of the present invention, a capacitor 16 is provided in parallel with the voltage resonance capacitor 4, and has a transistor 17 for switching whether or not the capacitor 16 is enabled. The transistor 17 is on / off controlled by the output of the comparator 13. Comparator 13
Is the terminal voltage of the current detection resistor 15 provided in the DC output circuit of the DC-DC converter and the reference voltage generator 14.
Is input. Since the current detection resistor 15 has a very small resistance value, the terminal voltage of the current detection resistor 15 is D
Voltage signal der proportional to C-DC converter output current
You . The resistor 18 is a resistor for driving the transistor 17. Next, the operation of this embodiment will be described. If the current flowing through the load resistor 10 is close to the rated current of the DC-DC converter and is sufficiently large, the voltage of the current detecting resistor 15 is set to be sufficiently higher than the voltage of the reference voltage generator 14. The transistor 17 does not output a signal, so the transistor 17 is off. Therefore, the voltage resonance circuit includes only the primary winding of the switching transformer 1 and the voltage resonance capacitor 4. FIG. 2A shows a voltage waveform between the drain and the source of the power switching transistor 2 in this case. As can be seen from FIG. 2A, when the output current is close to the rated current of the DC-DC converter and sufficiently large, the power switching transistor 2 performs a pulse width modulation (PWM) control IC The oscillation frequency of the oscillator and the constant of the voltage resonance circuit are set. Here, when the current of the load resistor 10 decreases, the voltage of the current detection resistor 15 is set to be smaller than the voltage of the reference voltage generator 14, so that the comparator 13 outputs a signal, and the transistor 17 outputs Turns on. Accordingly, the voltage resonance circuit includes the primary winding of the switching transformer 1, the voltage resonance capacitor 4, and the parallel capacitor 16. Therefore, the voltage resonance frequency is given by the following equation (1) .
The frequency is lower than the frequency of the voltage resonance circuit of the conventional DC-DC converter represented by Expression (2) . f = 1 / {2π} [Lp (C1 + C2)]} (1) f = 1 / {2π} [Lp (C1)]} (1) 2) 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
FIG. 2B shows the drain-source voltage waveform of FIG.
The curve shown by a broken line in FIG. 2B is that of a conventional example in which the capacitors 16 are not connected in parallel. The voltage waveform between the drain and the source of the power switching transistor 2 in the case of the present 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. Since the voltage resonance frequency is reduced 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 As described in detail above, the DC-DC converter according to the present invention can be made variable in a conventional pulse width modulation (PWM) DC-DC converter. The voltage resonance frequency that was not present can be arbitrarily set by the output current, and the DC-D
Since the control IC used in the C converter device is a pulse width modulation (PWM) control IC, even if the load current fluctuates, the switching operation of the power switching transistor can be switched to zero voltage, The switching loss of the switching transistor can be reduced, and a high-efficiency, low-noise and low-cost DC-DC converter can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of a DC-DC converter showing one embodiment of the present invention. FIG. 2 is an explanatory diagram showing a drain-source voltage waveform of a power switching transistor according to one embodiment of the present invention. FIG. 3 is a circuit diagram of a conventional DC-DC converter. FIG. 4 is a diagram illustrating the operation of a conventional DC-DC converter. [Description of Signs] 1 switching transformer 2 power switching transistor 3 flywheel diode 4 voltage resonance capacitor 5 IC for controlling pulse width modulation (PWM) 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

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H02M 3/28

Claims (1)

(57) Claims 1. A pulse width modulation control type DC-DC converter device including a voltage resonance circuit including a primary winding of a switching transformer and a first capacitor for voltage resonance, a DC output current detection means, connected in parallel with the first capacitor for voltage resonance of the voltage resonance circuit, transients of the second capacitor and the switching
Comparison to compare the series circuit of the static, the output of the direct-current output current detecting means with a predetermined value
And the output of the DC output current detection means has dropped below a predetermined value.
The second capacitor is connected to the first capacitor by the output of the comparator.
Turn on the switching transistor that connects the capacitors in parallel
Switching means for switching to the pulse width modulation control method.
C-DC converter device.