JPH11168876A - Dc/dc conversion circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make thin a DC reactor and to miniaturize a conversion circuit, by providing a switching element from an anode side of a diode being inserted in forward direction between the DC reactor and an output voltage terminal and turning on/off the switching element in synchronization with a main switching element. SOLUTION: A MOS-FET 20 that is provided along a diode 19 that is connected to a DC reactor 3 and an output terminal 6 and a MOS-FET 21 that is connected to an output terminal 18 are both connected in forward direction. When a main switch element 2 continues and a first DC output voltage V1 is controlled constantly by a stabilization control circuit B, the terminal voltage of the FET 21 is proportional to that of an FET 20 and resistance can be ignored, thus also making constant a second DC output voltage V2. When the FET 20 continues, the second output voltage V2 is separated from a first output voltage, and the first DC output voltage V1 cannot be affected by the output voltage, thus constantly maintaining the first output voltage V1 by operating the control circuit B and also the second output voltage V2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a two-output type DC / DC conversion circuit in which the first output voltage is +1.8 V and the second output voltage is +2.5 V and the output voltages have no significant difference.

[0002]

2. Description of the Related Art In a recent example in which elements requiring slightly higher voltages than the first output voltage are increasing, the first output voltage is + 1.8V and the second output voltage is + 1.8V.
Like 2.5V. In this case, two types of methods are used as a configuration method of the DC / DC conversion circuit. Such a circuit is used exclusively for obtaining a desired stable DC voltage from a power source such as a battery having a large voltage difference between the time of completion of charge and the end of discharge. These prior arts are shown in FIG.
This will be described with reference to FIG. 2 and 3 show an example of a circuit for converting a fluctuating DC power supply into a stable DC voltage using a switching method. FIG. 2 shows an example of a circuit for obtaining two stable DC voltages by using two similar switching circuits among them. FIG. 3 shows a circuit in which a DC reactor performs an operation of a transformer to obtain a desired second DC voltage. It is an example of a circuit for obtaining an output voltage. First, a schematic circuit operation will be described with reference to FIG. In the description of the circuit operation in FIG. 3, only the parts different from the operation in FIG. 2 will be described.

First, the circuit configuration will be described. "1" is a DC power supply having a fluctuation range such as a battery, and a switching element "2" (a MOS-FET or a bipolar element may be used for the (+) pole of this DC power supply, but an N-channel MOS-FET will be described here). ) Connect the drain terminal. The source terminal of the switching element '1' is connected to one end of the DC reactor '3' and the cathode terminal of the freewheel diode '4'. The other end of the DC reactor '3' is connected to the (+) terminal of the first output terminal '6'. Next, the (-) pole of the unstable DC power supply '1' is connected to the anode terminal of the freewheel diode '4' and the (-) terminal of the first output terminal '6'. A filtering capacitor '5' is connected to the (+) and (-) poles of the first output terminal '6'. Further, between the (+) and (-) terminals of the first output terminal '6', resistors '7' and '8' are connected in series as detecting means for stabilizing the output voltage. The point is connected to the (-) pole of the error amplifier '9'. The basic reference voltage '10' is connected to the (+) terminal of the error amplifier '9'. The output terminal of the error amplifier '9' is applied to the (+) input terminal of the comparison amplifier '11', and the triangular wave output of the oscillator '12' is applied to the (-) input terminal. The output of the comparison amplifier '11' is connected to the pulse amplifier '1'.
Apply to 3 'input terminal. The output of the pulse amplifier is applied to the gate terminal of switching element '2'. The control circuit required for these stabilizations is hereinafter referred to as a stabilization control circuit 'B'. DC / D circuit after switching element
It is assumed that the C conversion circuit is “A”. The other output voltage circuit has the same configuration as the block 'A' and obtains the second output voltage. Next, the circuit operation will be described.

A description will be given of an example of a method of stabilizing an output voltage when a change factor such as a load current or an input voltage occurs, taking a case where an input voltage fluctuates as an example. When the input DC voltage (Vin) rises, the output voltage (V1) also tries to rise, but the midpoint voltage of the resistors '7' and '8' connected between the output terminals also tries to rise. When this voltage deviates from the reference voltage '10', the output voltage of the error amplifier circuit '9' becomes equal to the deviation of the input voltage (Vk
−Vr) × appears as a voltage change of the amplification factor of the error amplifier '9'. Due to this voltage change, the output pulse width of the comparison amplifier '11' operates in a decreasing direction. This pulse voltage is applied as the gate voltage of the switching element '2'. The pulse width of this pulse voltage is narrower than before the fluctuation. In a system in which the output voltage is controlled by so-called pulse duty control with a constant switching frequency, the pulse width becomes narrow. In other words, the output voltage decreases as the duty decreases. This operation is continuously performed until the output voltage divided value (Vk) becomes equal to the reference voltage (Vr). As a result, it operates to keep the output voltage constant at all times in response to factors of output voltage fluctuation such as a change in input voltage and a change in output current. As a result, the output voltage is kept at a constant value.

The operation of stabilizing the second output voltage (V2) is exactly the same as the operation of the first output voltage (V1). The operation and theory of general switching circuits are described in Akira Hasegawa, switching regulator design know-how (CQ Publishing Company)
Please refer to.

Next, the circuit configuration and operation in FIG. 3 will be described. The (+) terminal of the unstable DC power supply '1' is connected to the drain terminal of the switching element '2'. The source terminal of the switching element '2' is connected to one end of the DC reactor '3' and the cathode terminal of the freewheel diode '4'. The other end of the DC reactor '3' is connected to the (+) terminal of the first output terminal '6'. Unstable DC power supply '
The (-) pole of 1 'is connected to the anode terminal of the freewheel diode' 4 'and the (-) terminal of the first output terminal' 6 '. A filtering capacitor '5' is connected to the (+) and (-) poles of the output terminal '6'. Here, another winding '14' is wound around the same magnetic core around the DC reactor '3', and the winding '15' has a transformer configuration. The polarities are configured to be opposite polarities.
The winding start side of the winding '14' is connected to the winding end side of the winding '15'. Diodes are placed at both ends of this winding '14'.
16 'and the filtering capacitor' 17 'are connected. An output terminal '18' is drawn out from a connection point between the capacitor '17' and the diode '16' and used as an output terminal for the second output voltage (V2). A control circuit for stabilization is indicated by a block 'B', which is the same as the control circuit shown in FIG. Next, the operation of this circuit will be described. Switching element '
2 'conducts (ON mode), so that the input voltage (Vi) is applied across the winding' 15 'of the DC reactor' 3 '.
n), an output voltage (V1), and a voltage (Vr1) having a difference between the output voltage (V1) and a voltage corresponding to a voltage drop at the switching element '2' are applied. At this time, a voltage (Vr2) proportional to the turns ratio of the winding '15' to the winding '15' is generated in the winding '14' of the DC reactor '3'. Output voltage (V2)
Is not supplied with energy. Next, when the switching element '2' is in the non-conducting (OFF) mode, the current energy stored in the winding '15' of the DC reactor '3' is converted to the freewheel diode '4', the filtering capacitor '5' and the output terminal ' 6 'and the first voltage load'
Continue flowing through 22 '. At this time, a voltage corresponding to the voltage drop of the freewheel diode '4' from the output voltage (V1) is generated in the winding '15' of the DC reactor '3'. The voltage generated across the other winding '14' in the DC reactor '3' is a voltage corresponding to the turns ratio between the winding '15' and the winding '14'. Since this voltage (V2) has a polarity such that the direction of e becomes (+), the diode '16' conducts and the voltage generated in the winding '14' passes through the diode '16' to the filtering capacitor. '17' and the output terminal '18' and the second voltage load '23'. Here, the first output voltage (V1)
Is kept constant by the operation of the DC / DC stabilization control circuit. If the voltage drop of the freewheel diode '4' is constant, the voltage generated in the winding '14' is also substantially constant. In fact, it can be considered that this voltage is almost constant in a diode using a silicon junction. As a result, the second output voltage (V2) is also substantially constant. This method is often used as a method for obtaining two output voltages with a simple configuration. The MO used here as the switching element
The drive voltage of the gate terminal of the S-FET needs to be several volts higher than the source terminal. The voltage for this needs to be prepared separately. However, an integrated circuit having a built-in bias voltage for performing such an operation is also commercially available. Such a DC / DC conversion circuit mounting requirement does not have its own mounting area as an independent power supply unit circuit, but is often required to be mounted near a logic circuit. The demand for a thinner type (particularly, a notebook-type logic board is particularly demanding) is strict, and is often 3 mm or less. The DC / DC conversion circuit is no exception. The problem here is the thinning of the winding part. Since the winding portion is formed by winding a conductive wire around a magnetic core, the smaller the number of windings and the simpler the winding configuration, the smaller the size can be. However, in the circuit configuration shown in FIG. In order to improve the accuracy of the output voltage, the number of turns needs to be increased more than usual so as to obtain an optimum turns ratio. In addition, two windings need to be wound in order to form a transformer. For these reasons, the winding parts become a bottleneck, and it is impossible to make the semiconductor device thinner corresponding to the thinner semiconductor.

[0007]

FIGS. 2 and 3 show examples of conventional circuits. In the example of FIG. 2, two identical circuits are required, so that the number of components used is large, and a large mounting area is required. In addition, the reliability is low because the number of components is large. Yes, for the same reason it will be expensive. In the example of FIG.
The number of parts used is overwhelmingly smaller than that of the example of FIG. 2, so that the mounting area may be small, the reliability is high, and the price is low. When a power supply is used, it is difficult to reduce the thickness of a DC reactor having two windings. (As described above, in order to improve the accuracy of the second output voltage, the number of turns is more than usual so as to have an optimum turns ratio. It is necessary to use a transformer,
Two windings need to be wound. ), And there is a problem that the height cannot be reduced to a value equal to or less than the equivalent value of the semiconductor element.

[0008]

Means for Solving the Problems The structure of a DC reactor having a problem in achieving a reduction in thickness is simplified, and the second voltage output is reduced.
All are realized by a circuit configuration using a semiconductor. In particular,
A diode is inserted between the DC reactor and the output terminal in the forward direction, and a switching element is provided from the anode side of the diode. The switching element is turned on when the switching element is turned on.
F. A switching element is similarly connected to both ends of the diode, and the main switching element is turned off when conducting and turned on when not conducting.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The circuit configuration and operation will be described with reference to FIG. 1, which is an embodiment of the present invention.

(+) Of an unstable DC power source '1' such as a battery
Connect the drain terminal of switching element '2' to the pole,
The source terminal of the switching element '2' is connected to one end of the DC reactor '3' and the freewheel diode '4'.
To the cathode terminal. The other end of the DC reactor '3' is connected to the anode terminal of the diode '19' and the MO terminal.
The source terminal of the S-FET (here, N-channel type) '21' is connected. The cathode terminal of the diode '19' is connected to the (+) terminal of the output terminal '6', and the diode '1'
MOS-FET between anode terminal and cathode terminal of 9 '
(Here, P-channel type) '20' is connected. MO
The drain terminal of the S-FET '21' is connected to the (+) terminal of the output terminal '18' of the second output voltage (V2) and the filtering capacitor '17'. The other end of the filtering capacitor '17' is connected to the (+) terminal of the first output terminal (V1). The (-) electrode of the DC power supply '1' is connected to the anode terminal of the freewheel diode '4', and to the filtering capacitor '5' and the (-) terminal of the first output terminal '6'. The first output voltage (V1) is supplied from the (+) terminal and the (-) terminal of the output terminal '6' and the second output voltage (V1).
2) is the (+) terminal of the output terminal '18' and the output terminal '6'
(-) Terminals. Here two MOS-FET '
Both 20 'and' 21 'are connected so that the built-in diodes are in the forward direction. The control circuit required for the stabilizing operation has a configuration shown by a block 'B', but since this circuit is the same as the configuration of the conventional circuit, the description is omitted.
Next, the operation of the circuit will be described in detail. Switching element '
When 2 ′ conducts (ON mode), the equivalent circuit shown in FIG. 4 is formed. Assuming that the DC voltage (V1) is controlled to be constant, the voltages Va, V2, and Ia are as follows: Va = V1 + Vd V2 = Va−Vb Ia = (Vin−V1−Vd) / L × Ton Here, when a MOS-FET is used as a control element, Vb becomes channel resistance × current, the channel resistance is several tens of mΩ, and when the output current is small, it can be made negligibly small compared to Vd. Eventually, V2 = V1 + Vd, and Vd is almost constant if the diode is a silicon junction, so that V2 also has a constant value. Here, a diode having a voltage drop of Vd = 0.7 V may be used. Next, the main switching element
2 'is off (OFF mode), the control element N-channel MOS-FET' 21 'is also turned off, and conversely, the control element P-channel MODS-FET' 20 'connected to both ends of the diode' 19 '. Is controlled to be ON, the circuit at this time is an equivalent circuit shown in FIG. That is, the freewheel diode '4', the DC reactor '3', and the control element P-channel MOS-FET (SW
2) A closed loop circuit consisting of '20' and output terminal '6'.
Since the control element N-channel MOS-FET '21' is in the OFF mode, the circuit to the second output terminal '18' uses the built-in diode of the control element '21' for the output terminal '
6 '. However, if the value of (channel resistance × current) is made smaller than the above-mentioned voltage drop of the built-in diode when the control element '20' is conducting, the built-in diode will be in a reverse bias state after all and the second output voltage (V2) will be reduced. The circuit is in a state of being disconnected from the circuit of the first output voltage (V1). DC reactor at this time
The current flowing through 3 ′ is as follows: Ia = Ia ′ − (V1 + Vd) / L × Toff At this time, the current mode becomes the same as the well-known operation mode of the DC / DC power supply due to the action of the DC reactor '3', and the first output voltage (V1) is changed by the control element P-channel MOS-FET '20'. The output voltage is not affected by conduction (ON) / non-conduction (OFF). The second output voltage (V2) is the DC reactor '3'
This is proportional to the pulse width increase,
This results in V2−V1 = √ (load resistance (R2) × L × Id × Id). The pulse voltage required for the operation of the two control elements is
It can be easily obtained as follows. First, the same voltage as that of the main switching element '2' is applied to the gate terminal of the control element N-channel MOS-FET '21'. A pulse voltage obtained by inverting the gate terminal voltage of the element '2' may be applied.
This voltage can be easily obtained by inserting a one-stage inverting circuit into the gate terminal voltage of the main switching element '2'. The first output voltage (V1) is controlled by the control circuit
The second output voltage (V2) also becomes constant as a result because it is always kept constant by the operation of B '. When this circuit is used, there is only one main DC reactor, despite the fact that there is a two-output circuit. Can be reduced in height because they are all semiconductor components. In the embodiment of the present invention, all the MOS-
Although the operation in the circuit using the FET has been described, it is obvious that the same operation can be obtained even when using a bipolar element.

[0011]

According to the present invention, the height of the DC / DC conversion circuit can be reduced and the number of parts used can be reduced.

Reasons why the height can be reduced: A configuration using semiconductor elements (excluding a capacitor) except for a DC reactor. A DC reactor to be used has a very simple configuration.

As a result, the notebook personal computer can be mounted at high density and thinned.

[Brief description of the drawings]

FIG. 1 is a DC / DC conversion circuit showing one embodiment of the present invention.

FIG. 2 shows a DC / DC circuit having two identical circuits in a conventional circuit.
One embodiment of a conversion circuit.

FIG. 3 shows an embodiment of a DC / DC conversion circuit using a reactor having two windings in a conventional circuit.

FIG. 4 is an equivalent circuit in an ON mode according to the embodiment of the present invention.

FIG. 5 is an equivalent circuit in an OFF mode according to the embodiment of the present invention.

FIG. 6 is an operation timing chart of a control element.

[Explanation of symbols]

1: DC power supply, 2: MOS-FET, 3: DC reactor, 4: Diode, 5: Capacitor,
6 output terminal, 7, 8 resistance, 9 error amplifier, 10 reference voltage, 11 comparison amplifier,
12 ... oscillator, 13 ... pulse amplifier, 14,
15 ... winding, 16 ... diode, 17 ... capacitor, 18 ... output terminal, 19 ... diode,
20, 21: MOS-FET, 22: load of first output voltage, 23: load of second output voltage,
A: DC / DC converter, B: Stabilization control circuit.

Claims (1)

[Claims] 1. DC power supply and DC reactor, ON / OFF
In a so-called DC / DC conversion circuit, which is composed of a switching element that performs an operation, a freewheel diode, and an output filtering capacitor and controls an operation duty of the switching element to control an output voltage to be constant, a diode is provided between a DC reactor and a reactor wave capacitor. Are connected in the forward direction, and one end of each of two switching elements operating in synchronization with the operation of the switching element is connected from a connection point between the reactor and the diode. The other end of the first switching element is connected to the cathode of the diode. And a DC / DC conversion circuit having two outputs connected to the other end of the second switching element and a second output terminal.