JP2010011563A - Dc power supply device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a DC power supply device which normally starts a system without causing a malfunction of a control IC even if an input voltage is temporally turned off and then turned on. <P>SOLUTION: The DC power supply device includes a power conversion transformer (T1) having an auxiliary winding, a control circuit (12) which controls a current of a primary-side winding of the transformer, and a rectification circuit (D3) having a rectification means for converting an alternate current induced by the auxiliary winding into a direct current, and a smoothing capacity. The control circuit can be operated by a voltage generated at the rectification circuit as a power supply voltage. The DC power supply device is provided with voltage drop means (R3, R4 and Q2) which drop voltages supplied to the control circuit by extracting an electric charge of the smoothing capacity of the rectification circuit when an input voltage (Vin) of the primary-side winding of the transformer drops below a predetermined potential. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a start-up stabilization technique for an insulated DC power supply device including a voltage conversion transformer, and relates to a technique that is effective when used for a primary side control circuit used in a power supply device such as an AC adapter.

  The AC adapter includes a diode bridge circuit that rectifies an AC power supply, and a DC-DC converter that steps down a DC voltage rectified by the circuit and converts the voltage to a DC voltage having a desired potential. As this DC-DC converter, for example, a switching power supply device is used which controls the voltage induced in the secondary winding by controlling the current flowing in the primary winding of the voltage conversion transformer. Yes.

  The AC adapter is being reduced in size and cost, and for that purpose, it is important to reduce the number of parts. Therefore, a control IC for controlling a switching transistor that causes a current to flow through the primary winding has been developed. Conventional switching control ICs have a relatively large number of external elements such as photocouplers, capacitors, and resistors for feeding back the output voltage on the secondary winding side to the control IC.

In view of this, an invention relating to a power supply apparatus in which an auxiliary winding is provided in a voltage conversion transformer and a switching transistor for supplying a current to a primary winding based on a voltage induced in the auxiliary winding is proposed. ing. Examples of such an invention include those disclosed in Patent Document 1 and Patent Document 2.
Japanese Patent Laid-Open No. 5-130773 JP-A-8-126307

  By the way, the AC adapter is required to have a function of converting an AC voltage of about 100V AC in Japan and about 200V overseas into a DC voltage of several tens of volts or 10V or less. As described above, in the AC adapter having the primary side control IC, the withstand voltage of the IC is considerably lower than the input voltage. Therefore, in the invention disclosed in Patent Document 1, the primary side input voltage is stepped down by a resistor and supplied to the control IC as a power supply voltage.

  However, when the primary side input voltage is stepped down by a resistor and supplied to the primary side control IC as a power supply voltage, there is a problem that the voltage is not stable. Therefore, as shown in FIG. 4, the voltage conversion transformer T1 is provided with the auxiliary winding Nb, and the alternating current induced in the auxiliary winding is converted into direct current by the diode D3 and smoothed by the capacitor C3. The power supply device is configured to supply the voltage Vb to the primary-side control IC 12 as a power supply voltage and to control the switching transistor Q1 that causes a current to flow through the primary-side winding. Then, at the time of start-up, after the voltage stepped down by the resistor R1 is supplied to the primary side control IC, the switching transistor Q1 is operated to pass a current through the primary side winding Np, and after the voltage is induced in the auxiliary winding Nb, Then, a technique for operating the primary side control IC with the DC voltage Vb obtained by rectifying the AC voltage induced in the auxiliary winding without using the input voltage was examined.

  As a result, it was found that if the plug of the AC adapter is removed from the outlet and immediately re-inserted, the primary side control IC may malfunction and the system may not start normally. This is because the primary control IC for the AC adapter is designed to operate normally when the input voltage rises from 0V in the initial state, but if the plug is removed from the outlet as described above, it is immediately turned on again. Since the voltage comes in before the potential of the node somewhere in the control IC (probably the power supply switching unit) drops sufficiently, the power supply voltage input node does not switch from the induced voltage of the auxiliary winding to the primary side input voltage. This is probably because of this.

  The present invention has been made paying attention to the problems as described above. The purpose of the present invention is to prevent the control IC from malfunctioning even when the input voltage is once reduced and then re-input. An object of the present invention is to provide a DC power supply device that can be activated.

  In order to achieve the above object, the present invention provides a voltage converting transformer having an auxiliary winding, a control circuit for controlling a current of a primary side winding of the transformer, and an alternating current induced in the auxiliary winding as a direct current. A rectifier circuit having a rectifier circuit and a smoothing capacitor, wherein the control circuit is configured to operate with a voltage generated by the rectifier circuit as a power supply voltage. When the input voltage of the line becomes lower than a predetermined potential, voltage drop means is provided to draw out the charge of the smoothing capacitor of the rectifier circuit and drop the voltage supplied to the control circuit.

  According to the above means, when the input voltage falls, the voltage drop means draws out the charge of the smoothing capacitor of the rectifier circuit and quickly drops the voltage supplied to the control circuit. It is possible to prevent the control circuit from malfunctioning when it falls and the input voltage rises again.

  Preferably, the voltage drop means includes a voltage dividing resistor connected in series between a terminal to which the input voltage is applied and a reference potential point, and a connection node between the rectifier means and the smoothing capacitor. And a three-terminal switching element connected between the reference potential point, and a voltage depending on the voltage generated by the voltage dividing resistor is applied to the control terminal of the three-terminal switching element. Configure. As a result, when the input voltage drops below a certain level, the three-terminal switching element becomes conductive, and the voltage supplied to the control circuit can be quickly lowered by extracting the charge of the smoothing capacitor of the rectifier circuit.

  More preferably, the three-terminal switching element is a bipolar transistor, and the voltage generated by the voltage dividing resistor is applied to the base terminal of the bipolar transistor via the voltage protection element. Configure. As a result, when the input voltage drops below a certain level while reducing the current flowing through the voltage dividing resistor, the charge supplied to the smoothing capacitor of the rectifier circuit can be extracted and the voltage supplied to the control circuit can be quickly reduced.

  Preferably, the voltage drop means includes a voltage dividing resistor connected in series between a terminal to which the input voltage is applied and a reference potential point, and a connection node between the rectifier means and the smoothing capacitor. A two-terminal switching element connected between the voltage dividing resistor connection nodes. As a result, a voltage drop means is configured by adding a small number of elements, and when the input voltage drops below a certain level, the charge of the smoothing capacitor of the rectifier circuit is drawn and the voltage supplied to the control circuit is quickly dropped. It becomes possible.

  Further preferably, the control circuit includes a switching element for intermittently passing a current through the primary winding of the transformer, and a switching control circuit for driving the switching element on and off. Thereby, voltage conversion efficiency can be improved by driving a switching element by PWM control or the like.

  The switching element and the switching control circuit are preferably configured as a semiconductor integrated circuit on one semiconductor chip. As a result, the number of parts of the primary circuit can be reduced and the apparatus can be miniaturized.

  According to the present invention, there is an effect that it is possible to realize a DC power supply device that can start up the system normally without causing a malfunction of the control IC even when the input voltage is once lowered and then turned on again.

DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the drawings. In the following, an example in which the present invention is applied to an AC adapter will be described as an example of an effective DC power supply device, but the DC power supply device to which the present invention can be applied is not limited to this.
<Embodiment 1>
FIG. 1 is a circuit configuration diagram showing an AC adapter according to a first embodiment of the present invention.

  The AC adapter 10 of this embodiment includes a diode bridge circuit 11 and a smoothing capacitor C1 that rectifies an alternating voltage (AC) and converts it into a direct voltage, a primary coil Np, an auxiliary coil Nb, and a secondary coil Ns. It has a voltage conversion transformer T1, a switching transistor Q1 connected in series with the primary coil Np of the transformer T1, and a switching control circuit 12 for driving the switching transistor Q1. In this embodiment, the switching control circuit 12 is formed as a semiconductor integrated circuit (hereinafter referred to as a control IC) on a single semiconductor chip such as single crystal silicon.

  On the secondary side of the transformer T1, a rectifying diode D2 connected in series with the secondary coil Ns, and a smoothing connected between the cathode terminal of the diode D2 and the other terminal of the secondary coil Ns. Capacitor C2 is provided, and the primary side coil Np and the secondary side coil Ns are rectified by rectifying and smoothing the alternating current induced in the secondary side coil Ns by passing a current intermittently through the primary side coil Np. A DC voltage Vout corresponding to the winding ratio is output.

  Moreover, the AC adapter 10 of this embodiment is connected on the primary side between the rectifying diode D3 connected in series with the auxiliary coil Nb, and the cathode terminal of the diode D3 and the other terminal of the auxiliary coil Nb. And a smoothed voltage Vb is applied to a predetermined terminal of the control IC 12. At the same time, the input voltage Vin is applied to the other terminal of the control IC 12 via the resistor R1. The smoothing capacitor C3 has a capacitance value of about 150 μF.

  Although not shown, the control IC 12 is provided with a voltage switching circuit that supplies the smoothing voltage Vb or the input voltage Vin on the auxiliary coil side to the internal circuit as a power supply voltage. The voltage switching circuit supplies the input voltage Vin as the power supply voltage so that the internal circuit can operate at the initial stage immediately after the power is turned on and the auxiliary coil Nb cannot yet supply sufficient power, and the control IC 12 performs switching.・ After the transistor Q1 is operated to pass a current through the primary coil and the voltage is induced in the auxiliary coil Nb, the input voltage is not used and the AC voltage induced in the auxiliary winding Nb is controlled with a rectified voltage. The internal circuit of the IC 12 is operated.

  The control IC 12 samples the smoothing voltage Vb on the auxiliary coil side and amplifies a potential difference from a predetermined reference voltage, and an output of an oscillator that generates a waveform signal (triangular wave) having a predetermined frequency. A comparator or the like that compares the output of the error amplifier circuit is provided, and is configured to generate and output a control pulse for controlling on / off of the switching transistor Q1 by, for example, a PWM method.

  Further, in the AC adapter 10 of this embodiment, a pnp bipolar transistor Q2 having an emitter connected to a connection node N3 between the diode D3 and the smoothing capacitor C3, and a collector connected to the ground GND via a resistor R2, Voltage dividing resistors R3 and R4 connected in series are provided between the output node N1 of the diode bridge 11 and the ground GND. In the AC adapter 10 of this embodiment, the base terminal of the transistor Q2 is connected to the connection node N2 of the resistors R3 and R4 via the diode D4 connected in the reverse direction. The diode D4 functions as a withstand voltage protection element that protects the base terminal of the transistor Q2 from being applied with a voltage higher than the withstand voltage.

  In the embodiment of FIG. 1, the switching transistor Q1 for supplying current to the primary coil is illustrated as a separate element from the control IC 12. However, the transistor Q1 can be incorporated into the control IC 12. It is.

  Next, the operation of the transistor Q2 will be described.

  In the AC adapter of this embodiment, the winding ratio of the primary winding Np and the auxiliary winding Nb is set so that the smoothing voltage Vb of the auxiliary coil becomes a voltage such as 24V, and in a steady state. The resistance ratio between the resistors R3 and R4 is set so that the potential V2 of the node N2 is about 30V, which is slightly higher than Vb (= 24V). Further, in order to reduce the current consumption of the primary circuit in the steady state, the resistance values of the resistors R3 and R4 are determined to be several MΩ in total.

  When the AC adapter is unplugged and the input voltage Vin falls and the potential V2 of the node N2 becomes Vb-2Vf or lower (Vf is the forward voltage of the PN junction), the transistor Q2 is turned on and the charge of the smoothing capacitor C3 becomes Q2. And Vb falls quickly by being pulled out to the ground GND through the resistor R2. Specifically, the resistance value of the resistor R2 is set to about 800Ω so that a collector current of about 300 mA flows through Q2 and Vb is lowered.

By setting the resistors R2 to R4 to the values as described above, the AC adapter according to this embodiment can reduce wasteful current consumption in the steady state even when the input voltage Vin is a high voltage of 180V or higher. While suppressing, the power supply voltage (Vb) of the control IC 12 can be quickly lowered when the input voltage is off. When the power supply voltage of the control IC 12 falls, the potential of the node of the internal circuit also falls, so that the control IC 12 can be prevented from malfunctioning when the power is turned on again.
<Embodiment 2>
In this embodiment, the charge extracting transistor Q2 and its emitter resistor R2 in the first embodiment are omitted, and only the diode D4 ′ is used. When the input voltage Vin falls, the smoothing capacitor C3 is connected through the diode D4 ′. The electric charge is extracted to the ground GND, and Vb is lowered. In this embodiment, the diode D4 ′ functions not as a withstand voltage protection element but as a two-terminal switching element.

  When configured as described above, if the resistance values of the resistors R3 and R4 are the same as those in the first embodiment, the falling of the voltage Vb of the smoothing capacitor C3 is delayed, but the current flowing through the resistors R3 and R4 If the allowable value of the voltage Vb is large, or the fall time of the voltage Vb is not so severe, the resistance value of R4 is reduced and the charge of the smoothing capacitor C3 is extracted through the diode D4 ′ as in this embodiment. The potential Vb of the node N3 can be lowered.

  In the second embodiment, the series resistors R5 and R6 for voltage division are connected between the output terminals, and the overvoltage detection circuit 13 for detecting the output voltage on the secondary side is provided. When an overvoltage is detected, the driving of the switching transistor Q1 by the control IC 12 is stopped. Since the power supply device of this embodiment is an insulation type using a transformer, that is, the reference potentials on the primary side and the secondary side are separated, the signal from the overvoltage detection circuit 13 is transmitted to the primary side via the photocoupler PC. It is configured to be input to the control IC 12.

  FIG. 3 shows a modification of the AC adapter according to the first embodiment. In this modification, an overvoltage detection circuit 13 is provided in the same manner as in the second embodiment, and a current detection resistor Rsns is provided in the middle of one output line, and an output current is detected from the voltage across the resistor Rsns. A current detection circuit 15 is provided so that the driving of the switching transistor Q1 by the primary side control IC 12 is stopped even when an output overcurrent is detected.

  Further, in this modification, one photocoupler PC is shared by taking the logical sum of the output of the overvoltage detection circuit 13 and the output of the overcurrent detection circuit 15 and supplying it to the photocoupler PC. The voltage detection by the overvoltage detection circuit 13 and the current detection by the overcurrent detection circuit 15 are not limited to those shown in FIG. 3 and can be variously changed.

  Although the invention made by the inventor has been specifically described based on examples, the present invention is not limited to the above-described embodiment, and various modifications can be made. For example, in the above-described embodiment, the bipolar transistor is used as the switching transistor Q1 for supplying current to the primary coil and the transistor Q2 for extracting the charge of the smoothing capacitor C3. However, a MOSFET (field effect transistor) can also be used. It is. In this case, the diode D4 in FIG. 1 can be omitted if the MOSFET used has a sufficiently high breakdown voltage.

  In the above embodiment, the resistor R2 in series with the charge extracting transistor Q2 is connected to the collector side of Q2. However, it can be connected to the emitter side of Q2.

  In the above description, the case where the invention made mainly by the present inventor is applied to the AC adapter which is the field of use behind it has been described. However, the present invention is not limited to this, and a transformer is used to induce the auxiliary winding. The present invention can be widely used in a DC power supply device that rectifies the AC voltage generated to generate the power supply voltage of the control circuit.

It is a circuit block diagram which shows the AC adapter of the 1st Embodiment of this invention. It is a circuit block diagram which shows the AC adapter of the 2nd Embodiment of this invention. It is a circuit block diagram which shows the modification of the AC adapter of 1st Embodiment. It is a circuit block diagram which shows an example of the conventional AC adapter provided with the transformer for voltage conversion which has an auxiliary | assistant winding.

Explanation of symbols

10 AC Adapter 11 Diode Bridge Circuit 12 Switching Control Circuit (Primary Side Control IC)
13 Overvoltage detection circuit 14 Photocoupler 15 Overcurrent detection circuit T1 Transformer Q1 Switching transistor Q2 Charge extraction transistor (3-terminal switching element)
R3, R4 Voltage dividing resistor D2, D3 Rectifier diode D4 Withstand voltage protection diode D4 'Charge extraction diode (2-terminal switching element)

Claims (6)

A transformer for voltage conversion having an auxiliary winding, a control circuit for controlling the current of the primary winding of the transformer, a rectifying means for converting alternating current induced in the auxiliary winding into direct current, and a smoothing capacitor A rectifier circuit, and the control circuit is a DC power supply device configured to be operable with a voltage generated by the rectifier circuit as a power supply voltage,
Voltage drop means for drawing out the charge of the smoothing capacitor of the rectifier circuit and dropping the voltage supplied to the control circuit when the input voltage of the primary winding of the transformer becomes lower than a predetermined potential. DC power supply device characterized.   The voltage drop means includes a voltage dividing resistor connected in series between a terminal to which the input voltage is applied and a reference potential point, a connection node between the rectifier means and the smoothing capacitor, and a reference potential point. And a voltage depending on a voltage generated by the voltage dividing resistor is applied to a control terminal of the three-terminal switching element. Item 4. The DC power supply device according to Item 1.   The three-terminal switching element is a bipolar transistor, and a voltage generated by voltage division by the voltage dividing resistor is applied to a base terminal of the bipolar transistor via a withstand voltage protection element. The DC power supply device according to claim 2.   The voltage drop means includes a voltage dividing resistor connected in series between a terminal to which the input voltage is applied and a reference potential point, a connection node between the rectifying means and the smoothing capacitor, and the voltage dividing resistor. The DC power supply device according to claim 1, comprising: a two-terminal switching element connected to the connection node.   5. The control circuit includes a switching element for causing a current to flow intermittently through the primary winding of the transformer, and a switching control circuit for driving the switching element on and off. The DC power supply device according to any one of the above.   6. The DC power supply device according to claim 5, wherein the switching element and the switching control circuit are formed as a semiconductor integrated circuit on one semiconductor chip.

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