JP2013150513A - Power-supply circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve downsizing of a power-supply circuit including a switching element and a control IC for driving the switching element at the secondary winding side of a transformer.SOLUTION: A power-supply circuit 1a includes a transformer, a switching element Q connected to a secondary winding T of the transformer, and a control IC 10 outputting a driving signal for the switching element, and outputs power supplied between two terminals at both ends of the secondary winding to a load with a predetermined voltage value. The control IC operates using the power supplied between the two terminals of both ends of the transformer as a power source, and a charge pump circuit CP is formed toward the two terminals and a power-supply terminal 10vcc of the control IC. The switching element is used as a switching element constituting the charge pump circuit.

Description

  The present invention relates to an electric circuit including a switching element on a secondary winding side of a transformer, such as a synchronous rectifier circuit, and a control IC for driving the switching circuit. Specifically, the present invention relates to an improvement in a circuit for supplying power to the control IC.

  The synchronous rectifier circuit is a rectifier circuit using a switching element such as a MOSFET, and the rectifier circuit is often provided on the secondary side of the transformer. Since a load is connected to the secondary side of the transformer, the winding ratio with the primary winding of the transformer is set according to the voltage required by the load.

  By the way, a control circuit for driving a switching element is usually made into an IC and is provided as a control IC for synchronous rectification, a driving IC for a switching element (hereinafter referred to as a control IC) or the like. And, in a circuit having a switching element such as a synchronous rectifier circuit on the secondary winding side of the transformer and a control IC for driving the same, the power for operating the control IC itself is on the secondary winding side of the transformer. Will be supplied from. When the voltage required by the load is lower than the power supply voltage of the control IC, in addition to the main winding for securing the voltage applied to the load on the secondary winding side of the transformer, the power supply voltage of the control IC It is necessary to provide an auxiliary winding for securing For example, if the circuit connected as a load is a circuit mainly composed of a logic IC, the load voltage is about 3.3 V to 5 V, but a MOSFET that is a general switching element for a power supply circuit has a voltage of 12 V. The above operating voltage is required, and a power supply voltage of 12 V or more is also required for the control IC that drives the MOSFTE. When the switching element is a MOSFET, a recommended voltage value between the gate and the source is determined, and an auxiliary winding is similarly provided when the recommended value is lower than the power supply voltage of the control IC.

  For reference, in FIG. 3, the voltage output to the secondary winding T side of the transformer in the synchronous rectifier circuit on the secondary winding side of the transformer as the power supply circuit 1 b according to the conventional example is the power supply voltage of the control IC 10. The circuit configuration employed in the lower case is illustrated. Further, FIG. 4 shows the components related to the load and the power source for the control IC 10 in the circuit shown in FIG. In FIG. 3, terminals other than the power supply terminal 10vcc, the ground terminal 10gnd of the control IC 10 and the drive signal output terminal (output terminal) 10out of the switching element Q are irrelevant to the essence of the present invention. A description of the specific functions related to the above will be omitted, but schematically, FIG. 3 shows a synchronous rectifier circuit configured on the secondary winding T side of a transformer in an isolated flyback converter or the like. The control IC 10 detects the voltage between the drain D and the source S of the switching element Q made of a MOSFET, determines the current level, and operates to turn on and off the switching element Q in the vicinity where the current change becomes zero. To do. Thereby, in the power supply circuit 1b, the switching element Q is turned on and off in synchronization with the waveform of the high-frequency current flowing on the secondary winding T side, and the high-frequency current is efficiently converted into a direct current.

  As shown in FIG. 3, the secondary winding T of the transformer is used in addition to the winding (main winding) Tm for generating the voltage of the output power supply Vout for finally outputting a DC power supply as a power supply for the load. In this configuration, a winding (auxiliary winding) Ts for generating a voltage for the power source Vcc of the control IC 10 is added.

  As shown in FIG. 4, a capacitor C1 connected in parallel with the main winding Tm is connected to the main winding Tm, and this capacitor C1 is charged and discharged by a switching element Q made of a MOSFET. A synchronous rectifier circuit is connected, and power from the main winding Tm is output as a DC power source (hereinafter referred to as load power source) Vout for the load. In this example, the source S of the switching element Q made of a MOSFET is connected to the ground GND to constitute a low-side type synchronous rectifier circuit. Further, a capacitor C3 for the purpose of noise removal or the like is connected between the load power supply Vout and the ground GND.

  On the other hand, a direct current originating from the power supplied from the secondary winding T in which the auxiliary winding Ts is added to the main winding Tm is supplied to the power supply terminal 10 vcc of the control IC 10. Specifically, the alternating current induced from the primary winding side of the transformer to the entire secondary winding T is converted into a direct current by a known rectifying and smoothing circuit including a diode D1 and a smoothing capacitor C2 that also serves as a noise removing application. The direct current is rectified, and this direct current is used as the power source Vcc of the control IC 10. The following Non-Patent Document 1 describes in detail a specific internal circuit configuration, usage method, and the like of the control IC 10 provided as a product.

International Rectifier Japan KK, “Application Note AN-1087“ Application Design Example of SmartRectifierTM Control IC “IR1167” for Secondary Side Synchronous Rectification ””, [online], [December 1, 2011 search ], Internet <URL: http://www.irf-japan.com/technical-info/appnotes/AN-1087.pdf>

  In recent years, miniaturization is required for various electronic devices. However, a power circuit essential for an electronic device includes a transformer that has a winding structure and is difficult to reduce in size in principle, which makes it difficult to further reduce the size of the device. Of course, even if the power supply circuit is not built in the device, it can be used to charge the secondary battery built in the device or as an auxiliary power supply that enables the device to operate for a long time. ", And even if it is a portable electronic device, the AC adapter is often carried. In any case, further miniaturization is required for a power supply circuit built in a portable device itself or a device that may be carried with the device.

  In addition, electronic devices are required to be reduced in size and power consumption, and electronic circuits of electronic devices, that is, ICs that constitute loads, etc., that operate at a low voltage are actively adopted. . For this reason, as described above, there is a possibility that the power supply voltage of the control IC constituting the power supply circuit becomes higher than the voltage applied to the load.

  As described above, in the power supply circuit that supplies the power from the secondary winding of the transformer to the load, the power supply circuit including the switching element and the control IC for driving the switching element is applied to the load. When the voltage is lower than the power supply voltage of the control IC, or when the switching element is a MOSFET, if the voltage is lower than the recommended voltage between the gate and the source, power for the load is supplied to the secondary winding of the transformer. In addition to the main winding, an auxiliary winding for supplying power to the control IC is required together with the main winding, and the transformer becomes large. Therefore, further downsizing of the electronic device is contrary to the downsizing of the configuration of the power supply circuit essential for the electronic device, particularly the secondary winding side of the transformer.

  Accordingly, an object of the present invention is to provide a power supply circuit capable of further miniaturizing a power supply circuit including a switching element and a control IC for driving the switching element on the secondary winding side of the transformer. It is said.

To achieve the above object, the present invention includes a transformer, a switching element connected to the secondary winding of the transformer, and a control IC that outputs a drive signal for the switching element. A power supply circuit that outputs power supplied between two terminals at both ends of the power supply to a load at a predetermined voltage value,
The control IC operates using power supplied between the two terminals at both ends of the transformer as a power source, and a charge pump circuit is formed toward the two terminals and the power source terminal of the control IC.
The switching element is a power supply circuit that also serves as a switching element constituting the charge pump circuit.

  The load power supply may be a power supply circuit that is a DC power supply rectified by a synchronous rectification circuit including the switching element.

  The predetermined voltage value of the load power supply is lower than the power supply voltage of the control IC, or the switching element is a MOSFET, and the predetermined voltage value of the load power supply is equal to the drain of the MOSFET A power supply circuit that is lower than the recommended voltage value between the sources is more suitable.

  According to the present invention, it is possible to downsize a power supply circuit including a switching element and a control IC for driving the switching element on the secondary winding side of the transformer.

It is a figure which shows the outline of the power supply circuit which concerns on the Example of this invention. It is a figure which shows the structure part regarding the load power supply and the power supply of control IC in the power supply circuit of the said Example. It is a figure which shows the outline of the power supply circuit which concerns on a prior art example. It is a figure which shows the structure part regarding the power supply of a load power supply and control IC in the power supply circuit of the said prior art example.

=== Configuration of Power Supply Circuit ===
FIG. 1 shows a power supply circuit according to an embodiment of the present invention. The power supply circuit 1a is a synchronous rectifier circuit on the secondary winding T side in a flyback converter type switching power supply or the like, similar to the power supply circuit 1b according to the conventional example shown in FIG. The voltage output to the line T side is assumed to be lower than the power supply voltage of the control IC 10. However, in the power supply circuit 1a according to the present embodiment, the power supply voltage of the control IC 10 can be secured without providing the auxiliary winding Ts on the secondary winding T side of the transformer. The configuration and operation of the power supply circuit 1a according to the embodiment of the present invention will be described below. Also in FIG. 1, since the terminals of the control IC 10 other than the power supply terminal 10vcc, the ground terminal 10gnd, and the output terminal 10out are unrelated to the essence of the present invention, their specific functions will be described. Is omitted.

  In the power supply circuit 1a of the present embodiment, the capacitor C1 connected in parallel to the main winding Tm of the secondary winding T repeatedly charges and discharges by the on / off operation of the switching element Q and flows through the main winding Tm. The current is converted into a direct current, which becomes the load power supply Vout. Further, the source S of the switching element Q made of MOSFET is connected to the ground GND. Thereby, the configuration from the secondary winding T to the load power supply Vout is a typical low-side synchronous rectifier circuit. That is, this component is the same as the power supply circuit 1b of the conventional example shown in FIG.

  On the other hand, the power supply circuit for the control IC 10 is greatly different from the configuration of the power supply circuit 1b according to the conventional example. First, there is no auxiliary winding Ts, and the secondary winding T is provided with only the main winding Tm. The cathode of the diode D2 is connected to the end of the secondary winding T on the load power supply Vout side, and the capacitor C4 is connected to the anode of the diode D2. The diode D2 and the capacitor C4 connected in series are connected in parallel between the two terminals that are both ends of the coil constituting the secondary winding T. That is, both the capacitor C4 and the capacitor C1 constituting the circuit for the load power supply Vout are configured to repeatedly charge and discharge by turning on and off the same switching element Q.

  Further, a diode D3, a resistor R3, and a capacitor C5 are connected in series in this order between the connection point P1 of the diode D2 and the capacitor C4 connected in series and the ground GND. A connection point P2 between the resistor R3 and the capacitor C5 is connected to the power supply terminal 10vcc of the control IC 10. A Zener diode ZD is forward-connected from the ground GND to the power supply terminal 10 vcc.

=== Boost Circuit for Control IC ===
FIG. 2 shows components related to the power supply (Vout, Vcc) to the load and the control IC 10 in the power supply circuit 1a shown in FIG. As described above, the secondary winding T of the transformer is formed of only the main winding Tm, and the load power supply Vout and the power supply Vcc of the control IC 10 are supplied from the secondary winding T. As can be seen by comparing FIG. 2 and FIG. 4, the circuit from the secondary winding T (main winding Tm) to the load power supply Vout in this embodiment has the same configuration as the power supply circuit 1b according to the conventional example. It has become.

  On the other hand, the power supply circuit for the control IC 10 in the present embodiment is greatly different in configuration from the conventional example. In order to reduce the size of the transformer, the auxiliary winding Ts of the secondary winding T is omitted even if the power supply voltage of the control IC 10 is higher than the voltage of the load power supply Vout. Instead, a booster circuit is configured in a path from the secondary winding T to the power supply terminal 10vcc of the control IC. Of course, if the reduction in size of the power supply circuit that can be achieved by omitting the auxiliary winding Ts of the transformer is offset by adding a booster circuit, or if the size of the power supply circuit is increased, it will end up falling. Therefore, in this embodiment, a charge pump circuit is employed as the booster circuit, and the switching element Q for obtaining the load power supply Vout is also used as the switching element Q for the booster circuit including the charge pump circuit CP.

  Specifically, as shown in FIG. 2, a diode D2 and a capacitor C4 connected in series are connected in parallel with the secondary winding T, and the capacitor C4 is connected to the load power supply Vout capacitor C2 from the MOSFET. It becomes the structure connected to the drain D of the switching element Q which becomes. Furthermore, a diode D3 and a resistor R3 connected in the forward direction from the connection point P1 between the diode D2 and the capacitor C4 connected in series toward the power supply output Vcc of the control IC 10 are connected in series. Further, a capacitor C5 is connected between an end point P2 on the power supply output Vcc side of the control IC 10 and the ground GND in the resistor R3. That is, an electrode on the ground GND side of the capacitor C5 is connected to the source S of the switching element Q made of a MOSFET and the ground GND. Thereby, a charge pump circuit CP including two sets of diodes and capacitors (D2 and C4, D3 and C5) connected in series is configured.

  The two capacitors (C4, C5) constituting the charge pump circuit CP are in a parallel state when the switching element Q is turned on, and are charged by the current flowing through the secondary winding T, and the switching element Q is turned off. Then, a serial connection state is established, and the sum of the charging voltages of the capacitors (C4, C5) becomes the power supply voltage of the control IC 10. The resistor R3 connected in series to the diode D3 on the rear stage side of the charge pump circuit CP, that is, the power supply output Vcc side of the control IC 10 is a current for limiting the current input to the power supply terminal 10vcc of the control IC 10. A resistor having a resistance value that functions as a limiting resistor and that allows a current required by the control IC 10 to flow is employed. Further, the Zener diode ZD is connected in the reverse direction from the power supply output Vcc of the control IC 10 to the ground GND, so that the voltage applied to the power supply terminal 10vcc does not become excessive.

  As described above, in the power supply circuit 1a of the present embodiment, even if the power supply voltage of the control IC 10 included in the circuit configuration on the secondary winding T side of the transformer is higher than the additional voltage output for addition, the secondary circuit There is no need to provide the auxiliary winding Ts in the winding T. Further, since the switching element Q driven by the control IC 10 functions as a switching element Q for the charge pump circuit CP for boosting the voltage of the load power supply to the power supply voltage of the control IC, the switching element Q is added separately. There is no need. As can be seen by comparing the power supply circuit (1a, 1b) of the embodiment shown in FIG. 1 and FIG. 3 with the conventional power supply circuit (1a, 1b), the power supply circuit 1a according to the present embodiment Instead of omitting the auxiliary winding Ts, substantially one diode D3, one capacitor C4, one resistor R3 and one Zener diode ZD are added, but these elements (D3, C4, R3 and ZD) are both extremely small and inexpensive. Therefore, compared to the mounting area required to provide the auxiliary winding Ts in the conventional power supply circuit 1b, it can be mounted in a much smaller area, and the size of the power supply circuit 1a can be greatly reduced. Become. It can also be expected to provide the power supply circuit 1a at a lower cost.

=== Other Embodiments ===
In the power supply circuit 1a according to the above-described embodiment, the mounting locations of the elements (D1, D2, C4, C5, R3) included in the booster circuit CP and the elements (zener diodes) ZD added to the conventional power supply circuit For example, in the booster circuit CP, elements (D2, C4) other than the elements (D2, C5) substantially equivalent to the elements (D1, C2) included in the power supply circuit 1b of the conventional example are included. If R3 and ZD) are mounted in the vicinity of the transformer, it is almost unnecessary to change the mounting positions of the printed wiring and individual elements for the same components as those of the conventional power supply circuit 1b. Therefore, an increase in initial cost due to the design change can be extremely reduced.

  In the power supply circuit 1a according to the embodiment, the switching element Q is a part of the synchronous rectifier circuit. However, if the switching element Q and the control IC 10 are included on the secondary winding T side of the transformer, the switching element Q is switched. The use of the element Q is not limited. Including the switching element Q on the secondary winding T side of the transformer and the control IC 10 that outputs a drive signal for the switching element Q, the power supplied between the two terminals at both ends of the secondary winding T is used as a load. On the other hand, as long as it is configured to output at a predetermined voltage value, it can be applied to a power supply circuit having any configuration.

  The present invention can be applied to, for example, an insulating flyback converter type switching power supply device.

1a, 1b power supply circuit, 10 control IC, 10gnd ground terminal,
10out output terminal, 10vcc power supply terminal, C1-C5 capacitor,
CP charge pump circuit, D1-D3 diode, GND ground,
Q switching element, R1-R3 resistance, T secondary winding, Tm main winding,
Ts Auxiliary winding, Vcc control IC power supply, Vout load power supply,
ZD Zener diode

Claims (4)

Power supplied between two terminals at both ends of the secondary winding, including a transformer, a switching element connected to the secondary winding of the transformer, and a control IC that outputs a drive signal for the switching element A power supply circuit that outputs a predetermined voltage value to a load,
The control IC operates using power supplied between the two terminals at both ends of the transformer as a power source, and a charge pump circuit is formed toward the two terminals and the power source terminal of the control IC.
The switching element also serves as a switching element constituting the charge pump circuit.
A power supply circuit characterized by that.   The power supply circuit according to claim 1, wherein the load power supply is a DC power supply rectified by a synchronous rectification circuit including the switching element.   3. The power supply circuit according to claim 1, wherein the predetermined voltage value of the load power supply is lower than a power supply voltage of the control IC.   3. The power supply circuit according to claim 1, wherein the switching element is a MOSFET, and the predetermined voltage value of the load power supply is lower than a recommended voltage value between a drain and a source of the MOSFET.

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