JP5097609B2 - Booster circuit - Google Patents

Description

  The present invention relates to a booster circuit, and more particularly to a circuit for improving operation efficiency.

  In recent years, in portable electronic devices such as mobile phones and PDAs (personal digital assistants), white LEDs are frequently used as backlights for liquid crystal display units. In the case of 20 mA, a driving voltage of about 3.0V to 4.0V is required. Lithium ion batteries often used in portable electronic devices vary in their output voltage depending on the state of charge, and are usually about 2.7 V to 4.2 V, so that they can be used directly as a power source for obtaining the LED driving voltage described above. Since the battery voltage is not sufficient, the battery voltage is generally boosted by a booster circuit and supplied as a drive voltage.

  In addition, the light emission luminance of the LED is determined according to the value of the current flowing through the LED. However, when driving a plurality of LEDs, they are connected in series in order to equalize the light emission luminance. The forward voltage is multiplied by the number of LEDs connected in series, and the voltage is obtained by a booster circuit. Therefore, when many LEDs are driven in series using the battery voltage, a booster circuit capable of generating a higher boosted voltage is required.

As a boost circuit capable of obtaining a high boost ratio, a boost chopper method using an inductor and a switching element is generally known. However, in this method, the higher the boost voltage, the higher the withstand voltage rating required for the switching element, leading to an increase in the size and cost of the element.
Therefore, a booster circuit having the configuration shown in FIG. 3 is known as a circuit for solving such a problem.

In the booster circuit shown in FIG. 3, the primary winding n1 of the autotransformer T1 and the switching element Q1 are provided in series with the input power source 1A, while the intermediate tap and the ground of the autotransformer T1 are connected. The switching element Q1 is provided between them, and the boosted voltage VOUT is obtained on the secondary winding n2 side of the autotransformer T1 by switching of the switching element Q1.
Then, the boosted voltage VOUT is supplied to the plurality of LEDs 1 to LEDn connected in series, and the magnitude of the voltage drop generated in the load resistor R1 is compared with the voltage of the reference voltage source 4A in the error amplifier 3A, thereby switching control. By being input to the circuit 2A, a drive signal VG for controlling on / off of the switching element Q1 is output.

  However, when the autotransformer T1 is used, immediately after the switching element Q1 is turned off due to the leakage inductance generated between the primary winding n1 and the secondary winding n2, the switching element Q1 and the autotransformer T1 A surge voltage is generated at the connection point with the intermediate tap, and the voltage VD at this connection point changes as shown by the dotted line in FIG.

  Therefore, as a measure against such a surge voltage, as shown in FIG. 4, a snubber circuit 6 is provided to suppress the peak value of the surge voltage so as not to exceed the breakdown voltage between the drain and source of the switching element Q1. Generally, it is configured (see, for example, Patent Document 1).

The operation of the conventional booster circuit provided with the snubber circuit 6 will be generally described below with reference to FIGS.
First, when a predetermined drive signal VG is applied from the switching control circuit 2A to the gate of the switching element Q1 and the switching element Q1 is turned on (see FIGS. 5A and 5B), the voltage VIN is output. A current flows from the input power source 1A to the primary winding n1 of the autotransformer T1, and magnetic energy is accumulated.

  When the switching element Q1 is turned off after a predetermined time by the switching control circuit 2A, the magnetic energy stored in the autotransformer T1 is released as described above, and the voltage at the intermediate tap of the autotransformer T1 to which the switching element Q1 is connected. VD rises. When a MOS field effect transistor is used as the switching element Q1, the voltage VD is the drain voltage of the switching element Q1.

Simultaneously with the rise of the voltage VD, a current also flows through the secondary winding n2 due to the magnetic coupling effect of the autotransformer T1, and the voltage VA at the connection point between the secondary winding n2 of the autotransformer T1 and the diode D1 rises. (See FIGS. 5B and 5C).
The voltage VA is clamped to about the boosted voltage VOUT by the diode D1, but a surge voltage is generated at the connection point between the autotransformer T1 and the switching element Q1 due to the leakage inductance of the autotransformer T1.
However, the peak value of this surge voltage is suppressed by the snubber circuit 6 connected in parallel between the drain and source of the switching element Q1 (the solid line waveform indicated as “conventional example” in FIG. 5B). (See figure).
JP-A-6-113534 (page 3-4, FIGS. 1-8)

  However, when the snubber circuit as described above is used, while suppressing the peak value of the surge voltage as described above, power loss is caused at the rise and fall when the drain voltage of the switching element Q1 changes. There is a problem that the power efficiency is lowered.

  Further, in the conventional circuit shown in FIG. 4, since the diode D1 is reverse-biased when the switching element Q1 is turned on, the connection between one end of the secondary winding n2 of the autotransformer T1 and the diode D1. The point is open. As a result, immediately after the switching element Q1 is turned on, the voltage VA at the connection point between the one end of the secondary winding n2 of the autotransformer T1 and the diode D1 is not immediately stabilized, and a negative surge voltage is generated. (See FIG. 5C).

  Therefore, since the sum of the peak value of the surge voltage and the boosted voltage VOUT is applied to the diode D1, if the sum voltage exceeds the reverse breakdown voltage of the diode D1, the diode D1 may be destroyed. For this reason, when a diode having a high reverse breakdown voltage is used, there is a problem in that the size and cost of the device are increased.

  The present invention has been made in view of the above circumstances, and provides a booster circuit that can suppress the occurrence of unnecessary power loss while suppressing the peak value of a surge voltage.

In order to achieve the above object of the present invention, a booster circuit according to the present invention includes:
A booster configured such that a primary winding of an autotransformer and a switching element are connected in series to an input power supply, and a boosted voltage is obtained in the secondary winding of the autotransformer by switching of the switching element. A circuit,
A snubber circuit is provided between the intermediate tap of the autotransformer and one end of the secondary winding of the autotransformer.

According to the present invention, the peak voltage of one end of the secondary winding of the autotransformer is suppressed while suppressing the peak value of the surge voltage generated at the connection point between the switching element and the intermediate tap of the autotransformer when the switching element is turned on / off. Since extra power loss at the time of rising and falling is reduced, there is an effect that the operating efficiency is improved as compared with the conventional case.
Further, since the peak value of the negative surge voltage generated at one end of the secondary winding of the autotransformer can be suppressed, a diode having a lower reverse breakdown voltage can be used as a diode connected to one end of the secondary winding. In addition to reducing the component cost, the mounting space can be reduced.

Hereinafter, embodiments of the present invention will be described with reference to FIGS. 1 and 2.
The members and arrangements described below do not limit the present invention and can be variously modified within the scope of the gist of the present invention.
First, a configuration example of the booster circuit according to the embodiment of the present invention will be described with reference to FIG.
The booster circuit according to the embodiment of the present invention is an example of a circuit configuration suitable for generating drive voltages for a plurality of light-emitting diodes LED1 to LEDn connected in series. An autotransformer ("T1 in FIG. 7), the switching control circuit 2, the error amplifier 3, the switching element (indicated as “Q1” in FIG. 1) 11, and the snubber circuit 5 are configured as main components.

Such a booster circuit has basically the same circuit configuration as that of the prior art except for a connecting portion of a snubber circuit 5 described later.
More specifically, first, one end of the primary coil n1 of the autotransformer 7 is connected to the positive side of the input power source 1 that outputs the voltage VIN, and the negative side of the input power source 1 is connected to the ground. It has been made. An input smoothing capacitor (denoted as “C1” in FIG. 1) 12 is connected between one end of the primary side coil n1 of the autotransformer 7 and the ground.

The autotransformer 7 is formed by connecting a primary winding n1 and a secondary winding n2 in series, and the winding end of the primary winding n1 and the winding start of the secondary winding n2 are connected. It can be connected to the outside as an intermediate tap. Further, the winding start of the primary winding n1 and the winding end of the secondary winding n2 can be connected to the outside, respectively.
In the following description, for convenience of explanation, “one end of the primary winding n1” means one end of the primary winding n2 that can be connected to the outside, that is, in the embodiment of the present invention, the primary winding n1. The winding start of the winding n1 means "one end of the secondary winding n2", one end of the secondary winding n2 that can be connected to the outside, that is, in the embodiment of the present invention, the secondary winding n2. The end of the winding n2 is meant to mean each.

On the other hand, an anode of a diode (indicated as “D1” in FIG. 1) 16 is connected to one end of the secondary winding n2 of the autotransformer 7, and a plurality of diodes are connected between the cathode of the diode 16 and the ground. Light emitting diodes LED1 to LEDn and a load resistor (indicated as “R1” in FIG. 1) 13 are connected in series.
That is, the plurality of LEDs 1 to LEDn as loads of the booster circuit are connected in series, and the anode of LED 1 located on one end side of the series connection portion is connected to the cathode of the diode 16, while the other of the series connection portions A load resistor 13 is connected between the cathode of LEDn located on the end side and the ground.

An output smoothing capacitor 17 is connected between the cathode of the diode 16 and the ground.
On the other hand, the inverting input terminal of the error amplifier 3 is connected to the connection point between the LEDn and the load resistor 13, and the reference voltage source 4 is connected to the non-inverting input terminal of the error amplifier 3.
The output terminal of the error amplifier 3 is connected to the input stage of the switching control circuit 2.

The output stage of the switching control circuit 2 is connected to the control terminal of the switching element 11.
In the embodiment of the present invention, the switching element 11 is an N-channel MOS FET, and the output stage of the switching control circuit 2 is connected to a gate as a control terminal.
The drain of the switching element 11 is connected to the intermediate tap of the autotransformer 7, while the source is connected to the ground. As a result, the primary winding n1 and the switching element 11 of the autotransformer 7 are provided in series with the input power supply 1.

A snubber circuit 5 is connected between the intermediate tap of the autotransformer 7 and one end of the secondary winding n2.
That is, the snubber circuit 5 is configured by connecting a snubber resistor 14 and a snubber capacitor 15 in series, and one end of the snubber resistor 14 is connected to an intermediate tap of the autotransformer 7 as a switching element. 11, the snubber capacitor 15 has one end connected to one end of the secondary winding n <b> 2 of the autotransformer 7.

Next, the operation in the above configuration will be described with reference to the waveform diagram shown in FIG.
First, the booster circuit has a drive signal VG applied to the gate of the switching element 11 output from the switching control circuit 2 in accordance with the comparison result between the magnitude of the voltage drop in the load resistor 13 and the voltage of the reference voltage source 4. Is adjusted so that the booster circuit operates at a desired voltage by adjusting the on time and the off time of the switching element 11, and this operation is basically the same as that of the conventional circuit. It is.

  Thus, when the switching element 11 is turned on by the drive signal VG output from the switching control circuit 2 (see FIG. 2A), a current flows from the input power source 1 to the primary winding n1 of the autotransformer 1. Magnetic energy is accumulated in the next winding n1 (see FIGS. 2A and 2B). When the switching element 11 is turned off by the switching control circuit 2 after a predetermined time, the accumulated magnetic energy is released and the voltage VD of the intermediate tap rises (see FIGS. 2A and 2B). At the same time, due to the magnetic coupling effect of the autotransformer 7, a current flows through the secondary winding n2, the voltage VA at one end of the secondary winding n2 rises, the diode 16 conducts, and energy is output to the output smoothing capacitor 17. Is transmitted and the boosted voltage VOUT is obtained.

The boosted voltage VOUT is applied to the light emitting diodes LED1 to LEDn connected in series, and a current flows through the light emitting diodes LED1 to LEDn. The current is detected as a voltage drop in the load resistor 13, and the inverting input terminal of the error amplifier 3 To be input.
The voltage input to the inverting input terminal of the error amplifier 3 is compared with the voltage of the reference voltage source 4 at the non-inverting input terminal, and an output signal corresponding to the comparison result is output from the error amplifier 3 to the switching control circuit 2. . In the switching control circuit 2, the ratio of the on / off time of the switching element 11 is adjusted in accordance with the signal input from the error amplifier 3, and the switching element 11 is subjected to switching control.

In such an operation, when the switching element 11 is turned on / off, surge voltages are generated at the intermediate tap of the autotransformer 7 and one end of the secondary winding n2, respectively. Since it is absorbed by the snubber circuit 5 connected between one end of the next winding n2, unlike the conventional case, a surge voltage having a large peak value does not appear (see FIGS. 2A to 2C). ).
2B and 2C, the change in the voltage VD at the intermediate tap of the autotransformer T1 in the conventional circuit having the configuration shown in FIG. 4 and the secondary winding n2 of the autotransformer T1. A change in the voltage VA at one end is indicated by a dotted line, and “conventional example 2” is indicated in the vicinity of the waveform.

  At the time of absorption of the surge voltage by the snubber circuit 5, the two voltages VD and VA transition almost simultaneously at the rise and fall of the voltage VD of the intermediate tap and the voltage VA at one end of the secondary winding n2. Unlike the conventional case, the power loss in the circuit 5 is greatly reduced.

  Furthermore, since the diode 16 is reverse-biased when the switching element 11 is turned on, one end of the secondary winding n2 is in an open state, so that a negative surge voltage is generated at one end of the secondary winding n2. However, the peak value is also suppressed by the snubber circuit 5 (see FIG. 2C and FIG. 5C).

As described above, the snubber circuit 5 in the embodiment of the present invention suppresses the peak value of the surge voltage generated at the intermediate tap of the autotransformer 7 when the switching element 11 is turned on / off, and at one end of the secondary winding n2. The power loss at the rise and fall when the voltage VA changes is reduced, so that the booster circuit according to the embodiment of the present invention is improved in operating efficiency as compared with the conventional circuit. .
Further, by suppressing the peak value of the surge voltage generated at the intermediate tap of the autotransformer 7 as described above by the snubber circuit 5, it is possible to use a diode 16 having a lower reverse breakdown voltage as compared with the conventional diode. Therefore, the component cost can be reduced and the mounting space can be reduced by downsizing the diode 16.

It is a block diagram which shows the structural example of the booster circuit in embodiment of this invention. FIG. 2A is a waveform diagram of a main part of the booster circuit shown in FIG. 1, FIG. 2A is a waveform diagram showing a change in a drive signal applied to the gate of the switching element, and FIG. FIG. 2C is a waveform diagram showing a change in the voltage VA at one end of the secondary winding of the autotransformer. It is a circuit diagram which shows the 1st structural example of the conventional booster circuit. It is a circuit diagram which shows the 2nd structural example of the conventional booster circuit. FIG. 5A is a waveform diagram of the main part of the conventional booster circuit shown in FIG. 3 and FIG. 4, and FIG. 5A is a waveform diagram showing changes in the drive signal applied to the gate of the switching element, and FIG. ) Is a waveform diagram showing a change in the voltage VD of the intermediate tap of the autotransformer, and FIG. 5C is a waveform diagram showing a change in the voltage VA at one end of the secondary winding of the autotransformer.

Explanation of symbols

2 ... Switching control circuit 3 ... Error amplifier 5 ... Snubber circuit 7 ... Autotransformer 11 ... Switching element

Claims (1)

A booster configured such that a primary winding of an autotransformer and a switching element are connected in series to an input power supply, and a boosted voltage is obtained in the secondary winding of the autotransformer by switching of the switching element. A circuit,
A booster circuit comprising a snubber circuit provided between an intermediate tap of the autotransformer and one end of a secondary winding of the autotransformer.

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