JP2008118816A - Insulating circuit of piezoelectric transformer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an insulating circuit of a piezoelectric transformer, in which leakage current flowing between primary and secondary terminals is reduced. <P>SOLUTION: A balun transformer 4 is arranged between an AC power supply 1 and piezoelectric transformers 2 and 3. A high voltage side of cold-cathode tubes 5 and 6 is connected to each secondary terminal of the piezoelectric transformers 2 and 3, and a low voltage side of a plurality of the cold cathode tubes 5 and 6 is secondarily grounded to a chassis of a unit in parallel. Since currents in opposite directions in a balance state flow in first and second windings 4a and 4b of the balun transformer 4, the windings 4a and 4b constituting the balun transformer 4 do not resist the currents flowing in the AC power supply 1 and the piezoelectric transformers 2 and 3. Since leakage current is not balanced like the current between the piezoelectric transformers 2 and 3 and the AC power supply 1, it is affected by inductance L of the windings 4a and 4b of the balun transformer 4 and the windings 4a and 4b of the balun transformer 4 cuts off leakage current. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a piezoelectric transformer used in a high-voltage generator such as a cold cathode tube (CCFL) driving device, a television receiver, an electronic copying machine, and a mobile phone, and more particularly, the primary of the piezoelectric transformer. The present invention relates to an insulating circuit for insulating a side and a secondary side.

  A piezoelectric transformer uses a resonance phenomenon of a piezoelectric vibrator to input a low voltage and output a high voltage. Compared with an electromagnetic transformer, the piezoelectric vibrator has a higher energy density and can be downsized. It has the feature of being possible. Therefore, it is used for lighting a cold cathode tube or a liquid crystal backlight, or for a small high voltage power source. For example, Patent Document 1 discloses a technique that uses a piezoelectric transformer as a backlight inverter for lighting a cold-cathode tube for a backlight of a liquid crystal television.

  FIG. 4 is a functional block diagram showing an example of a liquid crystal television using this type of backlight inverter (piezoelectric inverter). In the figure, 101 is an alternating current (AC) power supply, 102 is a power factor correction circuit (PFC), 103 is a switching power supply circuit (PS), 104 is a television circuit including a tuner and an audio circuit, 105 is a backlight inverter, 106 is It is a cold cathode tube.

  In FIG. 4, the predetermined voltage input from the AC power supply 101 is boosted to a predetermined voltage (for example, 400 V) by the power factor improvement circuit 102 and the power factor is improved and input to the switching power supply circuit 103. . In the switching power supply circuit 103, the boosted voltage is lowered to a low voltage (for example, 12 V) for driving the television circuit, and this is input to the television circuit 104 and the backlight inverter 105.

  In this case, insulation is ensured by electrically separating the primary side and the secondary side by a step-down electromagnetic transformer incorporated in the switching power supply circuit 103.

  In the backlight inverter 105, the low voltage dropped by the switching power supply circuit 103 is boosted by an electromagnetic transformer that constitutes the inverter, and then the voltage is further increased by a piezoelectric transformer (for example, 1000V). The cold cathode tube 106 is lit by applying to the cold cathode tube 106. On the other hand, the low voltage input to the television circuit 104 is used as a power source for driving each device in the television circuit 105.

  In the conventional technique as described above, the voltage is raised to a high voltage of 1000 V after being lowered to a low voltage of 12 V, for example, by the switching power supply circuit 103, so that a boost ratio of about 100 times is required. However, when a single-plate piezoelectric transformer is used alone, the step-up ratio is insufficient, so that an electromagnetic transformer for boosting is required and there is a problem that efficiency is poor. On the other hand, when a laminated piezoelectric transformer is used, the voltage can be boosted independently, but there is a drawback that a complicated and expensive piezoelectric transformer is required.

  Therefore, as shown in FIG. 5, a circuit has been proposed in which the output voltage of the power factor correction circuit 102 is directly input to the backlight inverter 105 without going through the step-down switching power supply circuit 103 to improve efficiency. Yes. In this circuit, since the output voltage (for example, 400 V) of the power factor correction circuit 102 is input to the backlight inverter 105, the step-up ratio is about 2.5 times, and a single-plate piezoelectric transformer is used. Can be used, and an electromagnetic transformer that has been conventionally required is not required, and the loss in the electromagnetic transformer and the switching power supply circuit 103 is eliminated, thereby improving the efficiency.

  However, this circuit has a problem that the primary side and the secondary side of the piezoelectric transformer provided in the inverter cannot be insulated because the output voltage of the power factor correction circuit 102 is directly input to the backlight inverter 105. In other words, in electronic devices such as television receivers and personal computers, the principle is to insulate the power supply from the equipment used by stepping down or boosting it from the viewpoint of user safety. The primary and secondary sides of the step-up / down transformer are insulated.

  In this case, in the circuit shown in FIG. 4, the primary side and the secondary side can be insulated by the electromagnetic transformer built in the switching power supply circuit 103, but the circuit shown in FIG. In this case, an electromagnetic transformer is unnecessary in the backlight inverter 105 portion, and only a piezoelectric transformer is used, so that it is difficult to insulate.

  That is, as shown in FIG. 6, the piezoelectric transformer 201 has a three-terminal structure having a pair of primary electrodes 201 a and 201 b connected to a power source 202 and a secondary electrode 201 c connected to a cold cathode tube 203. . Therefore, the return terminal 203a of the cold cathode tube 203 is shared with one primary electrode 201b of the piezoelectric transformer 201, and the primary side and the secondary side of the piezoelectric transformer 201 cannot be insulated.

  In order to improve this, the present applicant has proposed a circuit shown in FIG. In this circuit, the first and second piezoelectric transformers 201 and 204 are connected in series on the primary electrode side, and the return terminal of the cold cathode tube 203 is connected to the secondary electrode 204c of the second piezoelectric transformer 204. Is.

  In this circuit, since the return terminal of the secondary side terminal is independent of the primary side terminal, the insulation between the primary side and the secondary side is performed unless the load current of the cold cathode tube 203 leaks to another route. Can be secured. In this case, the outputs of the piezoelectric transformers 201 and 204 drive the cold cathode tube 203 with a balanced voltage with respect to a secondary ground such as a chassis of an electronic device.

  By the way, recently, with the increase in size of liquid crystal televisions and the like, the size of the cold cathode tube 203 has also been increased. In the circuit of FIG. 7, the cold cathode tube 203 can be a U-shaped tube, or two cold cathode tubes arranged in parallel can be connected in series to cope with an increase in size.

  However, in a larger television receiver or the like, as shown in FIG. 8, it is also required that a plurality of cold-cathode tubes 203 and 205 are arranged in parallel and the return terminal thereof is secondarily grounded. . The reason is that a connector and a terminal block are required when connecting to an adjacent cold cathode tube, but in the case of a chassis, only the processing of the terminal is required. By forcing the low voltage side of the cold cathode tube to the GND potential. This is because there is a demand that the surrounding noise level is reduced.

As a result, in the circuit of FIG. 8, the outputs of the piezoelectric transformers 201 and 204 individually drive the cold cathode tubes 203 and 205 with respect to the secondary ground.
JP-A-10-200194 International Publication Number WO2006 / 088176

  Indeed, even in the circuit of FIG. 8, when the two cold cathode tubes 201 and 204 are driven with a balanced voltage, the primary side and the secondary side of the piezoelectric transformer are the same as in the circuit of FIG. Insulation is ensured. However, in these circuits, even if the insulation related to the load current is ensured, the insulation function related to the leakage current (leakage current) generated according to the state of the load is not necessarily sufficient. This leakage current needs to be suppressed within a certain value from the safety standard of electronic equipment, and if it exceeds that value, there is a risk of electric shock, so it is not regarded as an insulating structure in the standard.

  That is, in the circuit of FIG. 8 as described above, the output of the piezoelectric transformer drives the two cold-cathode tubes individually with respect to the secondary ground, so the difference between the impedances of the two cold-cathode tubes is different. If there is a difference, a difference occurs in the tube currents ((a) and (b) in the figure) of the cold cathode tubes 203 and 205, and the difference current becomes a leakage current.

  In addition, as a reason that the impedances between a plurality of cold cathode tubes are different, a deviation in internal gas pressure or a deviation in mercury amount can be considered. In a large television, the temperature varies depending on the position of the screen (upper and lower), so the ambient temperature varies depending on the position where the cold cathode fluorescent lamp is also mounted. Since the cold cathode tube has temperature characteristics, the impedance is different.

  In addition, the currents flowing in the capacitances 206 and 207 between the piezoelectric transformer and the cold cathode tube and the chassis ((c) and (d) in the figure) are shifted in phase by 180 ° between the two piezoelectric transformers 201 and 204. Therefore, in principle, it will not be a leakage current, but if there is a deviation in the capacitance between the two piezoelectric transformers and the chassis, the current magnitudes in (c) and (d) will also differ. Current flows.

  The present invention has been proposed in order to solve the above-described problems of the prior art, and its purpose is to connect a cold cathode tube to a secondary terminal of a piezoelectric transformer connected to a power source, and An object of the present invention is to provide an insulation circuit for a piezoelectric transformer capable of insulating a primary side and a secondary side of a piezoelectric transformer in a piezoelectric transformer for driving a cold cathode tube with a low voltage side of the cold cathode tube grounded.

  Another object of the present invention includes a plurality of piezoelectric transformers connected to a power source and a plurality of cold cathode tubes connected to secondary terminals of the plurality of piezoelectric transformers. An object of the present invention is to provide an insulation circuit for a piezoelectric transformer which can insulate a primary side and a secondary side of a piezoelectric transformer in a piezoelectric transformer for driving a cold cathode tube grounded in parallel.

  In order to achieve the above object, an insulation circuit for a piezoelectric transformer according to the present invention connects the first and second primary terminals of a piezoelectric transformer to an AC power source, and connects the secondary terminal of the piezoelectric transformer to a high voltage of a cold cathode tube. A balun transformer having a first winding and a second winding disposed opposite to each other between the AC power source and the piezoelectric transformer, and connecting one end of the first winding The other end is connected to the AC power supply, the other end is connected to the first primary terminal of the piezoelectric transformer, one end of the second winding is connected to the AC power supply, and the other end is connected to the second primary terminal of the piezoelectric transformer. And

  The insulation circuit for a piezoelectric transformer according to the present invention includes a plurality of piezoelectric transformers connected in series via the first and second primary terminals, and the first primary terminal of the leading piezoelectric transformer is used as an AC power source. And the second primary terminal of the last piezoelectric transformer is connected to an AC power source, and the secondary terminals of the plurality of piezoelectric transformers are connected to the high-voltage side of a cold cathode tube, respectively, A balun transformer having a first winding and a second winding arranged opposite to each other is arranged between the piezoelectric transformer, one end of the first winding serving as an AC power source, and the other end serving as a head. The first primary terminal of the piezoelectric transformer is connected, one end of the second winding is connected to an AC power source, and the other end is connected to the second primary terminal of the last piezoelectric transformer.

  It is also an aspect of the present invention that the low-voltage side of the cold cathode tube connected to the piezoelectric transformer is secondarily grounded.

  According to the present invention, by disposing a balun transformer between the AC power source and the piezoelectric transformer, even when the secondary side is grounded, the leakage current flowing in the grounded portion such as the chassis is not affected by the winding of the balun transformer. By using the inductance of the piezoelectric transformer, insulation between the primary side and the secondary side of the piezoelectric transformer can be ensured.

  In particular, a balanced current between the piezoelectric transformer and the AC power source flows through the balun transformer, and the magnetic flux generated by the first and second windings of the balun transformer is canceled by this balanced current. Reduced. As a result, the efficiency of iron loss can be improved even if the copper loss has a similar value as compared with an insulation circuit using a conventional electromagnetic transformer.

(1) First Embodiment Hereinafter, a first embodiment of the present invention will be described in detail with reference to FIG. In FIG. 1, reference numeral 1 is an AC power source, and 2, 3 are two piezoelectric transformers connected thereto, and the primary side of the piezoelectric transformer is grounded. A balun transformer 4 is disposed between the AC power source 1 and the piezoelectric transformers 2 and 3. The secondary terminals of the plurality of piezoelectric transformers 2 and 3 are connected to the high-voltage side of the cold-cathode tubes 5 and 6, respectively, and the low-voltage side of the plurality of cold-cathode tubes 5 and 6 is connected in parallel to the chassis of the device. Next, it is grounded to form a cold cathode tube drive circuit.

  More specifically, the two piezoelectric transformers 2 and 3 are connected in series via the first and second primary terminals, and the first primary terminal of the piezoelectric transformer 2 is connected to the AC power source 1. The second primary terminal of the piezoelectric transformer 3 is connected to the AC power source 1.

  The balun transformer 4 includes a first winding 4a and a second winding 4b that are arranged to face each other between an AC power source and a plurality of piezoelectric transformers. One end of the first winding 4a of the balun transformer 4 is connected to the AC power source 1, the other end is connected to the first primary terminal of the piezoelectric transformer 2, and one end of the second winding 4b is connected to the AC power source. Is connected to the second primary terminal of the piezoelectric transformer 3.

  In FIG. 1, reference numerals 7 and 8 denote electrostatic capacitances formed between the secondary side of the piezoelectric transformers 2 and 3 and a chassis or the like that is a ground location.

  In the first embodiment having the above-described configuration, on the primary side of the piezoelectric transformers 2 and 3, the AC power source 1-the first winding 4 a of the balun transformer 4 -the primary terminal of the piezoelectric transformers 2 and 3- A circuit composed of the second winding 4b of the balun transformer 4 and the AC power source 1 is formed.

  In this case, a current in the opposite direction in an equilibrium state flows through the first and second windings 4a and 4b of the balun transformer 4, and the magnetic flux generated in both windings cancels each other. The windings 4 a and 4 b that do not become an inductance component for the current flowing through the AC power source 1 and the piezoelectric transformers 2 and 3. Therefore, the AC transformer 1 can excite the piezoelectric transformers 2 and 3 to function as an inverter and light the cold cathode tubes 5 and 6.

  On the other hand, since the cold-cathode tubes 5 and 6 are grounded on the secondary side of the piezoelectric transformers 2 and 3, there is a possibility that leakage current may occur as described above. This leakage current flows into the balun transformer 4 through the secondary ground and the primary ground, but this leakage current is not in an equilibrium state like the current between the piezoelectric transformer 2 and the 3-AC power supply 1. , Affected by the inductance L of the windings 4a and 4b of the balun transformer 4. As a result, since the windings 4a and 4b of the balun transformer 4 block the leakage current, insulation between the primary side and the secondary side of the piezoelectric transformers 2 and 3 can be ensured.

  In this case, the balun transformer 4 acts so that the divided value of the secondary side voltage of the two piezoelectric transformers 2 and 3 becomes the secondary ground. The partial pressure value can be considered as a combined impedance of the capacitances 7 and 8 with the chassis which is the grounding location and the cold cathode fluorescent tubes 5 and 6, and theoretically if the inductance L of the balun transformer 4 is L = ∞. No leakage current will flow. Actually, the inductance of the winding of the balun transformer 4 is determined in consideration of the magnitude of the leakage current that will be generated in the actual circuit and the magnitude of the leakage current that is allowed for safety measures.

(2) Other Embodiments The present invention is not limited to the embodiment as described above, and includes the following other embodiments.

(1) As shown in FIG. 2, in a drive circuit for a single piezoelectric transformer and a cold cathode tube connected thereto, even when the low voltage side of the cold cathode tube is grounded, the AC power source and the piezoelectric transformer are not connected. A balun transformer can be placed in

(2) A balun transformer is simply placed between the AC power supply and the piezoelectric transformer without secondary grounding of the low-voltage side of the cold cathode tube. In this case, it is only possible to block the leakage current flowing through the electrostatic capacitance generated between the secondary side of the piezoelectric transformer and the chassis.

(3) As shown in FIG. 3, a plurality of piezoelectric transformers 2 and 3 are connected in parallel to a balun transformer 4 connected to an AC power source 1.

(4) In the circuit of the first embodiment or (2) (3), the number of piezoelectric transformers and cold cathode tubes connected to the AC power supply is three or more.

1 is a block diagram showing a first embodiment of the present invention. The block diagram which shows other embodiment of this invention. The block diagram which shows other embodiment of this invention. The block diagram which shows the structure of the conventional television receiver etc. which connected the backlight inverter to the switching power supply circuit. The block diagram which shows the structure of the television receiver etc. which connected the backlight inverter to the power factor improvement circuit. The circuit diagram which shows the reason why insulation structure cannot be ensured with a piezoelectric transformer itself. The circuit diagram of the backlight inverter which secured the insulation by connecting two piezoelectric transformers and one cold cathode tube. The circuit diagram of the backlight inverter which shows the reason why insulation cannot be ensured when two piezoelectric transformers and two cold cathode tubes are connected and secondarily installed.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... AC power source 2, 3 ... Piezoelectric transformer 4 ... Balun transformer 5, 6 ... Cold cathode tube 7, 8 ... Electrostatic capacity

Claims (3)

The first and second primary terminals of the piezoelectric transformer are connected to an AC power source and the secondary terminal of the piezoelectric transformer is connected to the high voltage side of the cold cathode tube,
A balun transformer having a first winding and a second winding arranged opposite to each other is arranged between the AC power source and the piezoelectric transformer, and one end of the first winding is used as an AC power source, and the like. One end of the second winding is connected to the AC power source and the other end is connected to the second primary terminal of the piezoelectric transformer. Insulation circuit. A plurality of piezoelectric transformers are connected in series via the first and second primary terminals, the first primary terminal of the first piezoelectric transformer is connected to an AC power source, and the second first of the last piezoelectric transformer is connected. A secondary terminal connected to an AC power source, each secondary terminal of the plurality of piezoelectric transformers connected to the high voltage side of the cold cathode tube,
A balun transformer having a first winding and a second winding arranged opposite to each other between the AC power supply and the plurality of piezoelectric transformers is arranged, and one end of the first winding is used as the AC power supply. The other end is connected to the first primary terminal of the first piezoelectric transformer, one end of the second winding is connected to the AC power source, and the other end is connected to the second primary terminal of the last piezoelectric transformer. An insulation circuit for a piezoelectric transformer.   3. The insulation circuit for a piezoelectric transformer according to claim 1, wherein the cold cathode tube is a secondary grounded low voltage side.

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