JP5204988B2 - Piezoelectric transformer insulation circuit - Google Patents

Description

  The present invention relates to a transformer circuit used in a high voltage generator such as a cold cathode tube (CCFL) drive device, a television receiver, an electronic copying machine, or a mobile phone, and more particularly, the primary side of the transformer. And an insulation circuit of a piezoelectric transformer for ensuring a secondary withstand voltage characteristic and preventing leakage current.

  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. 3 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. 3, 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 is 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, the primary side and the secondary side are electrically separated by a step-down electromagnetic transformer built in the switching power supply circuit 103 to ensure the insulation (withstand voltage characteristics).

  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 104.

  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 of poor efficiency. 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. 4, 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, thereby improving 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. 3, 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 not required in the backlight inverter 105 portion, and only a piezoelectric transformer is used, so that it is difficult to ensure withstand voltage characteristics.

  That is, as shown in FIG. 5, 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, unless the load current of the cold cathode tube 203 leaks to other routes, the resistance of the primary side and the secondary side Voltage characteristics 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.
Japanese Patent Laid-Open No. 10-200194 International Publication Number WO2006 / 088176

  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. 6, 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. 7, it is also required to arrange a plurality of cold cathode tubes 203 and 205 in parallel and to ground their return terminals secondary. . 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.

  In the circuit of FIG. 7, the outputs of the piezoelectric transformers 201 and 204 individually drive the cold cathode tubes 203 and 205 with respect to the secondary ground. C12 is a capacitor connected between the primary circuit and the secondary circuit, and is used for noise reduction of the power supply circuit.

  Certainly, even in the circuit of FIG. 7, when the two cold cathode tubes 203 and 205 are driven with a balanced voltage, the primary side and the secondary side of the piezoelectric transformer are similar to the circuit of FIG. 6. Insulation is ensured. However, in these circuits, even if the insulation (withstand voltage characteristics) relating to the load current is ensured, the insulation function relating 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. 7 as described above, the output of the piezoelectric transformer drives the two cold-cathode tubes individually with respect to the secondary ground, so that 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 In a piezoelectric transformer circuit for driving a cold cathode tube in which the low-voltage side of the cold cathode tube is grounded, an insulation circuit for the piezoelectric transformer is provided which reduces leakage current and secures insulation between the power supply side and the load side.

To achieve the above object, together with the piezoelectric transformer isolation circuit of the present invention is provided with two piezoelectric transformers, connecting the first and second primary terminals of the piezoelectric transformer to an AC power source, each The secondary terminal of the piezoelectric transformer is connected to the high-voltage side of the cold cathode tube, the AC power source and the primary terminal of each piezoelectric transformer are connected via a balancing transformer, and the primary side of each piezoelectric transformer and 2 Insulation is ensured between the secondary side and / or the primary side and the secondary side of the balance transformer, and an impedance for generating a virtual voltage source is connected to the balance transformer portion . The current flows in a direction to cancel a part of the leakage current, and the impedance suppresses the voltage of the virtual voltage source generated between the primary winding and the secondary winding of the balance transformer. And

  Further, the impedance in the insulation circuit of the piezoelectric transformer of the present invention is a resistance existing between the primary winding and the secondary winding of the balance transformer (for example, existing between the primary winding and the secondary winding). Insulation resistance, external resistance arranged to connect the primary and secondary windings, the combined resistance of the insulation resistance and external resistance), and between the primary and secondary windings It is characterized by the parallel impedance with the coupling capacitance.

  FIG. 1 is a diagram for explaining a virtual voltage source in the insulation circuit of the present invention having the above-described configuration. In FIG. 1, E1 is a voltage source for driving a load, E2 is a virtual voltage source, and the leakage current I3 approaches I3 = 0. PZT1 and PZT2 are piezoelectric transformers arranged in parallel to the load driving voltage source E1, I1 and I2 are output currents of the piezoelectric transformers PZT1 and PZT2, and V1 and V2 are output voltages of the piezoelectric transformers PZT1 and PZT2.

  Z1 and Z2 are equivalent load impedances of the load, and these equivalent load impedances Z1 and Z2 are represented by a combined (parallel) impedance of the resistance values RL1 and RL2 of the load and parasitic capacitances CL1 and CL2, and Z1 = RL1 // CL1, Let Z2 = RL2 // CL2.

  In the circuit configuration of FIG. 1, when the piezoelectric transformers PZT1 and PZT2 have the same characteristics and the equivalent load impedances Z1 and Z2 are equal and Z1 = Z2, the leakage current I3 = 0. This state is a state in which the circuit is balanced, and satisfies the insulation characteristics without leakage current flowing.

  When the equivalent load impedances Z1 and Z2 have deviations and have different values, or when the step-up ratio of the piezoelectric transformer is different between PZT1 and PZT2 and the circuit is out of balance, or Z1, Z2, PZT1, and PZT2 are both Assume that there is a deviation. Even in such an unbalanced state, by setting the value of the virtual voltage source E2 to a predetermined value, the output current of the piezoelectric transformers PZT1 and PZT2 can be balanced with I1 = I2, and the leakage current I3 = 0. It is possible.

By the way, in FIG. 1, assuming that I3 = 0 is realized by the presence of E2, the following equation is established. All the voltages are voltage values from the primary GND.

Further, assuming that the step-up ratios of the piezoelectric transformers PZT1 and PZT2 are n1 and n2, the output voltages V1 and V2 of the piezoelectric transformer are as follows.

Therefore, substituting V1 and V2 of Equation 2 into Equation 1 and solving for E2, the virtual voltage source E2 is as follows.

  Here, assuming that Vc = 0, the leakage current I3 = 0 can be realized by inserting the voltage source E2.

  According to the present invention, by arranging a virtual voltage source between the AC power source and the piezoelectric transformer, even if the secondary side of the piezoelectric transformer is grounded, the leakage current that flows to the grounded location such as the chassis is reduced. By canceling with the current from the virtual voltage source, it is possible to ensure insulation between the power supply side and the load side. Further, the withstand voltage characteristics on the power supply side and the load side can be ensured by a balance transformer or a piezoelectric transformer.

(1) First Embodiment Hereinafter, a first embodiment of the present invention will be described in detail with reference to FIG. In FIG. 2, reference numeral 1 is an AC power source, and 2, 3 are two piezoelectric transformers connected in parallel thereto, and the primary side of the piezoelectric transformer is grounded. A balance 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 balance transformer 4 includes a primary winding 4 a connected to the AC power source 1 and a secondary winding 4 b connected to the primary side terminals of the piezoelectric transformers 2 and 3. . In parallel with the balance transformer 4, an external resistor 7 is connected so as to connect the terminals of the primary winding 4a and the secondary winding 4b.

  In FIG. 2, a composite value of the external resistance and the insulation resistance between the primary winding 4a and the secondary winding 4b is indicated by Rt. Similarly, a coupling capacitance existing between the primary winding 4a and the secondary winding 4b is denoted by Ct. Accordingly, in this balance transformer 4 portion, there exists a parallel impedance Zt = Rt // Ct based on the combined value Rt of the resistors and the coupling capacitance Ct.

  2, CL1 and CL2 are electrostatic capacitances formed between the secondary side of the piezoelectric transformers 2 and 3 and a chassis as a grounding location, and C12 is between the primary circuit and the secondary circuit. It is a connected noise reduction capacitor.

In the first embodiment having the configuration as shown in FIG. 2, when the inter-terminal voltage of the parallel impedance Zt formed in the balance transformer 4 is E3, the following expression 4 is established from the expression 3. However, the impedance of the noise reducing capacitor C12 is Z12.

Here, if E3 >> Vc, Equation 4 can be written as Equation 5 below.

  As can be seen from Equation 5, the inter-terminal voltage E3 of the parallel impedance Zt is a voltage drop of Zt due to the leakage current I3. However, since it functions as the virtual voltage source E2, the leakage current I3 is generated from the virtual voltage source E2. The state is such that the canceling current I4 flows. Therefore, by increasing the parallel impedance Zt, the values of the output currents I1 and I2 approach I1 = I2 and are balanced. That is, the value of the leakage current I3 can be suppressed by increasing the combined resistance value Rt for increasing the parallel impedance Zt.

  As described above, according to the present embodiment, the leakage current flowing in the primary side and the secondary side can be effectively canceled out by paying attention to the impedance Zt based on the resistance value and the coupling capacitance provided in the balance transformer part. Thus, insulation can be ensured. In particular, when an external resistor is connected to the balance transformer, the withstand voltage of the supply can be ensured by the piezoelectric transformer, so that it is not necessary to ensure the withstand voltage by the balance transformer.

  As a result, the resistance value for determining the parallel impedance Zt can be easily increased without increasing the size of the balance transformer itself, and an increase in size of the device itself can be avoided. Further, as the balance transformer, a boosting transformer provided between the piezoelectric transformer and the AC power supply can also be used. In this case, there is an advantage that the number of parts can be reduced.

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

(1) In the embodiment of FIG. 2, an external resistor is provided in the balance transformer, and the parallel impedance Zt is determined based on the combined value of the external resistor and the insulation resistance of the balance transformer. When the insulation resistance value is large, the impedance can be determined only by the insulation resistance without providing an external resistor.

  In this case, it is necessary to increase the insulation resistance of the balance transformer, which increases the size of the transformer, but there is an advantage that an external resistor is not required. In addition, since a large transformer with a high insulation resistance value has a high dielectric strength, it contributes not only to preventing leakage current but also to improving the dielectric strength characteristics of the primary side and the secondary side. Therefore, without obtaining the dielectric strength characteristics at the piezoelectric transformer portion, it is possible to simultaneously achieve the dielectric strength characteristics and prevent the leakage current with this balance transformer alone.

(2) An external transformer is simply provided between the AC power supply and the piezoelectric transformer without secondary grounding of the low-voltage side of the cold cathode tube (or the transformer itself has a large insulation resistance and there is no need to provide an external transformer). A balance transformer is placed. 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) In FIG. 2, a plurality of piezoelectric transformers 2 and 3 are connected in parallel to the balance transformer 4 connected to the AC power source 1, but the plurality of piezoelectric transformers are connected to the secondary winding of the balance transformer. It can also be applied to circuits connected in series.

(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 four or more (even number).

The block diagram explaining the principle by which a leakage current is canceled by the virtual voltage source of this invention. 1 is a block diagram showing a first embodiment of the present 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, E1 ... AC power supply E2 ... Virtual voltage source 2, 3, PZT, PZT2 ... Piezoelectric transformer 4 ... Balance transformer 5, 6, RL1, RL2 ... Cold cathode tube CL1, CL2, C12 ... Capacitance I1 ... Piezoelectric transformer Current I2 flowing through 3 ... current I3 flowing through the piezoelectric transformer 4 ... leakage current I4 ... current flowing through the virtual voltage source

Claims (5)

With two piezoelectric transformers,
Step of connecting the first and second primary terminals of the piezoelectric transformer to an AC power source, the second terminal of the piezoelectric transformer connected to the high voltage side of the cold cathode tube,
The AC power supply and the primary terminal of each piezoelectric transformer are connected via a balance transformer,
Ensuring insulation between the primary and secondary sides of each piezoelectric transformer and / or the primary and secondary sides of the balance transformer;
Connect an impedance for generating a virtual voltage source to the balance transformer part ,
The virtual voltage source is configured to flow a current in a direction that cancels a part of the leakage current, and the impedance suppresses the voltage of the virtual voltage source generated between the primary winding and the secondary winding of the balance transformer. An insulation circuit for a piezoelectric transformer.   The impedance is a parallel impedance of a resistance existing between the primary winding and the secondary winding of the balance transformer and a coupling capacitance existing between the primary winding and the secondary winding. An insulation circuit for a piezoelectric transformer according to claim 1. Claims resistance present between the primary winding and the secondary winding of the balancing transformer, characterized in that an insulation resistance existing between the transformer primary and secondary windings for balance Item 3. An insulation circuit for a piezoelectric transformer according to Item 2 . The resistance existing between the primary winding and the secondary winding of the balancing transformer is an external resistor connected in parallel with the primary winding and the secondary winding of the balancing transformer. An insulation circuit for a piezoelectric transformer according to claim 2 . The resistance existing between the primary winding and the secondary winding of the balancing transformer is the insulation resistance existing between the primary winding and the secondary winding of the balancing transformer, and the primary of the balancing transformer. 3. The insulation circuit for a piezoelectric transformer according to claim 2 , wherein the insulation circuit is a combined resistance of a winding and an external resistor connected in parallel with the secondary winding.

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