JP2812235B2 - Resonant DC / DC converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resonance type DC / DC converter, and more particularly to a resonance type DC / DC converter suitable for a power supply of a fluorescent display device.

[0002]

2. Description of the Related Art A fluorescent display device that displays various information by exciting a fluorescent substance applied to an anode electrode enclosed in a vacuum vessel has a heater voltage for generating an electron beam, A driving voltage or the like is required to make this electron beam incident on a desired electrode, and such a power source is conventionally supplied by a converter that converts a DC voltage into a predetermined DC voltage and an AC voltage.

FIG. 7 shows a conventional switching power supply serving as a power supply for a fluorescent display device. Vin is a DC power supply (rectified output of a battery or commercial power supply), Ci is a smoothing capacitor, and Tr is a bipolar transistor. The switching element T is a primary winding N ₁ , a secondary winding N ₂ , and a tertiary winding N
₃ is an output transformer. The AC voltage of the tertiary winding N ₃ is output as the heater voltage Ef of the fluorescent display tube,
The secondary winding N ₂ diode D1 and a smoothing capacitor C0
After the full-wave rectification, the DC voltage is converted into a DC voltage, and supplied as a driving voltage Ebb (Ecc) to a driving IC circuit of a fluorescent display device (not shown).

The driving voltage is fed back to the base electrode of the switching transistor Tr via the Zener diode D _Z1 to form a switching voltage for controlling the switching element Tr on / off, and is also formed by the Zener diode D _Z2 and the resistor R. Limited to a given voltage,
This voltage Ek is applied to the center tap CT of the heater voltage Ef.
, A bias voltage is applied to the cathode electrode. Rs is the starting resistance, D
₂ and C _{1, r} and c are the diode and a capacitor for protection.

[0005]

The above-mentioned switching power supply can obtain various voltages for the fluorescent display device by a single converter. However, since the switching operation is of a flyback type, the switching frequency is increased. Then, there is a problem when miniaturizing each component. In other words, in the flyback method, a considerably large flyback pulse is generated, so that a harmonic component becomes large,
There is a problem in that interference noise is output to other electronic devices and the cost is increased due to the need to increase the withstand voltage of the electronic components.

[0006]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides an output transformer having a primary winding and a secondary winding wound at a predetermined winding ratio, and an output transformer for the output transformer. A resonance capacitor connected in parallel to the primary winding to form a resonance circuit; and first and second field-effect transistors connected in series between both terminals of the resonance circuit. A voltage feedback circuit is formed for each gate electrode of the second field-effect transistor to feed back the voltage at both ends of the resonance circuit so as to be alternately turned on / off. And a connection point between the first and second field-effect transistors and a drive power supply via a current-limiting inductor to provide a resonant DC / DC converter.

Since the resonance type DC / DC converter outputs a sine wave, excessive switching noise is suppressed, and a field effect transistor is inserted by inserting a limiting element for limiting a predetermined level into a feedback circuit. (Hereinafter, also referred to as a MOSFET) and can be a high-frequency oscillation circuit. Also, a tertiary winding is provided in the output transformer, and a rectified voltage of the secondary winding is superimposed and output, whereby a converter suitable for a fluorescent display device can be obtained.

[0008]

By connecting a resonance capacitor in parallel with the primary winding, a sine-wave oscillating circuit is formed. The generation of harmonics is reduced, and the voltage at both ends of the resonance circuit is directly fed back to switch the MOSFET. And the drive circuit can be simplified.
Further, since the switching frequency can be set high, downsizing of the core and winding of the output transformer and the electric field capacitor can be achieved, and in particular, the mounting area as a power source of the fluorescent display device can be reduced.

[0009]

FIG. 1 shows a basic circuit of a resonant DC / DC converter according to the present invention. In this figure, Vin is a direct current power source for driving, C ₁ is a capacitor, L is an inductor (coil) associated with the resonant frequency. T is an output transformer having a primary winding NP and a secondary winding NS, each having a center tap CT. A resonance capacitor C ₀ is connected in parallel to the primary winding NP to form a resonance circuit, and two field effect transistors (FETs) TR 1, TR 2 for flowing a drive current through the resonance circuit.
Is connected. An inductor L is connected between the connection point of the transistors TR1 and TR2 and the center tap of the primary winding.
Is connected to the driving DC power source Vin.

The voltage induced in the primary winding NP, that is, the resonance circuit, is mutually fed back to the gate electrodes of the two transistors so as to provide a positive feedback, thereby forming an oscillation circuit. That is, waveform diagrams A, D, and
As seen from E, M, and N, the voltage at the point E has a positive polarity during the period when the transistor TR1 is on, the voltage at the point D becomes 0, and the transistor TR2 is off. Also,
During the period in which the transistor TR2 is on, the voltage at the point D has a positive polarity, and the voltage at the point E becomes 0, thereby forming a push-pull voltage resonance type oscillation circuit in which the transistor TR1 is driven off. Therefore, transformer T 2
From the secondary winding, 2√2 proportional to the winding ratio NP / NS
An AC voltage having a peak-to-peak value of (NS / NP) × Vin is obtained. The resonance frequency f ₀ is f ₀ = _0.
Pai√LP 'set by _{C 0 (H Z), LP} ' denotes an inductance component of the transformer primary for one transistor.

In the above circuit, the gate electrode of each transistor is directly driven by the resonance sine wave voltage of the primary winding NP, so that no extra circuit element is required. Further, the switching voltage is also inverted at the zero point of the half-wave of the sine wave, so that the switching loss is reduced, and the switching voltage can be sufficiently put to practical use as a low power converter such as a fluorescent display device. However, when the frequency of the resonance circuit is increased to further reduce the size of the transformer or the capacitor, or when a short circuit or the like occurs on the load side and an abnormal voltage is induced, the level of the gate drive voltage is limited. For example, it is preferable to reduce the amount of heat generated by the transistor by driving with a voltage close to a rectangular wave.

FIG. 3 makes the converter of the present invention more secure.
In addition, this embodiment shows an embodiment capable of coping with a heavy load, and shows a method of limiting a gate drive voltage of a transistor. (A) of this figure shows that a resistor R1 is inserted in the gate electrode driving circuit. For example, an abnormal high voltage generated between D and E at the time of overload or output short circuit occurs between the gate and source of the FET. When applied, the voltage is V _GSJ
Can be prevented from breaking the field effect transistor (FET). (FET is 2SK1
(In the case of 772, V _GSJ is about 20 V.) Therefore, the larger the resistance is, the higher the safety is. Switching loss increases.
(May lead to thermal destruction)

In particular, when the switching frequency is set to be high, the resistance R1 must be set to a low value, but in this case, the protective effect on the gate electrode is not sufficient. Therefore, as shown in FIG. 3B, the Zener diode Dz1
Is provided between the gate and the source of the transistor, and the voltage applied to the gate is limited to a constant value to prevent the breakdown of the transistor. Alternatively, as shown in FIG. 3C, a varistor ZNR1 is inserted in parallel between the gate and the source to block an abnormal high voltage. Further, in order to protect the transistor from an abnormal voltage when the power is turned on, a second varistor ZNR2 may be added to the power supply as shown in FIG. By providing the protection circuit as described above, the present invention can be used as a converter for a higher switching frequency and a heavy load.

FIG. 4 shows a more detailed circuit configuration when the DC / DC converter of the present invention is used as a power supply for a fluorescent display device. In this figure, the same parts as those in FIG. 1 are denoted by the same reference numerals. That is, the output transformer T is a primary winding N1a, and the secondary side of the output transformer T is a secondary winding N2a for outputting the heater voltage of the fluorescent display tube, and for supplying the fluorescent display tube anode and bias voltage. A secondary winding N2b for outputting a DC voltage is provided.

[0015] Voltage is applied to the anode is full-wave rectified by the secondary winding N2b diode D _1, D ₂ Ebb
Over Ecc is output from the smoothing capacitor C _3. Also,
This voltage Ebb-Ecc is divided by the resistor R1 and the Zener diode Dz1, and connected to the center tap CT of the secondary winding N2a as a bias voltage for the cathode. Accordingly, a positive bias voltage is applied to the filament voltage Ef to a drive IC circuit for the fluorescent display tube described later.

If a bipolar transistor is used as a switching element of the DC / DC converter, a fourth winding for driving the bipolar transistor for the output transformer T is required,
The transformer cannot be reduced in size and simplified.

As shown in FIG. 5, in general, when the switching frequency of both the bipolar transistor and the field-effect transistor becomes high, when the switching timing of on / off is FET, the bipolar transistor of FIG. In the case of (1), a time difference of TV is generated as shown in FIG. 5 (b). This difference is caused by a delay on the rising side in the case of the FET and a delay on the falling point in the case of the bipolar transistor. Therefore,
In the case of the bipolar transistor shown in FIG. 5B, there is a problem that a time during which both are turned on occurs during the period of Tv, a harmonic component is generated in the oscillation waveform, and the heat generation increases during this period. This problem can be avoided in the case of the FET shown in FIG.

FIGS. 6A and 6B show an example of a circuit when the DC / DC converter of the present invention is used as a power supply for a fluorescent display device. FIG. 6A shows a filament negative power supply system, and FIG. 6B shows a filament positive power supply system. Supply circuit. The portion of the resonant DC / DC converter of the present invention is shown as a converter unit 10, and its internal circuit is almost the same as that of FIG. 4, and a detailed description thereof will be omitted. Also, 20
Denotes an IC circuit for driving the display tube 30 (drive control unit CPU)
Depending on the type of the IC circuit, a circuit of a positive power supply system and a circuit of a negative power supply system are employed. In addition, each point A, B, J,
When the potentials of I, K, and L are at voltage levels Ek, Vbc, Ebb,
S (v) (5V), shown as Ef. IC circuit 20
Although not shown, according to the number of electrodes of the fluorescent display tube,
A signal terminal for inputting information for display as data and a signal input terminal for operation are provided. In addition, the fluorescent display tube 3
Numeral 0 denotes a heater in which a heater H, a grid electrode G, and an anode electrode P coated with a fluorescent substance are arranged in a segmented manner or a dot matrix manner in a vacuum vessel. 3 shows a display device.

[0019]

As described above, the resonance type D of the present invention is used.
In the C / DC converter, a field-effect transistor is push-pull connected to form a resonance type oscillation circuit, and a DC voltage and an AC voltage can be obtained from an output transformer thereof. The transformer can be formed small and the number of windings can be reduced to reduce the lamination area. In addition, since the withstand voltage and the capacitance of the capacitor can be set small, there is an effect that the DC-DC converter can be reduced in size and the mounting area can be reduced particularly when configuring the power supply of the fluorescent display device. Further, since power conversion can be performed by the resonance circuit, switching of the transistor is performed at the zero cross point, so that the withstand voltage of the transistor serving as a switching element can be reduced. Furthermore, since heat generation is suppressed, safety and reliability are improved, and a power supply suitable for a fluorescent display tube with a small power supply can be provided.

[Brief description of the drawings]

FIG. 1 is a basic circuit configuration diagram of a resonance type DC / DC converter of the present invention.

FIG. 2 is a waveform diagram of each point in FIG.

FIG. 3 is a circuit diagram showing a modification of the resonance type DC / DC converter of the present invention.

FIG. 4 is a circuit diagram showing an embodiment when the present invention is used as a power supply for fluorescent display.

FIG. 5 is an explanatory diagram of switching timing of a transistor.

FIG. 6 is a circuit diagram showing a connection example between a fluorescent display device and a DC / DC converter.

FIG. 7 is a diagram showing an example of a conventional converter circuit for fluorescent display.

[Explanation of symbols]

L Inductor C ₀ resonant capacitor TR1 TR2 MOSFET T output transformer Vin DC power supply 10 converter - motor unit 20 IC circuit 30 display tube

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI H02M 7/538 H02M 7/538 Z 7/5383 7/5383 (56) References JP-A-6-153532 (JP, A) JP-A-1-91673 (JP, A) JP-A-60-70973 (JP, A) JP-A-56-51493 (JP, U) JP-A-60-41988 (JP, U) JP-A-52-114222 (JP, A) JP, U) JP-A 4-101288 (JP, U) JP-B-36-4589 (JP, B1) (58) Fields investigated (Int. Cl. ⁶ , DB name) H02M 7/42-7/98 H02M 3/00-3/44 G09G 3/12 301 G09G 3/30 301

Claims (5)

(57) [Claims] 1. An output transformer having a primary winding and a secondary winding wound at a predetermined winding ratio, and a resonance capacitor connected in parallel to the primary winding of the output transformer to form a resonance circuit. And first and second field-effect transistors connected in series between both terminals of the resonance circuit, wherein the gate electrodes of the first and second field-effect transistors are connected in parallel with each other. A voltage feedback circuit for feeding back the voltage between both ends of the resonance circuit is formed so as to be alternately turned on / off, and the intermediate point of the primary winding and the first and second field effect transistors A resonance type DC / DC converter, wherein a driving power is supplied to a connection point via an inductor. 2. The resonant DC / DC converter according to claim 1, wherein a limiting element for limiting a feedback voltage to a predetermined level is inserted in the feedback circuit. 3. The resonance type DC / DC according to claim 1, wherein a tertiary winding is provided in the output transformer, and a rectified voltage of the secondary winding is superimposed and output.
converter. 4. A heater voltage of a fluorescent display tube and a DC drive voltage of a fluorescent display device are output from a secondary winding of an output transformer of the resonance type DC / DC converter. Or the resonance type DC / DC converter according to 3. 5. The resonance type DC / DC converter according to claim 4, wherein a voltage obtained by dividing the DC output voltage is applied to a center tap of a secondary winding for outputting the heater voltage.
DC converter.