JPH0613395U - Power supply for fluorescent light - Google Patents

Abstract

(57) [Abstract] [Purpose] By using FET (Field Effect Transistor) as the main switching element, even if the transformer is saturated without turning on the fluorescent lamp due to deterioration of the fluorescent lamp or forgetting to insert it, the main The breakdown of the switching element can be avoided and the reliability is improved. [Structure] A pair of FETs Q5 and Q6 that are connected in a pull-up manner to a pair of primary windings of a transformer T, and an auxiliary transistor Q that is connected between the gate and source of each FET.
3, Q4, and a current detection resistor R3 inserted on the source side of the FET, and the auxiliary transistor when the detection voltage across the current detection resistor proportional to the drain current of the FET reaches or exceeds a set value. To make the F
This is a configuration in which ETs are switched alternately.

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to a fluorescent lamp power source for lighting a fluorescent lamp used in a copying machine or a facsimile, or a household fluorescent lamp at a high frequency.

[0002]

[Prior art]

 Recently, there has been a strong demand for downsizing and cost reduction of power supplies for fluorescent lamps. Therefore, high-frequency switching and self-excited oscillation power supplies have come to be used.

FIG. 3 shows a power supply for a fluorescent lamp of this type proposed by the present applicant, in which the transformer primary side is a self-excited push-pull circuit. In this figure, A 1 and B 2 are input terminals connected to a commercial power source, and R 1, D 1 and C 1 are a current limiting resistor, a diode bridge and a capacitor which respectively form a rectifying circuit (direct current power source). T is a transformer, N1a and N1b are the primary winding (main winding) of the transformer T that makes a pair, N2 is a secondary winding, and N3a and N3b are a pair of feedback windings (equivalent to the feedback winding with a midpoint tap). ). One end of each of the two primary windings N1a and N1b is connected to a positive end 3 which is one output end of the rectifier circuit, and the other end of each primary winding N1a and N1b is connected to a main transistor (bipolar transistor) Q1. , Q2, respectively, and the emitters of the main transistors Q1, Q2 are connected to the negative side end 4, which is the other output end of the rectifier circuit, via the common current detection resistor R3. The base of the main transistor Q1 is connected to one end of the feedback winding N3a via current limiting resistors R6 and R7, and the base of the main transistor Q2 is connected to one end of the feedback winding N3b by a current limiting resistor R8. , R9.

Q3 and Q4 are auxiliary transistors inserted between the bases of the main transistors Q1 and Q2 and the negative side terminal 4, respectively, and are mainly used when the current flowing through the current detection resistor R3 reaches a certain set value. It also serves to turn off the transistors Q1 and Q2. D4 and D5 are diodes inserted between the emitters of the main transistors Q1 and Q2 and the bases of the auxiliary transistors Q3 and Q4, respectively, and R4 and R5 are between the bases of the auxiliary transistors Q3 and Q4 and the negative side end 4, respectively. These resistors are inserted so that they are provided so that the main transistors Q1 and Q2 have good off characteristics, and they are not always necessary.

The resistor R2 is a starting resistor that connects the positive side end 3 of the rectifier circuit and a common tap (intermediate tap) 7 of the feedback windings N3a and N3b, and is an emitter of the main transistors Q1 and Q2. The diode D6 provided between the tap 7 and the tap 7 constitutes a path through which the base current of the main transistors Q1 and Q2 flows, and the bias current flowing through the starting resistor R2 is negative through the current detecting resistor R3. It serves to prevent the flow toward the side edge 4.

D2 and D3 are diodes connected in parallel and in opposite polarities to the series circuit of the main transistors Q1 and Q2 and the resistor R3, respectively, and are currents due to the counter electromotive force generated when the main transistors Q1 and Q2 are off. To bypass.

The fluorescent lamp 1 is connected to the secondary winding N2 of the transformer T via a choke coil L1, and the filament 2 of the fluorescent lamp 1 is energized for a predetermined period when the fluorescent lamp 1 is initially turned on. Switch SW is provided.

Next, the operation of the above conventional circuit will be described. When a direct current voltage is generated between the positive and negative terminals 3 and 4 of the rectifier circuit by turning on a power switch (not shown), it passes through the starting resistor R2 and further the feedback winding N3a and the resistors R6 and R7 or the feedback winding N3b. A base current flows through the main transistors Q1 and Q2 through the resistors R8 and R9. Then, the main transistor with the larger base current value at the beginning turns on and the other turns off. For example, if the main transistor Q1 is turned on and the main transistor Q2 is turned off immediately after startup, the collector current of the main transistor Q1 is monotonic to a degree determined by the reactance L1 of the load and the reactance values of the windings N1a to N3b. Will increase. When the collector current reaches a certain upper limit value, the voltage across the current detection resistor R3, which is proportional to the collector current, is the sum of the forward voltage of the diodes D4 and D5 and the ON voltage V _BE of the auxiliary transistors Q3 and Q4. Then, the auxiliary transistors Q3 and Q4 are turned on, the base current of the main transistor Q1 is bypassed by the auxiliary transistor Q3, and the main transistor Q1 is turned off.

As a result, the current flowing through the primary winding N1a sharply decreases, so that the counter electromotive force generated in the primary winding N1a causes the reverse winding voltage to be generated in the feedback windings N3a and N3b, and the main transistor Q2. Then, the base current flows, and the collector current of the main transistor Q2 monotonically increases. Then, when the collector current of the main transistor Q2 reaches a similar upper limit value, the voltage across the current detection resistor R3 proportional to the collector current exceeds the set value, and the auxiliary transistors Q3 and Q4 are turned on. The base current of the transistor Q2 is bypassed by the auxiliary transistor Q4 and the main transistor Q2 is turned off. By repeating such an operation, the DC voltage on the primary side of the transformer is switched, an AC voltage is generated on the secondary winding N2 of the transformer, and is applied to the fluorescent lamp 1 via the choke coil L1. The SW is closed and the filament 2 is energized to turn on the fluorescent lamp 1. After the discharge of the fluorescent lamp 1 is stabilized, the fluorescent lamp 1 continues to light even if the switch SW is opened.

[0010]

[Problems to be solved by the device]

 By the way, although the conventional circuit of FIG. 3 realizes cost reduction with a simple circuit configuration, if the fluorescent lamp 1 does not light up due to deterioration of the fluorescent lamp 1 or forgetting to insert the connector for mounting the fluorescent lamp 1, The ON period of the main transistors Q1 and Q2 becomes long, the transformer T saturates, and the collector current of the main transistors Q1 and Q2 suddenly rises. FIG. 4 shows the collector-emitter voltage and collector current of the main transistor Q1 (or Q2) when the fluorescent lamp 1 is not lit. Since the main transistors Q1 and Q2 are general bipolar transistors and have a considerable delay time (storage time) at turn-off, the auxiliary transistors Q3 and Q4 bypass the base currents of the main transistors Q1 and Q2. Even if Q2 is turned off, the phenomenon in which the collector current of the main transistor rapidly increases just before the turn-off as in the waveform portion P in FIG. 4 cannot be avoided. For this reason, the current rating of the main transistors Q1 and Q2 is exceeded, which is liable to cause damage.

In view of the above points, the present invention uses a FET (Field Effect Transistor) as a main switching element so that when the fluorescent lamp is not lit and the transformer is saturated due to deterioration or forgetting to insert the fluorescent lamp. Even in such a case, it is an object of the present invention to provide a power supply for a fluorescent lamp, which can prevent the destruction of the main switching element and improve reliability.

[0012]

[Means for Solving the Problems]

 In order to achieve the above object, the fluorescent lamp power supply of the present invention is connected to and corresponds to a transformer having a pair of primary windings, a feedback winding, and a secondary winding in series with each of the primary windings. A pair of FETs whose gates are connected to the ends of the feedback winding, auxiliary transistors connected between the gate and source of each FET, a current detection resistor inserted in the source side of the FET, and the primary winding. A direct current power source for applying a direct current voltage to a series circuit of a wire, the FET, and the current detection resistor; and a fluorescent lamp connected to the secondary winding via a coil, and the proportional to the drain current of the FET. The auxiliary transistor is made conductive when the detection voltage across the current detection resistor reaches a set value or higher.

[0013]

[Action]

 In the fluorescent lamp power supply of the present invention, since the FET with a very small delay time (storage time) at turn-off is used as the main switching element for switching the current on the primary side of the transformer, the drain current of the FET has a certain upper limit value. When the detection voltage across the current detection resistor, which is proportional to the drain current, exceeds the set value, the FET can be quickly turned off via the auxiliary transistor. As a result, even if the transformer is saturated without turning on the fluorescent lamp due to deterioration or forgetting to put it in the fluorescent lamp, the FET is turned off before the drain current becomes excessive. Will be done.

[0014]

【Example】

 Hereinafter, an embodiment of a fluorescent lamp power source according to the present invention will be described with reference to the drawings.

FIG. 1 shows an embodiment of a fluorescent lamp power source according to the present invention, in which Q5 and Q6 are MOSFETs as main switching elements for switching currents flowing through a pair of primary windings N1a and N1b of a transformer T. Is. The gate of the MOSFET Q5 is connected to one end of the feedback winding N3a via a current limiting resistor R7, and the gate of the MOSFET Q6 is connected to one end of the feedback winding N3b via a current limiting resistor R9. Further, resistors R10 and R11 are connected between the gates and sources of the MOS FETs Q5 and Q6. Further, a detection voltage proportional to the drain current of the MOSFET Q5 (voltage across the current detection resistor R3) is applied to the base of the auxiliary transistor Q3 for turning off the MOSFET Q5 via the coil L2 and the diode D4. A detection voltage proportional to the drain current of the MOSFET Q6 is applied to the base of the auxiliary transistor Q4 for turning off the MOSFET Q6 via the coil L3 and the diode D5. The coils L2 and L3 are for eliminating the malfunction of the MOSFETs Q5 and Q6 caused by noise and parasitic vibration and improving the switching operation characteristics. The other circuit configurations are the same as those of the conventional circuit of FIG.

Next, the operation of the embodiment will be described. When a DC voltage is generated between the positive and negative side ends 3 and 4 of the rectification circuit by turning on a power switch (not shown), it passes through the starting resistor R2, and further the feedback winding N3a and the resistor R7 or the feedback winding N3b and the resistor R9. A gate voltage is applied to the MOSFETs Q5 and Q6 as the main switching elements through the. Then, the MOSFET whose gate voltage value has increased initially is turned on, and the other MOSFET is turned off. For example, if the MOSFET Q5 is turned on and the MOSFET Q6 is turned off immediately after startup, the drain current of the MOSFET Q5 increases monotonically to the extent determined by the reactance L1 of the load and the reactance values of the windings N1a to N3b. Go. When the drain current reaches a certain upper limit value, the voltage across the current detection resistor R3 proportional to the drain current is the sum of the forward voltage of the diodes D4 and D5 and the ON voltage V _BE of the auxiliary transistors Q3 and Q4. Then, the auxiliary transistors Q3 and Q4 are turned on, the gate voltage of the MOSFET Q5 is short-circuited to zero level by the auxiliary transistor Q3, and the MOSFET Q5 is turned off.

As a result, the current flowing through the primary winding N1a sharply decreases, so that the counter electromotive force generated in the primary winding N1a generates a voltage of opposite polarity in the feedback windings N3a and N3b, and the MOS FET Q6. The gate voltage is added, and the drain current of MOSFET Q6 increases monotonically this time. When the drain current of the MOSFET Q6 reaches a similar upper limit value, the voltage across the current detection resistor R3 proportional to the drain current exceeds the set value, the auxiliary transistors Q3 and Q4 are turned on, and the MOSFET The gate voltage of Q6 is short-circuited to zero level by the auxiliary transistor Q4, and the MOSFET Q6 is turned off. By repeating such an operation, the direct current voltage on the transformer primary side is switched, an AC voltage is generated in the transformer secondary winding N2, which is applied to the fluorescent lamp 1 via the choke coil L1, and at the same time, is switched for a certain period of time. SW is closed and the filament 2 is energized to turn on the fluorescent lamp 1. After the discharge of the fluorescent lamp 1 is stabilized, the fluorescent lamp 1 continues to light even if the switch SW is opened.

FIG. 2 shows the drain-source voltage and the drain current of the MOSFET Q5 (or Q6) as the main switching element when the fluorescent lamp 1 is not turned on. Since the delay time (storage time) at turn-off of MOSFET Q5 and Q6 is very short, the auxiliary transistors Q2 and Q3 detect that the voltage across the current detection resistor R3, that is, the detection voltage, exceeds the set value, and the MOSFETs are detected. When the gate voltage of Q5 and Q6 is dropped to zero level, the MOSFETs Q5 and Q6 perform turn-off operation quickly (about 2 to 4 μsec faster than the conventional case of FIG. 4). As a result, even when the transistor T becomes saturated, the MOSFETs Q5 and Q6 can be turned off before the drain current becomes excessively large, so that the breakdown due to the overcurrent can be surely avoided.

In the circuit of the embodiment, the diode D6 may be omitted if the circuit portions connected to the feedback windings N3a and N3b of the transformer T are well balanced.

Further, in the above embodiment, the case of the two-wire system in which the pair of primary windings N1a and N1b are connected to the common DC power source is illustrated, but + V (positive voltage) is applied to one of the primary windings N1a. The present invention is also applicable to a three-wire system in which the voltage is applied and -V (negative voltage) is applied to the other primary winding N1b. In this case, the current detection resistor is provided corresponding to the source side of each MOSFET.

[0021]

[Effect of device]

 As described above, according to the fluorescent lamp power supply of the present invention, by using the FET (Field Effect Transistor) as the main switching element for switching the current on the primary side of the transformer, the fluorescent lamp is deteriorated or forgotten, and the fluorescent light is emitted. Even if the lamp is not turned on and the transformer is saturated, destruction of the main switching element can be avoided without fail, and reliability can be improved. Moreover, as a result of the improvement of the switching speed of the main switching element, the efficiency is improved by about several percent.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a fluorescent lamp power source according to the present invention.

FIG. 2 is a waveform diagram illustrating the operation of the embodiment.

FIG. 3 is a circuit diagram showing a conventional fluorescent lamp power source.

FIG. 4 is a waveform diagram illustrating the operation of a conventional fluorescent lamp power supply.

[Explanation of symbols]

 1 Fluorescent Lamp 2 Filament D1 to D6 Diode L1 to L3 Coil Q1 to Q4 Transistor Q5, Q6 MOSFET R1 to R11 Resistor T Transformer N1a, N1b Primary Winding N2 Secondary Winding N3a, N3b Feedback Winding

Claims (2)

[Scope of utility model registration request] 1. A transformer having a pair of primary windings, a feedback winding, and a secondary winding, and a pair of Fs connected in series to each of the primary windings and having a gate connected to a corresponding feedback winding end.
ET, an auxiliary transistor connected between the gate and source of each FET, a current detection resistor inserted on the source side of the FET, a series circuit of the primary winding, the FET, and the current detection resistor. A DC power supply for applying a DC voltage and a fluorescent lamp connected to the secondary winding via a coil are provided, and the detection voltage across the current detection resistor, which is proportional to the drain current of the FET, has reached a set value or higher. A power supply for a fluorescent lamp, characterized in that the auxiliary transistor is sometimes made conductive. 2. The power source for a fluorescent lamp according to claim 1, wherein the detection voltage is applied to each of the auxiliary transistors via a series circuit of a diode and a coil.