JP2009158423A - Overcurrent protection circuit of high pressure discharge lamp electronic ballast - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an overcurrent protection circuit of a high pressure discharge lamp electronic ballast capable of preventing an overcurrent flowing to a switching element, and preventing a failure of an inverter or the like. <P>SOLUTION: The overcurrent protection circuit of a high pressure discharge lamp electronic ballast includes: a transistor Q2 connected to an input side of the gate of an FET Q41 used for the high pressure discharge lamp electronic ballast; a diode D1 having an anode connected to the source or drain of the FET Q41, and a cathode connected to the base of the transistor Q2; and a current detection resistor R1 having one end connected between the anode of the diode D1 and the source or drain of the FET Q41. The output side of the transistor Q2 is connected to the other end of the current detection resistor R1. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an overcurrent protection circuit for a high pressure discharge lamp electronic ballast.

  FIG. 5 shows a conventional overcurrent protection circuit. A drive signal for driving Q1 (MOSFET: Metal Oxide Field Effect Transistor) is input to an input terminal (IN) of the overcurrent protection circuit from a control circuit (not shown) or the like. The drive circuit 40 converts the input weak drive signal into a strong drive signal for driving Q1. The output (OUT) of the drive circuit 1 is connected to the gate of Q1 through the gate resistor R5. A load L1 (inductance) is connected to the drain of Q1. The source of Q1 is grounded via the current detection resistor R1. One end of the base resistor R2 is connected between the source of Q1 and the current detection resistor R1. The other end of the base resistor R2 is connected to the base of the transistor Q2 for limiting (feeding back) the drain current Id (substantially the same as the source current Is) of Q1. One end of a discharge resistor R3 for discharging the stray capacitance of the transistor Q2 is connected between the base resistor R2 and the base of the transistor Q2. The emitter of the transistor Q2 and the other end of the discharge resistor R3 are grounded. The collector of the transistor Q2 is connected between the output (OUT) of the drive circuit 40 and the gate of Q1.

  Next, the operation of the overcurrent protection circuit of FIG. 5 will be described. The overcurrent protection circuit limits the drain current Id of Q1 because Q1 is forcibly turned off when the transistor Q2 is turned on.

The conditions for turning on the transistor Q2 are as follows. When the collector current of the transistor Q2 is Ic, the direct current amplification factor is _hFE , and the base current of the transistor Q2 is Ib,
Ic <h _FE × Ib (1)
When the above relationship is satisfied, the transistor Q2 is turned on.

  The collector current Ic is substantially the same as the maximum output current of the drive circuit 40.

The base current Ib is expressed as follows: V1 is the voltage at the connection point between the current detection resistor R1 and the base resistor R2, and Vbe is the base-emitter voltage of the transistor Q2.
Ib = (V1-Vbe) / R2-Vbe / R3 (2)
Here, V1 = Is × R1. Is is the source current of Q1.

Substituting equation (2) for Ib in equation (1) omits the details,
Ic <α × Is (3)
It becomes. Here, α is a constant. That is, when α × Is is larger than Ic, Q1 is turned off to protect Q1 (load L1).
JP 2004-032032 A JP 2006-166142 A

  FIG. 6 is a diagram showing waveforms of voltage Vgs (Q1 gate-source voltage) and current Id (Q1 drain current) when a conventional overcurrent protection circuit is operated. When the overcurrent protection circuit of FIG. 5 was actually operated, as shown in FIG. 6, the transistor Q2 repeatedly turned on and off, so that the drain current Id of Q1 could not be cut off. Vgs in FIG. 5 is a voltage between the gate and the source of Q1. The current detection resistor R1 has a resistance value of about 1Ω or less in order to suppress heat generation. The reason why the transistor Q2 cannot repeatedly turn on and off finely to cut off the drain current Id of Q1 is that the resistance value of the current detection resistor R1 is small, so that Vbe is discharged instantaneously when Q1 is off. it is conceivable that.

  The present invention has been made to solve the above-described problems, and can prevent an overcurrent flowing through a switching element such as a MOSFET, and can prevent failure of an inverter or the like. It is an object to provide an overcurrent protection circuit for a battery.

An overcurrent protection circuit for a high-pressure discharge lamp electronic ballast according to the present invention is a high-pressure discharge lamp electronic for lighting a high-pressure discharge lamp having a rectifier circuit connected to a commercial power source, a step-up inverter, and a step-down inverter including a switching element. In the overcurrent protection circuit of the ballast,
A first switching element used in a high pressure discharge lamp electronic ballast;
A second switching element whose input side is connected to the gate of the first switching element;
A diode having an anode connected to the source or drain of the first switching element and a cathode connected to the base of the second switching element;
A current detection element having one end connected between the anode of the diode and the source or drain of the first switching element, and the output side of the second switching element and the other end of the current detection element are connected And

  An overcurrent protection circuit for a high-pressure discharge lamp electronic ballast according to the present invention includes a base resistor connected between a cathode of a diode and a base of a second switching element.

  An overcurrent protection circuit for a high-pressure discharge lamp electronic ballast according to the present invention includes a charging capacitor having one end connected to the cathode of a diode and the other end connected to the output side of the second switching element. And

  The overcurrent protection circuit of the high voltage discharge lamp electronic ballast according to the present invention can prevent an overcurrent flowing through the switching element, and can prevent a failure of the inverter or the like.

Embodiment 1 FIG.
1 to 4 show the first embodiment. FIG. 1 is a circuit diagram of a high-pressure discharge lamp electronic ballast 100. FIG. 2 is a circuit diagram of an overcurrent protection circuit of the high-pressure discharge lamp electronic ballast. FIG. 4 is a partial circuit diagram showing an example in which the overcurrent protection circuit of FIG. 2 is applied to the FET Q41 of the step-down inverter 4 of the high-voltage discharge lamp electronic ballast 100. FIG. FIG.

  The configuration of the high-pressure discharge lamp electronic ballast 100 will be described with reference to FIG. The high-pressure discharge lamp electronic ballast 100 is a circuit before incorporating an overcurrent protection circuit.

  The rectifier circuit 2 takes in AC power from the commercial power source 1 and performs rectification. The rectifier circuit 2 includes a diode bridge including a diode D21, a diode D22, a diode D23, and a diode D24, and a capacitor 21 connected between both output terminals of the diode.

  The step-up inverter 3 includes an inductance L31 having one end connected to the output terminal of the high potential side of the rectifier circuit 2, a diode D31 having the other end connected to the anode of the inductance L31, a cathode of the diode D31, and a low potential side of the rectifier circuit 2. FET Q31 (field effect transistor, one example of a first switching element) to which the drain and source are connected to the output terminal, and a capacitor C31 connected between the cathode of the diode D31 and the low potential side output terminal of the rectifier circuit 2 Composed.

  The step-down inverter 4 includes a FET Q41 (an example of a first switching element) to which a cathode and a drain of a diode D31 are connected, a diode to which the source of the FET Q41 and the low potential side output terminal of the rectifier circuit 2 are connected to the cathode and the anode, respectively. D41, an inductance L41 connected to one end of the source of the FET Q41, and a capacitor C41 connected between the other end of the inductance L41 and the low potential side output end of the rectifier circuit 2.

  The lighting circuit 5 of the high-pressure discharge lamp 7 is connected in parallel with a series-connected FET Q51 (an example of a first switching element), an FET Q52 (an example of a first switching element), and the FET Q51 and FET Q52 connected in parallel with a capacitor C41. A series-connected FET Q53 (an example of a first switching element), an FET Q54 (an example of a first switching element), and a start pulse generating circuit 6.

  The control circuit 10 has a function of controlling supply of predetermined DC power from the step-down inverter 4 to the lighting circuit 5 by duty control of the FET Q41 of the step-down inverter 4 during normal lighting.

  Next, an overcurrent protection circuit which is a characteristic part of the present embodiment will be described. As shown in FIG. 2, a drive signal for driving Q1 (MOSFET, an example of the first switching element) is input to an input terminal (IN) of the overcurrent protection circuit from a control circuit (not shown) or the like. The drive circuit 40 converts the input weak drive signal into a strong drive signal for driving Q1. The output (OUT) of the drive circuit 40 is connected to the gate of Q1 through the gate resistor R5.

  An inductance L1 is connected to the drain of Q1. The source of Q1 is grounded via the current detection resistor R1. The anode of the diode D1 is connected between the source of Q1 and the current detection resistor R1 (an example of a current detection element). One end of a charging capacitor C1 and one end of a base resistor R2 are connected to the cathode of the diode D1. The other end of the base resistor R2 is connected to the base of a transistor Q2 (an example of a second switching element) for limiting (feeding back) the drain current Id (substantially the same as the source current Is) of Q1. The other end of the charging capacitor C1 is grounded. The other end of the charging capacitor C1 is connected to the output side of the transistor Q2. The anode of the diode D1 may be connected to the drain of the source of Q1.

  One end of a discharge resistor R3 for discharging the stray capacitance of the transistor Q2 is connected between the base resistor R2 and the base of the transistor Q2. The emitter of the transistor Q2 and the other end of the discharge resistor R3 are grounded. The collector of the transistor Q2 is connected between the output (OUT) of the drive circuit 40 and the gate of Q1.

  Next, the operation will be described. When an overcurrent (Is) flows through Q1, V1 becomes a voltage of R1 × Is. V1 charges the charging capacitor C1 through the diode D1. When the voltage of the charging capacitor C1 exceeds a specified voltage, the transistor Q2 is turned on. When the transistor Q2 is turned on, it is possible to turn off Q1 and limit the overcurrent flowing through Q1.

  When transistor Q2 is turned on and Q1 is turned off, Is does not flow, so V1 becomes 0 (v). The voltage of the charging capacitor C1 is not discharged to the current detection resistor R1 because the diode D1 is turned off. Therefore, the transistor Q2 is kept on and Q1 can be continuously turned off until the charging capacitor C1 is discharged.

  The conventional overcurrent protection circuit shown in FIG. 5 can solve the problem that the drain current Id of Q1 cannot be cut off because the transistor Q2 is repeatedly turned on and off.

  The overcurrent protection circuit of FIG. 2 needs to include at least a diode D1 and a current detection resistor R1.

  The overcurrent protection circuit of FIG. 2 is characterized in that it can be used where there is a potential at the source of Q1 (where it is not ground). In FIG. 2, the source of Q1 is grounded via the current detection resistor R1, but it can also be used where it is not grounded. The transistor Q2 is turned on / off with reference to a line connecting the emitter of the transistor Q2 and the other end of the discharge resistor R3. Therefore, the overcurrent protection circuit of FIG. 2 operates even when the reference line is not grounded.

  For example, the control circuit 10 shown in FIG. 1 cannot perform overcurrent protection of the FET Q41 of the step-down inverter 4 as it is. In order to perform overcurrent protection of the FET Q41 of the step-down inverter 4 by the control circuit 10 shown in FIG. 1, the source current of the FET Q41 is detected. In that case, a current detection resistor is connected to the source of the FET Q41, and a potential difference between both ends of the current detection resistor is captured. However, if a voltage is directly taken in, a voltage exceeding the allowable voltage of the control circuit 10 is taken in, so that it is taken in via a dividing resistor. When taking in through the dividing resistor, the potential difference between both ends of the current detection resistor becomes about 0.01 (v). Since this potential difference is very small, the control circuit 10 cannot accurately detect it.

  The overcurrent protection circuit of FIG. 2 can be used where the source of Q1 has a potential (where it is not ground). For example, it can be used as an overcurrent protection circuit of the FET Q41 of the step-down inverter 4 of FIG. it can.

  FIG. 3 is a partial circuit diagram showing an example in which the overcurrent protection circuit of FIG. 2 is applied to the FET Q41 of the step-down inverter 4 of the high-pressure discharge lamp electronic ballast 100. In FIG. 3, Q <b> 1 in FIG. 2 is replaced with FET Q <b> 41 of the step-down inverter 4.

  Although not shown, the overcurrent protection circuit of FIG. 2 can also be used as an overcurrent protection circuit for the FET Q31 of the step-up inverter 3 in the high voltage discharge lamp electronic ballast 100.

  Further, although not shown, the overcurrent protection circuit of FIG. 2 can also be used as an overcurrent protection circuit for the FET Q51, FET Q52, FET Q53, and FET Q54 of the lighting circuit 5 in the high pressure discharge lamp electronic ballast 100.

  FIG. 4 is a circuit diagram of an overcurrent protection circuit of a modified high pressure discharge lamp electronic ballast. Connected to the half-bridge drive circuit 50 is an overcurrent protection circuit that simultaneously performs overcurrent protection of Q1 (MOSFET) having a potential at the source and Q3 (MOSFET) having no potential at the source.

  In FIG. 4, the configuration of the overcurrent protection circuit of Q1 is the same as that of FIG. The only difference is that there is a potential at the source of Q1.

In FIG. 4, the configuration of the overcurrent protection circuit of Q3 is the same as that of FIG. 2, and each electronic component constituting the overcurrent protection circuit corresponds to FIG. 2 as follows.
(1) R10 corresponds to R5.
(2) Q4 corresponds to Q2.
(3) R7 corresponds to R2.
(4) R8 corresponds to R3.
(5) C2 corresponds to C1.
(6) D2 corresponds to D1.
(7) R6 corresponds to R1.

FIG. 3 shows the first embodiment, and is a circuit diagram of a high-pressure discharge lamp electronic ballast 100. FIG. 3 is a diagram showing the first embodiment, and is a circuit diagram of an overcurrent protection circuit of the high-pressure discharge lamp electronic ballast. FIG. 3 shows the first embodiment, and is a partial circuit diagram showing an example in which the overcurrent protection circuit of FIG. 2 is applied to the FET Q41 of the step-down inverter 4 of the high-pressure discharge lamp electronic ballast 100; FIG. 5 shows the first embodiment, and is a circuit diagram of an overcurrent protection circuit of a high-pressure discharge lamp electronic ballast according to a modification. The circuit diagram of the conventional overcurrent protection circuit. The figure which shows the voltage Vgs (gate-source voltage of Q1) and electric current Id (drain current of Q1) when the conventional overcurrent protection circuit is operated.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Commercial power supply, 2 Rectifier circuit, 3 Boost inverter, 4 Buck inverter, 5 Lighting circuit, 6 Start pulse generation circuit, 10 Control circuit, 7 High pressure discharge lamp, 21 Capacitor, 40 Drive circuit, 50 Half bridge drive circuit, 100 High voltage Discharge lamp electronic ballast.

Claims (3)

In an overcurrent protection circuit for a high-pressure discharge lamp electronic ballast for lighting a high-pressure discharge lamp having a rectifier circuit connected to a commercial power source, a step-up inverter, and a step-down inverter including a switching element,
A first switching element used in the high pressure discharge lamp electronic ballast;
A second switching element having an input connected to the gate of the first switching element;
A diode having an anode connected to a source or drain of the first switching element and a cathode connected to a base of the second switching element;
A current detection element having one end connected between the anode of the diode and the source or drain of the first switching element, and connecting the output side of the second switching element and the other end of the current detection element; An overcurrent protection circuit for a high-pressure discharge lamp electronic ballast characterized by that.   2. The overcurrent protection circuit for a high-pressure discharge lamp electronic ballast according to claim 1, further comprising a base resistor connected between a cathode of the diode and a base of the second switching element.   3. The high pressure discharge lamp electron according to claim 1, further comprising a charging capacitor having one end connected to the cathode of the diode and the other end connected to the output side of the second switching element. Ballast overcurrent protection circuit.

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