JPH07123077B2 - Lamp drive circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention (Field of Industrial Application) The present invention relates to, for example, a vehicle-mounted lamp drive circuit having a soft start function.

(Prior Art) As a conventional lamp drive circuit having a soft start function, there is one as shown in FIG. 9, for example. In the figure,
Reference numeral 1 is a power supply for vehicles, etc.
It is connected to the lamp 3 through the MOSFET 2, and the other end of the lamp 3 is grounded.

Since the lamp 3 is cold at the beginning of lighting, its filament resistance is a fraction of that at the time of lighting. Then, after lighting, MOSFET2 increases due to the increase in filament resistance.
Source potential rises. Therefore, in order to drive MOSFET2 appropriately, a high voltage is required as the gate voltage.
_A boosted version of _DD is used. Therefore, the lamp drive circuit is first provided with the booster circuit 4.

On the other hand, when a high voltage is applied to the gate electrode of MOSFET 2 from the beginning of lighting, the gate-source voltage V _GS increases,
Rush current of several tens of times that at normal lighting flows, and MOSFET
In No. 2, the internal temperature may rise and thermal destruction may be induced. Therefore, in the drive circuit, in order to detect the current flowing through the MOSFET 2, a circuit of the mirror MOSFET 22 and the shunt resistor 23 is connected in parallel to this. And MO
The current flowing in the SFET2 is detected by the current detection circuit 24 as a voltage drop due to the shunt resistor 23, and the detection signal is fed back to the control circuit 25 to control the booster circuit 4 and MOSF.
The voltage applied to the gate electrode of ET2 is gradually increased from a low voltage to control the current flowing through MOSFET2.

The drain-source voltage of MOSFET2 is V _DS , the mirror MOSFET
When the internal resistance of 22 is R _M and the resistance value of the shunt resistor 23 is R, the current I _M flowing in the mirror MOSFET 22 is expressed by the following equation.

I _M = (V _DS −I _M · R) / (R _M + R) (1) Then, the current I _M flowing through this mirror MOSFET 22 is
The current I _D flowing in 2 is set so that I _M = I _D / 100 to I _D / 1000 (2).

(Problems to be Solved by the Invention) In the conventional lamp drive circuit, in order to increase the voltage drop I _M · R due to the shunt resistor 23 and increase the S / N ratio, it is necessary to increase the value of the shunt resistor 23. However, if the value of the shunt resistor 23 is increased, the current I _M flowing through the mirror MOSFET 22 changes, and the current I _D flowing through the MOSFET 2 cannot be detected accurately, so that the current flowing through the lamp 3 can be accurately controlled. There was a problem that I could not.

The present invention has been made based on the above circumstances, and an object of the present invention is to provide a lamp drive circuit capable of accurately controlling a current flowing through a lamp without using a shunt resistor.

[Structure of the Invention] (Means for Solving the Problems) In order to solve the above problems, the present invention relates to a current control MOSFET connected between a driven lamp and a power supply, and the MOSFET.
A booster circuit for boosting the voltage applied to the gate of the
It has voltage control means for controlling the voltage applied to the gate of the ET based on the source potential of the MOSFET, and switching means for switching the voltage applied to the gate of the MOSFET between the output of the voltage control means and the output of the booster circuit. That is the summary.

(Operation) In the above configuration, the output voltage from the voltage control means is applied to the gate of the MOSFET as a gate voltage in the initial stage of lighting of the lamp, and the gate voltage gradually rises according to the change of the source potential due to the change of the filament resistance. To be controlled. As a result, the lamp is lit with a soft start
Thermal destruction of the MOSFET is prevented. When the filament resistance reaches a steady state and the source potential rises after the lamp is turned on, the switching means acts on the gate of the MOSFET by the action of the switching means.
A predetermined boosted voltage from the booster circuit is applied as a gate voltage. Therefore, the lamp has a
The lighting state is appropriately maintained by controlling so that a constant current flows.

Thus, the lamp drive circuit of the present invention does not require a shunt resistor, and the current flowing through the lamp is accurately controlled even after the steady state is reached from the initial lighting.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

1 to 3 are views showing a first embodiment of the present invention.

In FIG. 1 and the drawings showing the respective embodiments described later, the same or equivalent parts as the devices and elements in FIG. 9 are designated by the same reference numerals, and the duplicated description will be omitted.

First, the configuration of the lamp drive circuit will be described with reference to FIG. 1. The source potential detection circuit 5 for detecting the source potential of the MOSFET 2 is configured by a non-inverting amplifier composed of an operational amplifier 6 and resistors R _{1 to} R ₄ . The gate voltage control circuit 7 is constituted by the non-inverting adder circuit formed of another operational amplifier 8 and resistor R ₅ to R _8. Reference numeral 9 is an addition terminal for a certain constant voltage V _X. The source potential detection circuit 5 and the gate voltage control circuit 7 constitute a voltage control means for controlling the voltage applied to the gate of the MOSFET 2 based on the source potential.

Reference numeral 11 is an output switch of the voltage control means, and 12 is a changeover switch as a changeover means.

Next, the operation of the lamp drive circuit configured as described above will be described with reference to FIGS. 2 and 3.

The MOSFET has a voltage-current characteristic as shown in FIG.
When in the saturation region, the drain current I _D
It is almost unaffected by the source-to-source voltage V _DS and is determined by the value of the gate-source voltage V _GS . Therefore, even if the source potential V _S changes at the initial lighting of the lamp 3, V _GS
The drain current I _D can be maintained at a constant value by controlling the voltage constant so that the lamp 3 can be turned on by soft start. In this example,
As described below, this is achieved by the voltage control means.

Now, when the changeover switch 12 is switched to the contact a side, the output switch 11 is turned on, and the power supply voltage V _DD is applied to the lamp drive circuit, the filament resistance of the lamp 3 is small at the initial stage of lighting, so that FIG. As shown, the source potential V _S becomes lower and the source potential V _S rises as the filament resistance increases.

This change in the source potential V _S is input to the source potential detection circuit 5, and an output proportional to the input value is input to the gate voltage control circuit 7. Since the gate voltage control circuit 7 is an adder circuit, a voltage proportional to the sum of this input and V _X is output and applied to the gate of the MOSFET 2. Therefore, MOSFE
A constant gate-source voltage V _GS = K · V _X (constant) is applied between the gate and source of T2 even if the source potential V _S changes in the initial stage of lighting, and the drain current I _D remains constant. Controlled by the value of
Lamp 3 is lit by soft start.

After the lamp 3 is turned on, when the filament resistance reaches a steady state and the source potential V _S rises to a substantially constant value, the changeover switch 12 is changed over to the contact b, and the gate of the MOSFET 2 receives the voltage from the booster circuit 4. A predetermined boosted voltage is applied as the gate voltage. Therefore, the lamp is controlled so that a constant current I _D flows even after reaching the steady state, and the lighting state is appropriately maintained.

Next, FIG. 4 shows a second embodiment of the present invention.

In this embodiment, the gate-source voltage V _GS of the MOSFET 2 is directly detected, and in the initial stage of lighting, the gate-source voltage V _GS is controlled to a constant value according to the change of the source potential V _S. It is the one.

In FIG. 4, 51 is a gate-source voltage detection circuit composed of a non-inverting amplifier similar to that in FIG. 1, 13 is a subtraction circuit composed of an operational amplifier 14 and resistors R _{9 to} R _11. The gate voltage control circuit is configured, and 15 is an input terminal of the subtracted voltage V _T. And the gate above
Both the source-to-source voltage detection circuit 51 and the gate voltage control circuit 13 constitute voltage control means for controlling the voltage applied to the gate of the MOSFET 2 based on the source potential thereof.

Since the lamp drive circuit of this embodiment is configured as described above, in the initial stage of lighting, the changeover switch 12 is switched to the contact a side, the output switch 11 is turned on, and the power supply voltage V _DD is applied to the lamp drive circuit. The gate of MOSFET2
The source-to-source voltage V _GS is input to the gate-source voltage detection circuit 51, and an output proportional to the input value is input to the gate voltage control circuit 13. Since the gate voltage control circuit 13 is a subtraction circuit, the difference voltage between the voltage V _T applied to the terminal 15 and the input is output and applied to the gate of the MOSFET 2. Therefore, even if the source potential V _S changes in the initial stage of lighting between the gate and source of the MOSFET 2, a substantially constant gate-source voltage V _GS V _T / 2 is applied and the drain current I _D becomes constant. Controlled by value,
Lamp 3 is lit by soft start.

When the filament resistance of the lamp 3 reaches a steady state and the source potential V _S rises to a substantially constant value after the lamp 3 is turned on, the changeover switch 12 causes the contact point b to change, as in the case of the first embodiment.
Switched to the side.

5 to 7 show a third embodiment of the present invention.

In this embodiment, the Zener diode 16 is used as a voltage control means for controlling the voltage applied to the gate of the MOSFET 2 based on its source potential, and the voltage applied to the gate of the MOSFET 2 is set by the Zener diode 16 described above. A timer 17 and a switch element 18 composed of a MOSFET are used as switching means for switching the voltage to the output voltage of the booster circuit 4. Reference numeral 19 is a drive signal input terminal, and R ₁₂ is a resistor connected to the output line of the booster circuit 4.

Next, the operation of the lamp driving circuit configured as described above will be described with reference to FIGS. 6 and 7.

The power supply voltage V _DD is applied to the lamp drive circuit and the input terminal 19
When a drive signal is sent to, the booster circuit 4 and the timer 17 are started. Since the timer 17 receives the output voltage of the booster circuit 4 as a power source, the output voltage of the timer 17 becomes the same as the gate voltage applied to the MOSFET 2, and as shown in FIG. ) Has
The switch element 18 is fully turned on.

Therefore, in the initial stage of lighting, even if the source potential of the MOSFET 2 is low, the gate-source voltage V _GS is equal to the constant voltage V _{Z obtained} by adding the voltage drop of the switch element 18 to the Zener voltage V _Z of the Zener diode 16. _{Clamped to GS} V _Z. As a result, the drain current I of MOSFET 2
_As shown in FIG. 7, _D , that is, the current I flowing through the lamp 3, is controlled to a constant current and is turned on by soft start.

When the lamp 3 is turned on and the filament resistance reaches a steady state after the preset time of the timer 17, the output of the timer 17 is turned off and the switch element 18 is turned off. Therefore, after the lapse of the initial lighting, a predetermined boosted voltage from the booster circuit 4 is applied as a gate voltage to the gate of the MOSFET 2, and the lamp 3 reaches a steady state.
The lighting state is appropriately maintained by controlling so that a predetermined current flows.

FIG. 8 shows a fourth embodiment of the present invention.

In this embodiment, as the voltage control means for controlling the voltage applied to the gate of the MOSFET 2 based on its source potential, instead of the Zener diode in the above-mentioned third embodiment,
Series-connected diode 20 in which n diodes are connected in series
Is used. The other structure is similar to that of the third embodiment.

In this embodiment, in the initial stage of lighting, even if the source potential of the MOSFET 2 is low, the gate-source voltage V
_GS is clamped to a constant voltage V _GS 0.6n that is the forward voltage of the series-connected diode 20 plus the voltage drop of the switch element 18. As a result, the drain current of MOSFET2
I _D , that is, the current I flowing through the lamp 3 is controlled to a constant value and is turned on by soft start.

After turning on the lamp 3, as in the case of the third embodiment,
The gate voltage of the MOSFET 2 is switched to a predetermined boosted voltage from the booster circuit 4.

[Effects of the Invention] As described above, according to the present invention, the output voltage from the voltage control means is applied to the gate of the MOSFET as the gate voltage at the initial stage of lighting of the lamp, and after the lamp is lit, its filament resistance is After the steady state is reached, the gate voltage can be switched to a predetermined boost voltage from the booster circuit.Therefore, in the initial stage of lighting, the gate voltage of the MOSFET depends on the change in the source potential due to the change in the filament resistance. The effect is that the current flowing through the lamp is accurately controlled, the lamp is lit by soft start, and the thermal destruction of the MOSFE is accurately prevented.

Further, according to the third embodiment and the fourth embodiment, in addition to the common effects described above, a current detection circuit, a control circuit, and the like configured by an operational amplifier and the like are unnecessary, so that the chip area of the semiconductor chip is reduced. The effect is that it can be made smaller.

[Brief description of drawings]

1 to 3 show a first embodiment of a lamp driving circuit according to the present invention. FIG. 1 is a circuit diagram and FIG. 2 is a MO.
FIG. 3 is a characteristic diagram showing voltage / current characteristics of the SFET, FIG. 3 is a characteristic diagram showing changes in the source potential of the MOSFET, FIG. 4 is a circuit diagram showing a second embodiment of the present invention, and FIGS. FIG. 5 shows a third embodiment of the present invention. FIG. 5 is a circuit diagram and FIG. 6 is
FIG. 7 is a characteristic diagram showing changes in voltage applied between the gate and source of the MOSFET, FIG. 7 is a characteristic diagram showing changes in current flowing in the lamp, and FIG. 8 is a circuit diagram showing a fourth embodiment of the present invention.
The figure is a circuit diagram showing a conventional lamp driving circuit. 1: Power supply, 2: MOSFET, 3: Lamp, 4: Booster circuit, 5: Source potential detection circuit, 7: Gate voltage control circuit that constitutes a voltage control means together with the source potential detection circuit. 12: Changeover switch (changeover means), 51: Gate-source voltage detection circuit, 13:
Gate voltage control circuit that constitutes voltage control means together with gate-source voltage detection circuit, 16: Zener diode (voltage control means), 17: Timer, 18: Switch element that constitutes switching means together with timer, 20: Series connection diode (Voltage control means).

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location H03K 17/06 C 9184-5J 17/687

Claims (1)

[Claims] 1. A MOSFET for current control connected between a driven lamp and a power supply, a booster circuit for boosting a voltage applied to the gate of the MOSFET, and a source potential of the MOSFET for applying a voltage applied to the gate of the MOSFET. A lamp drive circuit comprising: a voltage control means for controlling the voltage applied to the gate of the MOSFET; and a switching means for switching the voltage applied to the gate of the MOSFET between the output of the voltage control means and the output of the booster circuit.