DE102010028795A1 - Circuit for operating a light unit and luminaire with such a circuit - Google Patents

Abstract

The invention relates to a circuit for operating a lighting unit, comprising a current mirror (Q2, Q3) which can be pretensioned via a reference voltage source (110), the current mirror (Q2, Q3) being connected to a controlled system (Q1), by means of which a current is adjustable from an energy source (120) to the lighting unit (130). Furthermore, a lamp is proposed comprising such a circuit.

Description

The invention relates to a circuit for operating a lighting unit and a lamp with such a circuit.

Conventional LED torches often use only a resistive resistor to adjust the LED current. If the nominal supply voltage is not significantly higher than the LED voltage, there is a strong dependence of the LED current and thus the brightness of the supply voltage. Thus, the condition of the battery or the accumulator has a noticeable effect on the brightness of the LED flashlight. As a result, a user replaces the batteries early. Thus, the capacity of the chemical cell is rarely exploited until the possible lower voltage.

There are also known electronic constant current sources that use special circuits and require a relatively high supply voltage for operation. This results in significant losses, for example, shorten the battery life of the flashlight accordingly.

The object of the invention is to avoid the disadvantages mentioned above and in particular to provide a possibility for a simple (linear) constant current source, for example for the operation of a flashlight or lighting unit with at least one semiconductor element.

This object is achieved according to the features of the independent claims. Further developments of the invention will become apparent from the dependent claims.

• – umfassend einen Stromspiegel, der über eine Referenzspannungsquelle vorspannbar ist,
• – wobei der Stromspiegel mit einer Regelstrecke verbunden ist, mithilfe derer ein Strom von einer Energiequelle zu der Leuchteinheit einstellbar ist.

To achieve the object, a circuit for operating a light unit is specified,

• Comprising a current mirror, which can be biased by a reference voltage source,
• - Wherein the current mirror is connected to a controlled system, by means of which a current from an energy source to the lighting unit is adjustable.

The power source may include at least one (rechargeable) battery.

The proposed solution has the advantage that the electrical energy stored in a battery is used efficiently.

The constant current source proposed here can be operated with a small voltage drop and has a good constant current characteristic.

A development is that the lighting unit comprises at least one semiconductor element.

Another development is that the controlled system is connected in series with the lighting unit.

In particular, it is a development that a current measuring resistor is connected in series with the controlled system and the lighting unit, wherein the current measuring resistor is connected to the current mirror.

It is also a development that the energy source is arranged parallel to the series circuit of current measuring resistor, controlled system and lighting unit.

Furthermore, it is a further development that the current mirror has two bipolar transistors, wherein the bipolar transistors are connected to each other via their base terminals and to the collector terminal of one of the bipolar transistors.

In the context of an additional development of a bipolar transistor of the current mirror is arranged in base circuit, wherein the current detectable by the current measuring current can be coupled as a voltage drop to the emitter of this bipolar transistor. The other bipolar transistor of the current mirror is arranged in emitter circuit and connected to its collector with the controlled system, in particular an input of the controlled system.

A next development is that a switch between the controlled system and the power source is arranged.

In particular, the switch can disconnect the controlled system from the energy source. For example, the switch can be connected directly, via one or more resistors to the base of the transistor, which can be used as a controlled system. If the switch is opened, the transistor blocks, the circuit is inactive.

An embodiment is that the controlled system comprises a transistor.

In particular, it can be in the controlled system to a bipolar transistor, z. B. a pnp transistor act. Also, the transistors mentioned herein may be replaced by other transistors (eg field effect transistors) or electronic components for switching or amplifying electrical signals.

An alternative embodiment is that the base of the transistor is connected as a controlled system to the current mirror.

Thus, the transistor allows a controlled system and is driven via the biased from the reference voltage current mirror.

A next embodiment is that the transistor is a bipolar transistor with low collector-emitter saturation voltage.

Such a low saturation bipolar transistor is also referred to as a "low saturation" bipolar transistor.

It is also an embodiment that the reference voltage source provides a reference voltage that can be generated by the energy source.

A development is that a diode, z. B. a zener diode and / or a light emitting diode, and / or a band-gap reference for adjusting the reference voltage is provided.

An additional embodiment is that the current mirror has two pnp transistors and in which the controlled system comprises a pnp transistor.

The above object is also achieved by a luminaire comprising the circuit as described herein.

The lamp may be a flashlight or a flashlight.

Embodiments of the invention are illustrated and explained below with reference to the drawing.

It shows:

1 a schematic diagram of an LED linear constant current source.

1 shows a schematic circuit diagram for a LED linear constant current source. For example, such a circuit in a mobile light, for. As a flashlight, are used.

A reference voltage source 110 provides a DC voltage Vref of, for example, 2.5V and is across its positive pole with a node 101 and with her negative pole with a knot 102 connected.

The reference voltage source 110 can by means of a diode, z. As a light emitting diode, a Zener diode or a band-gap reference can be set.

The knot 101 is further connected via a resistor R3 to the emitter of a pnp transistor Q2. The base of the transistor Q2 is connected to the base as well as to the collector of a pnp transistor Q3. Furthermore, the collector of the transistor Q3 is connected to the node via a resistor R4 102 connected. The two transistors Q2 and Q3 represent a current mirror.

The collector of the transistor Q2 is connected to the base of a pnp transistor Q1, the base of the transistor Q1 continuing to be connected to the node via a resistor R5 102 connected is.

The knot 101 is connected via a resistor R1 both to the emitter of the transistor Q3 and to the emitter of the transistor Q1. The collector of transistor Q1 is via a light unit 130 comprising a light-emitting diode D5 with the negative pole of a battery 120 connected. The cathode of the LEDs D5 points in the direction of the negative pole of the battery 120 , The positive pole of the battery 120 is with the node 101 connected. The battery provides a voltage equal to Vbat (eg, 4.5 volts). Instead of the LED D5 several LEDs, z. B. in series, as a lighting unit 130 be provided.

The lighting unit 130 may comprise at least one semiconductor element. For example, a white light-emitting diode (LED) can be used as the semiconductor light-emitting element.

The knot 102 is a resistor R6 and a switch S1 with the negative pole of the battery 120 (Ground potential) connected. The switch S1 can also be designed as an electronic switch (eg as a transistor, a mosfet, an IGBT, etc.), so that by means of a known pulse width modulation (PWM) a brightness control (dimming) of the lighting unit 130 can be achieved.

If the switch S1 is closed, the lighting unit 130 over the battery 120 supplied with electrical energy, the current through the lighting unit 130 is adjusted by means of the transistor Q1 so that a voltage drop of the battery voltage Vbat is compensated for as long as possible.

In the 1 The components shown can be dimensioned as follows: R1 = 1Ω, R3 = 100Ω, R4 = 2.2kΩ, R5 = 1kΩ, R6 = 0.5kΩ. The light emitting diode D5 may be a Golden Dragon type white LED, the transistors Q2 and Q3 may be a BCV62, and the transistor Q1 may be a PBSS5250X type.

The current mirror comprising the transistors Q2 and Q3 is set by means of the resistor R3 via the emitter of the transistor Q2 to a reference potential Vref. The resistor R1 serves as a current measuring resistor (shunt) at the emitter of the transistor Q3.

wobei

I_C

den Kollektorstrom,

I_S

einen Sperrstrom,

T

die Temperatur,

U_CE

eine Kollektor-Emitter-Spannung,

U_BE

eine Basis-Emitter-Spannung,

U_T

eine Temperaturspannung

bezeichnen.The collector current is

in which

I _C

the collector current,

I _S

a reverse current,

T

the temperature,

U _CE

a collector-emitter voltage,

U _BE

a base-emitter voltage,

U _T

a temperature voltage

describe.

The temperature voltage U _T is approximately 26 mV at 23 ° C. The current mirror comprises the transistors Q2, Q3, z. B. in the form of a double transistor with almost identical reverse current I _S and almost identical collector current I _C. Thus, for the ratio of the collector currents of the two transistors Q2 and Q3 follows:

The transistor Q1 implements a controlled system and is controlled by the current mirror biased from the reference voltage Vref.

In particular, a so-called low-saturation bipolar transistor Q1 is used as transistor Q1, which can be operated up to very small voltage drops (eg 100 mV) across its emitter-collector path.

The LED current is measured via the resistor R1 (shunt resistor) in the emitter circuit of the transistor Q1 and coupled to the base-commutated and biased by the reference voltage source Vref transistor Q3 of the bipolar current mirror. The other transistor Q2 of the current mirror is arranged in emitter circuit and connected with its collector to the base of the transistor Q1. This will close the loop.

In active operation of the circuit (that is, when switch S1 is closed) takes place at a node 103 a distribution of the provided current such that a portion flows through the transistor Q2 and another part is supplied to the transistor Q1. The proposed dimensioning of the resistors R1, R3 and R5 of the current mirror ensures that a decreasing current through the light-emitting diode D5 is approximately proportional to the current through the resistor R3. Thus, with decreasing current, the current that rises above the node increases 103 to the base of transistor Q1. Thus, a decrease in performance of the battery 120 be at least temporarily compensated, without this leading to a strong loss of brightness of the lighting unit 130 would lead.

Preferably, the switch S1 serves to turn on and off the whole system with low power consumption without having to disconnect the battery. Nevertheless, it is ensured that no power is consumed when switch S1 is open.

For example, at a supply voltage Vbat in a range of 0 V to 5 V, even at a low voltage drop across the resistor R1 and the transistor Q1 in the amount of 280 mV, an LED current of about 142 mA (this already corresponds to approximately 97, 5% of the maximum at the 5V supply voltage).

With this cost-effective circuit, a very good current constancy of the LED with minimal voltage drop, so up to very small battery voltages possible. As a result, the battery is used efficiently, as long as possible, a nearly constant brightness of the LED is guaranteed.

It should be noted that the in 1 shown circuit can also be constructed according to npn transistors (dual).

LIST OF REFERENCE NUMBERS

101

node

102

node

103

node

110

Reference voltage source

120

Battery (power supply)

130

light unit

Ri

resistance

Qi

PNP transistor

S1

switch

Vref

reference voltage

Vbat

battery voltage

Claims (13)

Circuit for operating a lighting unit ( 130 ), Comprising - a current mirror (Q2, Q3), which via a reference voltage source ( 110 ), wherein the current mirror (Q2, Q3) is connected to a controlled system (Q1), by means of which a current is supplied from an energy source (Q1) 120 ) to the light unit ( 130 ) is adjustable. Circuit according to Claim 1, in which the lighting unit ( 130 ) comprises at least one semiconductor element (D5). Circuit according to one of the preceding claims, in which a current measuring resistor (R1) is connected in series with the controlled system (Q1) and the lighting unit ( 130 ), wherein the current measuring resistor (R1) is connected to the current mirror (Q3). Circuit according to Claim 3, in which, parallel to the series connection of current measuring resistor, controlled system and lighting unit, the energy source ( 120 ) is arranged. A circuit according to any one of claims 3 or 4, wherein the current mirror comprises two bipolar transistors (Q2, Q3), the base terminals of the bipolar transistors being connected to each other and to the collector terminal of one of the bipolar transistors (Q3). Circuit according to Claim 5, - In which a bipolar transistor (Q3) of the current mirror is arranged in a base circuit, wherein the current from the current measuring resistor (R1) detectable current to the emitter of this bipolar transistor (Q3) is coupled, - In which the other bipolar transistor (Q2) of the current mirror is arranged in emitter circuit and connected to its collector with the controlled system (Q1). Circuit according to one of the preceding claims, wherein a switch (S1) between the controlled system (Q2) and the energy source ( 120 ) is arranged. Circuit according to one of the preceding claims, in which the controlled system comprises a transistor (Q1). A circuit according to any one of the preceding claims, wherein the reference voltage source provides a reference voltage which is producible by the power source. A circuit according to claim 9, wherein a band-gap reference is provided for adjusting the reference voltage. Circuit according to Claim 9, in which a diode, in particular a Zener diode or a light-emitting diode, is provided for setting the reference voltage. Circuit according to one of the preceding claims, in which the current mirror has two pnp transistors and in which the controlled system comprises a pnp transistor. Luminaire comprising the circuit according to one of the preceding claims.

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