CN102340906B - Dimming device for light emitting diode (LED) driving circuit - Google Patents

Abstract

The invention discloses a dimming device for a light emitting diode (LED) driving device. The dimming device comprises a sampling circuit, a square wave circuit, an oscillation period generation circuit, a reference voltage selection module, a pulse width modulation controller and a switch tube, wherein the sampling circuit is used for sampling a direct current input voltage to obtain a sampling voltage; the square wave circuit is used for obtaining a square wave signal according to a reference voltage and the sampling voltage; high level time ton of the square wave signal is in direct proportion to the sampling voltage and in inverse proportion to the reference voltage; the oscillation period generation circuit is used for generating an oscillation period T; the reference voltage selection module is used for determining a size relation between the high level time ton and the oscillation period T, switching and selecting the reference voltage when the high level time ton is greater than the oscillation period T due to drop of direct current input voltage, and sending an enable signal to close the pulse width modulation controller; and the pulse width modulation controller is used for generating a pulse wave to control the switch-on and switch-off of the switch tube according to the high level time ton and the oscillation period T when the high level time ton is smaller than the oscillation period T; and the switch tube is used for driving a flyback transformer to work.

Description

Light modulating device for LED drive circuit

Technical field

The present invention relates to power domain, in particular to a kind of light modulating device for LED drive circuit.

Background technology

In the common illuminators such as the electrically driven (operated) fluorescent lamp in city, by switch being carried out from the state of opening to once " pass " of (in several milliseconds), " opening " action fast, can switch the brightness of lamp.Mechanical switch is realized for this method, such as relay etc.

Because mechanical switch exists the shortcoming that failure rate is higher, more easily cause that circuit is unstable, cause the brightness of lamp to switch.

Summary of the invention

The invention provides a kind of light modulating device for LED drive circuit that can overcome the above problems.

The invention provides a kind of light modulating device for LED drive circuit, LED drive circuit comprises primary part and the sub section being coupled by flyback transformer, primary part provides DC input voitage, and disconnect and closed DC input voitage in setting-up time, sub section provides output voltage, comprise: sample circuit, for DC input voitage is sampled, obtains sampled voltage; Circuit and square-wave, for obtaining square-wave signal according to reference voltage and sampled voltage, the high level time t of square-wave signal wherein _onbe directly proportional to sampled voltage, be inversely proportional to reference voltage; Produce circuit cycle of oscillation, for generation of T cycle of oscillation; Reference voltage is selected module, for judging high level time t _onwith the magnitude relationship of T cycle of oscillation, when declining, DC input voitage causes high level time t _onbe greater than cycle of oscillation during T, reference voltage switched to selection, and and then send enable signal and close Pwm controller; Pwm controller, for working as high level time t _onbe less than cycle of oscillation during T, according to high level time t _onswitch with T generation cycle of oscillation impulse wave control switch pipe; Switching tube, for driving flyback transformer work.

Preferably, in above-mentioned light modulating device, reference voltage selects module to comprise: judging unit, and for judging high level time t _onwith the magnitude relationship of T cycle of oscillation, when declining, DC input voitage causes high level time t _onbe greater than cycle of oscillation during T, send enable signal and close Pwm controller; Counter unit, for judgment result is that high level time t when judging unit _onbe greater than cycle of oscillation during T, control reference voltage selected cell a plurality of reference voltage sources are switched; Reference voltage selected cell, for selecting a plurality of reference voltage sources according to the control signal of counter unit; Reference voltage source unit, for providing a plurality of reference voltage supply reference voltage selected cells to select.

Preferably, in above-mentioned light modulating device, primary part comprises the first switch, bridge rectifier and the first electric capacity, and wherein, the first switch is connected between common civil power and bridge rectifier; The first electric capacity is connected between the output and ground of bridge rectifier.

Preferably, in above-mentioned light modulating device, primary part comprises second switch and the 3rd electric capacity, and wherein, second switch is connected between DC power supply and one end of the 3rd electric capacity, the other end ground connection of the 3rd electric capacity.

Preferably, in above-mentioned light modulating device, sub section also comprises ancillary coil circuit, is used to light modulating device that operating voltage is provided, and it comprises second electric capacity of connecting with ancillary coil.

Preferably, in above-mentioned light modulating device, setting-up time t _lifecomputing formula be:

$t_{\mathrm{life}}=\frac{{\mathrm{<figure-callout\; id="2"\; label="C"\; filenames="HSA00000205101100011.png"\; state="\{\{state\}\}">C</figure-callout>}}_2\&CenterDot;(V_0-V_{\mathrm{thl}})}{I_Q}$

Wherein, C ₂be the electric capacity of the second electric capacity, V ₀initial voltage while moving for switching tube shutdown switch on the second electric capacity, V _thlfor allowing reference voltage, select the minimum service voltage of module Memory control signal, I _qstatic working current for light modulating device.

Preferably, in above-mentioned light modulating device, switching tube is MOSFET (MetalOxide Semicoductor Field Effect Transistor, metal oxide semiconductor field effect tube).

The above embodiment of the present invention realizes the brightness regulation to LED circuit by the mode of circuit, improved the stability of circuit, reduced the failure rate that the brightness of LED is regulated, overcome in prior art because mechanical switch exists the shortcoming that failure rate is higher, more easily cause that circuit is unstable, the problem that causes the brightness of lamp to switch.

Accompanying drawing explanation

Below with reference to accompanying drawings specific embodiment of the invention scheme is described in detail, in the accompanying drawings:

Fig. 1 is according to an embodiment of the invention for the light modulating device schematic diagram of LED drive circuit;

Fig. 2 is according to the light modulating device schematic diagram of the direct employing DC power supply of Fig. 1 embodiment.

Embodiment

Fig. 1 is according to an embodiment of the invention for the light modulating device schematic diagram of LED drive circuit.As shown in Figure 1, common civil power alternating current becomes DC input voitage V after bridge rectifier _in-dc.Module U1 (t _on∝ (V _ref/ V _in-dc)) by DC input voitage V _in-dcafter sampling by sample circuit, draw a square-wave signal, its high level time t _onwith DC input voitage V _in-dcbe inversely proportional to, with reference voltage V _refbe directly proportional, module U1 delivers to this square-wave signal in module U2, U7.Module U3 is the circuit (T Generator) that produces cycle of oscillation, and it is sent to the signal of the cycle of oscillation of generation in module U7 and U2.Module U7 is Pwm controller (PWM Controller), the t that it is obtaining being sent by U1 _onsignal and by U3, sent cycle of oscillation T signal after, produce an impulse wave and drive external NMOS pipe M3, the high level time of this pulse signal is t _on, the cycle is T.NMOS manages M3 as switch drive primary coils, thereby flyback transformer is started working, within each cycle of oscillation, and the peak current I reaching on primary coils _pkby t _ondetermine:

$I_{\mathrm{pk}}=\frac{V_{\mathrm{in}-\mathrm{dc}}}{L_p}\&CenterDot;t_{\mathrm{on}}---\left(1\right)$

Wherein, L _pinductance value for primary coils.Due to high level time t _onwith DC input voitage V _in-dcbe inversely proportional to, with reference voltage V _refbe directly proportional, that is:

$t_{\mathrm{on}}=k_1\&CenterDot;\frac{V_{\mathrm{ref}}}{V_{\mathrm{in}-\mathrm{dc}}}---\left(2\right)$

K wherein ₁represent proportionality coefficient.Therefore can obtain the expression formula of primary coils peak current:

$I_{\mathrm{pk}}=\frac{V_{\mathrm{in}-\mathrm{dc}}}{L_p}\&CenterDot;k_1\&CenterDot;\frac{V_{\mathrm{ref}}}{V_{\mathrm{in}-\mathrm{dc}}}=k_1\&CenterDot;\frac{V_{\mathrm{ref}}}{L_p}---\left(3\right)$

Therefore the energy that, primary coils stores within each cycle is:

$\mathrm{Ep}=\frac{1}{2}\&CenterDot;{I_{\mathrm{<figure-callout\; id="2"\; label="pk"\; filenames="HSA00000205101100011.png"\; state="\{\{state\}\}">pk</figure-callout>}}}^2\&CenterDot;L_p=\frac{{k_1}^2\&CenterDot;{{\mathrm{<figure-callout\; id="2"\; label="V\; ref"\; filenames="HSA00000205101100011.png"\; state="\{\{state\}\}">V</figure-callout>}}_{\mathrm{ref}}}^2}{2L_p}---\left(4\right)$

The coupling coefficient of supposing transformer is 100%, i.e. energy free of losses, and the average current at secondary end high-powered LED lamp is so:

$I_{\mathrm{out}}=\frac{E_p}{V_{\mathrm{LED}}\&CenterDot;T}=\frac{{k_1}^2\&CenterDot;{{\mathrm{<figure-callout\; id="2"\; label="V\; ref"\; filenames="HSA00000205101100011.png"\; state="\{\{state\}\}">V</figure-callout>}}_{\mathrm{ref}}}^2}{2L_p\&CenterDot;V_{\mathrm{LED}}\&CenterDot;T}---\left(5\right)$

From formula (5), at main end, the average current of LED lamp is by V _refdetermine, work as V _refwhile determining, the average current of LED lamp is constant; When switching V _refvalue time, the average current of LED lamp also can be switched.

Module U5 (V _ref<0:2>) provide 3 reference voltage V _ref<0>, V _ref<1> and V _ref<2>, and be input in module U6, U6 is reference voltage selector (V _refselector); Module U6 by module U4, (counter, select and output in module U1 3 of U5 reference voltages by the control of count signal Counter).As cut-off switch K1, when input AC electricity Vin-ac disconnects, the direct voltage Vin-dc after bridge rectifier starts to decline, because ton and direct voltage Vin-dc are inversely proportional to, and t _onstarting increases.Module U2 (ton > T?) to signal t _oncompare with T, work as t _onduring > T, first U2 sends signal controlling module U4, thereby the output of U4 is overturn, makes the output voltage of module U6 at V _ref<0>, V _ref<1> and V _refin <2>, once switch; Secondly U2 can send enable signal, closing module U5 and U7, the quiescent current I of reduction system _q.Now, due to the action of NMOS pipe M3 shutdown switch, secondary coil and ancillary coil have also all stopped switch motion, and the voltage storing in the capacitor C 2 of ancillary coil end is that chip continues power supply, and the control signal of U4 and U6 is memorized.Memory time t _lifefor:

$t_{\mathrm{life}}=\frac{{\mathrm{<figure-callout\; id="2"\; label="C"\; filenames="HSA00000205101100011.png"\; state="\{\{state\}\}">C</figure-callout>}}_2\&CenterDot;(V_0-V_{\mathrm{thl}})}{I_Q}---\left(6\right)$

Wherein, C ₂for the electric capacity of the capacitor C 2 of ancillary coil end, V ₀initial voltage while moving for switching tube shutdown switch in capacitor C 2, V _thlfor allowing reference voltage selector U6 to remember the minimum service voltage of control signal, I _qstatic working current for light modulating device.By rational setting, for example, get C ₂=10uF, V ₀=12V, V _thl=5V, I _q=30uA can obtain memory time and be:

$t_{\mathrm{life}}=\frac{10u\&times;(12-5)}{30u}\&ap;2.3S---\left(7\right)$

Closing Switch K1 during this period of time, i.e. incoming transport electricity V _in-ac, each module works under the prerequisite of not losing memory.As direct voltage V _in-dcbe increased to some values, make t _onduring < T, U2 sends enable signal and reopens module U5 and U7, and NMOS pipe M3 starts to do switch motion, and driving transformer continues as the power supply of LED lamp.The reference voltage V being now only switched _refchanged, so the average current on LED lamp has also been switched once.Therefore, by K switch 1 at t _lifeonce " disconnection, closure " operation in time, can realize the light modulation to LED lamp.

Module U4 has the function of cycle count, therefore can carry out cyclic switching to 3 reference voltages, and the average current on LED lamp just can constantly switch on following three gears:

$I_{\mathrm{out}0}=\frac{E_p}{V_{\mathrm{LED}}\&CenterDot;T}=\frac{{k_1}^2\&CenterDot;{V_{\mathrm{ref}0}}^2}{2L_p\&CenterDot;V_{\mathrm{LED}}\&CenterDot;T}---\left(8\right)$

$I_{\mathrm{out}1}=\frac{E_p}{V_{\mathrm{LED}}\&CenterDot;T}=\frac{{k_1}^2\&CenterDot;{V_{\mathrm{ref}1}}^2}{2L_p\&CenterDot;V_{\mathrm{LED}}\&CenterDot;T}---\left(9\right)$

$I_{\mathrm{out}2}=\frac{E_p}{V_{\mathrm{LED}}\&CenterDot;T}=\frac{{k_1}^2\&CenterDot;{{\mathrm{<figure-callout\; id="2"\; label="V\; ref"\; filenames="HSA00000205101100011.png"\; state="\{\{state\}\}">V</figure-callout>}}_{\mathrm{ref}}}^2}{2L_p\&CenterDot;V_{\mathrm{LED}}\&CenterDot;T}---\left(10\right)$

In the embodiment in figure 1, also can directly adopt DC power supply that DC input voitage is provided.Fig. 2 is according to the light modulating device schematic diagram of the direct employing DC power supply of Fig. 1 embodiment.As shown in Figure 2, primary part comprises DC power supply VDC, K switch 2 and capacitor C 20, and wherein, K switch 2 is connected between DC power supply VDC and one end of capacitor C 20, the other end ground connection of capacitor C 20.Other module of Fig. 2 embodiment is identical with Fig. 1 embodiment, by K switch 2 at t _life" disconnection, closure " operation in time, realizes the light modulation to LED lamp.

Obviously, do not departing under the prerequisite of true spirit of the present invention and scope, the present invention described here can have many variations.Therefore, all changes that it will be apparent to those skilled in the art that, within all should being included in the scope that these claims contain.The present invention's scope required for protection is only limited by described claims.

Claims (7)

1. the light modulating device for LED drive circuit, it is characterized in that, described LED drive circuit comprises primary part and the sub section being coupled by flyback transformer, described primary part provides DC input voitage, and disconnect and closed described DC input voitage in setting-up time, described sub section provides output voltage, comprising:

Sample circuit, described DC input voitage, by described sample circuit, obtains square-wave signal, the high level time t of wherein said square-wave signal _onbe directly proportional to reference voltage, be inversely proportional to described DC input voitage;

Produce circuit cycle of oscillation, for generation of T cycle of oscillation;

Reference voltage is selected module, for judging described high level time t _onwith the magnitude relationship of described cycle of oscillation of T, when declining, described DC input voitage causes described high level time t _onbe greater than described cycle of oscillation during T, described reference voltage switched to selection, and and then send enable signal and close Pwm controller;

Described Pwm controller, for working as described high level time t _onbe less than described cycle of oscillation during T, according to described high level time t _onswitch with described T generation cycle of oscillation impulse wave control switch pipe;

Described switching tube, for driving described flyback transformer work.

2. light modulating device according to claim 1, is characterized in that, described reference voltage selects module to comprise:

Judging unit, for judging described high level time t _onwith the magnitude relationship of described cycle of oscillation of T, when declining, described DC input voitage causes described high level time t _onbe greater than described cycle of oscillation during T, send enable signal and close Pwm controller;

Counter unit, for the described high level time t that judgment result is that when described judging unit _onbe greater than described cycle of oscillation during T, control reference voltage selected cell a plurality of reference voltage sources are switched;

Reference voltage selected cell, for selecting described a plurality of reference voltage sources according to the control signal of described counter unit;

Reference voltage source unit, for providing a plurality of reference voltages to select for described reference voltage selected cell.

3. light modulating device according to claim 1, is characterized in that, described primary part comprises the first switch, bridge rectifier and the first electric capacity, wherein

Described the first switch is connected between common civil power and described bridge rectifier;

Described the first electric capacity is connected between the output and ground of described bridge rectifier.

4. light modulating device according to claim 1, is characterized in that, described primary part comprises second switch and the 3rd electric capacity, wherein

Described second switch is connected between DC power supply and one end of described the 3rd electric capacity, the other end ground connection of described the 3rd electric capacity.

5. light modulating device according to claim 1, is characterized in that, described sub section also comprises ancillary coil circuit, is used to described light modulating device that operating voltage is provided, and it comprises second electric capacity of connecting with ancillary coil.

6. light modulating device according to claim 5, is characterized in that, described setting-up time t _lifecomputing formula be:

$t_{\mathrm{life}}=\frac{C_2\&bull;(V_0-V_{\mathrm{thl}})}{I_Q};$

Wherein, C ₂for the electric capacity of described the second electric capacity, V ₀initial voltage described in during for described switching tube shutdown switch action on the second electric capacity, V _thlfor allowing described reference voltage, select the minimum service voltage of module Memory control signal, I _qstatic working current for described light modulating device.

7. light modulating device according to claim 1, is characterized in that, described switching tube is MOSFET.

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