CN102340906A - 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

The light modulating device that is used for led drive circuit

Technical field

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

Background technology

In the illuminators such as fluorescent lamp that common civil power drives,, can switch the brightness of lamp through switch is carried out once " pass " of (in several milliseconds), " opening " action fast from the state of opening.This method realizes with mechanical switch, such as relay etc.

Because there is the higher shortcoming of failure rate in mechanical switch, be prone to 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 that is used for led drive circuit that can overcome the above problems.

The invention provides a kind of light modulating device that is used for led drive circuit; Led drive circuit comprises that primary part provides DC input voitage through transformer-coupled primary part of inverse-excitation type and sub section, and in setting-up time, breaks off and closed DC input voitage; Sub section provides output voltage; Comprise: sample circuit, be used for DC input voitage is taken a sample, obtain sampled voltage; Circuit and square-wave is used for obtaining square-wave signal according to reference voltage and sampled voltage, wherein the high level time t of square-wave signal _OnBe directly proportional with sampled voltage, be inversely proportional to reference voltage; Produce circuit cycle of oscillation, be used to produce T cycle of oscillation; Reference voltage is selected module, is used to judge high level time t _OnWith the magnitude relationship of T cycle of oscillation,, DC input voitage causes high level time t when descending _OnGreater than cycle of oscillation during T, reference voltage is switched selection, and and then send enable signal and close Pwm controller; Pwm controller is used for as high level time t _OnLess 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 is used to drive the work of inverse-excitation type transformer.

Preferably, in above-mentioned light modulating device, reference voltage selects module to comprise: judging unit is used to judge high level time t _OnWith the magnitude relationship of T cycle of oscillation,, DC input voitage causes high level time t when descending _OnGreater than cycle of oscillation during T, send enable signal and close Pwm controller; Counter unit, the judged result that is used for when judging unit is high level time t _OnGreater than cycle of oscillation during T, control reference voltage selected cell switches a plurality of reference voltage sources; The reference voltage selected cell is used for according to the control signal of counter unit a plurality of reference voltage sources being selected; The reference voltage source unit is used to provide a plurality of reference voltage supply reference voltage selected cells to select.

Preferably, in above-mentioned light modulating device, primary part comprises first switch, bridge rectifier and first electric capacity, and wherein, first switch is connected between common civil power and the bridge rectifier; 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 the end of DC power supply and 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 the ancillary coil circuit, is used to light modulating device 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:

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

Wherein, C ₂Be the electric capacity of second electric capacity, V ₀Initial voltage when moving on second electric capacity for the switching tube shutdown switch, V _ThlSelect the minimum service voltage of module memory control signal, I for allowing reference voltage _QStatic working current for light modulating device.

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

The above embodiment of the present invention realizes the brightness regulation to led circuit through the mode of circuit; Improved the stability of circuit; Reduced the failure rate that the brightness of LED is regulated; Overcome in the prior art because there is the higher shortcoming of failure rate in mechanical switch, be prone to cause that circuit is unstable, the problem that causes the brightness of lamp to switch.

Description of drawings

Below with reference to accompanying drawings specific embodiments of the present invention is explained in more detail, in the accompanying drawings:

Fig. 1 is the light modulating device sketch map that is used for led drive circuit according to an embodiment of the invention;

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

Embodiment

Fig. 1 is the light modulating device sketch map that is used for led drive circuit according to an embodiment of the invention.As shown in Figure 1, common civil power alternating current becomes DC input voitage V through behind the bridge rectifier _In-dcModule U1 (t _On∝ (V _Ref/ V _In-dc)) with DC input voitage V _In-dcAfter the sample circuit sampling, 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 among module U2, the 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 that produces among module U7 and the U2.Module U7 is Pwm controller (PWM Controller), and it is at the t that obtains being seen off by U1 _OnSignal and see off by U3 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 the switch drive primary coils, thereby the inverse-excitation type transformer is started working, in each cycle of oscillation, and the peak current I that reaches on the primary coils _PkBy t _OnDecision:

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

Wherein, L _pInductance value for primary coils.Because high level time t _OnWith DC input voitage V _In-dcBe inversely proportional to, with reference voltage V _RefBe directly proportional, that is:

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

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

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

Therefore, the primary coils energy stored is in each cycle:

$\mathrm{Ep}=\frac{1}{2}\·{I_{\mathrm{pk}}}^2\·L_p=\frac{{k_1}^2\·{V_{\mathrm{ref}}}^2}{2L_p}---\left(4\right)$

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

$I_{\mathrm{out}}=\frac{E_p}{V_{\mathrm{LED}}\·T}=\frac{{k_1}^2\·{V_{\mathrm{ref}}}^2}{2L_p\·V_{\mathrm{LED}}\·T}---\left(5\right)$

Visible by formula (5), at the main end, the average current of LED lamp is by V _RefV is worked as in decision _RefWhen confirming, the average current of LED lamp is constant; When switching V _RefValue the time, the average current of LED lamp also can be switched.

Module U5 (V _Ref<0:2>) 3 reference voltage V are provided _Ref<0>, V _Ref<1>And V _Ref<2>, and be input among the module U6, U6 is reference voltage selector (V _RefSelector); (counter, select and output among the module U1 3 reference voltages of U5 by the control of count signal Counter) through module U4 for module U6.As cut-off switch K1, when promptly input AC electricity Vin-ac broke off, the direct voltage Vin-dc behind the bridge rectifier began to descend, because ton and direct voltage Vin-dc are inversely proportional to t _OnBeginning increases.Module U2 (ton＞T?) to signal t _OnCompare with T, work as t _OnDuring＞T, U2 at first sends signal controlling module U4, thereby the output voltage that the output of U4 is overturn make module U6 is at V _Ref<0>, V _Ref<1>And V _Ref<2>In once switch; Secondly U2 can send enable signal, closing module U5 and U7, the quiescent current I of reduction system _QAt this moment, because the action of NMOS pipe M3 shutdown switch, secondary coil and ancillary coil have also all stopped switch motion, and the voltage that stores on the capacitor C 2 of ancillary coil end is that chip continues power supply, makes that the control signal of U4 and U6 is memorized.Memory time t _LifeFor:

$t_{\mathrm{life}}=\frac{C_2\&CenterDot;(V_0-V_{\mathrm{thl}})}{I_Q}---\left(6\right)$

Wherein, C ₂Be the electric capacity of the capacitor C 2 of ancillary coil end, V ₀Initial voltage when moving on the capacitor C 2 for the switching tube shutdown switch, V _ThlLive the minimum service voltage of control signal, I for allowing reference voltage selector U6 memory _QStatic working current for light modulating device.Through rational setting, for example get C ₂=10uF, V ₀=12V, V _Thl=5V, I _Q=30uA can obtain memory time and be:

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

Close 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 opening module U5 and U7 again, and NMOS pipe M3 begins to do switch motion, and driving transformer continues as the power supply of LED lamp.Has only the reference voltage V that is switched this moment _RefChanged, so the average current on the LED lamp has also been switched once.Therefore, through K switch 1 at t _LifeOnce " disconnection, closure " operation in time can realize the light modulation to the LED lamp.

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

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

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

$I_{\mathrm{out}2}=\frac{E_p}{V_{\mathrm{LED}}\&CenterDot;T}=\frac{{k_1}^2\&CenterDot;{V_{\mathrm{ref}2}}^2}{2L_p\&CenterDot;V_{\mathrm{LED}}\&CenterDot;T}---\left(10\right)$

In the embodiment in figure 1, also can directly adopt DC power supply that DC input voitage is provided.Fig. 2 is the light modulating device sketch map according to 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 the end of DC power supply VDC and capacitor C 20, the other end ground connection of capacitor C 20.Other module of Fig. 2 embodiment is identical with Fig. 1 embodiment, through K switch 2 at t _Life" disconnection, closure " operation in time realizes the light modulation to the LED lamp.

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

Claims (7)

1. light modulating device that is used for led drive circuit; It is characterized in that; Said led drive circuit comprises that said primary part provides DC input voitage through transformer-coupled primary part of inverse-excitation type and sub section, and in setting-up time, breaks off and closed said DC input voitage; Said sub section provides output voltage, comprising:

Sample circuit is used for said DC input voitage is taken a sample, and obtains sampled voltage;

Circuit and square-wave is used for obtaining square-wave signal according to reference voltage and said sampled voltage the high level time t of wherein said square-wave signal _OnBe directly proportional with said sampled voltage, be inversely proportional to said reference voltage;

Produce circuit cycle of oscillation, be used to produce T cycle of oscillation;

Reference voltage is selected module, is used to judge said high level time t _OnWith the magnitude relationship of said cycle of oscillation of T,, said DC input voitage causes said high level time t when descending _OnGreater than said cycle of oscillation during T, said reference voltage is switched selection, and and then send enable signal and close Pwm controller;

Said Pwm controller is used for as said high level time t _OnLess than said cycle of oscillation during T, according to said high level time t _OnSwitch with said T generation cycle of oscillation impulse wave control switch pipe;

Said switching tube is used to drive the work of said inverse-excitation type transformer.

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

Judging unit is used to judge said high level time t _OnWith the magnitude relationship of said cycle of oscillation of T,, said DC input voitage causes said high level time t when descending _OnGreater than said cycle of oscillation during T, send enable signal and close Pwm controller;

Counter unit, the judged result that is used for when said judging unit is said high level time t _OnGreater than said cycle of oscillation during T, control reference voltage selected cell switches a plurality of reference voltage sources;

The reference voltage selected cell is used for according to the control signal of said counter unit said a plurality of reference voltage sources being selected;

The reference voltage source unit is used to provide a plurality of reference voltages to supply said reference voltage selected cell to select.

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

Said first switch is connected between common civil power and the said bridge rectifier;

Said first electric capacity is connected between the output and ground of said bridge rectifier.

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

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

5. light modulating device according to claim 1 is characterized in that said sub section also comprises the ancillary coil circuit, is used to said light modulating device 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, said setting-up time t _LifeComputing formula be:

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

Wherein, C ₂Be the electric capacity of said second electric capacity, V ₀Initial voltage when moving on said second electric capacity for said switching tube shutdown switch, V _ThlSelect the minimum service voltage of module memory control signal, I for allowing said reference voltage _QStatic working current for said light modulating device.

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

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