CN103025021A - Step-down light emitting diode (LED) drive circuit based on electrical inductance discharge time modulation - Google Patents

Abstract

The invention provides a step-down light emitting diode (LED) drive circuit based on electrical inductance discharge time modulation and mainly solves the problem that internal and peripheral circuits of existing LED drive circuits are complicated. The step-down LED drive circuit comprises a rectifier bridge, a power switch device M, an output diode D, an inductor L, an output capacitor (COUT) and a control circuit (1). A forward direction end of the rectifier bridge is connected to one end of the inductor L, the other end of the inductor L is connected with a drain electrode of the power switch device M and a forward direction end of the output diode D, the control circuit (1) is in a bridge joint between the drain electrode and a grid electrode of the power switch device M, the discharge time of the inductor L is modulated by controlling turn-off time of the power switch device M, a negative direction end of the output diode D is connected to a load LED, the load LED is in a parallel connection with the output capacitor (COUT) to be in the bridge joint between the output diode D and the forward direction end of the rectifier bridge, and the output power can be adjusted by controlling the discharge time of the inductor. The step-down LED drive circuit based on the electrical inductance discharge time modulation has the advantages that the internal and peripheral circuits are simple, and the step-down LED drive circuit can be applied to an LED bulb lamp and an LED light tube.

Description

Step-down led drive circuit based on the inductive discharge time-modulation

Technical field

The invention belongs to the electronic circuit technology field, especially relate to a kind of step-down led drive circuit based on the inductive discharge time-modulation, can be used for the design of analog integrated circuit.

Background technology

LED because it has luminous efficiency height, life-span length, high brightness, energy-saving and environmental protection and the characteristics such as durable, is considered to one of light source of current tool potentiality in illumination is used.In order to give full play to its advantage, and fully compatible with former generation illuminating product such as incandescent lamp, power saving fluorescent lamps etc., it is little to need to be equipped with volume, and the life-span is long, and cost is low, and direct led drive circuit take electric main as power supply, to bring into play to greatest extent the LED intrinsic advantage.Led drive circuit take electric main as power supply is of a great variety at present, and wherein voltage-dropping type AC-DC Switching Power Supply led drive circuit is low with its voltage, safe, and the advantage that isolation is good is occupied critical positions in LED.Voltage-dropping type AC-DC Switching Power Supply led drive circuit mainly can be divided into two classes.

With reference to figure 1, the first is non-isolation type AC-DC converter, or claims Buck type AC-DC converter, comes from classical adjusting and voltage-reduction switch Power Management Design.When switch MOS FET pipe S conducting, the sustained diode cut-off, AC power is load LED power supply simultaneously by being induction charging behind the rectifier bridge; When switch MOS FET pipe S turn-offs, inductive discharge, the sustained diode conducting continues as load LED power supply.Repeatedly conducting and the shutoff of controller C control switch MOSFET pipe S discharge and recharge inductance L, with the driving LED load.This type of led drive circuit advantage is that efficient is high; But obvious shortcoming is arranged also, the source potential that is switch MOS FET pipe S is floated, this causes controller C to drive it with boostrap circuit or transformer isolation type of drive, increased the complexity of controller internal circuit, so the example of driving LED is considerably less in fact by this way.

With reference to figure 2, the second is isolated form AC-DC converter, or claims inverse-excitation type AC-DC converter, is widely used in now LED and drives the field.During switch MOS FET pipe S conducting, alternating current input power supplying is transformer main inductance Lp charging, sustained diode cut-off this moment, and output capacitance Co is load LED power supply.S disconnects moment, transformer main inductance Lp discharge, and polarity inversion, secondary coil L1 polarity inversion, the sustained diode conducting, main inductance is powered by the transformer coupled load LED of being, is output capacitance Co charging simultaneously.Feedback coil L2 is corresponding with secondary coil L1 Same Name of Ends, and the coupled voltages of L2 and L1 both end voltage are proportional, by resistance R 1, feeds back to controller C after the R2 dividing potential drop.Controller C changes the parameters such as duty cycle of switching, frequency by the value that detects feedback voltage V FB, with according to V _FbRegulate V _Out, realize constant current output.The advantage of isolated form AC-DC converter is the isolation between the high-low pressure, can accomplish less output voltage and larger output current, meets some specific safety standards; Its shortcoming also clearly, the existence of transformer is so that its conversion efficiency can't be accomplished very highly, under the low-power applications even do not reach 80%, and peripheral components is various, debugging is complicated.

Protection to the LED device also is a very important problem.When driving cascaded structure LED with traditional buck configuration, in case surging appears in input voltage, above-mentioned two kinds of structures can directly be passed to output with this surging, the LED device is formed damage.

Summary of the invention

The object of the invention is to the defective for above-mentioned existing power supply System on Chip/SoC power supply technique, a kind of step-down led drive circuit based on the inductive discharge time-modulation is provided, with facilitating chip internal circuit and peripheral circuit, improve conversion efficiency, the establishment abrupt voltage wave is on the impact of LED device.

For achieving the above object, the present invention is based on the step-down led drive circuit of inductive discharge time-modulation, comprise rectifier bridge, device for power switching M, output diode D, inductance L and output capacitance C _OUTIt is characterized in that: the rectifier bridge forward end is connected to an end of inductance L, the other end of inductance L links to each other with the drain electrode of device for power switching M and the forward end of output diode D, cross-over connection has control circuit between the drain electrode of device for power switching M and the grid, this control circuit is by the turn-off time of power ratio control switching device M, to modulating the discharge time of inductance L; The negative end of output diode D is connected to load LED, this load LED and output capacitance C _OUTBe connected across after the parallel connection between output diode D and the rectifier bridge forward end.

Above-mentioned step-down led drive circuit based on the inductive discharge time-modulation, wherein said control circuit comprise current detection circuit, discharge timing circuit and logical circuit;

Described peak current detection circuit, the current sampling signal of its input termination inductance L, its output termination logical circuit; Be used for the sampled signal of inductive current L is judged, surpass the current threshold I that sets when the inductive current sampled signal _LIMThe time, output logic signal turn-offs device for power switching M to logical circuit by logical circuit, and is constant with the peak current that guarantees inductance;

Described discharge timing circuit, its output termination logical circuit; Be used for the turn-off time of switch M is carried out timing, as the timing time T that reaches setting _SetThe time, output logic signal makes again conducting of device for power switching M to logical circuit by logical circuit, to guarantee that power output is constant;

Described logical circuit, two input are respectively the output of peak current detection circuit and the output of discharge timing circuit, the grid of its output termination device for power switching M; Carry out logic synthesis by two logical signals to peak detection circuit and discharge timing circuit output, the conducting of control switch M and shutoff realize the inductive discharge time-modulation by the turn-off time of power ratio control switching device M.

The present invention compared with prior art has following advantage:

(1) the present invention is owing to adopt the voltage-dropping type framework, and load LED both end voltage is lower than input voltage, so does not need transformer in the peripheral circuit, has simplified peripheral circuit;

(2) the present invention modulated the inductive discharge time, the size of control output current, and constant output current then load LED both end voltage is constant, does not therefore need output voltage is sampled, and has simplified peripheral circuit;

(3) the present invention is because the source ground of device for power switching M, therefore need not bootstrap type or transformer coil removes to drive the high-voltage MOSFET with unsteady source electrode, simplified driving circuit structure;

(4) the present invention is because the inductive discharge timing time is long, and then power output is little, and the discharge timing time is short, and then power output is large, and the user can customize according to demand, and power output is flexible and changeable.

Below by drawings and Examples, technical scheme of the present invention is described in further detail.

Description of drawings

Fig. 1 is that existing non-isolation type AC-DC converter is as the schematic diagram of led drive circuit;

Fig. 2 is that existing isolated form AC-DC converter is as the schematic diagram of led drive circuit;

Fig. 3 is schematic diagram of the present invention;

Fig. 4 is the control circuit block diagram among the present invention;

Fig. 5 is the inductive discharge loop equivalence configuration diagram among the present invention;

Fig. 6 is the induction charging loop equivalence configuration diagram among the present invention;

Fig. 7 is work schedule oscillogram of the present invention.

Embodiment

The invention will be further described below in conjunction with accompanying drawing and embodiment.

With reference to figure 3, the step-down led drive circuit that the present invention is based on the inductive discharge time-modulation comprises: rectifier bridge, device for power switching M, output diode D, inductance L, output capacitance C _OUTWith control circuit 1; Wherein the rectifier bridge input connects electric main, its negative end ground connection, and forward end is connected to an end of inductance L; The other end of inductance L is connected to the drain electrode of device for power switching M, is used for storage power; The source ground of switching device M is for the charging and discharging of control inductance L; The input a of control circuit 1 is connected to the drain electrode of switch power device M, and output b is connected to the grid of switching device M, is used for conducting and the shutoff of control switch device M; The forward end of output diode D is connected to the drain electrode of switch control device M, and its negative end is connected to output capacitance C _OUTWith load LED forward end, be used for preventing output capacitance C _OUTDischarging current pour in down a chimney to ground by switching device M; Output capacitance C _OUTBe connected across between the forward end of output diode D and rectifier bridge load capacitance C after load LED is in parallel _OUTBe used between the inductance L charge period, being load LED power supply, assurance output voltage and current constant.

With reference to Fig. 4, described control circuit 1 comprises peak current detection circuit 11, discharge timing circuit 12, logical circuit 13; Wherein the peak current detection circuit 11, the current sampling signal of its input termination inductance L, its output termination logical circuit 13; Be used for the sampled signal of inductive current L is judged, surpass the current threshold I that sets when the inductive current sampled signal _LIMThe time, the logical signal of output control peak current turn-offs device for power switching M to logical circuit 13 by logical circuit 13, and is constant with the peak current that guarantees inductance; Discharge timing circuit 12, its output termination logical circuit 13 is used for the turn-off time of switch M is carried out timing, as the timing time T that reaches setting _SetThe time, the logical signal of output controlled discharge time makes again conducting of device for power switching M to logical circuit 13 by logical circuit 13, to guarantee that power output is constant; The grid of logical circuit 13 output termination device for power switching M; Carry out logic synthesis by two logical signals to peak detection circuit 11 and 12 outputs of discharge timing circuit, the conducting of control switch M and shutoff realize the inductive discharge time-modulation by the turn-off time of power ratio control switching device M.

Fig. 5 provides the inductive discharge loop equivalence configuration diagram among the present invention, and wherein dotted line frame inside is the discharge loop of inductance, and inductive current is load LED power supply by output diode D, gives simultaneously output capacitance C _OUTInductance is flowed back in charging at last.

The voltage at load LED two ends is:

V _out=V _L+V _in-V _in=V _L 1)

V wherein _LInduced electromotive force during for inductive discharge, V _InBe input voltage, therefore led drive circuit of the present invention is the reduction voltage circuit that load LED negative pole floats, and this structure only need adopt less output capacitance to get final product regulated output voltage.

If ignore efficiency of transmission, the power output of any switching power circuit can represent that with inductance average current and inductance value power output of the present invention also can be drawn by following formula:

$P_{\mathrm{out}}=\frac{1}{2}\·L\·I_L^2---2)$

Wherein Pout is power output, and L is the inductance value of inductance L, I _LBe the current average in the inductance L.

Fig. 6 provides the induction charging loop equivalence configuration diagram among the present invention, and wherein dotted line frame inside is the induction charging loop; Rectifier bridge negative end ground connection, electric current flows through inductance L from the rectifier bridge forward end, and M flow back into ground by the switch power device.

Making peak inductive current at the peak current detection circuit is a fixed value, is made as I _MaxThe time, because the characteristic that inductance discharges and recharges:

$V=L\frac{\mathrm{di}}{\mathrm{dt}}---3)$

Wherein V is the inductance both end voltage, and i is inductive current, and t is the time, and inductive current i is linear change rule, then I under constant voltage _LValue just can be drawn by following formula:

$I_L=\frac{1}{2}\·(I_{\max }+I_{\min })\·(1-D)---4)$

I wherein _MinBe the minimum inductance electric current, D is that inductive current is time of 0 to account for the duty ratio in master switch cycle; At this moment as can be known, as peak inductive current I _MaxWhen constant, this power circuit power output and minimum inductance electric current I _MinD is relevant with duty ratio.Be t if establish the device for power switching M turn-off time _Off, at inductive current i from peak inductive current I _MaxBeginning has in the linear decline process:

$I_{\min }=I_{\max }-\frac{V}{L}\·t_{\mathrm{off}},I_{\min }>0---5)$

Drop to after zero at inductive current i, have:

$D=\frac{t_{\mathrm{off}}-\frac{L}{V}{I}_{\max }}{t_{\mathrm{on}}+t_{\mathrm{off}}},I_{\min }=0---6)$

T wherein _OnBe device for power switching M ON time, obviously at peak inductive current I _MaxOne regularly, the ON time t of device for power switching M _OnBe definite value; Can be drawn the minimum inductance electric current I by upper two formulas _MinWith duty ratio D all are turn-off time t of device for power switching M _OffFunction, change the turn-off time t of device for power switching M _Off, also just changed the minimum inductance electric current I _MinWith duty ratio D, thereby changed average inductor current I _L, finally realized power output P _OutControl; Because the I-V characteristic of LED itself, this power supply output voltage in large load current scope remains unchanged substantially very much, thereby has further guaranteed the turn-off time t of switch power device M _OffTo power output P _OutModulation.Turn-off time t _OffLarger, inductive current mean value I _LLess, power output P _OutAlso just less, for the LED load, it is less to be embodied in load current.If can come by changing external parameter the turn-off time t of by-pass cock device M _Off, then realized the adjusting to LED load brightness.

With reference to figure 7, in conjunction with the time ordered pair principle of the invention be further elaborated: switching signal is the signal of logical circuit 13 outputs, the turn-on and turn-off of power ratio control switching device M, switching signal is that high level represents device for power switching M conducting, and switching signal is that low level represents that device for power switching M turn-offs; In the minimum inductance electric current I _MinIn=0 situation, inductive current i linearity rises to peak inductive current I during the conducting of device for power switching M _Max, inductive current i was linear when device for power switching M turn-offed descends, and continues for some time after dropping to zero; In the minimum inductance electric current I _MinIn 0 the situation, inductive current i linearity rises to peak inductive current I during the conducting of device for power switching M _Max, inductive discharge when device for power switching M turn-offs, inductive current i is linear to descend, and drops to the turn-off time t of device for power switching M before zero at inductive current i _OffFinish, begin next turn-on cycle; For the LED load, can i drop to zero and only depend on t _OffSize, thereby to inductance average current I _LModulate, realize the control to power output.

The above; it only is preferred embodiment of the present invention; be not that the present invention is imposed any restrictions, every according to the technology of the present invention essence to any simple modification, change and equivalent structure transformation that above embodiment does, all still belong in the protection range of technical solution of the present invention.

Claims (4)

1. the step-down led drive circuit based on the inductive discharge time-modulation comprises rectifier bridge, device for power switching M, output diode D, inductance L and output capacitance C _OUTIt is characterized in that: the rectifier bridge forward end is connected to an end of inductance L, the other end of inductance L links to each other with the drain electrode of device for power switching M and the forward end of output diode D, cross-over connection has control circuit (1) between the drain electrode of device for power switching M and the grid, this control circuit (1) is by the turn-off time of power ratio control switching device M, to modulating the discharge time of inductance L; The negative end of output diode D is connected to load LED, this load LED and output capacitance C _OUTBe connected across after the parallel connection between output diode D and the rectifier bridge forward end.

2. step-down led drive circuit according to claim 1 is characterized in that described control circuit (1), comprises current detection circuit (11), discharge timing circuit (12) and logical circuit (13);

Described peak current detection circuit (11), the current sampling signal of its input termination inductance L, it exports termination logical circuit (13); Be used for the sampled signal of inductive current L is judged, surpass the current threshold I that sets when the inductive current sampled signal _LIMThe time, output logic signal turn-offs device for power switching M to logical circuit (13) by logical circuit (13), and is constant with the peak current that guarantees inductance;

Described discharge timing circuit (12), it exports termination logical circuit (13); Be used for the turn-off time of switch M is carried out timing, as the timing time T that reaches setting _SetThe time, output logic signal makes again conducting of device for power switching M to logical circuit (13) by logical circuit (13), to guarantee that power output is constant;

Described logical circuit (13), two input are respectively the output of peak current detection circuit (11) and the output of discharge timing circuit (12), the grid of its output termination device for power switching M; Carry out logic synthesis by two logical signals to peak detection circuit (11) and discharge timing circuit (12) output, the conducting of control switch M and shutoff realize the inductive discharge time-modulation by the turn-off time of power ratio control switching device M.

3. step-down led drive circuit according to claim 2 is characterized in that described current threshold I _LIM, customizing according to user's request, its maximum is no more than the saturation current of inductance L.

4. step-down led drive circuit according to claim 2 is characterized in that described timing time T _Set, its size determines load current size, customizes timing time T according to the size of the required load current of user _SetLarger, load current is less, timing time T _SetLess, load current is larger.

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