CN103025021B - 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

Based on the step-down LED drive circuit of inductive discharge time-modulation

Technical field

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

Background technology

LED in illumination applications, because of its have that luminous efficiency is high, life-span length, high brightness, energy-saving and environmental protection and the feature such as durable, be considered to one of light source of current most potentiality.In order to give full play to its advantage, and with former generation illuminating product as incandescent lamp, power saving fluorescent lamps etc. are fully compatible, need to be equipped with volume little, the life-span is long, and cost is low, and take directly electric main as the LED drive circuit of power supply, to play LED intrinsic advantage to greatest extent.The LED drive circuit with electric main being power supply is at present of a great variety, and wherein voltage-dropping type AC-DC Switching Power Supply LED drive circuit is low with its voltage, and fail safe is high, and the advantage that isolation is good occupies critical positions in the 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 the S conducting of switch MOS FET pipe, sustained diode is ended, and AC power is induction charging after passing through rectifier bridge, simultaneously for load LED powers; When switch MOS FET pipe S turns off, inductive discharge, sustained diode conducting, continues as load LED and powers.Conducting repeatedly and the shutoff of controller C control switch MOSFET pipe S make inductance L discharge and recharge, with driving LED load.This type of LED drive circuit advantage is that efficiency is high; But also there is obvious shortcoming, namely the source potential of switch MOS FET pipe S is floating, this causes controller C to need to drive it by boostrap circuit or transformer isolation type of drive, add 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 field of LED drive now.During the S conducting of switch MOS FET pipe, alternating current input power supplying is that transformer main inductance Lp charges, and now sustained diode cut-off, output capacitance Co is that load LED powers.S disconnects moment, and transformer main inductance Lp discharge, polarity inversion, secondary coil L1 polarity inversion, sustained diode conducting, and main inductance is powered for load LED by transformer coupled, is that output capacitance Co charges simultaneously.Feedback coil L2 is corresponding with secondary coil L1 Same Name of Ends, and coupled voltages and the L1 both end voltage of L2 are proportional, by feeding back to controller C after resistance R1, R2 dividing potential drop.Controller C changes the parameter such as duty cycle of switching, frequency by the value detecting 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 high-low pressure, can accomplish less output voltage and larger output current, meets some specific safety standards; Also clearly, it is very high that the existence of transformer makes its conversion efficiency to accomplish to its shortcoming, even do not reach 80%, and peripheral components is various under low-power applications, and debugging is complicated.

Also be a very important problem to the protection of LED component.When driving cascaded structure LED by conventional buck structure, once surging appears in input voltage, this surging directly can be passed to output by above-mentioned two kinds of structures, is formed damage LED component.

Summary of the invention

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

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 _oUT; It is characterized in that: rectifier bridge forward end is connected to one end of inductance L, the other end of inductance L is connected with the forward end of output diode D with the drain electrode of device for power switching M, between the drain electrode of device for power switching M and grid, cross-over connection has control circuit, this control circuit, by controlling the turn-off time of device for power switching M, is modulated 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 parallel connection between output diode D and rectifier bridge forward end.

The above-mentioned step-down LED drive circuit based on inductive discharge time-modulation, wherein said control circuit comprises current detection circuit, electric discharge timing circuit and logical circuit;

Described peak current detection circuit, the current sampling signal of its input termination inductance L, it exports termination logical circuit; For judging the sampled signal of inductive current L, when inductive current sampled signal exceedes the current threshold I of setting _lIMtime, output logic signal, to logical circuit, makes device for power switching M turn off by logical circuit, constant to ensure the peak current of inductance;

Described electric discharge timing circuit, it exports termination logical circuit; For carrying out timing to the turn-off time of switch M, when reaching the timing time T of setting _settime, output logic signal, to logical circuit, makes device for power switching M conducting again by logical circuit, to ensure that power output is constant;

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

The present invention compared with prior art tool has the following advantages:

(1) the present invention is owing to adopting buck mode architecture, and load LED both end voltage, lower than input voltage, does not therefore need transformer in peripheral circuit, simplifies peripheral circuit;

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

(3) the present invention is due to the source ground of device for power switching M, therefore removes to drive the high-voltage MOSFET with floating source without the need to bootstrap type or transformer coil, simplifies driving circuit structure;

(4) the present invention due to inductive discharge timing time long, then power output is little, and electric discharge timing time is short, then power output is large, and 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.

Accompanying drawing explanation

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

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

Fig. 3 is schematic diagram of the present invention;

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

Fig. 5 is the inductive discharge loop equivalent architectures schematic diagram in the present invention;

Fig. 6 is the induction charging loop equivalent architectures schematic diagram in the present invention;

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

Embodiment

Below in conjunction with accompanying drawing and embodiment, the invention will be further described.

With reference to figure 3, the step-down LED drive circuit that the present invention is based on 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 rectifier bridge input connects electric main, its negative end ground connection, and forward end is connected to one end of inductance L; The other end of inductance L is connected to the drain electrode of device for power switching M, for storage power; The source ground of switching device M, for controlling the charging and discharging of inductance L; The input a of control circuit 1 is connected to the drain electrode of switching power devices M, and output b is connected to the grid of switching device M, 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, 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 output diode D and the forward end of rectifier bridge after in parallel with load LED, load capacitance C _oUTfor between inductance L charge period for load LED powers, ensure output voltage and current constant.

With reference to Fig. 4, described control circuit 1, comprises peak current detection circuit 11, electric discharge timing circuit 12, logical circuit 13; Wherein peak current detection circuit 11, the current sampling signal of its input termination inductance L, it exports termination logical circuit 13; For judging the sampled signal of inductive current L, when inductive current sampled signal exceedes the current threshold I of setting _lIMtime, the logical signal exporting control peak current, to logical circuit 13, makes device for power switching M turn off by logical circuit 13, constant to ensure the peak current of inductance; Electric discharge timing circuit 12, it exports termination logical circuit 13, for carrying out timing to the turn-off time of switch M, when reaching the timing time T of setting _settime, the logical signal exporting the controlled discharge time, to logical circuit 13, makes device for power switching M conducting again by logical circuit 13, to ensure that power output is constant; Logical circuit 13 exports the grid of termination device for power switching M; Carry out logic synthesis, the conducting of control switch M and shutoff by two logical signals exported peak detection circuit 11 and electric discharge timing circuit 12, realize inductive discharge time-modulation by the turn-off time controlling device for power switching M.

Fig. 5 provides the inductive discharge loop equivalent architectures schematic diagram in the present invention, and wherein dotted line frame inside is the discharge loop of inductance, and inductive current is that load LED powers by output diode D, gives output capacitance C simultaneously _oUTcharging, finally flows back to inductance.

The voltage at load LED two ends is:

V _out=V _L+V _in-V _in=V _L1)

Wherein V _lfor induced electromotive force during inductive discharge, V _infor 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 needs to 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 with inductance average current and inductance value, and 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 _lfor the current average in inductance L.

Fig. 6 provides the induction charging loop equivalent architectures schematic diagram in the present invention, and wherein dotted line frame inside is induction charging loop; Rectifier bridge negative end ground connection, electric current flows through inductance L from rectifier bridge forward end, flow back into ground by switching power devices M.

Make peak inductive current be a fixed value at peak current detection circuit, be set to I _maxtime, the characteristic due to inductance discharge and recharge:

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

Wherein V is inductance both end voltage, and i is inductive current, and t is the time, inductive current i linearly Changing Pattern under constant voltage, then I _lvalue just can be drawn by following formula:

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

Wherein I _minfor minimum inductance electric current, D to be inductive current be 0 time account for the duty ratio in master switch cycle; Now known, as peak inductive current I _maxtime constant, this power circuit power output and minimum inductance electric current I _minrelevant with duty ratio D.If set the device for power switching M turn-off time as t _off, at inductive current i from peak inductive current I _maxstart, in linear decline process, to have:

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

Drop to after zero at inductive current i, have:

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

Wherein t _onfor device for power switching M ON time, obviously at peak inductive current I _maxone timing, the ON time t of device for power switching M _onfor definite value; Can be drawn by upper two formulas, minimum inductance electric current I _minwith the turn-off time t that duty ratio D is all device for power switching M _offfunction, change the turn-off time t of device for power switching M _off, also just change minimum inductance electric current I _minwith duty ratio D, thus change average inductor current I _l, finally achieve power output P _outcontrol; Due to the I-V characteristic of LED itself, this power supply output voltage within the scope of very large load current remains unchanged substantially, thus further ensures the turn-off time t of switching power devices M _offto power output P _outmodulation.Turn-off time t _offlarger, inductive current mean value I _lless, power output P _outalso less, for LED load, be embodied in load current less.If the turn-off time t of by-pass cock device M can be carried out by changing external parameter _off, then the adjustment to LED load brightness is achieved.

With reference to figure 7, in conjunction with time the ordered pair principle of the invention be further elaborated: switching signal is the signal that logical circuit 13 exports, control the turn-on and turn-off of device for power switching 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 turns off; In minimum inductance electric current I _minin=0 situation, during the conducting of device for power switching M, inductive current i linearly rises to peak inductive current I _max, when device for power switching M turns off, inductive current i linearly declines, and continues for some time after dropping to zero; In minimum inductance electric current I _minwhen >0, during the conducting of device for power switching M, inductive current i linearly rises to peak inductive current I _max, inductive discharge when device for power switching M turns off, inductive current i linearly declines, and drops to the turn-off time t of device for power switching M before zero at inductive current i _offterminate, start next turn-on cycle; For LED load, can i drop to zero and only depend on t _offsize, thus to inductance average current I _lmodulate, realize the control to power output.

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

Claims (3)

1., based on a step-down LED drive circuit for inductive discharge time-modulation, comprise rectifier bridge, device for power switching M, output diode D, inductance L, output capacitance C _oUTwith control circuit (1); It is characterized in that: rectifier bridge forward end is connected to one end of inductance L, the other end of inductance L is connected with the drain electrode of device for power switching M, for storage power; The source ground of device for power switching M, for controlling the charging and discharging of inductance L; Between the drain electrode of device for power switching M and grid, cross-over connection has control circuit (1), and this control circuit (1), by controlling the turn-off time of device for power switching M, is modulated the discharge time of inductance L; The forward end of output diode D is connected to the drain electrode of device for power switching M, and its negative end is connected to load LED forward end, for preventing output capacitance C _oUTdischarging current pour in down a chimney to ground by device for power switching M; Load LED and output capacitance C _oUTload capacitance C between output diode D and rectifier bridge forward end is connected across after parallel connection _oUTfor between inductance L charge period for load LED powers, ensure output voltage and current constant;

Described control circuit (1), comprises peak current detection circuit (11), electric 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); For judging the sampled signal of inductive current L, when inductive current sampled signal exceedes the current threshold I of setting _lIMtime, output logic signal, to logical circuit (13), makes device for power switching M turn off by logical circuit (13), constant to ensure the peak current of inductance;

Described electric discharge timing circuit (12), it exports termination logical circuit (13); For carrying out timing to the turn-off time of device for power switching M, when reaching the timing time T of setting _settime, output logic signal, to logical circuit (13), makes device for power switching M conducting again by logical circuit (13), to ensure that power output is constant;

Described logical circuit (13), two input is respectively the output of peak current detection circuit (11) and the output of electric discharge timing circuit (12), and it exports the grid of termination device for power switching M; Logic synthesis is carried out by two logical signals exported peak current detection circuit (11) and electric discharge timing circuit (12), control conducting and the shutoff of device for power switching M, realize inductive discharge time-modulation by the turn-off time controlling device for power switching M.

2. step-down LED drive circuit according to claim 1, is characterized in that described current threshold I _lIMcustomize according to user's request, its maximum is no more than the saturation current of inductance L.

3. step-down LED drive circuit according to claim 1, is characterized in that described timing time T _setsize determine load current size, needed for user, the size of load current customizes, timing time T _setlarger, load current is less, timing time T _setless, load current is larger.