Inductor current detection circuit and LED driver

The present invention pertains to an inductor current detection circuit in a switching mode power supply, and a light-emitting diode (LED) driver thereof. In one embodiment, an inductor current detection circuit configured in a switching mode power supply under discontinuous conduction mode, can include: (i) a voltage detection circuit configured to generate a sampling voltage based on a drain-source voltage of a power switch in the switching mode power supply; (ii) a voltage holding circuit configured to receive the sampling voltage, and to generate a holding voltage through a sampling and holding operation; and (iii) a comparison circuit configured to compare the sampling voltage against the holding voltage, and to generate a zero-crossing signal when the sampling voltage is less than the holding voltage, where the zero-crossing signal is configured to represent an inductor current ending time of the switching mode power supply.

RELATED APPLICATIONS

This application claims the benefit of Chinese Patent Application No. 201210233135.3, filed on Jul. 5, 2012, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention pertains to the field of electronics, and specifically to an inductor current detection circuit, and a light-emitting diode (LED) driver.

BACKGROUND

With continuous innovation and rapid development in the lighting industry, and an increased importance of energy-saving and environmental protection, light-emitting diode (LED) lighting is quickly developing as a revolutionary energy-saving lighting technology. However, LED is more sensitive to current than voltage due to its volt-ampere and temperature characteristics, so LED may not be directly powered by traditional power supplies. Therefore, such power supply may need to be addressed prior to utilizing LED as a lighting source. Although traditional LED drivers can regulate the luminance of an LED, power factor correction may not be achieved. In addition, an input power factor may be relatively low, with increased harmonic components.

SUMMARY

In one embodiment, an inductor current detection circuit configured in a switching mode power supply under discontinuous conduction mode, can include: (i) a voltage detection circuit configured to generate a sampling voltage based on a drain-source voltage of a power switch in the switching mode power supply; (ii) a voltage holding circuit configured to receive the sampling voltage, and to generate a holding voltage through a sampling and holding operation; and (iii) a comparison circuit configured to compare the sampling voltage against the holding voltage, and to generate a zero-crossing signal when the sampling voltage is less than the holding voltage, where the zero-crossing signal is configured to represent an inductor current ending time of the switching mode power supply.

In one embodiment, a light-emitting diode (LED) driver can include: (i) an inductor current detection circuit; (ii) a control circuit configured to receive the zero-crossing signal, and to generate a control signal configured to control a switching operation of the power switch; (iii) where the control signal is configured to control the power switch to turn on in a switching period after a delay time of the zero-crossing signal; and (iv) where the control signal is configured to control the power switch to turn off after a conduction time interval after the power switch is turned on, where the conduction time interval is proportional to an error between a present output current of the LED driver and an expected output current.

Embodiments of the present invention can advantageously provide several advantages over conventional approaches. For example, particular embodiments can provide an inductor current detection circuit for a switching mode power supply that can precisely obtain output current information, and an LED driver with high efficiency and a high power factor. Other advantages of the present invention may become readily apparent from the detailed description of preferred embodiments below.

DETAILED DESCRIPTION

FIG. 1shows a schematic diagram of an example light-emitting diode (LED) driver. An AC input power supply can be rectified and filtered through an EMI anti-electromagnetic interference circuit, a rectifier circuit, and filter capacitor Cin, to generate DC input voltage Vin. A power stage circuit can include power switch Q0, diode D0, and inductor L0to receive DC input voltage Vin, and to generate substantially constant output current Ioto drive an LED load. A control circuit can include a current sampling circuit, comparator CMP, and an RS flip-flop.

The current sampling circuit can connect with power switch Q0to obtain sampling voltage Vsense, and then sampling voltage Vsensecan be compared against reference value Vref. The comparison result can be input to the reset terminal of the RS flip-flop, and the set terminal of the RS flip-flop can receive fixed frequency clock signal CLK. In each switching period, clock signal CLK can set the RS flip-flop to control power switch Q0to turn on. After a predetermined period, when sampling voltage Vsenseis greater than reference value Vref, the RS flip-flop is reset to control power switch Q0to turn off. By repeating this operation, power switch Q0can be turned on or off periodically according to clock signal CLK and sampling voltage Vsense, so that output current Iocan be maintained as substantially constant to drive the LED load.

The example LED driver shown inFIG. 1can provide a substantially constant current for the LED load. However, this approach may have some drawbacks, including difficulty in achieving “power factor correction.” In AC circuits, loads that have capacitance or inductance may produce electrical current that is not in phase with the applied voltage. The phase shift can reduce an amount of the applied power that is usable in the circuit by a fraction known as the power factor. By adding other components to the circuit, the power factor can be restored to a normal value, and this may reduce or eliminate the adverse effect of the phase shift. If power factor correction cannot be achieved, a relatively low input power factor and relatively high harmonic components may result.

FIG. 2shows a schematic diagram of another LED driver. A power stage circuit can include power switch Q1, diode D1, and inductor L1to receive DC input voltage Vin, and to generate substantially constant output current Ioto drive an LED load. A control circuit can include a current sampling circuit, an input voltage sampling circuit, comparator CMP, and an RS flip-flop. The working principle of the control circuit is that the input voltage sampling circuit (e.g., including resistors R1and R2) can sample DC input voltage Vinto obtain input voltage sampling signal Vin—sense.

The current sampling circuit can connect with power switch Q1to obtain sampling voltage Vsenseby sampling the current of power switch Q1. The comparison result obtained by taking input voltage sampling signal Vin—senseas a reference signal to compare against sampling voltage Vsensecan be used to turn on or off power switch Q1via the RS flip-flop, so as to control the driving current of the LED load and to realize power factor correction. However, by adopting this implementation, the control precision of the output current may not be high since the current of power switch Q1is sampled rather than the load current. In addition, the power factor correction of this particular LED driver may not suffice to achieve a high enough power factor for certain applications.

In particular embodiments, an inductor current detection circuit can detect an ending time of inductor current under discontinuous conduction mode by precisely sampling and holding an induced voltage. The induced voltage can represent or express the inductor current, so that the inductor current holding time can be precisely attained. Also in particular embodiments, an LED driver can calculate an average value of the inductor current by using the inductor current detection circuit. Current control precision can be improved by utilising the output current information of the LED driver.

In addition, quasi-resonant driving for the power switch can be achieved by accurate detection of the ending time of the inductor current. This can decrease switching losses and improve working efficiency. Also, by controlling the inductor current, power factor correction can be achieved to obtain a higher power factor, and requirements for the EMI suppression circuit can be lowered. Further, volume and weight of the power supply can be decreased accordingly. The high power factor, high efficiency LED driver in particular embodiments can meet electro-magnetic compatibility (EMC) standards, and a series of other requirements, such as high current control precision, high reliability, small volume, and low cost.

In one embodiment, an inductor current detection circuit configured in a switching mode power supply under discontinuous conduction mode, can include: (i) a voltage detection circuit configured to generate a sampling voltage based on a drain-source voltage of a power switch in the switching mode power supply; (ii) a voltage holding circuit configured to receive the sampling voltage, and to generate a holding voltage through a sampling and holding operation; and (iii) a comparison circuit configured to compare the sampling voltage against the holding voltage, and to generate a zero-crossing signal when the sampling voltage is less than the holding voltage, where the zero-crossing signal is configured to represent an inductor current ending time of the switching mode power supply.

FIG. 3Ashows a schematic diagram of an example inductor current detection circuit in accordance with embodiments of the present invention. In this particular example, the inductor current detection circuit can be applied in a buck LED driver to detect an ending time of an inductor current. For example, the buck LED driver can operate in a discontinuous conduction mode (e.g., DCM inductor current discontinuous conduction mode).

In this example, the inductor current detection circuit can include voltage detection circuit302, voltage holding circuit303, and comparison circuit304. Voltage detection circuit302can be coupled with inductor L of the buck LED driver to generate an induced voltage based on the current of the inductor. Sampling voltage Vscan be obtained by dividing (e.g., via a resistor divider circuit) the induced voltage. Thus, the induced voltage can be used to express or represent the drain-source voltage of power switch Q.

Voltage holding circuit303can connect with voltage detection circuit302, and may be used to receive sampling voltage Vsand to generate a holding voltage VHby sampling and holding sampling voltage Vs. Comparison circuit304can connect to voltage detection circuit302and voltage holding circuit303, and may be used to receive and compare sampling voltage VSand holding voltage VH. For example, when sampling voltage VSis less than holding voltage VH, zero-crossing signal SZcan be generated. Further, zero-crossing signal SZcan express or represent an ending time (e.g., when inductor reaches zero) of inductor current.

Referring now to an example operation waveform diagram of the inductor current detection circuit ofFIG. 3AinFIG. 3B, working principles of the inductor current detection circuit of particular embodiments can be introduced below. In a switching period T (e.g., from time t0to time t3), during an active period (e.g., from time t0to time t1) of control signal Vctrl, inductor current iLcan rise from zero to an inductor current peak value. During an inactive period (e.g., from time t1to time t3) of control signal Vctrl, inductor current iLcan decrease from the inductor current peak value to zero until time t2. After the inductor current is reduced to zero, resonance between inductor L and the parasitic capacitor of power switch Q may increase, so sampling voltage VSmay begin to decrease at time t2.

By holding sampling voltage Vsthrough voltage holding circuit303, the rate of decline of holding voltage VHcan be less than that of sampling voltage VS. Comparison circuit304can be used to compare sampling voltage Vsagainst holding voltage VH. During a time interval form time t1to time t2, when the inductor current is not zero, sampling voltage VScan be equal to holding voltage VH. When inductor current iLdrops to zero at time t2, sampling voltage VScan be less than holding voltage VH. The start time when sampling voltage VSis less than holding voltage VHcan be configured as the ending time of inductor current iL. Therefore, zero-crossing signal SZcan be generated by comparison circuit304at time t2.

In the particular example shown inFIG. 3A, specific implementations of voltage detection circuit302, voltage holding circuit303, and comparison circuit304are shown. However, those skilled in the art will recognize that other circuit structures can also be utilized in certain embodiments. In this particular example, voltage detection circuit302can include auxiliary inductor LSand a dividing resistor network. Auxiliary inductor LScan couple with inductor L of the LED driver, and may generate an induced voltage according to the inductor current.

Resistor Rf1and resistor Rf2connected in series between two terminals of auxiliary inductor LScan form the dividing resistor network to divide the induced voltage of auxiliary inductor LSto generate sampling voltage VSat common node A of resistor Rf1and Rf2for expressing the inductor current. In this particular example, the induced voltage that represents the current of inductor L in the LED driver can be in the same phase with the drain-source voltage of power switch Q. However, the voltage detection circuit can be not limited to the very particular implementation shown above. Other suitable voltage detection circuitry can also be used in the inductor current detection circuit of particular embodiments. For example, the drain-source voltage of the power switch may be directly sampled in some cases.

Voltage holding circuit303can include resistor RHand capacitor CHconnected between common node A and ground, and switch SWwhich can connect in parallel with resistor RH. The switching stage of switch SWcan be controlled by an inverted version of control signal Vctrl. When control signal Vctrlgoes inactive, switch SWis turned on, and holding voltage VHcan rapidly follow sampling voltage VS. After a predetermined holding time (e.g., about 2 us), switch SWcan be turned off, and holding voltage VHcan be held through resistor RHand capacitor CH.

Before the inductor current reaches zero (e.g., before time t2), sampling voltage VScan be maintained as substantially constant, and holding voltage VHcan be almost the same as sampling voltage VS. After the inductor current reaches zero (e.g., after time t2), sampling voltage VSmay begin to decline due to resonance. Because of the holding function of capacitor CH, the voltage at common node B of resistor RHand capacitor CH(holding voltage VH) may decline relatively slowly with a rate of decline that is less than that of sampling voltage VS. Thus, holding voltage VHcan be higher than sampling voltage VS.

Comparison circuit304can include comparator CMP that has a non-inverting input terminal to receive holding voltage VH, and an inverting input terminal to receive sampling voltage VS. When holding voltage VHis greater than sampling voltage VS, the output signal of the comparator can change to generate zero-crossing signal SZ(e.g., as a one-shot pulse), so that the ending time of the inductor current can be precisely detected.

Other alternative implementations for the comparison of holding voltage VHand sampling voltage VScan also be supported in particular embodiments. For example, when sampling voltage VSis less than holding voltage VHand the difference between them is greater than a threshold value, zero-crossing signal SZcan be generated to represent the ending time of the inductor current. In this case, comparison circuit304can be implemented as a hysteresis comparator. Also, the threshold value can be the hysteresis width of the hysteresis comparator.

By adopting the inductor current detection circuit of certain embodiments, the ending time of the inductor current can be precisely detected. Therefore, the inductor current detection circuit can be widely used in isolated or non-isolated (e.g., flyback converter) topologies. Based on detecting the ending time of the inductor current precisely, various advantageous aspects can be provided for the implementation of the control circuit. For example, the output current can be obtained precisely to provide convenience for precise current control.

Particular embodiments also support a high efficiency and high power factor LED driver with an inductor current detection circuit. In one embodiment, an LED driver can include: (i) an inductor current detection circuit; (ii) a control circuit configured to receive the zero-crossing signal, and to generate a control signal configured to control a switching operation of the power switch; (iii) where the control signal is configured to control the power switch to turn on in a switching period after a delay time of the zero-crossing signal; and (iv) where the control signal is configured to control the power switch to turn off after a conduction time interval after the power switch is turned on, where the conduction time interval is proportional to an error between a present output current of the LED driver and an expected output current.

Referring now toFIG. 4A, shown is a block schematic diagram of an example LED driver in accordance with embodiments of the present invention. In this particular example, the LED driver can include a power stage circuit, inductor current detection circuit401, control circuit405, and driver406. The power stage circuit can include power switch Q, diode D, output capacitor C, and inductor L to receive, e.g., half sine wave DC input voltage Vin, and to generate DC output voltage Voutand output current Ioto drive the LED load. Inductor current detection circuit401can be any suitable type of inductor current detection circuit to generate zero-crossing signal SZas shown inFIGS. 3A and 3B. Control circuit405can receive zero-crossing signal SZto generate corresponding control signal Vctrl, so as to control a switching operation of power switch Q of the power stage circuit.

In a given switching period, after a delay time (e.g., a predetermined delay after a rising or a falling edge) of zero-crossing signal SZ, control signal Vctrlcan control power switch Q to turn on. This operation can realize quasi-resonant driving for power switch Q. After conduction time interval tonof power switch Q, control signal Vctrlcan control power switch Q to turn off. For example, conduction time interval toncan be proportional to an error between the present output current and an expected output current of the LED driver.

Driver406can generate a corresponding driving signal according to control signal Vctrl, to drive the switching operation of power switch Q to maintain the output current as substantially constant. In addition, the input current can be maintained in the same phase with half sine wave DC input voltage Vin.FIG. 4Bshows an example operation waveform diagram of the LED driver shown inFIG. 4A. Inductor current iLcan be discontinuous, and the inductor current peak can be expressed as an Equation 1 below.

As conduction time interval ton, output voltage Vout, and inductor L are substantially constant, inductor current peak ipkcan be proportional to half sine wave input voltage Vinand the inductor current peak envelope can be in a sinusoidal shape as shown. In this way, input current iINcan be proportional to half sine wave input voltage Vinand the LED driver in this particular example can achieve a relatively high power factor.

Also, in the particular LED driver example shown inFIG. 4A, a specific implementation of control circuit405provided can include on-signal generating circuit402, off-signal generating circuit403, and logic circuit404. On-signal generating circuit402can receive zero-crossing signal SZto generate an on-signal after a predetermined delay time, in order to achieve quasi-resonant driving for power switch Q. For example, the delay time can be set (e.g., programmed by a user) according to various applications or factors (e.g., product test or characterization).

Off-signal generating circuit403can receive zero-crossing signal SZ, detection voltage VCSthat represents inductor current iL, control signal Vctrl, and reference voltage source Vrefthat represents an expected output current. Off-signal generating circuit403can generate conduction time interval tonto represent an error between the present output current and the expected output current of the LED driver. Thus, the off-signal can be generated at the ending time of the conduction time interval to control power switch Q of the power stage circuit to turn off. In this way, constant conduction time control of power switch Q can be achieved.

Inductor current iLcan be obtained by detection resistor RCSwhich can connect in series with power switch Q. When power switch Q is on, detection voltage VCSat the common node of detection resistor RCSand power switch Q can express the current of power switch Q (inductor current iL). Logic circuit404can connect with on-signal generating circuit402and off-signal generating circuit403to receive an on-signal and an off-signal, such that control signal Vctrlcan be generated.

Referring now toFIG. 5, shown is a block schematic diagram of another LED driver in accordance with embodiments of the present invention. In this particular example, example implementations of on-signal generating circuit402, off-signal generating circuit403, and logic circuit404are shown. On-signal generating circuit402can include AND-gate501and single impulse delay time generating circuit502. AND-gate501can receive zero-crossing signal SZand an inverted version of control signal Vctrl. When both of zero-crossing signal SZand the inverted version of control signal Vctrlare active, the output signal of AND-gate501may trigger single impulse delay time generating circuit502. After a certain predetermined delay time, the on-signal can be generated.

In order to regulate the output electric signal under small load applications, and avoid inadvertently having power switch Q off, OR-gate503can be used to receive max off-time Toff—maxof power switch Q, and the output signal of single impulse delay time generating circuit502. When sampling voltage VSreaches a localized low or valley value, or the off-time of power switch Q reaches the maximum off-time Toff—max, the on-signal can be output via OR-gate503. Off-signal generating circuit403can include inductor current average value calculation circuit504, error operation circuit505, and fixed time generating circuit506. For example, inductor current average value calculation circuit504can receive zero-crossing signal SZ, control signal Vctrl, and detection signal VCS, to generate average voltage Vavg. Average voltage Vavgcan express the average value of the present inductor current or the present output current of the LED driver.

Error operation circuit505can receive average voltage Vavgand reference voltage source Vrefthat represents the expected output current, to obtain an error signal. Fixed time generating circuit506can generate conduction time interval tonaccording to the error signal after turning on power switch Q. After power switch Q is turned on for conduction time interval ton, power switch Q can be turned off. In this way, the output current can be maintained as substantially constant through a closed loop and can also ensure that the input current in phase with the half sine input voltage. Logic circuit404can include RS flip-flop507having a set terminal S to receive the on-signal, and a reset terminal R to receive the off-signal. The output signal of RS flip-flop507can be configured as control signal Vctrl.

Various implementations of on-signal generating circuit402, logic circuit404, and off-signal generating circuit403can be supported in particular embodiments. For example,FIG. 6shows a block diagram of an example off-signal generating circuit in accordance with embodiments of the present invention. In this particular example, off-signal generating circuit403can include an inductor current average value calculation circuit, error operation circuit610, and fixed time generating circuit612. The inductor current average value calculation circuit can include inductor current peak value sampling and holding circuit601, an inductor current holding time generating circuit, and a peak-average conversion circuit. Inductor current peak sampling and holding circuit601can receive sampling voltage VCSthat represents the inductor current, and control signal Vctrl, to generate peak value detection voltage VCS—peakto express the inductor current peak value.

The inductor current holding time generating circuit can include D-type flip-flop605receiving control signal Vctrland zero-crossing signal SZ, and providing inductor current holding time Tdis. Inductor current holding time Tdismay start from a beginning time of control signal Vctrland go to an ending time of zero-crossing signal SZ. The peak-average conversion circuit can include a buffer circuit, a chopper circuit, and a filter circuit, to receive peak value detection voltage VCS—peakand inductor current holding time Tdis, to obtain average voltage Vavgexpressing an average value of the inductor current.

When control signal Vctrlis active, detection voltage VCScan be used to generate peak value detection voltage VCS—peakthrough inductor current peak sampling and holding circuit601. The buffer circuit that includes amplifier602, resistor604, and transistor603can be configured as a unity-gain buffer. Peak value detection voltage VCS—peakcan connect to the non-inverting input terminal of comparator602, and the output terminal of comparator602can connect to the control terminal of switch603(e.g., N-type transistor). Resistor604can connect between the second power terminal of switch603and ground. The common node of resistor604and transistor603can connect to the inverting input terminal of comparator602.

Reset terminal RST of D-type flip-flop605can receive control signal Vctrl, and clock terminal CLK can receive zero-crossing signal SZ. The active-state interval of inductor current holding time Tdisgenerated at the output terminal (e.g., from time t0to time t2inFIG. 3B) can be configured as the inductor current holding time. Switches606and607can connect in series between a common node of resistor604and switch603, and ground, so as to form the chopper circuit. Inductor current holding signal Tdiscan control the switching state of switch606. The inverted signal of inductor current holding signal Tdiscan control a switching state of switch607.

After the chopping operation of switches606and607, and the filter operation of resistor608and capacitor609, average voltage Vavgrepresenting the average value of the inductor current can be obtained at common node D of resistor608and capacitor609. Also, error operation circuit610have a non-inverting input terminal coupled to average voltage Vavg, and an inverting input terminal to receive reference voltage source Vref, so as to generate an error signal at its output terminal. After being compensated by capacitor611, compensation signal Vcompcan be obtained based on the error signal. Fixed time generating circuit612can receive compensation signal Vcompto generate a signal with a time interval as an off-signal. People skilled in the art will recognize that fixed time generating circuit612can be implemented via any suitable type of existing technologies or modifications, including analog and/or digital circuitry.

In this way, an inductor current detection circuit in accordance with embodiments of the present invention can precisely obtain an ending time of inductor current, and can also be used in other suitable types of switching mode power supplies (e.g., an LED driver).

The foregoing descriptions of specific embodiments of the present invention have been presented through images and text for purpose of illustration and description of the start-up circuit and method. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching, such as the variable number of the current mirror and the alternatives of the type of the power switch for different applications.

The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.