Driving system with inductor pre-charging for LED systems with PWM dimming control or other loads

A method includes receiving a control signal associated with a load, where the control signal is to cause a load change from a perspective of a switching-mode power supply. The method also includes causing the power supply to adjust a current through an inductor of the power supply in response to the control signal. The method further includes delaying delivery of the control signal in order to delay a time of the load change, where the current through the inductor increases during the delay. The control signal could include a request to turn on one or more LEDs. The load could include a current regulator. The method could further include providing the request to the current regulator after the delay, such as after the current through the inductor reaches a specified level. Voltage spikes and audible noise in a capacitor coupled to an output of the power supply can be minimized.

TECHNICAL FIELD

This disclosure is generally directed to driving systems. More specifically, this disclosure relates to a driving system with inductor pre-charging for light emitting diode (LED) systems with pulse width modulation (PWM) dimming control or other loads.

BACKGROUND

Many systems use light emitting diodes (LEDs) to generate light. These systems often include an LED driver that controls a current through one or more LEDs, thereby controlling a brightness of the LEDs. An LED driver can also receive pulse width modulation (PWM) signals used to control the dimming of the LEDs.

Many LED drivers use ceramic capacitors to filter output voltages generated by power supplies. However, PWM dimming can create voltage spikes in the output voltage of a power supply. When a ceramic capacitor is used, these voltage spikes can create audible noise in the ceramic capacitor. Conventional systems often attempt to solve this problem using feed-forward control loops (which suffer from robustness problems) or fast transient controllers (which can still create audible noise).

DETAILED DESCRIPTION

FIG. 1illustrates an example driving system100with inductor pre-charging for light emitting diodes (LEDs) or other loads according to this disclosure. In this example, the driving system100includes a switching-mode power supply102and a load formed by one or more LED strings104a-104nand a current regulator106. The power supply102generally receives an input voltage VIN, and generates a regulated output voltage VOUT. The power supply102includes any suitable structure for generating a regulated output voltage, such as a buck, boost, buck-boost, SEPIC, or flyback converter. The input voltage VINcan be provided by any suitable source, such as a battery.

In this example, the power supply102represents a boost converter that generates an output voltage VOUTlarger than an input voltage VIN. In this particular implementation, the power supply102includes an inductor108coupled to a switch110and a diode112, which is coupled to an output capacitor114. The inductor108includes any suitable inductive structure having any suitable inductance. The switch110represents any suitable switching device, such as a power transistor. The diode112represents any suitable structure for substantially limiting current flow to one direction. Note that the diode112could be replaced by a switch that allows bi-directional current flow. The output capacitor114includes any suitable capacitive structure having any suitable capacitance, such as a ceramic capacitor. The power supply102generally operates by opening and closing the switch110using a gate drive signal, where the duty cycle of the gate drive signal can be adjusted to provide a desired output voltage VOUT.

Each LED string104a-104nincludes one or more LEDs116. Each LED116includes any suitable semiconductor structure for generating light. In this example, the LEDs116are coupled in series to form a string, and multiple strings104a-104nare coupled in parallel. However, any other configuration involving the serial and/or parallel connection of LEDs116could be used.

The current regulator106regulates a current I flowing through the LEDs116. In some embodiments, the current regulator106regulates the current to ensure that an equal amount of current flows through each string104a-104n(although equal currents need not be used). The current regulator106includes any suitable structure for regulating current through one or more LEDs, such as a linear current regulator.

In this example, the current regulator106supports the use of pulse width modulation (PWM) dimming control. A PWM dimming control signal118can be used to adjust the brightness of the LEDs116. For instance, the current regulator106can turn the current I through the LEDs116on and off, adjusting the average current through the LEDs116(and therefore adjusting the brightness of the LEDs). This could be done based on a duty cycle of the PWM control signal118.

The change in brightness of the LEDs116effectively appears as a load change to the power supply102. Voltage spikes can therefore appear in the output voltage VOUTof the power supply102. These voltage spikes can create audible noise in the output capacitor114. This problem may be particularly noticeable when ceramic output capacitors114are used.

The audible noise can be created due to a transient state of the switching-mode power supply102. The current ILthrough the inductor108can be nearly 0 A when the LEDs116are turned off and much higher when the LEDs116are turned on. The transient state occurs when the inductor current ILincreases rapidly from about 0 A towards a steady-state value ILED. During the transient state, at least part of the LED current I is provided by discharging the output capacitor114. This leads to the creation of large voltage spikes in the output capacitor114, causing audible noise.

In accordance with this disclosure, the driving system100can reduce or minimize audible noise created in the output capacitor114by reducing or minimizing the discharge of the output capacitor114during the transient state of the power supply102. This can be accomplished, for example, by delaying the time that the LEDs116are turned on, which delays the increase of the LED current I. This allows the inductor current ILto increase without requiring much (if any) discharge of the output capacitor114, reducing or eliminating the audible noise. Example details of this operation are shown inFIG. 2.

FIG. 2illustrates example waveforms200associated with the driving system100ofFIG. 1according to this disclosure. InFIG. 2, a waveform202represents the PWM dimming control signal118, and a waveform204represents the LED current I. Also, a waveform206represents the inductor current IL, and a waveform208represents the voltage across the capacitor114.

As shown inFIG. 2, the waveform202goes high at time t1. Ordinarily, this would turn on the LEDs116, causing the LED current I to increase right away. In this case, however, rather than turning on the LEDs116immediately, the driving system100provides a short delay and turns on the LEDs116at time t2. At time t2, the LED current I increases rapidly to a steady-state level ILEDand causes the LEDs116to generate light.

The delay in turning on the LEDs116may correspond approximately or exactly to the transient state of the switching-mode power supply102. During the transient state, the inductor current ILincreases rapidly from about 0 A to near or at a steady-state value that is ideally expressed as ILED/(1−d) (note that power losses can affect this expression). During the transient state, the output capacitor114discharges somewhat, but not as much as it would have if the LEDs116were turned on at time t1. As a result, discharge of the output capacitor114can be reduced or minimized, which can also reduce the severity of voltage spikes in the output capacitor's voltage.

After the transient state, the switching-mode power supply102enters a steady-state where the inductor current ILideally ripples around the value ILED/(1−d). During this time, the voltage on the output capacitor114can also ripple around some point, but these ripples may be small and high in frequency and therefore cause little or no audible noise.

In this way, the driving system100can reduce the number and/or severity of voltage spikes in the output voltage VOUT. Since these voltage spikes are related to audible noise in the output capacitor114, this can reduce or even eliminate audible noise caused by the output capacitor114. This may be particularly useful, for instance, when a ceramic capacitor is used as the output capacitor114.

In some embodiments, the value of ILEDis often known ahead of time, and the value of d in ILED/(1−d) is often a function of the input and output voltages VINand VOUTand/or parameters of the power supply. The input and output voltages VINand VOUTcan typically be measured on-line, meaning the value of ILED/(1−d) can often be determined adaptively in the system100.

To support the delay in turning on the LEDs116, the driving system100includes a controller120. The controller120could perform various options to help reduce or minimize the discharge of the output capacitor114during the transient state of the switching-mode power supply102. For example, the controller120could measure the input and output voltages and calculate the value of ILED/(1−d). The controller120could also receive the control signal118. In response to detecting the control signal118going high (as inFIG. 2), the controller120can cause the power supply102to begin increasing the inductor current IL. Once the inductor current ILreaches a threshold point (such as the steady-state value ILED/(1−d)), the controller120can provide the high pulse in the control signal118to the current regulator106. This causes the current regulator106to turn on the LEDs116. The controller120effectively delays the control signal118and therefore delays the time that the current regulator106turns on the LEDs, allowing the inductor current ILto increase and reducing the discharge of the output capacitor114.

Note that the delay in turning on the LEDs116may only be a few microseconds, which can be insignificant when the dimming frequency is relatively slow in comparison (such as about 200 Hz to about 1 kHz). Also note that this approach can be highly robust over the entire range of dimming cycles. In addition, note that decreasing the capacitance of the output capacitor114would normally affect both the transient and steady-state responses of the system100. Here, however, decreasing the capacitance of the output capacitor114could significantly affect only the steady-state response of the system100. The voltage drop during the transient state can be reduced or minimized, resulting in less voltage undershoot. As a result, the nominal output voltage of the power supply102can be set lower, higher efficiency can be obtained, and smaller or cheaper output capacitors114could be used.

While the above description has described the controller120as determining the value of ILED/(1−d), this need not be the case. For example, the controller120could cause the power supply102to begin increasing the inductor current ILin response to a pulse in the PWM dimming control signal118, and the controller120could then provide the PWM dimming control signal118to the current regulator106after a fixed or variable delay. The fixed delay could represent a short delay estimated based on the expected VINand VOUTvalues. The variable delay could be based on any other suitable characteristic(s).

The controller120includes any suitable structure for controlling a driving system to reduce or minimize spikes in a power supply's output voltage. The controller120could, for example, represent a microprocessor, microcontroller, digital signal processor, field programmable gate array, or application specific integrated circuit.

In the example shown inFIG. 1, the load being driven by the power supply102includes LEDs116and a current regulator106. Also, audible noise is reduced or minimized by delaying a control signal118for the current regulator106, which (from the perspective of the power supply102) effectively delays a load change until the inductor current ILhas increased. However, this technique of delaying a load change to allow an inductor current ILto increase can be used with any suitable load and is not limited to use with just LEDs. Also, while delaying a control signal118for the current regulator106is used here to delay the load change, any suitable technique could be used to delay a load change. In addition, the above description has described how to delay a load change in order to allow the inductor current ILto increase. To allow the inductor current ILto decrease (such as when the LEDs116are being turned off), the controller120can stop the switching of the switch110to allow the LED current I to drop to about 0 A before turning off the LEDs116.

AlthoughFIG. 1illustrates one example of a driving system100with inductor pre-charging, various changes may be made toFIG. 1. For example, the system100could include any number of power supplies, LEDs, LED strings, current regulators, and controllers. Also, the functional division shown inFIG. 1is for illustration only. Various components inFIG. 1could be combined, further subdivided, or omitted and additional components could be added according to particular needs. As a particular example, while a boost converter is shown inFIG. 1, the power supply102could implement other switching converters. As another particular example, the controller120could be incorporated into the current regulator106or the power supply102. AlthoughFIG. 2illustrates examples of waveforms202-208associated with the LED driving system100ofFIG. 1, various changes may be made toFIG. 2. For instance,FIG. 2merely illustrates waveforms that might appear in the driving system100. The signals represented by the waveforms202-208could vary and have other characteristics during operation of the driving system100.

FIG. 3illustrates an example method300for driving of LEDs or other loads using inductor pre-charging according to this disclosure. As shown inFIG. 3, a control signal for adjusting a load is received at step302. This could include, for example, the controller120in the system100receiving the PWM dimming control signal118. The control signal118could pulse high when the LEDs116are to be turned on. Note, however, that the control signal could be associated with any other load change.

In response, a power supply is operated to begin changing an inductor current at step304. This could include, for example, the controller120generating a gate drive signal for controlling the switch110. The gate drive signal can cause the switch110to open and close in order to begin generating the necessary output voltage VOUT, which increases the inductor current IL.

The delivery of the control signal to the load is altered at step306. This could include, for example, the controller120waiting for the inductor current ILto increase to some threshold value before sending the pulse in the PWM dimming control signal118to the current regulator106. The threshold value could be determined adaptively. For example, each time the LED driving system100receives a new value of the PWM dimming control signal118, the controller120could measure the input and output voltages VINand VOUTand then determine the duty cycle d needed to maintain the output voltage VOUT. The duty cycle d can be used to calculate the value of ILED/(1−d) for the new value of the PWM dimming control signal118. The next time the value of the PWM dimming control signal118is received, the controller120can retrieve the calculated threshold value ILED/(1−d).

The control signal is provided to the load at step308, and the system is operated to provide a current through the load at step310. This could include, for example, the controller120providing the high pulse in the PWM dimming control signal118to the current regulator106. The current regulator106can then operate to allow the LED current I to flow through the LEDs116, turning on the LEDs116. However, the LEDs116are turned on after the delay, which allows the inductor current ILto increase without significantly discharging the output capacitor114. This helps to avoid large voltage spikes that might otherwise create audible noise in the output capacitor114.

AlthoughFIG. 3illustrates one example of a method300for LED driving using inductor pre-charging, various changes may be made toFIG. 3. For example, while shown as a series of steps, various steps inFIG. 3may overlap, occur in parallel, occur in a different order, or occur multiple times. As a particular example, the controller120could delay the control signal118when the LEDs116are being turned on in order to allow the inductor current ILto increase. The controller120could also take steps to allow the inductor current ILto decrease before turning off the LEDs116. In either case, the controller120is adjusting the delivery of the control signal118to the current regulator106in order to reduce or minimize voltage spikes in the output capacitor114.

While this disclosure has described certain embodiments and generally associated methods, alterations and permutations of these embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not define or constrain this invention. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this invention as defined by the following claims.