Circuit and method for driving a light-emitting diode

Circuits and methods for driving an LED from a secondary side of a transformer are disclosed herein. An embodiment of the method includes monitoring an input voltage to determine the power level intended to drive the LED. The current flow through the primary side of the transformer is adjusted to make the power actually driving the LED equal to the power intended to drive the LED.

BACKGROUND

Light-emitting diodes (LEDs) emit light when a forward current is passed through them. The light intensity, which may be referred to as the luminous flux, radiant flux, or simply the lumens, output by an LED is proportional to the forward current. As with most diodes, LEDs have a forward voltage, which, in ideal circumstances, the forward voltage remains constant, so the light intensity is proportional to the forward current, which will also be constant. Therefore, in ideal circumstances, the intensity of light output by an LED is very predictable and constant because it is related to a constant forward current.

In real circumstances, the forward voltage of an LED is not constant over time or from one LED to another. For example, as an LED ages, its forward voltages may decrease. Likewise, as the temperature of an LED increases, its forward voltage decreases. When the forward voltage of an LED decreases and the forward current is maintained constant, the light intensity emitted by the LED decreases. The opposite occurs when the forward voltage of an LED increases. With a constant forward current, more power is delivered to the LED and the intensity of light emitted by the LED increases.

SUMMARY

Circuits and methods for driving an LED from a secondary side of a transformer are disclosed herein. An embodiment of the method includes monitoring an input voltage to determine the power level intended to drive the LED. The current flow through the primary side may be related to the voltage input to the circuit from a dimmer. The current flow through the primary side of the transformer is adjusted to make the power driving the LED equal to the power intended to drive the LED.

DETAILED DESCRIPTION

Circuits and methods for driving light-emitting diodes (LEDs) are disclosed herein. The circuits receive power from a power source, such as a sinusoidal line voltage. In some embodiments, the line voltage is clipped due to the affects of dimming. For example, a conventional dimmer using a triac may clip the sinusoidal line voltage. The circuits and methods analyze the line voltage to determine the power that is intended to be delivered to the LEDs. The power intended to be delivered to the LEDs is may be set by a user who moves a rotary or slide switch of the type commonly used with dimmers. The intended power may then be related to the voltage output by the switch. For example, the intended power may be related to the clipped voltage output by a conventional light dimmer. The circuits output the intended power to the LEDs irrespective of the forward voltage or current of the LEDs. By outputting a specific power to the LEDs, the intensity of light emitted by the LEDs is maintained, irrespective of changes in the forward voltages of the LEDs.

A block diagram of an embodiment of a circuit100for driving LEDs102is disclosed inFIG. 1. The circuit100receives a line voltage104by way of an input106. The line voltage104may be a sinusoidal line voltage such as a 110 v, 60 Hz or 220, 50 Hz AC voltage. The line voltage104may be associated with a dimmer (not shown), such as a triac. The triac or other dimmer may clip the line voltage104so that the sinusoidal AC voltage only conducts for a portion of each cycle. This portion of the cycle where the line voltage104conducts is referred to as the conduction angle. At a conduction angle of 180 degrees, the LEDs102are intended to operate at full power without any dimming. At lower conduction angles less power is intended to be delivered to the LEDs102so their light intensity is reduced.

The input106is connected to a rectifier110, which is shown inFIG. 1as being a full wave rectifier. The rectified voltage is output from the rectifier110onto a line112, which is referenced to ground. The line112is connected to a resistor RACand to the primary side of a transformer T1. In some embodiments, other components may be connected to the line112or in series with the line112. The function of the resistor RACis described in greater detail below. The primary side of the transformer T1is connected to ground by way of a current regulator that is referred to as switch QSW. A current sensing resistor RSis connected in series with the switch QSWand the primary side of the transformer. In the embodiment described herein, the switch QSWis a field effect transistor (FET), but it could be any electronic switching device. Current flow through the primary side of the transformer T1is controlled by the switch QSW.

The secondary side of the transformer T1is connected to the LEDs102. The transformer T1may have a gain (n) that is dependent on the primary and secondary windings. A unique element of the circuit100is that the power delivered to the LEDs102is controlled by the switch QSW, which controls the primary side of the transformer T1. By controlling the power on the primary side of the transformer T1, the power delivered to the LEDs102is maintained substantially constant irrespective of changes in the forward voltage of the LEDs102or the current flow through the LEDs102. Therefore, the intensity of light emitted by the LEDs102is able to be maintained at a constant or predetermined intensity. It is noted that the power to the LEDs102may also be maintained constant even with fluctuations in the level of the line voltage104.

The resistor RACis connected to an input116of a feedforward circuit118. The feedforward circuit118has an output120and a second input122. The input122is referred to as the control line and has a voltage that is proportional to the duty cycle of the switch QSW. The feedforward circuit118may have other inputs and outputs that are used for angle detection and other signals that are described in greater detail below. The output120is sometimes referred to as the feedforward output. In order to protect the circuitry from high current, the current at the output120is reduced. In the embodiment ofFIG. 1, the current is reduced by one tenth. The current at the output120is also multiplied by the duty cycle of the switch QSW. The voltage on the output120is the product of the duty cycle of the switch QSWand the rectified line voltage received at the input116.

The output120is connected to a resistor RFFand a capacitor CFFthat are connected in parallel to ground. The output120is also connected to the inverting input of an amplifier126. The resistor RFFis used in series with the resistor RACto set a power reference of the input as described below. The resistor RFFalso determines a voltage on the inverting input of the amplifier126that is compared to the voltage on the non-inverting input of the amplifier126. The capacitor CFFserves to form a low-pass filter with the resistor RFF. The low-pass filter is used to attenuate twice the frequency of the line voltage104, which is the frequency of the rectified voltage on the line112. In some embodiments, a pole is set at between 10 Hz and 12 Hz to achieve approximately 20 dB attenuation at twice the frequency of the line voltage104. Accordingly, the voltage at the inverting input to the amplifier126is the DC component, or RMS value, of the rectified line voltage104multiplied by the duty cycle of the switch QSW.

The amplifier126may be a GM error amplifier that has an output130. The inverting input of the amplifier126is connected to the output120of the feedforward circuit118. The non-inverting input is connected to a reference generator132. The reference generator132is described in greater detail below. In summary, the reference generator132generates a voltage that is proportional to the power intended to be delivered to the LEDs102, which is the light intensity intended to be output by the LEDs102. By monitoring the power intended to be delivered to the LEDs102, the power output to the LEDs102is regulated accordingly. When voltage at the input106has not been dimmed, the reference voltage on the non-inverting input of the amplifier126may be set to a predetermined voltage, such as one volt.

The output130of the amplifier126is connected to the non-inverting input of a comparator140and to a capacitor CCOMP. The inverting input of the comparator140is connected to a ramp generator142. The ramp generator142generates a ramp wave wherein the voltage at the non-inverting input is within the high voltage and low voltage of the ramp wave. The voltage at the output144of the amplifier140is a pulse width modulated (PWM) signal wherein the duty cycle is proportional to the voltage at the output130of the amplifier126. The capacitor CCOMPprovides slow integral compensation. The capacitor CCOMPmay have a value of between 4.7 μF and 10 μF to achieve a low bandwidth loop of 1 Hz to 10 Hz

The output144of the comparator140may be connected to a latch148, which in the embodiment ofFIG. 1is an SR latch148. More specifically, the output144is connected to the reset (R) of the latch148and the set (S) of the latch148is connected to other circuits that are described in greater detail below. Therefore, the output (Q) of the latch148is the PWM signal generated by the amplifier144so long as (S) is set appropriately. The output (Q) is connected to a buffer150that drives the gate of the switch QSW. It is noted that in some embodiments, the latch148is not used and the output144of the comparator140is connected directly to the buffer150or the gate of the switch QSW. The circuits that control the switch QSWand/or monitor the rectified voltage are sometimes referred to collectively as the control circuit.

Having described the basic circuit100for driving the LEDs102, its operation and design with no dimming will now be described. As described above, the purpose of the circuit100is to maintain a constant power to the LEDs102irrespective of changes in their forward voltages. Additionally, the circuit100maintains a constant power to the LEDs102even when the line voltage104varies.

In a conventional circuit, the power received at the primary side of the transformer T1is proportional to the product of the voltage and current at the primary side. If the current or voltage in the primary side drops, the power on the primary side drops accordingly. It follows that the power on the secondary side and the power delivered to devices connected to the secondary side also drops. In the case of LEDs, the reduced power causes the intensity of light emitted by the LEDs to drop. The reverse occurs if the voltage or current on the primary side of the transformer rises. The circuit100overcomes the above described problems by maintaining a predetermined power draw from the line voltage104, which is the power on the primary side of the transformer T1.

The voltage on the output120, which is the product of the DC component of the rectified line voltage and the control signal generated by the feedforward circuit118, is input to the inverting input of the amplifier126. The amplifier126compares the inverting input to the reference voltage, which may be set to one volt for non-dimming operation. A PWM signal is generated based on the output of the comparator140wherein the PWM signal drives the switch QSW. Because the switch QSWis driven with a PWM signal, the duty cycle of the primary side of the transformer T1is the duty cycle of the PWM signal. This PWM signal is used as the control signal that is at the input122of the feedforward circuit118. If the line voltage104drops, the voltage at the inverting input to the amplifier126will drop. This voltage drop causes the amplifier126to generate a longer duty cycle with the PWM signal. Thus, the current through the primary side of the transformer T1increases. The result is the input power to the primary side of the transformer T1is maintained, so the power on the secondary side is also maintained. The opposite occurs if the line voltage104increases.

The circuit100will now be described in greater detail. The line voltage104is rectified and the rectified current is passed through the resistor RACand to the feedforward circuit118where the current is reduced. In the embodiment ofFIG. 1, the current reduction is 10:1. The voltage at the input116of the feed forward circuit118is also multiplied by the duty cycle of the switch QSW. The low pass filter of CFFand RFFfilter the AC component of the voltage at the output120to yield the average DC component of the rectified line voltage multiplied by the duty cycle at the inverting input to the amplifier126.

The resistance ratio of RFF/RACmay be calculated using the following equation:

RFFRAC=π4⁢GFF⁢VREFLM⁢PIN⁢fsEquation⁢⁢(1)
where GFFis the feedforward gain, which is ten in the embodiment described herein; VREFis the reference voltage; LMis the magnetizing inductance of the transformer T1; PINis the input power of the circuit100; and fSis the switching frequency of the ramp generator142.

In order to maximize the power factor of the circuit100, the energy in the primary side of the transformer T1, or the magnetizing inductance LM, should be reset on every cycle. In addition, the circuit100should operate in a discontinuous conduction mode (DCM) for the power level PINover the range of input voltages. Based on the foregoing, the magnetizing inductance LMof the transformer T1may be selected as follows:

LM≤VREF4⁢⁢PIN⁢fs⁡(1nVOUT+1VREC⁡(PK,MIN))2,Equation⁢⁢(2)
L↓M≦V↓REF/(4P↓IN f↓S(1/(nV↓OUT)+1/V↓(REC(PK,MIN)↑2)
where n is the primary to secondary turns ratio in the transformer T1; VOUTis the string voltage on the LEDs102; and VREC(PK,MIN)is the minimum peak rectified input voltage.

As described above, the capacitor CFF is chosen to make a low-pass filter with the resistor RFF so as to filter the AC component of the rectified line voltage. The low pass filter may have a pole of between 10 Hz and 12 Hz to provide approximately 20 dB of attenuation at 120 Hz. Therefore, the value of the capacitor CFFmay be calculated as follows:

Slow integral compensation is achieved by way of the capacitor CCOMP, which may have a value of between 4.7 and 10.0 μF in order to have a low bandwidth loop of between 1 Hz and 10 Hz. The output of the amplifier126is input to the non-inverting input of the comparator140where the PWM signal is generated. The output of the amplifier140eventually drives the switch QSWas described above.

The switch QSWcontrols the current flow through the primary side of the transformer T1. By controlling the current flow through the transformer T1based on the above-described parameters, the power to the primary side of the transformer T1is maintained. The power output on the secondary side of the transformer T1is proportional to the power on the primary side, therefore, controlling the power input to the primary side of the transformer T1controls the power output on the secondary side. The secondary side of the transformer drives the LEDs102.

As briefly described above, the power factor of the circuit100is relatively high because the circuit uses a DCM flyback converter that behaves like a resistor. More specifically, the input current IIN(t) is calculated as follows:

where D is the duty cycle and TSis the switching period based on the ramp generated by the ramp generator142and valley detection circuit as described in greater detail below. As shown, both IIN(t) and VIN(t) are sinusoidal and in phase. Therefore, the power factor is close to unity when the circuit100is operated in a non dimming mode. The power factor is not as relevant when the circuit100is operated in a dimming mode.

The circuit100has been described above as operating in a non-dimming mode. The circuit100will now be described operating using dimming functions. The dimming is provided by a conventional dimmer (not shown) that may be incorporated into the line voltage104. The dimmer may operate by clipping a portion of the sinusoidal voltage used in a common AC voltage source. A conduction angle is the portion of the sinusoidal wave that is not clipped by the dimmer.

An example of a clipped rectified voltage200is shown inFIG. 2. This is the voltage that is present on the line112as a result of different levels of dimming. A wave202shows a rectified sine wave with no dimming. A wave204is the result of leading edge clipping. As shown, the conduction angle of the wave204is less than the conduction angle of the wave202. As more dimming is applied, the conduction angle decreases and a wave such as the wave206is generated, which has a conduction angle less than the wave204.

The voltage VREFoutput by the reference generator132is proportional to the conduction angle of the rectified line voltage. A block diagram of a circuit220for generating the reference signal VREFis shown inFIG. 3. The circuit220includes an input224wherein the input224is connected to the line112by way of the resistor RAC. The input consists of FETs Q2and Q3wherein the FET Q3draws current between its drain and source from a current mirror226. The current mirror226may be located within the feedforward circuit118ofFIG. 1. In the embodiment ofFIG. 3, the current mirror226is a 10:1 current mirror. The voltage at the input224is attenuated and recreated across a resistor R1, which may have a value of approximately 40 kΩ.

The voltage across the resistor R1is input to the non-inverting input of a comparator230. The inverting input of the comparator230is connected to a predetermined voltage V1, which may be approximately 280 mV. The voltage V1is used to set the lower threshold in which dimming is detected and to generate a PWM signal representative of the rectified line voltage. More specifically, the voltage V1is used to detect the lower threshold of the conduction angle where dimming will be applied to the LEDs102,FIG. 1. The comparator230has an output232that is connected to the input234of a driver or buffer236. The buffer236is powered by or has an output voltage that is limited to a voltage V2. In some embodiments, the voltage V2may be approximately 1.75V.

The buffer236has an output240that is connected to a low-pass filter242. In the embodiment ofFIG. 3, the low-pass filter242consists of a resistor R2and a capacitor C2. The pole of the low-pass filter242is set to attenuate the frequency of the rectified voltage, which is twice the frequency of the line voltage104. More specifically, the AC component of the voltage of the output240is attenuated so that only the DC component, or the RMS value, passes the low-pass filter242.

The output of the low-pass filter242is connected to the input244of an angle decoding circuit246. The angle decoding circuit246translates the voltage at the input244to a voltage VDIMat the output248which is linearly related to the voltage at the input244. The output248is connected to the input250of the reference generator132. The reference generator132has an output252where the reference voltage VREFis present. The reference generator132may also have an input256that is connected to a thermal feedback voltage VTFB. The voltage VTFBis representative of the operating temperature of the LEDs102,FIG. 1, and may be used to reduce the power output to the LEDs102if their operating temperature is above a predetermined threshold.

Having described the components of the circuit220, its operation and design will now be described. The circuit220receives the input current through RACand Q2. The current is replicated through Q3and scaled down by the current mirror226. In the embodiment described herein, the current mirror is a 10:1 current mirror. An attenuated representation of the input voltage is then replicated across the resistor R1. It is noted that the value of the resistor RACis proportional to the threshold dimming voltage divided by the current into the input224. The current may be the value of V1divided by the value of the resistor R1and multiplied by the value of the current mirror226. In some embodiments, the threshold voltage for 120V systems is set between 25V and 40V and for 220V or 230V systems, the threshold voltage is between 50V and 80V. The LEDs102may be turned off when the input voltage is below the threshold voltage.

The voltage at the non-inverting input to the comparator230is compared to the voltage V1. Reference is made toFIG. 2, where the rectified voltage200is compared to the voltage V1, which is shown as a dashed line. The output of the comparator230is a PWM signal wherein the duty cycle corresponds to the common area of the rectified voltage200and the voltage V1. The output of the comparator230is connected to the buffer236that saturates the input signal and produces a signal280as shown inFIG. 4.

The voltage ofFIG. 4passed through the low-pass filter242, wherein the result is a voltage V3that is shown inFIG. 5. The voltage V3has the AC component attenuated and is a DC representation of the PWM signal ofFIG. 4. The voltage V3is input to the angle decoding circuit246. The voltage V3represents the conduction angle of the rectified line voltage and the angle decoding circuit246converts the voltage V3to a linear relationship between the voltage V3and its output VDIM. Reference is made toFIG. 6which is a graph depicting the relationship between the voltage V3and the voltage VDIM. The voltage VDIMis linearly dependent on the voltage V3when the voltage V3is between a voltage V3Aand a voltage V3B. The voltage V3Amay be approximately 280 mV and the voltage VABmay be approximately 1.45 volts.

In the embodiment ofFIG. 6, no dimming occurs when the voltage V3is between zero and V3Avolts. In this situation, the conduction angle may be less than thirty degrees and the voltage VDIMmay be approximately 13 mV. As such, the LEDs102may be turned off. When the voltage V3is greater than V3B, the conduction angle may be from one hundred fifty degrees to one hundred eighty degrees. In this situation, no dimming may occur and the voltage VDIMis held constant, which in the embodiment ofFIG. 6is approximately one volt. In this range, the LEDs102may be powered at full power. In the linear range, when the voltage V3is between V3Aand V3B, the voltage V3is linearly related to the voltage VDIM, which may correspond to a conduction angle of thirty degrees to one hundred fifty degrees. Dimming occurs during this linear range.

The voltage VDIMis output to the reference generator132. The reference generator may change the voltage VDIMto generate the reference voltage VREF. The reference voltage VREFwas described above under no dimming conditions as being one volt. This corresponds to a conduction angle of between one hundred thirty degrees and one hundred eighty degrees where the voltage VDIMis saturated at one volt. The reference generator132may have an input256that is connected to a voltage VTFB. The voltage VTFBis a thermal feedback signal indicating the operating temperature of the LEDs102,FIG. 1. The voltage VTFBmay be connected to a resistor or other thermal transducer located proximate the LEDs102. When the voltage VTFBdrops below a predetermined level, the LEDs102have exceeded a predetermined operating temperature. At this time, the reference generator132reduces the reference voltage VREF, which reduces the power to the primary side of the transformer T1and reduces the power to the LEDs102. As the LEDs102cool, the voltage VTFBchanges and the reference voltage VREFrises to increase the power output to the LEDs102.

Having described the circuit100and its operation, methods of operating the circuit100will now be described. Reference is made to the flowchart320ofFIG. 7, which describes a broad method for driving the LEDs102. At step322, the conduction angle is measured and a signal representative of the conduction angle is generated. It is noted that generating a signal representative of the conduction angle may constitute measuring the conduction angle. The signal representative of the conduction angle is the voltage VREFgenerated by the reference generator132.

At step322, a signal representative of the input power to the primary side of the transformer T1is generated. This signal is located at the inverting input of the amplifier126. At step326, the signal from step322is compared to the signal at step324. The comparison is achieved by the amplifier126wherein the output of the amplifier126is representative of the difference. At step328, the current driving the primary side of the transformer T1is set or changed depending on the output of the amplifier126. This is accomplished by changing the duty cycle of the PWM signal driving the switch QSW.

The basic circuit100and its operation have been described above. Different embodiments of the circuit100will now be described.

The circuit100,FIG. 1, uses the transformer T1, which can induce ringing across the switch QSW, which may induce losses in the switch QSW. In order to lower the losses in the switch QSW, a valley detection circuit350,FIG. 1, may be used. The valley detection circuit350uses an auxiliary winding352on the primary side of the transformer T1wherein the voltage on this auxiliary winding352is monitored. An example of the voltage on the auxiliary winding350showing ringing in the monitored voltage VAUXis shown inFIG. 8. The voltage VAUXhas a rising edge occurring at T1followed by a falling edge at T2. A single ring290occurs between T2and T3. The period between T2and T3corresponds to the time in which the switch QSWis supposed to be on as determined by the ramp wave generated by ramp generator142.

In order to lower switch losses and reduce electromagnetic interference, the switch QSWcan be forced to turn on at the lowest point in the ring290. To determine the minimum value of the ring290, the circuit100may rely on the valley detection circuit350. During non-dimming operation, a ramp292corresponding to a fixed frequency set by the ramp generator142is generated. This frequency corresponds to the shortest duration of operation of the switch QSW, which is less than a period corresponding to the time between zero and T3. In the embodiments described herein, the ramp292has a period of approximately 14.5 μs.

In the embodiment ofFIG. 8, there are two valleys294and295detected during each period. The valley294is detected during the period of the ramp292and will not cause the switch QSWto turn on. After the period of the ramp292has ended, the second valley295is detected, which causes the switch QSWto turn on. When the switch QSWturns on, another ramp wave is generated. Therefore, the switch QSWis turned on at the detection of a valley and after the expiration of the period of the ramp192. The period of the ramp may be set by the ramp generator142.

It is noted that valley detection may introduce frequency jitter that can affect the output of the LEDs102during dimming. The frequency jitter may cause visible flicker in the LEDs102. For example, if a valley is detected at the end of the ramp192, the valley detection circuit350may detect the valley during or after the ramp period during alternating cycles, which will cause flicker. In order to overcome this problem, the valley detection may be disabled when a conduction angle below a predetermined value is detected. For example, referring toFIG. 6, when the value of VDIMis less than V3B, the valley detection may be disabled.

The circuit100,FIGS. 1 and 3, includes a reference generator132. As described above, the reference generator132generates the reference voltage used by the amplifier126. As shown inFIG. 3and as described above, the reference generator132may receive data on the input256related to the temperature of the LEDs102and may reduce the reference voltage VREFis the temperature of the LEDs102is above a predetermined value.

The reference generator132may be an analog OR circuit348as shown inFIG. 9. The analog OR circuit348includes a first input250, which is connected to the angle decoding circuit246. A second input256is connected to the temperature sensor and is the input for the voltage VTFB. The analog OR circuit348also includes a reference input350that may be connected to the output120of the feedforward circuit118,FIG. 1. The analog OR circuit348includes two outputs, a non-inverting output354and an inverting output356. The inverting and non-inverting outputs354,356are connected to the respective inputs of the amplifier126. It is noted that the use of the analog OR circuit348slightly changes the configuration of the circuit100, but the operation remains substantially the same. If the input250is less than the input256, then the output of354is the voltage of the input250; otherwise, the output is the voltage on the input256. In the embodiments as applied toFIGS. 1 and 3, the reference input350is connected directly to the inverting output356, which is connected to the inverting input of the amplifier126.

A schematic illustration of the analog OR circuit348is shown inFIG. 10. The circuit348includes a current source ISthat is connected to a current mirror360by way of a FET Q2and a FET Q3. The input250is the gate of the FET Q2and the input256is the gate of the FET Q3. The non-inverting output354is the source of the FETS Q2and Q3. The drain of the FET Q3is connected to a current mirror362, wherein the current flowing from the drain of the FET Q3to the current mirror362is the current I3minus the current I1. The reference input350is connected to the gate of a FET Q4and the inverting output is connected to the source of the FET Q4. If the voltage at the input250is less than the voltage at the input256, then the current I3is equal to the current I1; otherwise the current I3is equal to the current I2.

In some embodiments, the operation of the circuit may change from regulating the power driving the LEDs102to regulating the current driving the LEDs. For example, during dimming operation, the LEDs102may be driven by applying a regulated power based on the conduction angle of the line voltage104as described above. During full power operation that does not include dimming, the LEDs102may be driven by a regulated current.

Referring toFIG. 11, the current flow IPthrough the primary side of the transformer T1and the current flow IS through the secondary side of the transformer T1is shown. The current IS is proportional to the current IP multiplied by the ratio of T2to TS. The winding ratio n of the transformer T1will also affect the current IS. An LED estimation circuit400senses the voltage across the sensing resistor RS, wherein the voltage is equal to the resistance RSmultiplied by the current IP.

Referring to the block diagram ofFIG. 12, the output of the LED current estimation circuit400is connected to a comparator402where it is compared to a current reference voltage. A power estimation circuit404determines the power based on the input voltage on the line112multiplied by the duty cycle D. The output of the power estimation circuit404is input to a comparator406where it is compared to a power reference voltage. The outputs of the comparators402,406are input to an analog OR circuit410as described above. The analog OR circuit selects the lower of the two outputs to generate the driving signal for the switch QSW. The output of the analog OR410may be used as the non-inverting input to the comparator140,FIG. 1. In high power conditions of no or little dimming, the LED current estimation circuit400drives the switch QSWbased on current regulation. As dimming occurs, the power estimation circuit404drives the switch QSWas described above.

FIG. 13is a block diagram illustrating a method500for driving an LED from a secondary side of a transformer. The method includes, as shown at502, monitoring an input voltage to determine the power level intended to drive the LED. The method also includes as shown at504, adjusting the current flow through the primary side of the transformer to make the power driving the LED equal to the power intended to drive the LED.

The foregoing description of specific embodiments driving an LED has been presented for purposes of illustration and description. The specific embodiments described are not intended to be exhaustive or to suggest a constraint to the precise forms disclosed, and many modifications and variations are possible in light of the above teaching. The illustrated embodiments were chosen and described in order to best explain principles and practical application, to thereby enable others skilled in the art to best utilize the various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the language of the claims appended hereto be broadly construed so as to cover different embodiments of the structures and methods expressly disclosed here, except as limited by the prior art.