Generating a voltage feedback signal in non-isolated LED drivers

1. Field of the Invention

This disclosure relates to driving LED (Light-Emitting Diode) lamps and, more specifically, to generating a feedback signal indicating voltage across the inductor of the LED lamp.

2. Description of the Related Art

LEDs are being adopted in a wide variety of electronics applications, such as architectural lighting, automotive head and tail lights, backlights for liquid crystal display devices, and flashlights. Compared to conventional lighting sources such as incandescent lamps and fluorescent lamps, LEDs have significant advantages, including high efficiency, good directionality, color stability, high reliability, long life time, small size, and environmental safety.

The use of LEDs in lighting applications is expected to expand, as they provide significant advantages over incandescent lamps (light bulbs) in power efficiency (lumens per watt) and spectral quality. Furthermore, LED lamps represent lower environmental impact compared to fluorescent lighting systems (fluorescent ballast combined with fluorescent lamp) that may cause mercury contamination as a result of fluorescent lamp disposal.

However, conventional LED lamps cannot be direct replacements of incandescent lamps and dimmable fluorescent systems without modifications to current wiring and component infrastructure that have been built around incandescent light bulbs. This is because conventional incandescent lamps are voltage driven devices while LEDs are current driven devices, thus requiring different techniques for controlling the intensity of their respective light outputs.

Many dimmer switches adjust the RMS voltage value of the lamp input voltage by controlling the phase angle of the AC-input power that is applied to the incandescent lamp to dim the incandescent lamp. Controlling the phase angle is an effective and simple way to adjust the RMS-voltage supplied to the incandescent bulb and provide dimming capabilities. However, conventional dimmer switches that control the phase angle of the input voltage are not compatible with conventional LED lamps, since LEDs, and thus LED lamps, are current-driven devices.

One solution to this compatibility problem uses an LED driver that senses the lamp input voltage to determine the operating duty cycle of the dimmer switch and reduces the regulated forward current through an LED lamp as the operating duty cycle of the dimmer switch is lowered. In some cases, the LED driver delivers power to the LED lamp across a transformer, isolating the output of the LED lamp from the input. To regulate the current through the LED, the LED driver receives feedback about an output voltage or current through the LED. Many LED drivers sense the output using an auxiliary winding on the primary side of the transformer. However, sensing the output voltage via an auxiliary winding adds complexity to the LED driver, increasing both the cost and the size of the LED driver.

To reduce cost and complexity of an LED lamp, a feedback signal indicating voltage across an output of the inductor is generated without relying on an auxiliary transformer winding. An LED lamp according to various embodiments includes one or more LEDs and an inductive element (e.g., an inductor or a primary winding of a transformer) coupled to an input voltage source and the one or more LEDs. A switch is coupled to the inductive element such that current is generated in the inductor responsive to the switch being turned on and not generated responsive to the switch being turned off. A first current detector is coupled between the input voltage source and a ground node of the LED lamp, and a second current detector is coupled between the inductive element and the ground node. Current detected by the first current detector is proportional to a bulk voltage across the input voltage source, while current detected by the second current detector is proportional to a drain voltage across the switch.

In one embodiment, a comparator determines a difference between the current detected by the second current detector and the current detected by the first current detector. The current difference is converted to a voltage (e.g., based on the resistance of the first and second current detectors) and input to a switch controller as a feedback signal indicative of a voltage across the inductive element. When the current in the inductive element is not zero, the voltage is equal to the voltage across the LED. When the current is zero, the voltage will be oscillated due to the induction and capacitance of the inductive element, which can be used for valley mode detection to improve the efficiency of the LED driver. The switch controller controls switching of the switch based on the feedback signal to regulate output current through the one or more LEDs.

The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter.

The teachings of the embodiments of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings.

FIG. 1 illustrates an LED lamp circuit, according to one embodiment.

FIGS. 2A-2B are block diagrams illustrating components of an LED lamp, according to one embodiment.

FIG. 3 illustrates example waveforms of a bulk voltage and a drain voltage, according to one embodiment.

FIG. 4 illustrates example waveforms demonstrating relationships between bulk voltage, bulk current, drain voltage, and drain current, according to one embodiment.

The Figures (FIG.) and the following description relate to preferred embodiments by way of illustration only. It should be noted that from the following discussion, alternative embodiments of the structures and methods disclosed herein will be readily recognized as viable alternatives that may be employed without departing from the principles of the claimed invention.

Reference will now be made in detail to several embodiments of the present invention(s), examples of which are illustrated in the accompanying figures. It is noted that wherever practicable similar or like reference numbers may be used in the figures and may indicate similar or like functionality. The figures depict embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein.

As will be explained in more detail below with reference to the figures, a switching power supply providing a regulated output voltage with voltage feedback signal not requiring an auxiliary winding. For example, an LED lamp system and a method according to various embodiments generates a feedback signal indicating the regulated output voltage coupled to one or more LED devices without using an auxiliary transformer winding. As the auxiliary winding adds cost and complexity, generating a feedback signal independently of an auxiliary winding reduces a cost and complexity of the LED lamp system.

FIG. 1 illustrates an LED lamp system including an LED lamp 130 used with a conventional dimmer switch 120 . The LED lamp 130 according to various embodiments is a direct replacement of an incandescent lamp in a conventional dimmer switch setting. A dimmer switch 120 is placed in series with AC input voltage source 110 and LED lamp 130 . Dimmer switch 120 receives a dimming input signal 125 and uses the input signal 125 to set the desired light output intensity of LED lamp 130 . Dimmer switch 120 receives AC input voltage signal 115 and adjusts the V-RMS value of lamp input voltage 135 in response to dimming input signal 125 . In other words, control of the light intensity outputted by LED lamp 130 by dimmer switch 120 is achieved by adjusting the RMS value of the lamp input voltage 135 that is applied to LED lamp 130 . The LED lamp 130 controls the light output intensity of LED lamp 130 to vary proportionally to the lamp input voltage 135 , exhibiting behavior similar to incandescent lamps, even though LEDs are current-driven devices and not voltage driven devices. Dimming input signal 125 can either be provided manually (via a knob or slider switch, not shown herein) or via an automated lighting control system (not shown herein).

The dimmer switch 120 adjusts the V-RMS of lamp input voltage 135 by controlling the phase angle of the AC input voltage signal 115 . In particular, the dimmer switch 120 reduces the V-RMS of input voltage 135 by eliminating a portion of each half-cycle of the AC input signal 115 . Generally, the dimmer switch 120 increases the dimming effect (i.e., lowers the light intensity) by increasing the portion of each half-cycle that is eliminated and thereby decreasing the dimmer on-time.

FIGS. 2A-B are block diagrams illustrating components of the LED lamp 130 . In one embodiment, the LED lamp 130 comprises a bridge rectifier DB 1 , an input capacitor C 1 , an inductive element L 1 , an output capacitor C 2 , a switch S 1 , and a switch controller U 1 . Other embodiments of the LED lamp 130 may comprise different or additional components.

The bridge rectifier DB 1 rectifies the voltage signal 135 input to the LED lamp 130 by the dimmer switch 120 and provides the rectified voltage across the input capacitor C 1 . Inductive element L 1 , diode D 1 , capacitor C 2 , and switch S 1 form a buck boost type power converter providing a regulated current output to one or more LEDs, such as LED 1 shown in FIG. 2 . The controller U 1 controls on and off cycles of the switch S 1 to provide the regulated output current to LED 1 . When the switch S 1 is turned on, power input to the LED lamp 130 is stored in the inductive element L 1 because the diode D 1 is reverse biased. During off cycles of the switch S 2 , current is provided to LED 1 across the capacitor C 2 . In one embodiment, as shown in FIG. 2A , the inductive element L 1 comprises a primary winding of a transformer. In another embodiment, as shown in FIG. 2B , the inductive element L 1 is an inductor.

The controller U 1 controls switching of switch S 1 such that a substantially constant current is maintained through LED 1 . In one embodiment, the controller U 1 receives a feedback voltage Vsense indicating an output voltage across L 1 and controls switching of the switch S 1 in response to the feedback. Furthermore, in one embodiment, the controller U 1 receives a dimming signal from the dimmer switch 120 that is indicative of an amount of dimming for the LED lamp 130 . In this case, the controller U 1 controls current through LED 1 such that an output light intensity from LED 1 substantially corresponds to the amount of dimming for the LED lamp 130 . The controller U 1 can employ a number of modulation techniques, such as pulse-width modulation (PWM) or pulse-frequency modulation (PFM), to control the on and off states and duty cycles of the switch S 1 . PWM and PFM are techniques used for controlling switching power converters by controlling the widths and frequencies, respectively, of a drive signal generated by the controller U 1 for driving the switch S 1 to achieve output power regulation.

As shown in FIGS. 2A-B , LED 1 is coupled across the inductive element L 1 and is therefore a floating output (that is, not referenced to ground). Furthermore, because the rectified voltage input to the inductive element L 1 is a high voltage input, it is difficult to directly measure the input voltage. To measure the output voltage across L 1 , the LED lamp 130 includes two current detectors R 1 and R 2 , as shown in FIGS. 2A-B , which in one embodiment each comprise one or more resistors. The first current detector R 1 is coupled between the input voltage source and a ground node of the LED lamp 130 , while the second current detector R 2 is coupled between the inductive element L 1 and the ground node. A current I 1 detected by the first current detector R 1 is proportional to a bulk voltage V_bulk across the input capacitor C 1 (that is, the voltage of the rectified signal input to the LED lamp 130 by the bridge rectifier DB 1 ). A current I 2 detected by the second current detector R 2 is proportional to a drain voltage V_drain across the switch S 1 .

In one embodiment, the currents I 1 and I 2 are sensed (e.g., by ammeters 202 A and 202 B) and input to a comparator 204 . The comparator 204 generates a signal ΔI representing a difference between the current I 2 and the current I 1 . A current-to-voltage converter 206 receives the ΔI signal generated by the comparator 204 and determines the voltage across LED 1 based on ΔI. For example, if the current detectors are each a resistor, the current-to-voltage converter 206 determines the voltage across the LED based on ΔI and the resistance of the resistors R 1 and R 2 . The determined voltage across LED 1 is output to the controller U 1 as the voltage feedback signal Vsense. In another embodiment, the current-to-voltage converter 206 receives or detects the currents I 1 and I 2 , converts the currents to equivalent voltages V_bulk and V_drain, and determines a difference between the equivalent voltages. In this case, the determined voltage difference is equivalent to the voltage Vo across the inductive element L 1 and is output to the controller U 1 as the feedback signal Vsense. In yet another embodiment, the controller U 1 is configured to receive a signal representing the difference between currents I 2 and I 1 , determine the voltage Vo across L 1 based on the current difference, and control regulated output through LED 1 in response to the determined voltage.

FIG. 3 illustrates example waveforms of a bulk voltage V_bulk and a drain voltage V_drain measured by the current-to-voltage converter 206 . Illustrated in FIG. 3 is a portion of a cycle of the AC input signal V_in as well as switching of the switch S 1 during the cycle, measured values of V_bulk and V_drain, and a ΔV signal generated by subtracting V_bulk from V_drain. As shown in FIG. 3 , V_bulk measured by the current-to-voltage converter 206 is affected by the magnitude of the AC input voltage, increasing during off cycles of the switch S 1 in proportion to increases in the magnitude of the AC input voltage. V_drain is similarly affected by the magnitude of the AC input voltage, and also exhibits high frequency voltage oscillations during off cycles of the switch S 1 resulting from resonance of the inductive element L 1 and the output capacitor C 2 . By subtracting V_bulk from V_drain, the current-to-voltage converter 206 removes the low-frequency voltage changes in V_drain resulting from the AC input voltage and generates the signal ΔV.

FIG. 4 illustrates example waveforms demonstrating a relationship between the bulk voltage V_bulk and the current detected by the first current detector R 1 , as well as a relationship between the drain voltage V_drain and the current detected by the second current detector R 2 . As shown in FIG. 4 , the current I 1 detected by the first current detector R 1 is proportional to V_bulk and the current I 2 detected by the second current detector R 2 is proportional to V_drain. Accordingly, a signal ΔI generated by subtracting the current detected by the first current detector from the current detected by the second current detector is proportional to the signal ΔV representing the difference between the drain and bulk voltages. Thus, by measuring the current difference ΔI, the current-to-voltage converter indirectly measures the voltage across LED 1 .

A large difference in magnitude of the voltage of the two nodes, as there is in the case of the above example using V_bulk and V_drain to provide the voltage feedback signal, will tend to increase the inaccuracy of the resulting voltage feedback signal. For example, in the case of a buck-boost converter in which the turns ratio of the inductive element L 1 is 1:

I 1= V _bulk/ R 1 and I 2= V _drain/ R 2, or V drain= V bulk+ Vo.

Therefore:

I 2=( V bulk+ Vo )/ R 2

and

Δ I=I 2− I 1=( V bulk+ Vo )/ R 2− V bulk/ R 1.

If R 1 =R 2 , it simplifies to Vo/R 1 , but when R 2 is not equal to R 1 , then ΔI is

Δ I=V bulk/ R 2− V bulk/ R 1+ Vo/R 2.

With the first two terms not cancelling and considering that V_bulk is >>Vo, which it is in the above example, it corrupts the measurement of the output voltage (Vo) in a manner that is worsens as Vbulk increases. This problem can be solved by multiplying one of the terms by a normalizing factor (k), where

Δ I=I 2* k−I 1 or Δ I=I 2 −k*I 1.

The variable k can be easily calibrated by the controller U 1 when in the dead zone after the reset period of the switch, when V_drain=V_bulk. The normalizing factor “k” can be adjusted to calibrate the offset that is introduced by the common mode voltage. In one embodiment, the normalizing factor “k” is calibrated so that the difference output (voltage feedback) results in 0V.

The LED lamps according to various embodiments of the present disclosure have the advantage that the LED lamp can be a direct replacement of conventional incandescent lamps in typical wiring configurations found in residential and commercial lighting applications, and that the LED lamp can be used with conventional dimmer switches that carry out dimming by changing the input voltage to the lamps. Moreover, a feedback signal indicating voltage across the LED is generated without relying on an auxiliary winding, thereby reducing the cost and complexity of the LED lamp.

Upon reading this disclosure, those of skill in the art will appreciate still additional alternative designs for an LED lamp. Thus, while particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and components disclosed herein and that various modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus of the present invention disclosed herein without departing from the spirit and scope of the invention.