Secondary-side sensing of phase-dimming signal

Current is regulated in an LED lamp by sensing, in a manner electrically isolated from a primary side of a transformer, an LED current in an LED; creating a digital control signal based on the LED current; transmitting the digital control signal from the secondary side of the LED circuit to the primary side; and controlling power delivered to the primary side based at least in part on the transmitted digital control signal.

TECHNICAL FIELD

Embodiments of the current invention relate to lighting systems and, more specifically, to light-emitting diode (“LED”) driver circuits.

BACKGROUND

LED light sources are attractive alternatives to traditional incandescent, fluorescent, or halogen lamps because of their high light output and low power consumption. LED lamps, however, require specialized driver and/or control circuits in order to properly supply power to the LEDs (typically, via a regulated current) using traditional power sources (e.g., an AC line voltage). As a further constraint, safety standards created by organizations such as UL and CE require that hazardous voltages (e.g., voltages above approximately 50 volts) must be isolated from users.

A popular LED driver circuit that fulfills these requirements uses a flyback converter, which applies the input voltage to a primary side of a flyback transformer (i.e., a charge-storing transformer) to induce a current therein. A switch periodically shuts off the application of the input voltage, during which time the flyback transformer discharges its stored charge as a current through its secondary side. This secondary-side current is used to drive the LEDs. As an added benefit, the separation between the primary and secondary sides of the flyback transformer provides the electrical isolation required by the safety standards.

Because the LED drive current requires regulation, however, the flyback-converter circuit must also sense any variations in the secondary-side current and adjust the power delivered to the primary side of the flyback transformer accordingly. For example, if the secondary-side current is too low, the primary-side control circuit may increase the amount of time per cycle that the switch is on and thereby apply more input power to the primary side. The sensed secondary-side LED current must be therefore transmitted back to the primary side, and it must be done while preserving the primary/secondary electrical isolation.

An existing circuit100for feeding back the sensed LED current to the primary side of the circuit is shown inFIG. 1. An input line voltage102is applied to a filter and rectifier104and thereafter to a primary side106of a flyback transformer108. A primary-side control circuit110periodically shuts off a transistor switch112, at which time the secondary side114applies a current to the LEDs116(through a rectifier diode118and a filter capacitor120). A current-sense circuit122senses the current through the LEDs116and sends a corresponding sensing signal through an opto-isolator118. The opto-isolator118passes the signal across the isolation barrier using a light-emitting diode and a photodiode, thereby preserving the electrical isolation. The primary-side control circuit110receives the feedback signal from the opto-isolator118and adjusts the switching time of the switch112accordingly.

There are a number of drawbacks to the use of the opto-isolator118, however. The accuracy of the current regulation depends on the quality of the analog signal passed therethrough; if the opto-isolator118is poorly calibrated, out of specification, and/or changes or degrades over time, the light produced by the LEDs116may undesirably vary and/or the lifespan of the LEDs116may shorten. These issues may be mitigated (but not eliminated) by the use of a higher-quality opto-isolator118, but the higher cost of such a component may be undesirable or prohibitive.

The shortcomings of the use of the opto-isolator118are exacerbated if the LEDs116are to be used with a dimming signal. Any errors introduced into the feedback signal by the opto-isolator118produce erratic, inconsistent, or time-varying levels of light output by the LEDs116for a given dimmer setting. Furthermore, if one dimmer signal is used to control multiple LED lamps, differences in each of the opto-isolators in each lamp may produce different levels of output light from each lamp.

A need therefore exists for an LED lamp driver circuit that complies with safety standards while providing accurate and consistent current regulation and dimming control.

SUMMARY

In general, various aspects of the systems and methods described herein include a control circuit disposed on the secondary side of an LED driver transformer. The secondary-side control circuit samples the LED drive current and generates a digital control signal, which is transmitted back to the primary side. A switching circuit on the primary side receives the digital control signal and adjust current delivered to the primary accordingly. The secondary-side control circuit may also receive one or more dimming signals and/or sense the use of an upstream dimming signal and adjust the digital control signal accordingly.

These and other objects, along with advantages and features of the present invention herein disclosed, will become more apparent through reference to the following description, the accompanying drawings, and the claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations.

DETAILED DESCRIPTION

Described herein are various embodiments of methods and systems for accurately biasing and dimming one or more LEDs while complying with electrical isolation requirements mandated by safety organizations. A control circuit on the secondary side of a transformer senses one or more currents in the LEDs and determines whether the LED drive current requires adjustment. The secondary-side control circuit sends a digital control signal back to the primary side based on this determination through an opto-isolator. Because the current sense and regulation control occurs on the secondary side, the feedback signal need not contain the precise analog measurement of the LED current, but only a digital control value (encoded, for example, as a modulated signal such as a pulse-width modulation (“PWM”) signal or other data carrier). Thus, errors in the accuracy of the opto-isolator (i.e., fluctuations in its ability to translate analog values across the isolation barrier) do not affect the accuracy of the regulation. In various embodiments, the secondary-side control circuit receives one or more dimming signals on the secondary side of the transformer. The secondary-side control circuit adjusts the digital feedback signal in accordance with dimming information received in the dimming signals, thereby causing the LEDs to be dimmed accordingly. In other embodiments, a downstream dimming unit modifies a phase of the input voltage to the primary side; the secondary-side control circuit senses this modification and adjusts the digital control signal accordingly.

One embodiment of a circuit200for secondary-side control of LEDs is illustrated inFIG. 2. A secondary-side control circuit232includes a current-sensing circuit for sensing a drive current in one or more LEDs216. The current-sensing circuit may be analog or digital and operates in accordance with any method known in the art. In one embodiment, the current-sensing circuit includes a resistor236for converting the current to a voltage and a capacitor238for filtering the converted voltage. A current-control circuit analyzes the sensed current and generates a digital control signal236in response. In various embodiments, the current-control circuit compares the sensed current to an analog or digital reference and generates a data signal encoding the difference; for purposes of illustration and not limitation, the ensuing discussion assumes PWM encoding. For example, if the sensed current is too low, the PWM signal may be adjusted to have longer pulses (i.e., the PWM signal has a longer duty cycle) and if the sensed current is too high, the PWM signal is adjusted to have shorter pulses. If the sensed current is equal to (or within a range of tolerance with respect to) the reference, the current-control circuit makes no change to the PWM carrier signal.

An opto-isolator218transmits the digital control signal236across the primary/secondary side isolation barrier to the primary side of the transformer208. Because the control signal is digital, the absolute value of the digital signal (i.e., its potential difference relative to a reference or ground) does not affect the operation of the circuit200, and thus variations in the operation, performance, or tolerance of the opto-isolator218do not affect the operation of the circuit200. In other embodiments, a small-signal transformer (or any other isolating, signal-transmitting component) is used in place of the opto-isolator218.

A switching circuit230receives the primary-side version of the digital control signal via the opto-isolator218and switches a transistor switch212on and off accordingly, thereby regulating the power delivered to the primary side206of the transformer208. The switching circuit230may include a current-sense circuit for sensing current across a resistor234and a comparator for comparing the sensed current to the received digital signal.

In various embodiments, the circuit200adjusts the light emitted by the LEDs216in response to various types of dimming signals. In one embodiment, the dimming of the LEDs216is controlled by one or more dimming signals234input to the circuit200, such as signals conforming to the 0-10 V lighting control protocol or to the digital-addressable lighting interface (“DALI”) protocol. These signals may require electrical isolation from a high-voltage source (e.g., the input voltage202) as a part of their specifications. The input dimming-control signals234may therefore be received by the secondary-side control circuit232, which is electrically isolated from the primary side of the circuit200. In one embodiment, the secondary-side control circuit232adjusts the digital control signal in accordance with both the current sensed in the LEDs216and the received dimming control signals.

In another embodiment, the LEDs216are dimmed by a dimmer circuit upstream of the circuit200by changing the input power signal202. For example, a phase-based dimming circuit may adjust (i.e., “chop”) the phase of the input power signal202before it arrives at the input filter and rectifier204. When the phase dimmer chops the input voltage202, the current in the primary side206of the transformer208drops; the resulting dropping current in the secondary side214is detected by the secondary-side sensing circuit232, which adjusts the digital control signal accordingly. In one embodiment, the dropping secondary-side current causes a falling edge in the voltage across the resistor236; the secondary-side sensing circuit232detects the falling edge, converts the falling edge into a phase measurement (e.g., how much of the phase of the input voltage202was chopped by the phase dimmer) and adjusts the digital control signal accordingly to increase or decrease the drive current in the LEDs216.

One implementation of a primary-side circuit300is illustrated inFIG. 3. The secondary-side control circuit generates a PWM signal302, which is transmitted to the primary side of the circuit300by an opto-isolator304. A low-pass filter (including a resistor306and a capacitor308) filters the received PWM signal to smooth it into a signal more suitable for driving the LEDs216. The filtered PWM signal is compared, using a first comparator310, to a reference signal generated by a current sensor (which includes a second comparator312and a current-sensing resistor314). The output of the first comparator310drives a hysteretic-gate driver316, thereby controlling current in the transformer primary318. A startup circuit320may be used to ensure that the transformer starts in a discontinuous conduction mode; in this mode, the input impedance of the transformer is dominated by a resistive characteristic (which ensures the input current and input voltage are both sinusoidal and in phase).

The secondary-side sensing circuit232may be implemented using any components or methods known in the art. In one embodiment, a microcontroller, ASIC, or other digital-logic circuit or processor may be used to generate the digital control signal236based on the sensed LED current, received dimming signal234, and/or sensed phase-dimming signal. In one embodiment, the secondary-side sensing circuit232determines the type of the dimming signal (e.g., phase, 0-10 V, or DALI). The LEDs216may be controlled by more than one type of dimming signal; for example, a manual dimming controller may use phase dimming and an automatic dimming controller (which, for example, dims the LEDs216if a room in which the LEDs216are disposed is unoccupied for a certain amount of time, or based on sensed ambient light) may use DALI dimming. The secondary-side sensing circuit232may blend or sum the different types of dimming (i.e., cause the LEDs216to dim to an amount determined by both dimming controllers) or dim the LEDs216based on only one type of dimming in accordance with a pre-programmed or user-input precedence. The order of precedence, dimming-signal detection information, and any other information necessary for the implementation of the secondary-side sensing circuit232may be stored in an included volatile or non-volatile memory (e.g., RAM, ROM, flash, firmware, or other such circuitry or device).

The circuit200may be modified to reduce its cost (at the expense of accuracy) or to increase accuracy (by increasing its cost). A low-cost version of the circuit200may include a small output capacitor220(or no output capacitor200at all). A medium-cost version of the circuit200may reduce ripples or other noise in the LED drive current by increasing the size of the output capacitor220. A high-performance version of the circuit200may add an additional converter stage (making the circuit200a two-stage converter).