LEDS driver

This application claims the benefit of U.S. provisional application Ser. No. 60/434,550, filed Dec. 19, 2002, which the entire subject matter is incorporated herein by reference.

The technical field of this disclosure is power supplies, particularly, a power supply for LEDs.

Significant advances have been made in the technology of white light emitting diodes (LEDs). White light LEDs are commercially available which generate 10-15 lumens/watt. This is comparable to the performance of incandescent bulbs. In addition, LEDs offer other advantages such as longer operating life, shock/vibration resistance and design flexibility because of their small size. As a result, white light LEDs are replacing traditional incandescent sources for illumination applications such as signage, accenting, and pathway lighting. The white LEDs can be used alone or in conjunction with colored LEDs for a particular effect.

The electrical characteristics of LEDs are such that small changes in the voltage applied to the LED lamp will cause appreciable current changes. In addition, ambient temperature changes will also result in LED current changes by changing the forward drop across the LEDs. Furthermore, the lumen output of LEDs depends on the LED current. The existing electrical power supplies for LED light sources are not designed to precisely regulate the LED current to prevent luminous intensity variations due to input ac voltage variations and ambient temperature. Operation of LED lamps at excessive forward current for a long period can cause unacceptable luminous intensity variations and even catastrophic failure. In addition, current electrical power supplies do not minimize power consumption to maximize energy savings.

It would be desirable to have a power supply for LEDs that would overcome the above disadvantages.

One form of the present invention is a power supply for a LED light source that comprises a power converter and a LED control switch. The power converter operates to provide a regulated power including a LED current and a LED voltage. The LED control switch further operates to control a flow of the LED current through the LED light source. The LED control switch further operates to clamp a peak of the LED current during an initial loading stage of the LED light source. This prevents damage to the LED light source due to a field misapplication.

A second form of the present invention is a power supply for a LED light source further comprising a detection circuit operating to provide a detection signal indicative of an operating condition of the LED light source associated with the LED voltage. The detection signal has a first level representative of a load condition of the LED light source. The detection signal has a second level representative of a short condition or an open condition indicative of the LED light source.

A third form of the present invention is a power supply for a LED light source further comprising a LED current sensor or a LED voltage sensor. Each sensor includes a differential amplifier and means for adjusting a gain of the differential amplifier.

The foregoing forms as well as other forms, features and advantages of the present invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the present invention rather than limiting, the scope of the present invention being defined by the appended claims and equivalents thereof.

FIG. 1 illustrates a block diagram of a power supply for an LED light source in accordance with the present invention;

FIG. 2 illustrates a schematic diagram of one embodiment of the FIG. 1 power supply in accordance with the present invention;

FIG. 3 illustrates a timing diagram of one embodiment of a control circuit in accordance with the present invention;

FIG. 4 illustrates a schematic diagram of one embodiment of a short/open detection circuit in accordance with the present invention; and

FIG. 5 illustrates a schematic diagram of one embodiment of a differential amplification circuit in accordance with the present invention.

FIG. 1 illustrates a block diagram of a power supply 20 for powering an LED light source 10 including a variable number of LEDs wired in series and/or in parallel. A single-phase ac input 21 of power supply 20 provides a voltage V AC to an AC/DC converter 22 of power supply 20 whereby AC/DC converter 22 converts voltage V AC into a voltage V DC . AC/DC converter 22 provides voltage V DC to a power converter 23 of power supply 20 whereby power converter 23 generates a regulated power P REG including a LED current and a LED voltage V LED .

Power converter 23 provides regulated power P REG to LED light source 10 . In operation, LED control switch 24 controls a flow of the LED current through the LED light source 10 . A LED current sensor 25 of power supply 20 provides a sensed current I SE indicative of a magnitude of the LED current flowing through LED light source 10 . A LED voltage sensor 26 of power supply 20 provides a sensed voltage V SE indicative of a magnitude of the LED voltage V LED applied to LED light source 10 . Sensed current I SE and sensed voltage V SE are fed to a feedback controller 27 of power supply 20 . A signal reference 28 of power supply 20 provides a reference signal REF to a feedback controller 27 , whereby feedback controller 27 provides a feedback signal FB to power converter 23 based on sensed current I SE , sensed voltage V SE and reference signal REF.

LED control switch 24 further operates to clamp a peak of LED current flowing through LED light source 10 to thereby protect the LED light source 10 from electrical damage. LED control switch 24 is particularly useful when LED light source 10 transitions from an open operating state to a load operating state (i.e., an initial loading), such as, for example, a connection of LED light source 10 to power supply 20 subsequent to an energizing of power supply 20 . An LED dimmer 29 of power supply 20 operates to control a desired dimming of LED light source 10 by providing a control signal CS to LED control switch 24 . Control signal CS can be in one of many conventional forms, such as, for example, a pulse width modulation signal (“PWM”).

A short/open detection circuit 30 provides a detection signal DS as an indication of a short condition or an open condition of LED light source 10 based on the LED voltage V LED applied to LED light source 10 .

The configuration of each component 21 - 30 of power supply 20 is without limit. Additionally, coupling among the components 21 - 30 of power supply 20 can be achieved in numerous ways (e.g., electrically, optically, acoustically, and/or magnetically). The number of embodiments of power supply 20 is therefore essentially limitless.

FIG. 2 illustrates a schematic diagram of one embodiment 120 of power supply 20 ( FIG. 1 ) for one embodiment 110 of LED light source 10 ( FIG. 1 ) made in accordance with the present invention. Power supply 120 employs a flyback transformer with current feedback through a power factor corrector (“PFC”) IC to supply power to LED light source 110 . To this end, power supply 120 includes an EMI filter 121 , an AC/DC converter (“AC/DC”) 122 , a transformer 123 , a power factor corrector 124 , a feedback controller 125 , an optocoupler 126 , a LED control switch 127 , a LED PWM dimmer 128 resistors R 1 -R 7 , capacitors C 1 -C 5 , diodes D 1 -D 3 , zener diodes Z 1 -Z 3 and a MOSFET Q 1 as illustrated in FIG. 2 .

Voltage is supplied to power supply 120 at V IN to EMI filter 121 . The voltage can be an ac input and is typically 50/60 Hertz at 120/230 V RMS . EMI filter 121 blocks electromagnetic interference on the input. AC/DC 122 can be a bridge rectifier and converts the ac output of EMI filter 120 to dc. Transformer 123 includes a primary winding W 1 , W 4 and W 5 , and a plurality of secondary windings W 2 and W 3 . The windings W 1 /W 2 constitute the flyback transformer to power the LED light source 110 . The flyback transformer is controlled by PFC 124 , which is a power factor corrector integrated circuit, such as model L6561 manufactured by ST Microelectronics, Inc.

The flyback transformer transfers power to LED light source 110 where the LED current and the LED voltage are controlled by feedback control. The forward converter operation of windings W 1 /W 3 charge a capacitor C 3 and a reference current signal is generated between a series resistor R 4 and a zener Z 2 . The peak voltage across capacitor C 3 depends on the W 1 /W 3 turns ratio. The output dc voltage from flyback operation of windings W 1 /W 2 cannot be used to generate the reference current signal since the output dc voltage across LED light source 110 can have a wide range—from 2.6 Volts dc for one LED lamp to about 32 Volts dc for 8 LEDs in series. The forward converter operation of windings W 1 /W 3 can be used instead. The forward converter operation of the W 1 /W 5 windings can also be used to supply power to the integrated circuits, such as PFC 124 .

A sensed LED current I SE flows through resistor R 1 , which is in series with the LED light source 110 via LED control switch 127 . A voltage representative of sensed LED current I SE is applied to a non-inverting input of a comparator U 1 . A sensed LED voltage V SE is generated by zener diode Z 1 . Sensed LED current I SE and sensed LED voltage V SE as well as a voltage reference V REF are fed to feedback controller 125 , whereby a voltage feedback V FB from feedback controller 125 drives an optocoupler 126 via resistor R 7 . In generating voltage feedback V FB , feedback controller 125 employs a pair of comparators U 1 and U 2 , resistors R 8 -R 12 , and a capacitor C 6 as illustrated in FIG. 2 .

Feedback controller 125 is necessary since optocouplers have a wide range of current transfer ratio (CTR). Feedback controller 125 maintains an accurate voltage feedback V FB to thereby avoid large errors in LED current flowing through LED light source 110 . Optocoupler 126 isolates the dc circuit supplying the LED light source 110 from the ac circuit power supply at EMI filter 120 , the two circuits being on the opposite sides of the transformer 123 .

The output of the optocoupler 126 is connected to PFC 124 , which supplies a gate drive signal to MOSFET Q 1 . Control of MOSFET Q 1 adjusts the current flow through winding W 1 of transformer 123 to match the LED light source 110 power demand. The internal 2.5 V reference signal and an internal compensation circuit of PFC 124 maintains the voltage drop across a resistor R 6 at 2.5V. Although this example uses MOSFET Q 1 for adjusting the transformer current, alternate embodiments can use other types of transistors to adjust the current, such as an insulated gate bipolar transistor (“IGBT”) or a bipolar transistor. The input to PFC 124 at Z CD provides a reset signal powered from windings W 2 /W 4 .

Zener diode Z 1 also provides overvoltage protection for LED light source 110 . Specifically, zener diode Z 1 connects across the output connection to the LED light source 110 and clamps the output voltage to a specified maximum value. The nominal zener operating voltage is selected to be just over the maximum specified output voltage. In case of an output open circuit, the flyback operation of windings W 1 /W 2 of transformer 123 would continue to build the output voltage. The increasing output voltage turns on the zener diode Z 1 to thereby increase the amount of feedback to resistor R 6 from feedback controller 125 via resistor R 7 and optocoupler 126 . This limits the gate drive signal to MOSFET Q 1 , preventing the flyback converter from building the output voltage to the LED light source 110 beyond a specified maximum voltage. Similarly, zener diode Z 3 connected from the reset winding W 4 to resistor R 6 will prevent output overvoltage due to a malfunction of feedback controller 125 . In alternate embodiments, either zener diode Z 1 or zener diode Z 3 , or both zener diode Z 1 and zener diode Z 3 can be omitted depending on the degree of control protection required for a particular application.

LED control switch 127 includes a switch SW 1 in the form of a MOSFET and a switch SW 2 in the form of a bipolar transistor. Switches SW 1 and SW 2 can be in other conventional forms, such as, for example, an IGBT. As illustrated, a drain of MOSFET switch SW 1 is connected to LED light source 110 . A gate of MOSFET switch SW 1 is connected to a collector of bipolar switch SW 2 . A source of MOSFET switch SW 1 and a base of bipolar switch SW 2 are connected to zener diode Z 1 , resistor R 1 , and feedback controller 125 . An emitter of bipolar switch SW 2 is connected to ground. In operation, switch SW 1 is turned on and switch SW 2 is turned off when the LED current is below the desired peak. This mode permits a normal operation of the front-end components of power supply 120 . Conversely, switch SW 1 is turned off and switch SW 2 is turned on when the LED current exceeds the desired peak. This limits the peak of the LED current to a safe level whereby damage to LED light source 110 is prevented. As will be appreciated by one having skill in the art, LED control switch 127 is particularly useful upon a connection of LED light source 110 to an energized power supply 120 whereby capacitor C 2 discharges stored energy to LED light source 110 with a current having a peak clamped to thereby prevent damage to LED light source 110 .

MOSFET switch SW 1 can be operated by a conventional gate driver (not shown) or by an illustrated LED PWM dimmer 128 .

LED PWM dimmer 128 provides a PWM signal (not shown) to MOSFET switch SW 1 in response to an external dim command V DC . LED PWM dimmer 128 adjusts the duty cycle of the PWM signal to thereby produce a desired light output from LED light source 110 . LED PWM dimmer 128 is particularly useful in producing a precise and temperature sensitive minimum dim level for LED light source 110 .

LED PWM dimmer 128 includes a diode D 4 and a diode D 5 connected to the gate of MOSFET switch SW 1 . A comparator U 3 of LED PWM dimmer 128 is in the form of an operational amplifier having an output connected to diode D 4 and a non-inverting input for receiving a dimming command V DC . A conventional astable multivibrator circuit 129 of LED PWM dimmer 128 is connected to diode D 5 . A ramp generator of LED PWM dimmer 128 includes a resistor R 16 connected to diode D 5 and a gate of transistor Q 2 in the form of a MOSFET. Transistor Q 2 can be in other forms, such as, for example, an IGBT. The ramp generator further includes an operational amplifier U 4 . A resistor R 15 , a resistor R 17 , a drain of bipolar transistor Q 2 , a capacitor C 7 , and an inverting input of comparator U 3 are connected to a non-inverting input of operational amplifier U 4 . Resistor R 15 is further connected to an output of operational amplifier U 4 . A resistor R 13 is connected to the output and an inverting input of operational amplifier U 4 . A resistor R 14 is connected to the inverting input of operational amplifier U 4 and ground. The source of MOSFET transistor Q 2 and capacitor C 7 are connected to ground. Resistor R 17 is further connected to a DC voltage source.

In operation, LED PWM dimmer 128 achieves a precise and temperature insensitive minimum dim level for LED light source 110 . Specifically, astable multivibrator circuit 129 produces a minimum pulse width (e.g., T ON,MIN illustrated in FIG. 3 ). The duration of the minimum pulse width is a function of a resistance and capacitance of astable multivibrator circuit 129 . Thus, the minimum pulse width is accurate and temperature insensitive. The ramp generator produces a ramp signal (e.g., RS illustrated in FIG. 3 ), which is periodically reset by the minimum pulse width. The ramp signal is supplied to the inverting input of comparator U 3 whereby a comparison of the ramp signal and dim command V DC yields a target pulse width at the output of comparator U 3 (e.g., T ON illustrated in FIG. 3 ). The minimum pulse width and the target pulse width are combined to provide the PWM signal at the gate of MOSFET switch SW 1 . As such, the PWM signal consists of the target pulse width overlapping the minimum pulse width when the dim command V DC exceeds or is equal to the ramp signal. Conversely, the PWM signal exclusively consists of the minimum pulse width when the ramp signal exceeds the voltage dim command V DC .

In practice, a suitable range for voltage dim command V DC is 0 to 10 volts.

Short/Open Circuit Detection

FIG. 4 illustrates one embodiment of short/open detection circuit 130 . A LED voltage drop V LD across the LED light source 110 applied between a node N 1 and a node N 2 , and an input voltage V IN is applied between node N 2 and a common reference. The LED voltage drop V LD approximates zero (0) volts when LED light source 110 ( FIG. 2 ) is shorted, and approximates the LED voltage V LED of regulated power P REG ( FIG. 1 ) when LED light source 110 is an open circuit. The input voltage V IN is typically in the range of six (6) volts to sixteen (16) volts. A comparator U 5 in the form of an operational amplifier provides a detection signal V DS at a high level to indicate a “LED outage” condition of LED light source 110 and at a low level to indicate a normal operation of LED light source 110 . The “LED outage” condition is either indicative of a short or open of LED light source 110 .

Input voltage V IN in the illustrated embodiment is a dc voltage. A dc-dc type power converter can therefore be used to supply power to LED light source 110 ( FIG. 2 ). In alternative embodiments, detection circuit 130 can be adapted for use in ac to dc type power converters.

An emitter of a transistor Q 3 in the form of a bipolar transistor, and a zener diode Z 4 are also connected to node N 1 . Transistor Q 3 can be in other conventional forms, such as, for example, an IGBT. A resistor R 18 , a resistor R 21 , and a resistor R 22 are also connected to node N 2 . A base of bipolar transistor Q 3 is connected to resistor R 18 . Zener diode Z 4 , a resistor R 20 and resistor R 21 are connected to an inverting input of comparator U 5 . A collector of bipolar transistor Q 3 , a diode D 6 , and a resistor R 19 are connected to a node N 3 . Resistor R 19 and resistor R 20 are further connected to the common reference. Diode D 6 and resistor R 22 are connected to a non-inverting input of comparator U 5 .

For a normal operation of LED light source 110 , the LED voltage drop V LD is greater than the base-emitter junction voltage of transistor Q 3 whereby transistor Q 3 is on, diode D 6 is in a non-conductive state, and the voltage at the collector of transistor Q 3 exceeds the input voltage V IN . As a result, the input voltage V IN is applied to the inverting input of comparator U 3 . The conducting voltage of zener diode Z 4 is chosen to be above the LED voltage drop V LD and therefore zener diode Z 4 is in a non-conductive state. As a result, a voltage applied to the non-inverting input of comparator U 2 will equate the input voltage V IN reduced by a voltage divider factor established by resistor R 20 and resistor R 21 . The output of comparator U 5 will be low (e.g., close to ground) since the voltage applied to the inverting input exceeds the voltage applied to the non-inverting input.

For an open array condition of LED light source 110 , the LED voltage drop V LD approximates the LED voltage V LED of regulated power P REG , which is chosen to be higher than the voltage of zener diode Z 4 . The LED voltage drop V LD is greater than the base-emitter junction voltage of transistor Q 3 whereby transistor Q 3 is on and the voltage at the collector transistor Q 3 exceeds the input voltage V IN . As a result, the input voltage V IN is applied to the inverting input of comparator U 3 . The conducting voltage of zener diode Z 4 is lower than the LED voltage drop V LD and zener diode Z 4 is therefore in a conductive state. As a result, a voltage applied to the non-inverting input of comparator US will equate a summation of the input voltage V IN and the LED voltage drop V LD minus the conducting voltage of diode D 6 . The output of comparator US will be high (e.g., close to the input voltage V IN ) since the voltage applied to the non-inverting input exceeds the voltage applied to the inverting input.

For a short array condition of LED light source 110 , the LED voltage drop V LD approximates zero (0) volts. The LED voltage drop V LD is therefore less than the base-emitter junction voltage of transistor Q 3 whereby transistor Q 3 is off, the voltage at the collector transistor is pulled down by resistor R 19 and diode D 6 is conducting. As a result, a voltage applied to the inverting input of comparator US will equate the input voltage V IN reduced by a voltage divider factor established by resistor R 19 and resistor R 22 . The conducting voltage of zener diode Z 4 exceeds the LED voltage drop V LD and zener diode Z 4 is therefore in a non-conductive state. The output of comparator US will be high (e.g., close to the input voltage V IN ) since the voltage applied to the non-inverting input exceeds the voltage applied to the inverting input.

In an alternate embodiment, an additional zener diode or a voltage reference can be inserted in the emitter path of transistor Q 3 to detect a voltage level other than less that one base-emitter junction of transistor Q 3 .

FIG. 5 illustrates a differential amplification circuit having a voltage output V O that can be employed in LED current sensor 25 ( FIG. 1 ) or LED current sensor 26 ( FIG. 1 ). A resistor R 23 and a resistor R 25 are connected to an offset voltage source V OFF . Resistor R 25 , a resistor R 26 , and a resistor R 28 are connected to an inverting input of an operational amplifier U 6 . A resistor R 24 and a resistor R 27 are connected to a non-inverting input of operational amplifier U 6 . Resistor R 23 and resistor R 24 are connected. Resistor R 28 is further connected to an output of operational amplifier U 6 .

In operation, the voltages applied to the inputs of the operational amplifier U 6 are lower than the supply voltage V dd irrespective of the size of resistor R 23 . In one embodiment, resistors R 25 and R 26 are chosen to apply half of the offset voltage V OFF to the inverting input of operational amplifier U 6 , and resistors R 24 and R 27 are chosen to obtain a proper common mode rejection (e.g., resistor R 28 equaling a parallel combination of resistor R 26 and R 28 ). As a result, the gain of operational amplifier U 6 can be adjusted as desired.

It is important to note that FIGS. 2-5 illustrates specific applications and embodiments of the present invention, and is not intended to limit the scope of the present disclosure or claims to that which is presented therein. Upon reading the specification and reviewing the drawings hereof, it will become immediately obvious to those skilled in the art that myriad other embodiments of the present invention are possible, and that such embodiments are contemplated and fall within the scope of the presently claimed invention.

While the embodiments of the invention disclosed herein are presently considered to be preferred, various changes and modifications can be made without departing from the spirit and scope of the invention. The scope of the invention is indicated in the appended claims, and all changes that come within the meaning and range of equivalents are intended to be embraced therein.