Patent Application
20110175532
This invention relates generally to supplying power to luminous loads, and in particular, to a system and method of supplying substantially constant power to a luminous load within a defined input voltage range and temperature range. Additionally, the system and method are capable of adequately interfacing a dimmer circuit to a luminous load, such that the illumination or brightness of the luminous load may be controlled by the dimmer.
Light fixtures that use light emitting diode (LED) technology for illumination are gaining in popularity. These fixtures are now employed more frequently in commercial, residential and public settings. The main reasons that LED-based light fixtures are becoming more popular are that they generally have a longer operational life and operate at a much higher power efficiency. For example, LED-based light fixtures typically have an operational life of around 50 to 100 thousand hours; whereas, incandescent-based light fixtures typically have an operational life of only one to two thousand hours. Additionally, LED-based light fixtures typically have a light efficacy that is 5 to 10 times that of an incandescent light fixture.
Driving or supplying power to LED-based light fixtures, however, may need more consideration to ensure substantially constant illumination. In the past, LED-based light fixtures have been driven by constant output voltage and constant output current ballasts. However, these devices generally do not provide constant power to LED-based loads, and thus, cannot ensure constant illumination of the luminous loads.
Taking, as an example, a constant output voltage ballast, it employs output voltage feedback to ensure that the voltage across an LED-based load is substantially constant. However, the junction voltage of LED devices decreases as environment temperature increases. As a consequence, the current, as well as the power, supplied to the LED load increases with a rise in temperature. As the current increases, this, in turn, may create more heat, which results in even higher current delivered to the load. This, in effect, may result in a thermal runaway, which may eventually lead to a burn out of the LED-based load.
In the case of a constant output current ballast, it employs output current feedback to ensure that the current through the LED-based load is substantially constant. However, as discussed above, the junction voltage of LED devices decreases as environment temperature increases. This has the consequence of the output voltage, as well as the power, decreasing with a rise in temperature. In this case, the LED light output will decrease with rising temperature, which may be undesirable for lots of applications.
Another issue with constant output voltage and current ballasts is that they do not work well with phase control dimming circuits. A phase control dimming circuit controls the amount of power delivered to a luminous load by suppressing or cutting off a portion of the rectified input voltage. Accordingly, as the dimmer is controlled to reduce the brightness of the luminous load, the constant output voltage or current ballast would sense the output voltage or current reduction due to the dimmer, and try to increase the same to maintain the same output voltage or current. As a result, the brightness of the luminous load remains fairly the same, even though the dimmer is attempting to reduce the brightness. This renders the dimmer ineffective.
An aspect of the invention relates to an apparatus that is capable of delivering substantially constant power to a luminous load in response to variation in the input voltage and variation in the environment temperature. In another aspect, the apparatus is further adapted to vary the power supplied to the luminous load in response to changes in the input voltage produced by a dimmer circuit. In other words, during non-dimming applications, the apparatus is able to maintain substantially constant power supplied to the load even though the input voltage and environment temperatures are varying during typical daily operations. Additionally, if the input voltage is changed due to a user controlling a dimmer device to control the brightness of the luminous load, the apparatus is able to control the power delivered to the load in response to the dimmer device.
Other aspects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
The AC source 102 supplies power in the form of an alternating voltage (ac) (e.g., a substantially sinusoidal voltage) having defined or standardized parameters, such as the North American standard of 60 Hz, 110-120 Volt or the European standard of 50 Hz, 220-240 Volt. The optional dimmer may be a phase-control type dimmer circuit, which suppresses or cut-outs a portion of the ac voltage based on a user input device (e.g., a dimming knob) for the purpose of controlling the illumination or brightness of the luminous load 150. The EMI filter 106 reduces extraneous signal interference and noise that may be present on the ac voltage line. The input rectifier and DC filter 108 rectifies the ac voltage of the EMI filter 106 in order to generate an input voltage for the transformer circuit 110.
The control circuit 112 controls or modulates the current through the transformer circuit 110 in response to a voltage ˜Vin that is derived from the input voltage to the transformer circuit 110, and a current ˜Iin that is derived from a current flowing through the input winding of the transformer circuit 110. The control circuit 112 is adapted to control the current through the input winding of the transformer circuit 110 in order to control, regulate, or maintain the power delivered to the luminous load 150. The control circuit 112 may employ pulse width modulation at a substantially constant frequency to regulate the power delivered to the luminous load 150. More specifically, the control circuit 112 is adapted to maintain the power delivered to the luminous load 150 substantially constant given a defined range for the input voltage to the transformer circuit 110 and a defined temperature range. Additionally, as discussed in more detail below, the control circuit 112 may be, at least partially, insensitive to the dimmer control, allowing the dimmer to control the brightness of the luminous load 150 without compensating for the reduced power delivered to the load.
The output rectifier and DC filter 114 rectifies and DC filters the voltage developed across or partially across an output winding of the transformer circuit 110 in order to generate a regulated output voltage and current for the luminous load 150. Alternatively, as discussed in more detail below, the output rectifier and DC filter 114 may perform its rectifying and filtering operations based on the voltage across or partially across an input winding of the transformer circuit 110. The voltage clamp 116 protects the luminous load from voltages that may spike or surge above a defined threshold level. The voltage clamp 116 performs this by shunting the load when the output voltage exceeds the defined threshold.
The switch drive 204 develops a control signal CSo for driving (e.g., turning ON and OFF) the switch module 208 based on the input signal CSi. As an example, the control signal CSo may be a pulse-width modulated signal cycling substantially at a center operating frequency, and modulated based on the input signal CSi. As previously discussed, the switch drive 204 may generate the control signal CSo in order to regulate the power delivered to the luminous load 250. For instance, the control signal CSo may be set or adjusted to maintain the power delivered to the luminous load 150 substantially constant for a defined range of the input voltage Vin and/or the environment temperature. Additionally, the switch drive 204 may generate the control signal CSo such that it is at least partially insensitive to the dimmer control, allowing the dimmer to control the brightness of the luminous load 150 without compensating for the reduced power delivered to the load. The load interface 220 conditions (e.g., rectifies, filters, etc.) the voltage across or partially across the input winding (PW) or output winding SW of the transformer T to generate an output voltage Vo for the luminous load 250. The load interface 220 may further provide over-voltage protection of the luminous load 250.
The starting circuit 302 is adapted to generate a starting current in response to detecting the input voltage Vin so that the driver 316 generates a signal adapted to turn ON the MOSFET Q1. This produces a current to flow from the positive input voltage terminal Vin+ through the first primary winding PW1 of the transformer T1, MOSFET Q1, and current-sensing resistor R, and to the negative input voltage terminal Vin−. This causes energy to be stored in the primary winding PW1 of the transformer T1. In response to the transformer current, a voltage V3 develops across the current-sensing resistor R that is related (e.g., proportional) to the transformer current. Additionally, a voltage V1 develops across the second primary winding PW2 of the transformer T1 that is related or derived from the input voltage Vin by the equation, V1=Vin×N, where N is the turn ratio between the first primary winding PW1 and the second primary winding PW2. Through the diode D2, the voltage V1 is stored by the capacitor C1, and then scaled by the voltage divider 312 in order to generate a voltage V2. At the summing node 320, the voltages V2 and V3 are combined to generate a voltage V4, which may be related to the power delivered to the luminous load 340 for a defined range of the input voltage Vin.
The voltage V4 is applied to the negative input of the comparator 318, and a reference voltage Vr generated by the temperature-compensated voltage reference 322 is applied to the positive input of the comparator. Initially or upon start-up, the output of the comparator 318 is at a high logic level due to the voltage V4 being lower than the reference voltage Vr. Due to the rising transformer current V3 and the transformer voltage V2, the voltage V4 rises above the reference voltage Vr. When this occurs, the comparator 318 then generates a low logic level. As a consequence, the AND-gate 314 produces a low logic level, which the driver 316 outputs to cause the MOSFET Q1 to turn OFF. When this occurs, the windings of the transformer T1 reverse its voltage polarity (commonly referred to as a fly-back action).
During this time, the energy stored in the first primary winding PW1 of the transformer T1 is released to the luminous load 340 by way of the secondary winding SW of the transformer. Once all of the energy in the primary winding PW1 of the transformer T1 is released, the voltages on windings PW1-2 and S2 reverse again, and allow the MOSFET Q1 to turn ON again. This process continuously repeats causing the MOSFET Q1 to turn ON and OFF, and sustain its self oscillation at a particular or defined frequency. The duty cycle or pulse width of the signal driving the MOSFET Q1 is modulated by the voltage V2 which is related or derived from the input voltage Vin, and the voltage V3 which is related or derived from the current through the primary winding of the transformer T1. The duty cycle and frequency of the signal driving the MOSFET Q1 adjust for each cycle in order to maintain substantially a constant power delivered to the luminous load 340 even in view of fluctuations in the input voltage Vin and the environment temperature. By configuring the voltages V2 and V3, the control circuit 310 is capable of delivering substantially constant power to the luminous load 340 within a specific or defined voltage range of the input voltage Vin. The use of the temperature-compensated voltage reference 322 provides the temperature-compensation to maintain the load power substantially constant in view of temperature variation.
The third diode D3, second capacitor C2 and output voltage clamp 330, in this example, make up the load interface circuit. More specifically, the third diode D3 rectifies the alternating energy released from the transformer T1, and the second capacitor C2 dc filters the rectified energy to generate the output voltage across the luminous load 340. The duty cycle and frequency of the signal driving the MOSFET Q1 as well as the transformer T1 may be configured to provide a relatively high ac power factor (e.g., >80%) in delivering power to the luminous load 340. The control circuit 310 may be configured easily into an integrated circuit form, discrete circuit form, or a combination thereof.
In any non-normal operating condition that causes the output voltage across the luminous load 340 to exceed a defined level, the output voltage clamp will activate and automatically shunt the output voltage and reduce the power delivered to the load in order to prevent damage to the load and the apparatus 300. Additionally, the transient voltage clamp 304 is coupled in series with the first diode D1 to clamp leakage energy from the first primary winding PW1 of the transformer T1 to prevent excessive voltage present to the MOSFET Q1 when it is turned OFF. This clamp circuit 304 may contain transient voltage suppressor or other resistor, capacitor or combination thereof to achieve the voltage clamping function.
The control circuit 310 may also be configured to be insensitive to adjustment of the input voltage Vin due to it being controlled by a phase control dimmer circuit. As previously discussed, a phase control dimmer circuit suppresses or cuts-out a portion of the input rectified waveform Vin. If the portion of the input rectified waveform being suppressed is less than a half period or 180 degrees of the waveform, the peak of the input waveform is not affected. However, the received power or integration of the rectified waveform varies as a function of the waveform suppression. If the voltage V2 is configured to vary only as a function of the peak voltage of the input rectified voltage Vin, then the dimmer circuit is able to reduce the power delivered to the luminous load without the control circuit 310 reacting to the reduced power. Thus, the apparatus 300 is able to adequately interface a dimmer circuit to the luminous load 340, and at the same time maintain constant power to the load during normal or non-dimming operations.
More specifically, the apparatus 350 comprises the transient voltage clamp 304 and first diode D1 to clamp leakage energy from the first primary winding PW1 of the transformer T1 to prevent excessive voltage present to the MOSFET Q1 when it is turned OFF. The capacitor C3 and resistor R1, in combination, operate similar to the starting circuit 302, discussed above, to turn ON the MOSFET Q1 upon start-up. That is, upon start-up, the voltage across the capacitor C3 begins to rise. The voltage across the capacitor C3 is coupled to the gate of the MOSFET Q1 via the resistor R1. Once the voltage crosses the threshold of MOSFET Q1, the device turns ON allowing a current to flow through the primary winding PW1 of the transformer T1. The resistor R operates to generate a voltage V3 that is related to the current flowing through the primary winding PW1 of the transformer T1.
The second diode D2 and capacitor C1 operate to sample and hold the voltage V1, which is related to the input voltage Vin. The resistors R3 and R4 operate as the voltage divider 312 and summing node 320 to scale the voltage V2 with reference to the voltage V3 to generate the voltage V4. The thermistor R7 in conjunction with the base-emitter voltage Vbe of the bipolar-junction transistor (BJT) Q2 operate as the temperature-compensated voltage reference 322 discussed above. The BJT Q2 in conjunction with the second Zener diode Z2, capacitor C4 and resistor R5 operate as the AND-gate 314 and driver 316 discussed above.
The third diode D3 operate to rectify the alternating voltage received from the secondary winding SW of the transformer T1. The second capacitor C2 operate to DC filter the rectified voltage to generate the output voltage for the luminous load 340. The first Zener Z1 in conjunction with resistor R2 and silicon-controlled rectifier (SCR) operate as the output voltage clamp 340 discussed above to protect the luminous load 340 from harmful voltage levels.
While the invention has been described in connection with various embodiments, it will be understood that the invention is capable of further modifications. This application is intended to cover any variations, uses or adaptation of the invention following, in general, the principles of the invention, and including such departures from the present disclosure as come within the known and customary practice within the art to which the invention pertains.
