Abstract:
A power supply for supplying power to a lamp with functions of dimming, over-current protection, over-voltage protection, arcing protection, and low-temperature start-up is provided. When frequency of the output current exceeds a predetermined value, the power supply is turned off to accomplish a dimming goal and extend lifetime of the lamp. When abnormal statuses such as open-circuited status, short-circuited status, or arcing status occur, a surge current induced by the abnormal statuses may be eliminated to prevent the power supply from being damaged. A high-frequency current detection circuit is configured to detect whether a current supplied to the high-voltage load is a high-frequency current to prevent damage to the electronic elements in the high-voltage load. A current adjusting circuit is configured to adjust an alternating current outputted to a lamp set in response to an environment temperature to supply an adequate alternating current at a low temperature for starting the lamp set.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Taiwan Patent Application No. 096132331, filed on Aug. 30, 2007, Taiwan Patent Application No. 096135349, filed on Sep. 21, 2007, Taiwan Patent Application No. 096139818, filed on Oct. 24, 2007, Taiwan Patent Application No. 096143207, filed on Nov. 15, 2007, and Taiwan Patent Application No. 097112847, filed on Apr. 9, 2008, the disclosures of which are incorporated herein by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power supply. More particularly, the present invention relates to a power supply for supplying power to a lamp with functions of dimming, over-current protection, over-voltage protection, arcing protection, and low-temperature start-up. 
     2. Descriptions of the Related Art 
     Nowadays, lighting has become indispensable to people&#39;s life, and commonly using for lighting are still lamps. Generally, in order to drive a lamp to emit light, an alternating current (AC) power supply is required. A frequency of the AC power supply shall match the impedance characteristics of the lamp, i.e., be maintained in a range from 40 KHz to 80 KHz, for driving the lamp. Besides the range, either an excessively high or an excessively low frequency of the AC power supply would result in increased impedance, i.e. a decreased gain, of the lamp. 
     In order to match up requirements in different surroundings, the power supply of a lamp may also be designed to provide the lamp with adjustable output brightness, i.e., a dimming function. Since the power supply of a lamp typically adopts a pulse width modulation (PWM) design, the dimming function may be accomplished by adjusting a duty cycle of the power supply, or by increasing a frequency of the power supply output to reduce the gain of the lamp. 
     However, when dimming a lamp with the above methods, there is still energy outputting to the lamp. Since the service life of the lamp is positively related to the illumination duration of the lamp, and the purpose of dimming the lamp is to decrease the output brightness of the lamp, the energy output continued during the dimming process may not only unnecessarily consume electric power, but also shorten service life of the lamp. 
     Also, the power supply may suffer surge current when open-circuited status, short-circuited status, or arcing status occurs. If the power supply cannot turns off timely when the aforementioned statuses occur, the lamp or the power supply may be damaged by the surge current. 
     Almost every kind of electrical products has a certain number of electronic elements interconnected via circuits. If there are any defects in the interconnections, for example, two adjacent bare electrical wires, arcing would occur therebetween. The arc is essentially a high-frequency current, which may damage the electronic elements of an electrical product and may further lead to the complete failure of the product. 
     A popular way to drive multiple lamps simultaneously is to have the lamps connected in parallel and powered by a single power supply module. However, lamps are known to require a larger starting current at a lower environment temperature. Therefore, if the power supply module still supplies a fixed alternating current when the lamps are exposed to a much lower environment temperature, some of the lamps may fail to start. Particularly, the starting voltage of the lamps and the starting voltage differences between the individual lamps will both increase as the environment temperature decrease, thus, preventing the lamps from starting up simultaneously. As a result, the overall applicability of the lamps will be decreased. 
     Accordingly, efforts still have to be made in the art to provide a dimming circuit that is able to conserve electric power and prolong the service life of the lamp. 
     SUMMARY OF THE INVENTION 
     The primary objective of this invention is to provide a power supply comprising a dimming circuit for dimming a lamp. The dimming circuit is configured to gradually decrease output current from a power supply to a lamp when dimming, and turn off the power supply when a frequency of the output current exceeds a pre-determined value. As a result, a dimming function is accomplished and the service life of the lamp is prolonged. 
     Another objective of this invention is to provide a power supply with an over-current control circuit being configured to turns off the power supply when an over-current signal represents abnormal statuses such as open-circuited status, short-circuited status, or arcing status. Thus, a surge current induced by the abnormal statuses may be eliminated to prevent the power supply from being damaged. 
     Another objective of this invention is to provide a power supply with a high-frequency current detection circuit. The power supply comprises a current output module configured to provide a current to a high-voltage load; the high-frequency current detection circuit is configured to detect whether a current supplied to the high-voltage load is a high-frequency current to prevent damage to the electronic elements in the high-voltage load. To this end, the high-frequency current detection circuit comprises a first capacitive element, a resistor and a second capacitive element. 
     Yet an objective of this invention is to provide a power supply with a current adjusting circuit for adjusting an alternating current outputted to a lamp set in response to an environment temperature to supply an adequate alternating current at a low temperature for starting the lamp set. More particularly, according to the present invention, the alternating current is increased at a low temperature to increase the cross voltage of the capacitor (i.e., a passive element in series) connected in series with the lamps, so that the lamps that would otherwise fail to start will obtain an increased starting voltage and light up. In this way, it is possible to start a plurality of lamps, connected in parallel, simultaneously with a single transformer at a low temperature. 
     The detailed technology and preferred embodiments implemented for the subject invention are described in the following paragraphs accompanying the appended drawings for people skilled in this field to well appreciate the features of the claimed invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a power supply comprising a dimming circuit; 
         FIG. 2  is a schematic graph of gain versus frequency of a lamp; 
         FIG. 3  is a schematic current waveform of a lamp during a dimming process; 
         FIG. 4  is a schematic diagram of a dimming circuit, a photo coupler, a protection circuit, and a current adjusting circuit; 
         FIG. 5  is a schematic diagram of an over-current control circuit; 
         FIG. 6  is a schematic diagram of a high-frequency current detection circuit; and 
         FIG. 7  illustrates the gain versus frequency of a lamp. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     This invention relates to a power supply for supplying power to a lamp with functions of dimming, over-current protection, over-voltage protection, arcing protection, and low-temperature start-up. Embodiments will be described hereinbelow to explain this invention. However, these embodiments are not intended to limit that this invention can only be embodied in any specific context, applications or with particular methods described in these embodiments. Therefore, description of the embodiments is only intended to illustrate this invention, rather than to limit this invention. It should be noted that, in the following embodiments and attached drawings, elements not directly related to this invention are omitted from depiction, and the dimensional relationships among various elements are slightly exaggerated for ease of understanding. 
       FIG. 1  is a block diagram of a power supply comprising a dimming circuit. The power supply is configured to receive a DC power input Vin and supply an AC output to power a lamp set  18 . The power supply comprises an energy transfer element  11 , a power control circuit  12 , a dimming circuit  13 , a photo coupler  14 , a protection circuit  15 , an over-current control circuit  19 , and a high-frequency current detection circuit  20 , and the lamp set  18  may comprises a single lamp or a plurality of lamps mutually connected in parallel. In this embodiment, the energy transfer element  11  is a transformer comprising an energy transfer input end  111  and an energy transfer output end  112 , and is coupled to an output of the power supply. The power control circuit  12  comprises an energy control circuit  121  coupled to the energy transfer element  11  and a control circuit  122 , in which the energy control circuit  121  is configured to receive a control signal  101  from the control circuit  122 , in order to supply energy to the energy transfer element  11 . 
     Further, in this embodiment, the dimming circuit  13  comprises three terminals of a first terminal  131 , a second terminal  132  and a third terminal  133  respectively, a switching element  134  being a transistor in this embodiment, and a dimming control circuit  135 . The switching element  134  is coupled to the first terminal  131  and the second terminal  132 , in which the first terminal  131  is coupled to the power control circuit  12  of the power supply, and the second terminal  132  is coupled to a grounding terminal. The dimming control circuit  135  is coupled to the third terminal  133  and the switching element  134 , in which the third terminal  133  is adapted to receive a dimming signal  102 . The dimming control circuit  135  generates a dimming control signal  103  in response to the dimming signal  102 , and also switches the switching element  134  in response to the dimming control signal  103 . The power control circuit  12  increases a frequency of the AC output to a frequency range in response to switching of the switching element  134 , and turns off the power supply when the frequency exceeds a pre-determined value. 
     Explicitly, the dimming signal  102  from a system comprising the power supply is adapted to adjust brightness of the lamp set  18 . Refer to  FIG. 2  together, where a schematic graph of gain versus frequency of a lamp is depicted. It can be seen from this figure that, to keep the gain of the lamp at a maximum value, the lamp set  18  shall operate at an appropriate frequency, which generally ranges from 40 KHz to 80 KHz. When the frequency exceeds 40 KHz and keeps increasing continuously, the gain of the lamp will decline. Generally, when the frequency goes higher than 200 KHz, the gain will become very small, i.e., the lamp current will become very small and instable. Therefore, when it is desired to dim the light, the dimming control circuit  135  of the dimming circuit  135  switches the switching element  134  in response to the dimming signal  102 , so that the frequency of the AC output of the power control circuit  12  can be increased. At this point, current of the lamp set  18  will decrease gradually as the frequency increases, and once the frequency exceeds a pre-determined value which in this embodiment is 180 KHz, the power supply will be turned off. 
     Referring to  FIG. 3 , a schematic current waveform of the lamp set  18  during a dimming process is depicted. The time interval T 1  represents the soft-starting characteristics exhibited by the lamp set  18  when started. The time interval T 2  represents a current waveform of the lamp set  18  in normal operation, in which case the lamp operates with a substantially fixed frequency value. The time interval T 3  represents that a dimming operation is performed on the lamp set  18 , in which case the lamp current decreases gradually as the frequency increases from 40 KHz to 180 KHz. The time interval T 4  represents that the power supply is turned off when the frequency exceeds a pre-determined value (i.e., 180 KHz), in which case the lamp set  18  has a zero output. By modulating ratio of T 4  to T 1 +T 2 +T 3 , the output of the lamp set  18  may be controlled to achieve a dimming effect. For example, when the ratio of T 4  to T 1 +T 2 +T 3  is set at 1:4, the output brightness of the lamp set  18  will be adjusted to 80%. 
     It should be noted that, the frequency range described above is only taken for an example That is, in other embodiments, the frequency range may be varied in response to the manufacturing characteristics of the lamp, while the pre-determined frequency at which the power supply is turned off may vary accordingly. As the service life of the lamp is positively related to the operation duration thereof, turning off the power supply output may not only accomplish the dimming purpose, but also prolong the service life of the lamp. As manifested by the above embodiments, if the output brightness of the lamp set  18  is adjusted to 50%, the service life of the lamp set  18  will be extended by twice theoretically. 
     Referring back to  FIG. 1 , in this embodiment, the photo coupler  14  comprises a light emitting element  141  and a light receiving element  142 , in which the light emitting element  141  is coupled to the first terminal  131  of the dimming circuit  13 , and the light receiving element  142  is coupled to the power control circuit  12 . The dimming control circuit  135  is configured to turn on the switching element  134  in response to the dimming control signal  103 . The photo coupler  14  is adapted to generate a coupling signal  104  in response to a turn-on mode of the switching element  134 . In response to the coupling signal, the power control circuit  12  increases the frequency of the AC output to the aforesaid frequency range to gradually decrease the peak current of the AC output, and turns off the power supply when the frequency exceeds the aforesaid pre-determined value, i.e., 180 KHz. 
     The protection circuit  15  is configured to turn off the power supply in case of an excessive output of the power supply, so as to protect the lamp set  18  against damage. The protection circuit  15  is coupled to the output of the power supply via a voltage detection circuit  21  and a current detection circuit  22  for receiving an over-current signal  105  and an over-voltage signal  106 , and coupled to the photo coupler  14 . The protection  15  may also drive the photo coupler  14  to generate the coupling signal  104  in response to the output of the power supply, thereby to increase frequency of the AC output to the aforesaid frequency range to gradually decrease the peak current thereof and turn off the power supply once the frequency exceeds the aforesaid pre-determined value, i.e., 180 KHz. Embodiments of the protection circuit  15  will be described hereinafter. 
     Referring to  FIG. 4 , there is depicted a schematic diagram of parts of the dimming circuit  13 , parts of the photo coupler  14 , the protection circuit  15 , and a current adjusting circuit  41 . It should be emphasized that, the protection circuit  15  comprises an over-current protection circuit  151  and an over-voltage protection circuit  152 . Further, the dimming signal  102  is an AC signal, for example, a rectangular wave or a pulse signal. The dimming control circuit  135  comprises a comparator circuit  231  configured to compare the dimming signal  102  against a reference value, and generates a dimming control signal  103  in response to the comparing result. The comparator circuit  231  comprises a comparator  232  and two resistors  233 ,  234 . The two resistors  233 ,  234  form a voltage divider circuit, which is connected to a reference voltage V REF  and generates a reference value at one of the input terminals of the comparator  232 . The other input terminal of the comparator  232  is connected to the dimming signal  102 . When the dimming signal  102  is larger than the reference value, the comparator  232  outputs the dimming control signal  103  to turn on the switching element  134 , in which case the input voltage V 1  of the comparator  236  will be pulled low by the turn-on current of the switching element  134  through the resistor  235 , thus turning on the light receiving element  141  of the photo coupler  14 . 
     In other embodiments, the dimming control circuit  135  may also be amplitude detect circuit, which generates the dimming control signal  103  in response to amplitude of the dimming signal  102  when the amplitude becomes larger than a reference value. The implementation of the amplitude detect circuit may readily occur to those skilled in the art, and will not be described in detail herein. 
     The over-current protection circuit  151 , which comprises a resistor  237  and a capacitor  238 , is connected to one of the input terminals of the comparator  236 . Upon receiving an over-current signal  105 , the over-current protection circuit  151  will have the input voltage V 2  of the comparator  236  increased, thereby to turn on the light receiving element  141  of the photo coupler  14 . As a result, the lamp current is controlled to prevent damages caused by over-current. 
     Similarly, the over-voltage protection circuit  152 , which comprises a resistor  239  and a capacitor  240 , is connected to one of the input terminals of the comparator  241 . Upon receiving an over-voltage signal  106 , the over-voltage protection circuit  152  will have the input voltage V 3  of the comparator  241  increased, thereby to turn on the light receiving element  141  of the photo coupler  14 . As a result, the lamp voltage is controlled to prevent damages caused by over-voltage. 
     The current adjusting current  41  comprises a temperature sensing module  401 , and a feedback circuit. The temperature sensing module  401  is configured to sense an environment temperature and to generate a sensing voltage signal at the node  407  in response to the environment temperature. The feedback circuit is coupled to the temperature sensing module  401  to generate a feedback signal  107  in response to the sensing voltage signal, the feedback signal  107  is adapted to modulate the voltage V 1  by the resistor Rs. The feedback circuit comprises a comparator  403 , resistors R 1 , R 2 , R 3 , R 4 , and Rs. In the present embodiment, the temperature sensing module  401  is a negative temperature coefficient element. 
     With this configuration, the reference voltage input  400  will result in a voltage of 
                 R   ⁢           ⁢   4         R   ⁢           ⁢   3     +     R   ⁢           ⁢   4         ×     V   ref           
at nodes  405  and  407 , when temperature decrease, the resistance of the negative temperature coefficient element  401  will increase, and result in increase of the voltage at node  107 , wherein V ref  represents the reference voltage  400 . Thus, when the environment temperature decreases, the feedback signal  107  is adapted to decrease the frequency of the AC output.
 
     In other embodiments, the temperature sensing module  401  may be selected from one of a positive temperature coefficient element, a diode, and a combination thereof including the negative temperature coefficient element. 
     The over-current control circuit  19  is coupled to the AC output of the power supply via the current detection circuit  22 . The over-current control circuit  19  is configured to compare the over-current signal  105  with a first reference and a second reference, in which the first reference is larger than the second reference. When the over-current signal  105  is larger than the first reference or when the over-current signal is lower than the second reference, the over-current control circuit  19  generates a judgment signal  191  to the power control circuit  12 , and the power control circuit  12  turns off the power supply when the judgment signal  191  is larger than a judgment reference. Therefore the surge current occurred by the open-circuited status, short-circuited status, or arcing status can be prevented. Embodiments of the over-current control circuit  19  will be described hereinafter. 
       FIG. 5  a schematic diagram of the over-current control circuit  19  comprising a comparator  190 , a first resistor R 5 , a second resistor R 6 , a third resistor R 7 , a fourth resistor R 8 , and a fifth resistor R 9 . The comparator  190  has an inverting end, a non-inverting end, and an output end. The first resistor R 5  has a first end and a second end, wherein the first end coupled to a reference Vref, and the second end coupled to the output end of the comparator. The second resistor R 6  has a first end and a second end, wherein the first end is coupled to the output of the comparator, and the second end is coupled to the non-inverting end. The third resistor R 7  has a first end and a second end, wherein the first end is coupled to the second end of the second resistor, and the second end is coupled to the ground. The fourth resistor R 8  has a first end and a second end, wherein the first end is coupled to the first end of the third resistor, and the second end is coupled to the reference Vref. The fifth resistor R 9  has a first end and a second end, wherein the first end is coupled to the inverting end, and the second is coupled to the over-current signal  105 . The first reference is generated at the node B according to the fundamental principle of electrical divider, and similarly, the second reference is generated at the node A. By the aforementioned circuitry of the over-current control circuit  19 , the judgment signal is a square periodical signal. 
     The high high-frequency current detection circuit  20  is coupled to the AC output of the power supply via the current detection circuit  22 . 
     The high-frequency current detection circuit  20  is coupled to the current output module  2 , and is adapted to receive a current  201  of the AC output of the power supply and, in response to a frequency of the current  201 , to generate a high-frequency current detecting signal  202 . The power control circuit  12  is configured to turn off the power supply when the frequency exceeds a high-frequency reference. 
     Next, the structure of the high-frequency current detection circuit  20  will be described. As shown in  FIG. 6 , the high-frequency current detection circuit  20  comprises a first capacitive element  211 , a resistor  212 , a second capacitive element  213 , a first direction element  214  and a second direction element  215 . The first capacitive element  211  is coupled to the AC output of the power supply, and is adapted to receive the current  201 . The resistor  212  is connected to the first capacitive element  211  in series, and is connected to the second capacitive element  213  in parallel. The first capacitive element  211  and the second capacitive element  213  are adapted to generate the high-frequency current detecting signal  202  at a junction, where they are connected with the resistor  212 , in response to a frequency variation of the current  201 . Additionally, the first direction element  214  is coupled to the first capacitive element  211 , the resistor  212  and the second capacitive element  213 . The second direction element  215  is connected to the resistor  212  and the second capacitive element  213  in parallel and is also coupled to the first direction element  214  to transmit the high-frequency current detecting signal  202  to the feedback circuit  13 . In this preferred embodiment, both the first direction element  214  and the second direction element  215  are diodes, but do not necessarily have to be in other embodiments. 
     To detect a high-frequency current, the high-frequency current detection circuit  20  is configured to filter out the low-frequency components of the current to retain the high-frequency components thereof. Therefore, in this preferred embodiment, the first capacitive element  211  is a high-voltage capacitor. 
       FIG. 7  illustrates the gain versus frequency of a lamp. When the second capacitive element  213  has impedance much higher than that of the resistor  212 , the gain it produces will be increased as indicated by the line  40 . The increased impedance is unfavorable for filtering the high frequency components. In contrast, when the second capacitive element  213  has impedance much lower than that of the resistor  212 , it will produce a gain as indicated by the line  41 . The decreased impedance is favorable for filtering the high-frequency components successfully. In this embodiment, the second capacitive element  213  is designed to have impedance much lower than that of the resistor  212  to generate the high-frequency current detecting signal  202  by filtering the high-frequency components of the current. In other words, the high-frequency current detection circuit  20  may be considered as a high-pass filter for retaining the high-frequency components of the current. 
     It should be noted that, the over-current protection circuit and the over-voltage protection circuit are provided to detect excessive power output conditions of the power supply, in order to drive the photo coupler  14  to generate a coupling signal  104  in response to such an output of the power supply. Other embodiments for accomplishing substantially the same protection purpose may readily occur to those skilled in the art, and the scope of this invention is not just limited to the embodiments described above. 
     It follows from the above description that, this invention provides a dimming circuit capable of conserving electric power and prolonging service life of a lamp. The above disclosure is related to the detailed technical contents and inventive features thereof. People skilled in this field may proceed with a variety of modifications and replacements based on the disclosures and suggestions of the invention as described without departing from the characteristics thereof. Nevertheless, although such modifications and replacements are not fully disclosed in the above descriptions, they have substantially been covered in the following claims as appended.