200816869 九、發明說明： 【發明所屬之技術領域】 本發明係關於一種發光裝置，特別關於一種以交流驅 動之發光二極體。 【先前技術】 發光二極體一般係利用P型半導體以及N型半導體製 作而成，由於利用半導體元件所製作的發光二極體，具有 壽命長、點燈速度快、光電轉換效率高、體積小以及光譜 窄等優點，近來受到相當的重視，也因此加速了發光二極 體技術的成長。 請參照圖1所示，一般而言，發光二極體11係藉由 一直流電源12的驅動而使筹發光。另外，為了確保電路 的穩定，通常需要使用一電阻器13串聯於發光二極體11 以及一接地端G之間，以作為限流之用。然而，由於電阻 器13會消耗電路中的實功率（P=VI)，因此會造成額外的 功率消耗。 複數個前述發光二極體係可配置為發光陣列，並用於 液晶顯示器的背光模組中作為光源，然而，由於發光陣列 的發光二極體係以直流電源驅動，且各發光二極體的串聯 電阻器會消耗電路中的實功率，造成整個發光陣列產生額 外的功率消耗。 因此如何提供一種發光裝置，使其節省功率消耗，實 屬當前重要課題之一。 200816869 【發明内容】 有鑑於上述課題，本發明之目的為提供一種能夠節省 功率消耗之發光裝置。 緣是，為達上述目的，依據本發明之一種發光裝置， 包含一交流電源供應電路、一電容器、一第一發光二極體 以及一第二發光二極體。交流電源供應電路輸出一交流電 源；電容器係電性連接至交流電源供應電路以接收交流電 源；第一發光二極體之p極電性連接至電容器；第二發光 二極體之N極電性連接至電容器。 緣是，依據本發明之一種發光裝置包含一交流電源供 應電路以及複數個發光單元。交流電源供應電路輸出一交 流電源；各發光單元包含一電容器、一第一發光二極體、 一第二發光二極體以及一開關，其中電容器電性連接至交 流電源供應電路以接收交流電源，第一發光二極體之P極 與N極分別電性連接至電容器與開關，第二發光二極體之 P極與N極分別電性連接至開關與電容器。 緣是，依據本發明之一種發光裝置包含一交流電源供 應電路、一電容器、一全橋電路以及至少一發光二極體。 交流電源供應電路輸出一交流電源；電容器係電性連接至 交流電源供應電路以接收交流電源；全橋電路係電性連接 至電容器；發光二極體係電性連接至全橋電路。 承上所述，因依據本發明之發光裝置中，係以交流電 源來驅動發光二極體，並以電容器作為限流之用。因電容 器具有儲存能量之特性，其係消耗電路中之虛功率而非電 200816869 路中之實功率，故依據本發明之發光裝置能夠降低電路中 實功率的功率消耗，進而可提高效率。另外，在變壓器之 二次側設置有二個PN介面方向相反之發光二極體，或是 設置有全橋電路與發光二極體，因此，在前述情況下二次 側的發光二極體能夠被交流電源驅動而發光。 【實施方式】 以下將參照相關圖式，說明依據本發明較佳實施例之 一種發光裝置。 請參照圖2所示，一發光裝置2包含一交流電源供應 電路28、一電容器22、一第一發光二極體23、以及一第 二發光二極體24。 交流電源供應電路28係依據一電源訊號而輸出交流 電源，電容器22係電性連接至交流電源供應電路28以接 收交流電源，交流電源供應電路28係可將交流電流或電 壓提供至電容器22以驅動第一發光二極體23以及第二發 光二極體24。 第一發光二極體23之P極電性連接至電容器22，第 二發光二極體24之N極電性連接至電容器22，第一發光 二極體23之N極係與第二發光二極體24之P極電性連 接，這樣的二極體連接方式俗稱為二極體環（Ring Diode)〇 在本實施例中，交流電源供應電路28包含一變壓器 21以及一功率訊號產生電路25，變壓器21具有一 一次側 200816869 211與一二次側212，二次側212具有一第一端213與一 第二端214。功率訊號產生電路25係電性連接至變壓器 21之一次側211，電容器22電性連接至變壓器21之二次 側212之第一端213，第一發光二極體23之P極與N極 分別電性連接至電容器22與變壓器21之二次侧212之第 二端214，第二發光二極體24之P極與N極分別電性連 接至變壓器21之二次側212之第二端214與電容器22。 功率訊號產生電路25可以包含一全橋式、一半橋式 或一推挽式等型式之開關電路，開關電路的主要目的是依 據電源訊號切換出一個功率訊號，變壓器之一次側211係 接收功率訊號，藉以在二次側212輸出交流電源，俾使第 一發光二極體23與第二發光二極體24係由交流方式來驅 動，其中電源訊號可以是直流訊號或是交流訊號。在本實 施例中，功率訊號係可為一脈波訊號。 由於在電容器與變壓器之二次側之第二端之間，第一 發光二極體23與第二發光二極體24之PN介面方向相 反，因此，第一發光二極體23與第二發光二極體24能夠 被交流電源驅動而發光。 在交流電源之正半週期間，變壓器21之二次側212 係對第一發光二極體23與第二發光二極體24分別施以順 向偏壓（forward bias)與逆向偏壓（reverse bias)，因而 僅有第一發光二極體受驅動而發光；另外，於交流電源之 負半週期間，變壓器21之二次側212係對第一發光二極 體23與第二發光二極體24分別施以逆向偏壓與順向偏 200816869 壓，因而僅有第二發光二極體受驅動而發光。 第一發光二極體23與第二發光二極體24之發光強度 係受控於交流電源的電流強度，此二發光二極體若以較大 的電流驅動則其發光強度則越強。 電容器22係作為限流之用，避免過大的電流驅動第 一發光二極體23與第二發光二極體24而造成其損壞。另 外，由於電容器22係僅消耗電路中之虛功率而不會消耗 實功率，故發光裝置2因而能夠降低限流元件的功率消耗。 請參照圖3所示，與圖2不同之處在於一開關26以 及前述之電容器22、第一發光二極體23與第二發光二極 體24包含於一發光單元20中，發光裝置2包含複數個發 光單元20並更包含一開關控制電路27。 在發光單元20中，開關26係用來控制交流電源是否 驅動發光單元來發光，開關26之一端電性連接至變壓器 21之二次側212之第二端，開關26之另一端電性連接至 第一發光二極體23之N極與第二發光二極體24之P極。 開關控制電路27電性連接至各開關26以控制開關26 的導通與否，其中開關控制電路27係產生一功率調變訊 號（或是脈寬調變訊號）PWM，且其責任週期或頻率係經 過調變，並據以分別控制各開關26的導通頻率或導通時 間等，且配合變壓器21所輸出之交流電源的驅動，因而 可控制用來驅動各發光單元20的電流大小，以及控制驅 動各發光單元20的時間，俾使各發光單元20的發光強度 能夠分別受到控制。因此，各發光單元20可被控制發出 200816869 不同強度的光線，或是被控制產生閃光。 各發光單7L 20可被設定為發出不同顏色的光，例如 第一個發光單元20中的發光二極體23、24係分別為一紅 色發光一極體，第二個發光單元2〇中的發光二極體23、 24係分別為一綠色發光二極體，第三個發光單元2〇中的 發光二極體23、24係分別為—藍色發光二極體，如此— 來，每三個發光單元20即可混出白色光。 這些發光單元20可配置為發光陣列，並且發出紅色 光、綠色光與藍色光的發光單元2()均勻分佈於發光陣列 中，藉以形成白色面光源。 另外，形成白色光源的方式亦可以是各發光單元2〇 被設定為直接發出白色光，意即各發光單元Μ中的發光 一極體23 24係分別為_白色發光二極體，且這些白色 光的發光單元2〇可配置為發光陣列，藉以形成白色面光 另外’與前述不同的是，纖授^ 故 W i χ 文【态具有複數個二次側， 各二次侧分別電性連接至對岸 m ^ ^ w的毛先早兀。例如··變壓器 具有二個―:人側，各二次側相電 與藍色的發光單元，^條主、，工巴、、苯色 ,13A 欠側可僅電性連接一個發光單 70 ’或疋條連接魏個發光n 遴#個二攻锏，夂-l 4考疋，kM态具有 者是，變壓器具有複數個一句丨A邑的lx先早兀’或 一 一人側，部分二次側電性遠接 白色的發光單元，部公—a人w电性遷接至 .^ 一〜人側電性連接至其他顏色的發光 早兀。於此，值得注咅沾3 , nwu Μ、疋，由於複數個二次侧與各種顏 200816869 色的發光單元之組合方式眾多，在此僅列舉部份實施態樣 來說明，並非用以限定本發明僅能限定於以上的實施態 樣。 在以上實施例中，發光裝置2係可用於液晶顯示器的 背光模組中以作為光源，並且視液晶顯示器之尺寸設置不 同數量或不同陣列大小的發光單元20。 請參照圖4所示，依據本發明較佳實施例之另一發光 裝置3包含一交流電源供應電路38、一電容器32、一全 橋電路33、複數個發光二極體34、複數個開關36以及一 開關控制電路37，交流電源供應電路38包含一變壓器31 以及一功率訊號產生電路35,變壓器31具有——次側311 與至少一二次側312，二次側312具有一第一端313與一 第二端314。電容器32之一端係電性連接至變壓器31之 二次側312之第一端313。全橋電路33係電性連接至電容 器32之另一端。發光二極體34係電性連接至全橋電路33。 功率訊號產生電路35電性連接至變壓器31之一次侧 311，並輸出一交流電源至變壓器31之一次侧311。變壓 器31係將交流電源轉換後於二次側312輸出，全橋電路 33係將交流電源整流以驅動發光二極體34。因此，不論 交流電源於正半週期或是負半週期皆可驅動發光二極體 34發光。 開關36電性連接至發光二極體，開關控制電路37電 性連接至開關36以控制開關36導通，其中當開關36導 通時發光二極體34係被驅動而發光。開關控制電路37係 11 200816869 產生一功率調變訊號PWM，並據以控制開關36導通。 發光二極體34係為一紅色發光二極體、一綠色發光 二極體、一藍色發光二極體或一白色發光二極體。 由於本實施例中與圖2、圖3内相同名稱之元件具有 相同的功效與類似的組合變化，在該領域具有通常知識者 皆能夠輕易地明暸並實現各種類似的態樣，故於此不再贅 述。 综上所述，因依據本發明之發光裝置中，係以交流電 源來驅動發光二極體，並以電容器作為限流之用。而因電 容器具有儲存能量之特性，其係消耗電路中之虛功率而非 消耗電路中之實功率，故依據本發明之發光裝置，其係能 夠降低電路中實功率的消耗。另外，在變壓器之二次侧設 置有二個PN介面方向相反之發光二極體，或是設置有全 橋電路與發光二極體，因此，在前述情況下二次側的發光 二極體能夠被交流電源驅動而發光。 以上所述僅為舉例性，而非為限制性者。任何未脫離 本發明之精神與範疇，而對其進行之等效修改或變更，均 應包含於後附之申請專利範圍中。 【圖式簡單說明】 圖1為習知一種以直流驅動發光二極體之架構之一示 意圖， 圖2為依據本發明較佳實施例之一種發光裝置之一示 意圖； 12 200816869 圖3為依據本發明較佳實施例之發光裝置之另一示意 圖；以及 圖4為依據本發明較佳實施例之另一種發光裝置之一 示意圖。 元件符號說明： 11 :發光二極體 12 :直流電源 13 :電阻器 213 ··第一端 22 :電容器 2 :發光裝置 20 :發光單元 21 :變壓器 211 : —次側 212 :二次侧 214 :第二端 23 :第一發光二極體 25 :功率訊號產生電路 27 :開關控制電路 3:發光裝置 31 :變壓器 312 :二次側 314 :第二端 33 :全橋電路 35 :功率訊號產生電路 37 :開關控制電路 PWM :功率調變訊號 24 :第二發光二極體 26 :開關 28 :交流電源供應電路 311 : —次側 313 :第一端 32 :電容器 34 :發光二極體 36 :開關 38 :交流電源供應電路 13200816869 IX. Description of the Invention: [Technical Field] The present invention relates to a light-emitting device, and more particularly to a light-emitting diode driven by alternating current. [Prior Art] A light-emitting diode is generally fabricated using a P-type semiconductor and an N-type semiconductor, and has a long life, a high lighting speed, a high photoelectric conversion efficiency, and a small volume due to a light-emitting diode fabricated using a semiconductor element. As well as the advantages of narrow spectrum, it has recently received considerable attention, which has accelerated the growth of LED technology. Referring to Fig. 1, in general, the light-emitting diode 11 is driven by a DC power source 12 to emit light. In addition, in order to ensure the stability of the circuit, it is usually necessary to use a resistor 13 connected in series between the LED 11 and a ground terminal G for current limiting. However, since the resistor 13 consumes real power (P = VI) in the circuit, it causes additional power consumption. A plurality of the foregoing light-emitting diode systems can be configured as a light-emitting array and used as a light source in a backlight module of a liquid crystal display. However, since the light-emitting diode system of the light-emitting array is driven by a DC power source, and the series resistors of the light-emitting diodes It consumes real power in the circuit, causing additional power consumption across the entire array. Therefore, how to provide a light-emitting device to save power consumption is one of the current important issues. SUMMARY OF THE INVENTION In view of the above problems, it is an object of the present invention to provide a light-emitting device capable of saving power consumption. In order to achieve the above object, a light-emitting device according to the present invention comprises an AC power supply circuit, a capacitor, a first light-emitting diode, and a second light-emitting diode. The AC power supply circuit outputs an AC power source; the capacitor is electrically connected to the AC power supply circuit to receive the AC power; the p-pole of the first LED is electrically connected to the capacitor; and the N-pole of the second LED is electrically connected. Connect to the capacitor. The reason is that a light-emitting device according to the present invention comprises an AC power supply circuit and a plurality of light-emitting units. The AC power supply circuit outputs an AC power supply; each of the light emitting units includes a capacitor, a first light emitting diode, a second light emitting diode, and a switch, wherein the capacitor is electrically connected to the AC power supply circuit to receive the AC power. The P and N poles of the first LED are electrically connected to the capacitor and the switch, respectively, and the P and N poles of the second LED are electrically connected to the switch and the capacitor, respectively. The illuminating device according to the present invention comprises an AC power supply circuit, a capacitor, a full bridge circuit and at least one light emitting diode. The AC power supply circuit outputs an AC power source; the capacitor is electrically connected to the AC power supply circuit to receive the AC power; the full bridge circuit is electrically connected to the capacitor; and the LED system is electrically connected to the full bridge circuit. As described above, in the light-emitting device according to the present invention, the light-emitting diode is driven by an alternating current source, and the capacitor is used as a current limiting. Since the capacitor has the characteristic of storing energy, which consumes the virtual power in the circuit instead of the real power in the circuit of 200816869, the light-emitting device according to the present invention can reduce the power consumption of the real power in the circuit, thereby improving the efficiency. In addition, two light-emitting diodes having opposite PN interface directions are disposed on the secondary side of the transformer, or a full-bridge circuit and a light-emitting diode are disposed. Therefore, in the foregoing case, the secondary-side light-emitting diode can It is driven by an AC power source to emit light. [Embodiment] Hereinafter, a light-emitting device according to a preferred embodiment of the present invention will be described with reference to the related drawings. Referring to FIG. 2, a light-emitting device 2 includes an AC power supply circuit 28, a capacitor 22, a first LED 23, and a second LED 24. The AC power supply circuit 28 outputs an AC power source according to a power signal. The capacitor 22 is electrically connected to the AC power supply circuit 28 to receive AC power. The AC power supply circuit 28 can supply an AC current or voltage to the capacitor 22 for driving. The first light emitting diode 23 and the second light emitting diode 24 are provided. The P pole of the first LED 23 is electrically connected to the capacitor 22, the N pole of the second LED 24 is electrically connected to the capacitor 22, and the N pole and the second LED of the first LED 23 are The P pole of the pole body 24 is electrically connected. Such a diode connection is commonly referred to as a diode ring. In the present embodiment, the AC power supply circuit 28 includes a transformer 21 and a power signal generating circuit 25 . The transformer 21 has a primary side 200816869 211 and a secondary side 212. The secondary side 212 has a first end 213 and a second end 214. The power signal generating circuit 25 is electrically connected to the primary side 211 of the transformer 21, and the capacitor 22 is electrically connected to the first end 213 of the secondary side 212 of the transformer 21. The P and N poles of the first LED 23 are respectively Electrically connected to the capacitor 22 and the second end 214 of the secondary side 212 of the transformer 21, the P and N poles of the second LED 24 are electrically connected to the second end 214 of the secondary side 212 of the transformer 21, respectively. With capacitor 22. The power signal generating circuit 25 can include a full-bridge, half-bridge or push-pull type switching circuit. The main purpose of the switching circuit is to switch a power signal according to the power signal, and the primary side 211 of the transformer receives the power signal. The first light emitting diode 23 and the second light emitting diode 24 are driven by an alternating current mode, wherein the power signal can be a direct current signal or an alternating current signal. In this embodiment, the power signal can be a pulse signal. Since the first light emitting diode 23 and the second light emitting diode 24 have opposite PN interfaces between the capacitor and the second end of the transformer, the first light emitting diode 23 and the second light emitting body are opposite to each other. The diode 24 can be driven by an alternating current source to emit light. During the positive half cycle of the AC power source, the secondary side 212 of the transformer 21 applies a forward bias and a reverse bias to the first LED 23 and the second LED 24, respectively. Therefore, only the first light emitting diode is driven to emit light; in addition, during the negative half cycle of the alternating current power source, the secondary side 212 of the transformer 21 is paired with the first light emitting diode 23 and the second light emitting diode 24 The reverse bias and the forward bias are respectively applied to the 200816869 voltage, so that only the second light-emitting diode is driven to emit light. The luminous intensity of the first light-emitting diode 23 and the second light-emitting diode 24 is controlled by the current intensity of the alternating current power source, and the light-emitting intensity of the two light-emitting diodes is stronger when driven with a larger current. The capacitor 22 is used as a current limiting device to prevent the excessive current from driving the first light-emitting diode 23 and the second light-emitting diode 24 to cause damage. In addition, since the capacitor 22 consumes only the virtual power in the circuit and does not consume real power, the light-emitting device 2 can thereby reduce the power consumption of the current limiting element. Referring to FIG. 3 , the difference between FIG. 2 and FIG. 2 is that a switch 26 and the capacitor 22, the first LEDs 23 and the second LEDs 24 are included in a light-emitting unit 20, and the light-emitting device 2 includes A plurality of light emitting units 20 further include a switch control circuit 27. In the light-emitting unit 20, the switch 26 is used to control whether the AC power source drives the light-emitting unit to emit light, and one end of the switch 26 is electrically connected to the second end of the secondary side 212 of the transformer 21, and the other end of the switch 26 is electrically connected to The N pole of the first light emitting diode 23 and the P pole of the second light emitting diode 24. The switch control circuit 27 is electrically connected to each switch 26 to control whether the switch 26 is turned on or not. The switch control circuit 27 generates a power modulation signal (or pulse width modulation signal) PWM, and its duty cycle or frequency system. After being modulated, and accordingly, the on-frequency or on-time of each switch 26 is controlled, and the driving of the alternating current power source outputted by the transformer 21 is matched, thereby controlling the magnitude of the current used to drive each of the light-emitting units 20, and controlling the driving of each. The time of the light-emitting unit 20 is such that the light-emitting intensity of each of the light-emitting units 20 can be individually controlled. Therefore, each of the light-emitting units 20 can be controlled to emit light of different intensity in 200816869, or can be controlled to generate a flash. Each of the light-emitting sheets 7L 20 can be set to emit light of different colors. For example, the light-emitting diodes 23 and 24 in the first light-emitting unit 20 are respectively a red light-emitting diode, and the second light-emitting unit 2 The light-emitting diodes 23 and 24 are respectively a green light-emitting diode, and the light-emitting diodes 23 and 24 in the third light-emitting unit 2 are respectively a blue light-emitting diode, so that each three The light-emitting units 20 can mix white light. These light emitting units 20 may be configured as a light emitting array, and the light emitting units 2 () emitting red light, green light, and blue light are uniformly distributed in the light emitting array, thereby forming a white surface light source. In addition, the white light source may be formed in such a manner that each of the light-emitting units 2 is configured to directly emit white light, that is, the light-emitting diodes 23 in each of the light-emitting units are respectively _white light-emitting diodes, and these white lights The light-emitting unit 2〇 can be configured as a light-emitting array, thereby forming a white surface light. In addition to the foregoing, the fiber has a plurality of secondary sides, and each secondary side is electrically connected. To the other side of the m ^ ^ w hair first. For example, the transformer has two --- human side, each secondary side phase and blue light-emitting unit, ^ main, work, benzene, 13A underside can only be electrically connected to a luminous single 70 ' Or the 疋 连接 connection Wei 发光 n 遴 个 个 个 个 个 个 个 个 l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l The secondary side is electrically connected to the white light-emitting unit, and the male-a person is electrically connected to the .^ one-man side is electrically connected to other colors of the light-emitting early. Here, it is worthwhile to note that 3, nwu Μ, 疋, because of the multiple secondary side and various colors of the 200816869 color illuminating unit combination, there are only a few examples to illustrate, not to limit this The invention can only be limited to the above embodiments. In the above embodiment, the light-emitting device 2 can be used in a backlight module of a liquid crystal display as a light source, and different numbers or arrays of light-emitting units 20 are disposed depending on the size of the liquid crystal display. Referring to FIG. 4, another illuminating device 3 according to a preferred embodiment of the present invention includes an AC power supply circuit 38, a capacitor 32, a full bridge circuit 33, a plurality of LEDs 34, and a plurality of switches 36. And a switch control circuit 37, the AC power supply circuit 38 includes a transformer 31 and a power signal generating circuit 35. The transformer 31 has a secondary side 311 and at least one secondary side 312, and the secondary side 312 has a first end 313. And a second end 314. One end of capacitor 32 is electrically coupled to a first end 313 of secondary side 312 of transformer 31. The full bridge circuit 33 is electrically connected to the other end of the capacitor 32. The light emitting diode 34 is electrically connected to the full bridge circuit 33. The power signal generating circuit 35 is electrically connected to the primary side 311 of the transformer 31, and outputs an alternating current power source to the primary side 311 of the transformer 31. The transformer 31 converts the AC power to the secondary side 312, and the full bridge circuit 33 rectifies the AC power to drive the LEDs 34. Therefore, the light-emitting diode 34 can be driven to emit light regardless of whether the AC power source is in the positive half cycle or the negative half cycle. The switch 36 is electrically connected to the light emitting diode, and the switch control circuit 37 is electrically connected to the switch 36 to control the switch 36 to be turned on. When the switch 36 is turned on, the light emitting diode 34 is driven to emit light. The switch control circuit 37 generates a power modulation signal PWM and controls the switch 36 to be turned on. The light-emitting diode 34 is a red light-emitting diode, a green light-emitting diode, a blue light-emitting diode or a white light-emitting diode. Since the components of the same name in FIG. 2 and FIG. 3 have the same functions and similar combination changes in the present embodiment, those having ordinary knowledge in the field can easily understand and implement various similar aspects, and thus Let me repeat. As described above, in the light-emitting device according to the present invention, the light-emitting diode is driven by the alternating current power source, and the capacitor is used as the current limiting. Since the capacitor has the property of storing energy, which consumes the virtual power in the circuit instead of consuming the real power in the circuit, the light-emitting device according to the present invention can reduce the consumption of real power in the circuit. In addition, two light-emitting diodes having opposite PN interface directions are disposed on the secondary side of the transformer, or a full-bridge circuit and a light-emitting diode are disposed. Therefore, in the foregoing case, the secondary-side light-emitting diode can It is driven by an AC power source to emit light. The above is intended to be illustrative only and not limiting. Any equivalent modifications or alterations to the spirit and scope of the present invention are intended to be included in the scope of the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view showing a structure of a light-emitting diode for driving a DC; FIG. 2 is a schematic diagram of a light-emitting device according to a preferred embodiment of the present invention; 12 200816869 FIG. Another schematic diagram of a light emitting device in accordance with a preferred embodiment of the invention; and FIG. 4 is a schematic illustration of another light emitting device in accordance with a preferred embodiment of the present invention. Description of the component symbols: 11: Light-emitting diode 12: DC power supply 13: Resistor 213 · First end 22: Capacitor 2: Light-emitting device 20: Light-emitting unit 21: Transformer 211: - Secondary side 212: Secondary side 214: Second end 23: first light emitting diode 25: power signal generating circuit 27: switch control circuit 3: light emitting device 31: transformer 312: secondary side 314: second end 33: full bridge circuit 35: power signal generating circuit 37: switch control circuit PWM: power modulation signal 24: second light-emitting diode 26: switch 28: AC power supply circuit 311: - secondary side 313: first end 32: capacitor 34: light-emitting diode 36: switch 38: AC power supply circuit 13