LED照明的功率控制

　　智能照明是智能家居的關(guān)鍵之一。無線電和網(wǎng)絡(luò)控制可以用來改變照明水平，無論是直接從用戶控制，采用專用遠(yuǎn)程控制或智能手機(jī)應(yīng)用程序的幫助下，或自動(dòng)根據(jù)時(shí)間的一天。然而，LED照明電源控制需要特別考慮。傳統(tǒng)的調(diào)光控制電路開發(fā)不符合固態(tài)照明的要求。

　　大多數(shù)的調(diào)光器安裝在家里都是基于一個(gè)切相電路。這種類型的電路的優(yōu)點(diǎn)是，它是非常簡(jiǎn)單的設(shè)計(jì)和強(qiáng)大的。該電路的工作原理是簡(jiǎn)單地切斷電源的燈泡的交流周期的一部分，減少提供給燈絲的整體電壓。這反過來又減少燈絲的光輸出。由于相對(duì)較長(zhǎng)的時(shí)間，它需要燈絲加熱和冷卻，這種切換是不明顯的，除非用戶可能在最低的設(shè)置調(diào)光器。

　　然而，與LED，其中有更短的響應(yīng)時(shí)間，開關(guān)頻率低的主電源意味著試圖使用類似的策略來控制光輸出將導(dǎo)致顯著閃爍。因此，LED需要專門的電源電路，可以提供更一致的電壓和電流輸出，以提供功率，最大限度地減少閃爍而不影響效率。

　　Although it is possible to manipulate light levels from LEDs by altering the current supplied to them， this results in a drop in power efficiency at lower current levels. A power LED provides its best efficiency at the highest rated current. So， dimming behavior is still best obtained using pulse-width modulation （PWM） techniques， turning the LED on and off at higher frequencies than would be supported by traditional mains-based dimmer circuits.

　　In a PWM-based power converter， a reference signal fed back to the controller is used to determine whether the transistor that feeds power to the load should be on or off. The reference is compared to a periodic signal created by a ramp generator. While the reference signal is higher than the ramp， the transistor is switched on. When the reference signal falls below the ramp， the transistor is switched off. The integrated current delivered therefore follows the profile of the reference signal. Various techniques can be used to control the reference signal and therefore manipulate the power output. For smart-home use， power converters can integrate additional inputs that tune the reference signal based on the preferred dimming level.