傳統彩色發光二極體混光調色,通常採用三組固定直流電壓源,
分別串接紅,綠,藍發光二極體與功率電晶體。利用 PWM 切換調 整功率電晶體導通週期 時間,控制流入三原色發光二極體之平均 電流,間接控制三原色亮度,完成彩色調光。傳統 PWM 混光調色 方法,將功率電晶體等效為負載電阻。調整功率電晶體通週期時間,
亦即改變負載電阻等效阻值。功率電晶體將消耗部分輸入電源功率 產生發熱現象。當功 率電晶體導通時,流入發光二極體脈衝電流正 比於輸入直流電壓源電壓。直流電壓源電壓設計太大,造成發光二 極體脈衝電流過大,容易損傷發光二極體。在電源管理方面,若考 慮安規,隔離電源需求,遭成電源模組須備足三個獨立電源,彼此 電源無法相互支援。計畫提出順向式轉換器架構,在隔離變壓器二 次側並繞三組輸出。各組輸出線路,利用功率電晶體調所需輸出電 流至發光二極體。由於彩色紅,綠,藍發光二極體,具有不同順向 電壓與順向導通電阻。當發光二極體輸入電壓小於順向電壓時,輸 出阻抗趨近於無窮大;當發光二極體輸入電壓大於順向電壓時,輸 出阻抗為順向導通電阻。此極劇非線性特性負載,直接執行電流控 制,容易在輸出電壓大於順向電壓瞬間,產生過電流現象。若過電 流值超過發光二極體額定值,容易損毀發光二極體。計畫提出雙迴
路控制架構,只採用輸出電感電流回授。內迴路為輸出電感電流控 制,採用 PI 控制器,頻寬設計為 1 KHz;外迴路為輸出二極體電 流控制,從電感電流訊號,估測二極體電 流大小。控制器採用二自 由度前饋控制器,降低對發光二極體不同順向導通電壓所造成影響。
從 PSIM 模擬 中,雙迴路控制可以有效抑制過電流現象出現。
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