削減式臨界模式功率因數改善電路的特性分析

Abstract

在臨界模式CRM (critical mode) 中，當輸入弦波電壓波形在谷底的時候切換頻率會變高。在這段期間對輸入功因之改進本就沒太大影響，但切換損耗卻仍然高。因此本篇論文是針對在有限制最高頻率的情形下，將切換頻率高於一個上限時，停止在這段期間MOSFET作切換的動作，以減少開關損耗，提高輕載時的效率。並且在待機(standby) 的情況下希望能達到省能 (green mode) 的世界標準。因此使用此提倡之方法將在本論文中稱為削減型臨界模式 (truncated CRM) 。本論文中將以數學分析truncated CRM情形下的功因值、諧波成分、輸出功率、導通角度等等之間的關係。並將利用這樣的關係討論如何設計電感之大小以達到所要的功因值及諧波成份。 本論文也提到利用CRM之功因修正時可以以interleaving方式將兩組PFC合成以達到較大功率輸出。因此利用兩組各自輸出功率較小的PFC電路來並聯，來供應出一樣大小的功率可提升線路效率及降低電流漣波 (ripple) 。並利用推導的公式描繪出輸入與輸出電容電流漣波，以便了解兩組PFC做interleaving和單組PFC時電容之漣波(Ripple) 彼此間的差異。本論文將討論此漣波與電路其他之參數及工作情況之間的關係，這對設計時選擇電容將有助益。

In a Critical-Mode Power Factor Correction circuit, the switching frequency is variable. The frequency can be considerably higher when the input voltage waveform is near the valley point. This leads to higher switching loss during this period of time. Since the contribution to power factor correction and output power throughput during this period is insignificant but MOSFET switching loses are high, it’s better just to cut off the MOSFET switch during this period. In other words, one can set a frequency limit beyond which MOSFET is cut off. This is called Truncated CRM operation in this thesis. This method saves power especially when the circuit is under stand-by mode. There is a mandate to reduce this type of loss because in many house-hold applications the equipment are most of the time at stand-by mode. In the thesis, an analysis was conducted to find out how the various PFC performances such as power factor, harmonic current, and conduction angles depend on circuit parameter and working conditions. These relationships were also used to help design the Truncated-CRM PFC circuit in the thesis. Two CRM PFC circuits are often interleaved to provide higher output power. Because of interleaving, the ripple current of the capacitors can be reduced. An analysis was also conducted to find out the ripple information in terms of circuit parameters and operating conditions. This information is also useful to the designers.