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图1显示了使用NPN BRT的基本开关电路。
当NPN BRT在饱和区导通时,由于外部电阻(RL)和集电极电流(IC),其集电极电压降至GND电平。但是,实际上,集电极和GND(发射极)电位之间有一个称为集电极-发射极饱和电压(VCE(sat))的电压电平。可通过增加基极电流(IB)来降低VCE(sat)。
(由于当图1的电路处于“导通”状态时,VCE(sat) << VCC,故IC = VCC/RL,这几乎恒定不变。因此,增加IB会导致hFE(= IC/IB)减小。这意味着BRT达到更深的饱和度,从而降低了VCE(sat)。)
通常,只能将有限范围的电压施加至基极。因此,让我们考虑如何在特定的基极电压下使更多的基极电流(Ib)。
内部基极电流(Ib)表示如下:
Ib=IB–IR2=(VI–Vbe)/R1–Vbe/R2
其中,VI为输入电压,Vbe为内部晶体管的基极-发射极电压,可以将其视为恒定在大约0.7V*。
Ib等式表明可通过使用BRT来增加Ib:
1)R1小值
2)R2大值
这并不意味着电阻比(R1/R2)较小的BRT,而是R1值较小的BRT。这种BRT在特定输入电压(VI)下会向基极传输更大的电流。
但这种BRT的缺点是其会消耗更多的功率并进入更深的饱和度的状态,从而导致开关速度降低。同样,饱和电压随着温度的升高而增加,如图4所示。
需考虑这些因素以实现最佳设计。
*:图3显示了通用型2SC2712晶体管的VBE(sat)–IC曲线,该晶体管相当于BRT中使用的晶体管。在使用BRT的IC范围内,VBE的变化仅为数百毫伏。VBE的这种变化相对于输入电压可忽略不计。