The Art of Electronics 02

    (接上回)

    EXERCISE 2.3
    Design a +I0 volt regulated supply for load currents from 0 to 100mA; the input voltage is +20 to +25 volts. Allow at least 10mA zener current under all (worst-case) conditions. What power rating must the zener have? 

    习题 2.3

    设计一个+10v的稳压电源，其输出负载电流为0~10mA，输入电压是+20~+25v。在任何情况下（最坏的情况下）都能保证有至少10mA的齐纳电流流过。试问需选用至少多大功率的齐纳二极管。

    This simple zener-regulated supply is sometimes used for noncritical circuits, or circuits using little supply current. However, it has limited usefulness, for several reasons:

    这种简单的齐纳二极管稳压源常用在一些不太重要的，或者只汲取很少电流的电路中。因此，其应用范围较小，原因如下：

    1.Vout isn't adjustable, or settable to a precise value.

    1.Vout不可调整，或者说不可设置到某一精确值。

    2.Zener diodes give only moderate ripple rejection and regulation against changes of input or load, owing to their finite dynamic impedance.

    2.由于齐纳二极管只有有限的动态阻抗（dVzener/dIzener），所以只能对纹波进行抑制，或对由输入或负载引起的变化进行调整。

    3.For widely varying load currents a high-power zener is often necessary to handle the dissipation at low load current.

    3.在负载电流变化较大的场合，需要使用大功率的齐纳二极管来应付负载电流较低时齐纳二极管的（较大的）功率损耗（负载电流低时流过齐纳二极管的电流较大）。

    By using an emitter follower to isolate the zener, you get the improved circuit shown in Figure 2.12.  Now the situation is much better. Zener current can be made relatively independent of load current, since the transistor base current is small, and far lower zener power dissipation is possible (reduced by as much as l/hFE).The collector resistor Rc can be added to protect the transistor from momentary output short circuits by limiting the current, even though it is not essential to the emitter follower function. Choose Rc so that the voltage drop across it is less than the drop across R for the highest normal load current.

    如果使用射极跟随器来隔离齐纳二极管，就可以得到如图2.12所示的改进型电路。这种电路中，情况有所改善。因为晶体管基极电流很小，所以齐纳二极管电流与负载电流相对无关，这样做还可以进一步降低齐纳二极管的功率损耗（减少l/hFE）。集电极电阻Rc通过限制电流来保护晶体管不受输出瞬时短路的损害，当然，这不是射极跟随器必需的功能。请合理选择Rc，使得在输出最大工作电流时，Rc的压降仍小于R的压降。

   

    图 2.12 有助于提高输出电流的带射极跟随器的齐纳稳压电路，Rc通过限制最大输出电流来保护晶体管