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发表于 2017-10-7 20:08:12
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本帖最后由 shangdawei 于 2017-10-7 20:09 编辑
http://robotics.ong.id.au/2014/0 ... protection-circuit/
It turns out that the matched PNP transistors are used as voltage comparator.
When Vin > Vout, the PNPs are biased in such a way that the mosfet is switched on.
When Vin < Vout, the mosfet is switched off.
In another word, the circuit acts as a diode letting current to flow only in the forward direction.
The advantage of this "diode" over silicon or even schottky diode is the much lower voltage drop.
This diode functionality is important because when the RPI is connected to a computer USB port, the RPI must never source current upstream into VBus.
Without this PNP circuit, if there is voltage present on Vout, the mosfet will be turned on.
Voltage on Vout will be present if the RPI is powered through the GPIO expansion port.
If this applied voltage happens to be higher than VBus, then current will flow into VBus, potentially damaging the computer.
The way the circuit works is this:
if Vin=Vout, lets say 5.0V, the PNP transistors are biased in a current mirror configuration.
Vb of transistors U14 will be 5-0.6=4.4V, the mirroring configuration means that Vg will also be at 4.4V.
Vgs is 4.4-5.0=-0.6V. The mosfet will be in the off state (almost turned on).
If Vout is slightly lower than Vin, lets say 4.9V, U14a will still be biased as before, so Vb is still 4.4.
Vbe of U14b will now be 4.4-4.9=-0.5, U14b will be off and Vg will be 0V since no current flows through R3.
Vgs will be 0-4.9 = -4.9V, and the mosfet will be fully switched on.
If Vout is slightly higher that Vin, lets say 5.1V.
U14b will be turned on via the biasing resistor R2.
Transistor U14b will be in saturation mode and Vg will be 0.2 V lower than Vout,
Vgs = -0.2, and the mosfet will be off.
Conclusion
The protection circuit in the RPI model B+ functions as an ideal power diode.
结果是匹配的PNP晶体管用作电压比较器。
当Vin> Vout时，PNP被偏置，使得mosfet被接通。
当Vin <Vout时，mosfet关闭。
换句话说，电路用作使电流仅在正向流动的二极管。
这种“二极管”优于硅或甚至肖特基二极管的优点是电压降低得多。
这种二极管功能很重要，因为当RPI连接到计算机的USB端口时，RPI绝对不能将电流上游流向VBus。
没有这个PNP电路，如果Vout上存在电压，则mosfet将被打开。
如果RPI通过GPIO扩展端口供电，则会出现Vout上的电压。
如果这个施加的电压恰好高于VBus，则电流将流入VBus，从而潜在地损害计算机。
电路的工作原理是：
如果Vin = Vout，则说5.0V，PNP晶体管以当前的镜像配置偏置。
晶体管U14的Vb将为5-0.6 = 4.4V，镜像配置意味着Vg也将为4.4V。
Vgs为4.4-5.0 = -0.6V。 mosfet将处于关闭状态（几乎打开）。
如果Vout略低于Vin，则说4.9V，U14a仍然会像以前一样偏置，所以Vb仍然是4.4。
U14b的Vbe现在将为4.4-4.9 = -0.5，U14b将关闭，Vg将为0V，因为没有电流流经R3。
Vgs将为0-4.9 = -4.9V，并且mosfet将被完全打开。
如果Vout稍高于Vin，则说5.1V。
U14b将通过偏置电阻R2导通。
晶体管U14b将处于饱和模式，Vg将比Vout，
Vgs = -0.2低0.2 V，MOSFET将关闭。
结论
RPI型号B +中的保护电路用作理想功率二极管。 |
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