why can't a mosfet be directly driven by a microcontroller?

[vbsemi]

https://youtu.be/pbz5aMdxSEU

首先，微控制器 I/O 端口的负载能力有限，通常允许的电流约为 10-20 mA。因此，它们通常不用于直接驱动负载。

让我们简要比较一下驱动 BJT 和 MOSFET 的差异。

双极结型晶体管 （BJT）：

BJT 是电流控制器件。只要基极-发射极电压 （Ube） 超过阈值电压（通常为 0.7V），晶体管就会导通。对于 BJT，3.3V 肯定大于 Ube，基极电流 （Ib） 可以计算为 \（ Ib = \frac{（VO - 0.7V）}{R2} \）。通过将适当的电阻器与基座串联，BJT 可以在饱和状态下工作。微控制器通常以低功耗为目标，因此电源电压通常很低，约为 3.3V。

MOSFET：

MOSFET 是电压控制器件。栅源电压 （Vgs） 必须超过阈值电压才能开启，一般在 3-5V 左右，饱和驱动电压为 6-8V，高于 I/O 端口的 3.3V。如果用 3.3V 驱动，MOSFET 可能无法完全导通或以部分导通状态工作。在这种状态下，MOSFET 具有高内阻，这限制了其处理大电流负载的能力，从而导致功率耗散增加和潜在的损坏。

 

因此，通常最好使用微控制器来控制 BJT，而微控制器又会驱动 MOSFET。为什么使用 BJT 来驱动 MOSFET？这是因为与 MOSFET 相比，BJT 的负载能力较低，因此适用于控制应用。MOSFET 可以直接驱动吗？虽然某些低功率 MOSFET 是可能的，但通常不建议用于较大的负载。

 

 

[HarryA]

I think the post is worth reading. So lets try Google translator:

First, microcontroller I/O ports have limited load capability, typically allowing about 10-20 mA of current. Therefore, they are not usually used to drive loads directly.First, microcontroller I/O ports have limited load capability, typically allowing about 10-20 mA of current. Therefore, they are not usually used to drive loads directly.

 

https://youtu.be/pbz5aMdxSEU First, microcontroller I/O ports have limited load capability, usually allowing about 10-20 mA of current. Therefore, they are not usually used to drive loads directly. image.png.4e399c62899257b83f0e2103daa204e1.png Let's briefly compare the differences in driving BJTs and MOSFETs. Bipolar Junction Transistor (BJT): BJTs are current-controlled devices. As long as the base-emitter voltage (Ube) exceeds the threshold voltage (usually 0.7V), the transistor will turn on. For BJTs, 3.3V is definitely greater than Ube, and the base current (Ib) can be calculated as \( Ib = \frac{(VO - 0.7V)}{R2} \). By connecting an appropriate resistor in series with the base, the BJT can be operated in saturation. Microcontrollers are usually targeted for low power consumption, so the supply voltage is usually low, around 3.3V.

MOSFET:

MOSFET is a voltage controlled device. The gate-source voltage (Vgs) must exceed the threshold voltage to turn on, which is generally around 3-5V, and the saturation drive voltage is 6-8V, which is higher than the 3.3V of the I/O port. If driven with 3.3V, the MOSFET may not be fully turned on or operate in a partially turned-on state. In this state, the MOSFET has a high internal resistance, which limits its ability to handle high current loads, resulting in increased power dissipation and potential damage.

Therefore, it is usually better to use a microcontroller to control the BJT, which in turn drives the MOSFET. Why use a BJT to drive a MOSFET? This is because BJTs have lower load capabilities compared to MOSFETs, making them suitable for control applications. Can MOSFETs be driven directly? While it is possible with some low power MOSFETs, it is generally not recommended for larger loads.