When using MOS tubes to design switching power supply or motor drive circuits, most people will consider the MOS on-resistance, maximum voltage, maximum current, etc., and many people only consider these factors. Such a circuit may work, but it is not excellent. It is also not allowed as a formal product design.

    The following is my summary of MOSFET and MOSFET driver circuit foundation, which refers to some materials, not all original. It includes the introduction, characteristics, driver and application circuit of MOS transistor.

    MOSFET tube is one kind of FET (the other is JFET), which can be manufactured into enhanced or depleted type. There are four types of P-channel or N-channel, but only the enhanced N-channel MOS tube and the enhanced P-channel MOS tube are used in practice, so NMOS or PMOS are usually mentioned.

    As for why we don‘t use exhausted MOS tubes, it is not recommended to find out.

    For these two enhanced MOS tubes, NMOS is commonly used. The reason is that the on-resistance is small and easy to manufacture. Therefore, in the application of switching power supply and motor drive, NMOS is generally used. In the following introduction, NMOS is mainly used.

    There is parasitic capacitance between the three pins of the MOS tube, which is not what we need, but because of the manufacturing process limitations. The existence of parasitic capacitance makes it more difficult to design or select the drive circuit, but there is no way to avoid it, which will be described in detail later.

    As can be seen from the MOS tube schematic diagram, there is a parasitic diode between the drain and source. This is called body diode. It is very important when driving inductive loads (such as motors). By the way, the body diode only exists in a single MOS tube, and usually does not exist in the IC chip.

    Conduction means to act as a switch, which is equivalent to closing the switch.

    NMOS features that Vgs will conduct when it is greater than a certain value. It is suitable for source grounding (low end drive), as long as the grid voltage reaches 4V or 10V.

    The characteristic of PMOS is that Vgs will conduct when it is less than a certain value, which is suitable for the case when the source is connected to VCC (high-end drive). However, although PMOS can be easily used as a high-end driver, NMOS is usually used in high-end drivers due to high on-resistance, high price and few replacement types

    No matter NMOS or PMOS, there is a conduction resistance after conduction, so the current will consume energy on this resistance, which is called conduction loss. Selecting MOS tubes with small on-resistance will reduce the on-loss. At present, the on-resistance of low-power MOS transistor is generally around tens of milliohm, and even a few milliohm.

    When MOS is on and off, it must not be completed in an instant. The voltage at both ends of MOS has a decline process and the current flowing through it has a rise process. During this period, the loss of MOS tube is the product of voltage and current, which is called switch loss. Generally, the switching loss is much larger than the conduction loss, and the faster the switching frequency is, the greater the loss is.

    The product of voltage and current at the moment of conduction is very large, resulting in great losses. Shortening the switching time can reduce the loss of each conduction; Reducing the switching frequency can reduce the switching times per unit time. Both methods can reduce switching loss.

    Compared with bipolar transistor, it is generally believed that no current is required to make MOS transistor turn on, as long as the GS voltage is higher than a certain value. This is easy to do, but we still need speed.

    It can be seen from the structure of MOS tube that there is parasitic capacitance between GS and GD, and the driving of MOS tube is actually the charging and discharging of capacitance. The charging of the capacitor requires a current, because the capacitor can be regarded as a short circuit at the moment of charging, so the instantaneous current will be relatively large. The first thing to pay attention to when selecting/designing the MOS transistor driver is the size of the instantaneous short-circuit current that can be provided.

    The second thing to note is that the gate voltage is greater than the source voltage when conducting the NMOS commonly used for high-end drive. The source voltage is the same as the drain voltage (VCC) when the MOS transistor driven by the high end is on, so the gate voltage is 4V or 10V higher than the VCC. If in the same system, a special boost circuit is needed to obtain a voltage higher than the VCC. Many motor drivers are integrated with charge pumps. It should be noted that appropriate external capacitors should be selected to obtain sufficient short circuit current to drive MOS tubes.

    The 4V or 10V mentioned above is the turn-on voltage of the common MOS transistor. Of course, there should be a certain margin in the design. Moreover, the higher the voltage, the faster the conduction speed and the smaller the conduction resistance. Now there are MOS tubes with smaller conduction voltage used in different fields, but in the 12V automotive electronic system, generally 4V conduction is sufficient.

    The MOSFET driver circuit and its loss can be described in detail in Microchip‘s AN799 Matching MOSFET Drivers to MOSFETs.

    The most remarkable characteristic of MOS transistor is its good switching characteristics, so it is widely used in circuits requiring electronic switches, such as switching power supply and motor drive, as well as lighting dimming.