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

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

    MOSFET tube is a kind of FET (the other is JFET), can be manufactured into enhanced or depleted type, P channel or N channel a total of 4 types, but the actual application of only enhanced N channel MOS tube and enhanced P channel MOS tube, so usually referred to NMOS, or PMOS refers to these two kinds.

    As for why not use a depletion MOS tube, it is not recommended to get to the bottom of it.

    For these two enhanced MOS tubes, the more commonly used is NMOS. 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 needed, but due to manufacturing process limitations. The presence of parasitic capacitors makes it more difficult to design or select a drive circuit, but there is no way to avoid it, which is described in detail later.

    As can be seen in the schematic diagram of the MOS tube, there is a parasitic diode between the drain and the source. This is called the bulk diode, and in driving inductive loads (such as motors), this diode is important. By the way, body diodes are only present in a single MOS tube and are usually absent inside an integrated circuit chip.

    On means as a switch, equivalent to the switch closed.

    The characteristics of NMOS, Vgs greater than a certain value will be switched on, suitable for use when the source is grounded (low-end drive), as long as the gate voltage reaches 4V or 10V.

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

    Whether it is NMOS or PMOS, there is an on-resistance after the on-resistance, so that the current will consume energy on this resistance, and this part of the energy consumed is called the on-loss. Choosing MOS with small on-resistance will reduce on-loss. At present, the on-resistance of the low-power MOS tube is generally about tens of milliohm, and a few milliohm are also available.

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

    The product of voltage and current at the on-moment is large, and the loss caused is also large. Shorten the switching time, can reduce the loss of each conduction; Reducing the switching frequency can reduce the number of switching times per unit time. Both methods can reduce the switching loss.

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

    In the structure of the MOS tube, it can be seen that there is a parasitic capacitor between GS and GD, and the driver of the MOS tube is actually the charge and discharge of the capacitor. The charging of the capacitor requires a current, because the charging of the capacitor can be regarded as a short circuit, so the instantaneous current will be relatively large. The first thing to pay attention to when selecting/designing MOS tube drivers is the size of the instantaneous short-circuit current that can be provided.

    The second note is that NMOS, commonly used for high-end drives, need to be switched on when the gate voltage is greater than the source voltage. The source voltage and drain voltage (VCC) are the same when the high-end driven MOS tube is switched on, so the gate voltage is 4V or 10V larger than the VCC. If you want to get a voltage greater than VCC in the same system, you need a special booster circuit. Many motor drivers have integrated charge pumps, it should be noted that the appropriate external capacitor should be selected to get enough short circuit current to drive the MOS tube.

    The 4V or 10V mentioned above is the on-voltage of the commonly used MOS tube, and of course, there is a certain margin when designing. And the higher the voltage, the faster the switching speed, the smaller the switching resistance. There are also MOS tubes with lower on-voltage in different fields, but in 12V automotive electronics systems, 4V on-voltage is generally sufficient.

    The MOSFET driver circuit and its loss can be referred to Microchip‘s AN799 Matching MOSFET Drivers to MOSFETs. It‘s very detailed, so I‘m not going to write any more.

    The most significant feature of MOS tube is that it has good switching characteristics, so it is widely used in circuits that require electronic switches, such as switching power supplies and motor drives, and lighting dimming.