MOS tube use literacy

2018-11-19

When designing a switching power supply or a motor driving circuit using a MOS tube, most people will consider the on-resistance of the MOS, the maximum voltage, the maximum current, etc., and many people only consider these factors. Such a circuit may work, but it is not excellent, and it is not allowed as a formal product design. Below is a summary of my MOSFET and MOSFET driver circuit, including the introduction of MOS transistors, features, drivers, and application circuits.

 

1, MOS tube type and structure

 

The MOSFET is a type of FET (the other is a JFET), which can be fabricated as an enhancement or depletion type. There are four types of P-channel or N-channel, but only the enhanced N-channel MOS is actually used. Tubes and enhanced P-channel MOS transistors, so NMOS is usually mentioned, or PMOS refers to both.

 

As for why not use a depletion type MOS tube, it is not recommended to ask the bottom. For these two enhanced MOS transistors, the NMOS is more commonly used. The reason is that the on-resistance is small and easy to manufacture. Therefore, in switching power supply and motor driven applications, NMOS is generally used.

 

In the following introduction, NMOS is mostly used.

 

There is parasitic capacitance between the three pins of the MOS tube, which is not what we need, but is due to manufacturing process limitations. The presence of parasitic capacitance makes it a bit tricky when designing or selecting a driver circuit, but there is no way to avoid it, as described in more detail later. As you can see on the MOSFET schematic, there is a parasitic diode between the drain and the source. This is called a body diode, which is important for driving inductive loads such as motors. Incidentally, the body diodes are only present in a single MOS transistor and are usually not available inside the integrated circuit chip.

 

2, MOS tube conduction characteristics

 

Turn-on means as a switch, which is equivalent to a switch closure. The characteristics of the NMOS, Vgs greater than a certain value will be turned on, suitable for the case when the source is grounded (low-side drive), as long as the gate voltage reaches 4V or 10V.

 

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

 

3, MOS switch tube loss

 

Whether it is NMOS or PMOS, there is an on-resistance after conduction, so that the current will consume energy on this resistor, and this part of the energy consumed is called conduction loss. Selecting a MOS transistor with a small on-resistance reduces the conduction loss. The current low-power MOS tube on-resistance is generally in the range of several tens of milliohms, and several milliohms are also available. When MOS is turned on and off, it must not be completed in an instant. The voltage across the MOS has a falling process, and the current flowing through has a rising process. During this time, the loss of the MOS tube is the product of voltage and current, called switching loss. Usually the switching loss is much larger than the conduction loss, and the higher the switching frequency, the greater the loss.

 

The product of the voltage and current at the turn-on moment is large, and the loss is large. By shortening the switching time, the loss per turn-on can be reduced; reducing the switching frequency can reduce the number of switches per unit time. Both of these methods can reduce switching losses.

 

4, MOS tube driver

 

Compared with bipolar transistors, it is generally considered that no current is required to turn on the MOS transistor, 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 in the structure of the MOS transistor that there is a parasitic capacitance between GS and GD, and the driving of the MOS transistor is actually charging and discharging the capacitor. The charging of the capacitor requires a current, because the capacitor can be regarded as a short circuit when charging the capacitor, so the instantaneous current will be relatively large.

 

The first thing to note when selecting/designing a MOS tube driver is the amount of transient short-circuit current available. The second note is that NMOS, which is commonly used for high-side driving, needs to have a gate voltage greater than the source voltage when turned on. When the high-side driving MOS transistor is turned on, the source voltage is the same as the drain voltage (VCC), so the gate voltage is 4V or 10V larger than VCC. If you want to get a voltage larger than VCC in the same system, you need a special boost circuit. Many motor drivers have integrated charge pumps. It is important to note that a suitable external capacitor should be selected to get enough short-circuit current to drive the MOSFET.

 

The 4V or 10V mentioned above is the conduction voltage of the commonly used MOS tube, and it is of course necessary to have a certain margin when designing. Moreover, the higher the voltage, the faster the conduction speed and the smaller the on-resistance. There are also MOS tubes with smaller turn-on voltages used in different fields, but in 12V automotive electronic systems, 4V turn-on is generally sufficient.


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