[Repost] Detailed description of the five key points of MOS tube drive circuit

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[Repost] Detailed description of the five key points of MOS tube drive circuit [Copy link]

When using MOS tubes to design switching power supplies or motor drive circuits, most people will consider the on-resistance, maximum voltage, maximum current, etc. of MOS, 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.

1. Types and structures of MOS tubes

MOSFET tube is a kind of FET (the other is JFET), which can be manufactured into enhancement type or depletion type, P channel or N channel, there are 4 types in total, but in practice only enhancement type N channel MOS tube and enhancement type P channel MOS tube are used, so usually NMOS or PMOS refers to these two types.

Of these two types of enhancement MOS tubes, NMOS is more commonly used. The reason is that it has a small on-resistance and is easy to manufacture. Therefore, NMOS is generally used in switching power supplies and motor drive applications. In the following introduction, NMOS is also the main focus.

There is parasitic capacitance between the three pins of the MOS tube, which is not what we need, but is caused by manufacturing process limitations. The existence of parasitic capacitance makes it more troublesome when designing or selecting a drive circuit, but there is no way to avoid it. I will introduce it in detail later.

In the MOS tube schematic diagram, you can see that there is a parasitic diode between the drain and the source. This is called a body diode. This diode is very important when driving an inductive load (such as a motor). By the way, the body diode only exists in a single MOS tube and is usually not present inside an integrated circuit chip.

2. MOS tube conduction characteristics

Conduction means acting as a switch, which is equivalent to a closed switch.

The characteristic of NMOS is that it will turn on when Vgs is greater than a certain value, which is suitable for the situation when the source is grounded (low-end drive), as long as the gate voltage reaches 4V or 10V.

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

3. MOS switch tube loss

Whether it is NMOS or PMOS, there is an on-resistance after it is turned on, so the current will consume energy on this resistance, and this part of the energy consumed is called conduction loss. Choosing a MOS tube with a small on-resistance will reduce the conduction loss. The on-resistance of today's low-power MOS tubes is generally around tens of milliohms, and there are also several milliohms.

When MOS is turned on and off, it must not be completed instantly. The voltage across the MOS has a process of decreasing, and the current flowing through it has a process of increasing. During this period of time, the loss of the MOS tube is the product of the voltage and the current, which is called the switching loss. Usually the switching loss is much larger than the conduction loss, and the faster the switching frequency, the greater the loss.

The product of voltage and current at the moment of conduction is very large, and the loss caused is also very large. Shortening the switching time can reduce the loss at each turn-on; reducing the switching frequency can reduce the number of switches per unit time. Both methods can reduce switching losses.

4. MOS tube drive

Compared with bipolar transistors, it is generally believed that no current is required to turn on the MOS tube, as long as the GS voltage is higher than a certain value. 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 capacitance between GS and GD, and the driving of the MOS tube is actually the charging and discharging of the capacitor. Charging 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 tube driver is the size of the instantaneous short-circuit current that can be provided.

The second thing to note is that the NMOS commonly used for high-side drive needs to have a gate voltage greater than the source voltage when it is turned on. When the MOS tube of the high-side drive is turned on, the source voltage is the same as the drain voltage (VCC), so the gate voltage must be 4V or 10V greater than VCC. If you want to get a voltage greater than VCC in the same system, you need a dedicated boost circuit. Many motor drivers have integrated charge pumps. It should be noted that you should choose a suitable external capacitor to get enough short-circuit current to drive the MOS tube.

The 4V or 10V mentioned above is the commonly used MOS tube conduction voltage. Of course, a certain margin is required during design. And the higher the voltage, the faster the conduction speed and the smaller the conduction resistance. There are also MOS tubes with lower conduction voltage used in different fields, but in 12V automotive electronic systems, 4V is generally sufficient.

For the drive circuit and loss of MOS tubes, please refer to Microchip's AN799 Matching MOSFET Drivers to MOSFETs. It is described in great detail, so I don't plan to write more.

5. MOS tube application circuit

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

Source: Internet, please delete if infringed

btty038

The most important thing is to dissipate the heat of the MOS tube. It depends on the purpose. If the requirements are not high, you can do it at will as long as the indicators are met. But in situations with higher requirements, special treatment must be given. I have suffered a lot in this regard, haha

aidman111

If you need MOS, you can contact me, especially COOL MOS

shakencity

When using MOS tubes to design switching power supplies or motor drive circuits, most people will consider the MOS's on-resistance, 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.