Explain in detail the MOS tube drive circuit
When designing a switching power supply or a motor driving circuit using a MOS tube, most people will consider the on-resistance, maximum voltage, maximum current, etc. of the MOS tube, 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.
The following is a summary of the basics of MOS and MOS driver circuits. It refers to some materials, not original. Includes introductions, features, drivers, and application circuits for MOSFETs.
One of the MOSFET FETs (the other is JEFT) can be fabricated as an enhancement or depletion type, and there are four types of P-channel or N-channel, but only the enhanced N-channel MOS transistor is actually used. And enhanced P-channel MOS transistors, so the commonly mentioned NMOS, or PMOS refers to these two.
As for why it is not applicable to the 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 the application of switching power supply and motor drive, NMOS is generally used. In the following description, NMOS is mainly used.
There are parasitic capacitances between the three disciplines of the MOS tube. This is not what we need, but is caused by manufacturing process limitations. The existence of parasitic capacitance makes it troublesome when designing or selecting the driver circuit, but there is no way to avoid it. I will introduce it in detail later.
On the schematic diagram of the MOS transistor, it can be seen that there is a parasitic diode between the drain and the source. This is called a body diode, and it is important to drive an inductive load (such as a motor). Incidentally, the body diodes are only present in a single MOS transistor and are usually not available inside the integrated circuit chip.
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 where 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 where the source is 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 driving because of high on-resistance, high price, and low replacement.
MOS switch tube loss
Whether it is NMOS or PMOS, there is an on-resistance after the conduction, so that the point current will consume energy on this resistor, and this part of the energy consumed is called conduction loss. Choosing a MOS transistor with a small on-resistance will reduce the conduction loss. The on-resistance of the current low-power MOS transistor is generally around several tens of millivolts, and there are several ohms.
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 product of the voltage and current when the MOS transistor is lost 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 the voltage and current at the moment of conduction is large, and the loss is also large. By shortening the switching time, the loss per turn-on can be reduced, the switching frequency can be reduced, and the number of switchings per unit time can be reduced. Both of these methods can reduce switching losses.
MOS tube drive circuit
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 at the moment of charging, 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 and the drain voltage (Vcc) are the same, so this is that 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 MOSFETs with smaller turn-on voltages used in different fields, but in 12V automotive electronic systems, 4V turn-on is generally sufficient.
The drive circuit of MOS tube and its loss can refer to Microchip's AN799 matching MOSFET Drivers to MOSFETs. It is very detailed, so I don't plan to write more.
MOS tube application circuit
The most remarkable feature of MOS transistors is their good switching characteristics, so they are widely used in circuits that require electronic switches. Commonly used are switching power supplies and motor drive circuits, as well as lighting dimming.
There are several special requirements for the current MOS driver:
Low voltage application
When using a 5V power supply, if the conventional totem pole structure is used, since the voltage of the transistor is only about 0.7V, the voltage on the final final loading gate is only 4.3V. At this time, we use the nominal gate voltage of 4.5V. The MOS tube has certain risks. The same problem occurs when using 3V or other low voltage power supplies.
2. Wide voltage application
The input voltage is not a fixed value, it will change over time or other factors. This change causes the driving voltage supplied to the MOS transistor by the PWM circuit to be unstable.
In order to make the MOS tube safe under high gate voltage, many MOS tubes have a built-in voltage regulator to forcibly limit the amplitude of the gate voltage. In this case, when the supplied driving voltage exceeds the voltage of the Zener diode, a large static power consumption is caused.
At the same time, if the voltage of the resistor is used to reduce the gate voltage, the MOS transistor works well when the input voltage is relatively high, and the gate voltage is insufficient when the input voltage is lowered, causing the conduction to be insufficient, thus increasing the power consumption. .
3. Dual voltage application
In some control circuits, the logic uses a typical 5V or 3.3V digital voltage, while the power section uses a voltage of 12V or higher. The two voltages are connected in a common manner.
This raises a requirement that a circuit is required to allow the low-voltage side to effectively control the MOS transistor on the high-voltage side, while the MOS transistor on the high-voltage side also faces the problems mentioned in 1 and 2.
In these three cases, the totem pole structure can not meet the output requirements, and many off-the-shelf MOS driver ICs do not seem to contain the structure of the gate voltage limit.
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