[Repost] Summary of MOS tube drive circuit

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1. Overview of MOS tube drive circuits

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 tubes are a type of FET (the other is JFET), which can be made into enhancement type or depletion type, P channel or N channel, a total of 4 types, but only enhancement type N channel MOS tubes and enhancement type P channel MOS tubes are actually used, so usually NMOS or PMOS refers to these two types.

As for why depletion type MOS tubes are not used, it is not recommended to dig into the root of the matter.

Of these two types of enhanced MOS tubes, NMOS is more commonly used. The reason is that the on-resistance is small and it 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 the 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.

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

PMOS characteristics, Vgs less than a certain value will be turned on, suitable for the source connected to VCC (high-end drive). However, although PMOS can be easily used as a high-end drive, due to the large on-resistance, high price, few replacement types and other reasons, NMOS is usually used in high-end drives. 3. MOS switch tube loss No matter NMOS or PMOS, there is an on-resistance after conduction, 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 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 at both ends of MOS has a process of falling, and the current flowing through has a process of rising. During this period of time, the loss of MOS tube is the product of voltage and current, which is called switching loss. Usually the switching loss is much greater 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 large, and the loss caused is also large. Shortening the switching time can reduce the loss each time it is turned 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, which is commonly used for high-end driving, needs the gate voltage to be greater than the source voltage when it is turned on. When the high-side driven MOS tube is turned on, the source voltage is the same as the drain voltage (VCC), so the gate voltage should be 4V or 10V higher than VCC. If you want to get a voltage higher than VCC in the same system, you need a special boost circuit. Many motor drivers integrate 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 turn-on voltage. Of course, there needs to be a certain margin when designing. Moreover, the higher the voltage, the faster the turn-on speed and the smaller the turn-on resistance. There are also MOS tubes with lower turn-on voltages used in different fields, but in 12V automotive electronic systems, 4V turn-on is generally enough. For the drive circuit and loss of MOS tubes, you can 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 characteristics, so it is widely used in circuits that require electronic switches, such as switching power supplies and motor drives, as well as lighting dimming.

2. There are several special applications for current MOS drivers

1. Low voltage application

When using a 5V power supply, if the traditional totem pole structure is used, the voltage drop of the transistor's be is about 0.7V, resulting in the actual voltage applied to the gate being only 4.3V. At this time, we use a nominal gate voltage of 4.There are certain risks with 5V MOS tubes. The same problem also 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 with time or other factors. This change causes the driving voltage provided by the PWM circuit to the MOS tube to be unstable. In order to make the MOS tube safe under high gate voltage, many MOS tubes have built-in voltage regulators to forcibly limit the amplitude of the gate voltage. In this case, when the driving voltage provided exceeds the voltage of the voltage regulator, it will cause a large static power consumption. At the same time, if the gate voltage is simply reduced by the principle of resistor voltage division, the MOS tube will work well when the input voltage is relatively high, and the gate voltage will be insufficient when the input voltage is reduced, causing the conduction to be incomplete, thereby increasing power consumption. 3. Dual voltage application In some control circuits, the logic part uses a typical 5V or 3.3V digital voltage, while the power part uses a 12V or even higher voltage. The two voltages are connected in a common ground. This requires the use of a circuit that allows the low-voltage side to effectively control the MOS tube on the high-voltage side. At the same time, the MOS tube on the high-voltage side will also face the problems mentioned in 1 and 2. In these three cases, the totem pole structure cannot meet the output requirements, and many off-the-shelf MOS driver ICs do not seem to include a gate voltage limit structure.

3. Relatively common circuit

The circuit diagram is as follows:

[attach]363473 [/attach]

Here we only make a simple analysis of the NMOS drive circuit:

Vl and Vh are the low-end and high-end power supplies respectively. The two voltages can be the same, but Vl should not exceed Vh.

Q1 and Q2 form an inverted totem pole to achieve isolation and ensure that the two drive tubes Q3 and Q4 will not be turned on at the same time.

R2 and R3 provide the PWM voltage reference. By changing this reference, the circuit can work at a relatively steep position of the PWM signal waveform. Q3 and Q4 are used to provide driving current. When turned on, Q3 and Q4 have a minimum voltage drop of Vce relative to Vh and GND. This voltage drop is usually only about 0.3V, which is much lower than 0.7V Vce. R5 and R6 are feedback resistors, which are used to sample the gate voltage. The sampled voltage generates a strong negative feedback to the base of Q1 and Q2 through Q5, thereby limiting the gate voltage to a limited value. This value can be adjusted by R5 and R6. Finally, R1 provides base current limit for Q3 and Q4, and R4 provides gate current limit for MOS tubes, that is, Ice limit for Q3 and Q4. If necessary, an acceleration capacitor can be connected in parallel to R4. This circuit provides the following features: 1. Use low-end voltage and PWM to drive high-end MOS tubes. 2. Use small-amplitude PWM signals to drive MOS tubes with high gate voltage requirements. 3. Peak limit of gate voltage 4. Input and output current limit 5. By using appropriate resistors, very low power consumption can be achieved. 6. PWM signal inversion. NMOS does not need this feature, which can be solved by placing an inverter in front.

A low voltage and high frequency BiCMOS drive circuit using a bootstrap circuit

When designing portable devices and wireless products, improving product performance and extending battery life are two issues that designers need to face. DC-DC converters have the advantages of high efficiency, large output current, and low quiescent current, and are very suitable for powering portable devices. The main trends in the development of DC-DC converter design technology are:

(1) High frequency technology: As the switching frequency increases, the size of the switching converter also decreases, the power density is greatly improved, and the dynamic response is improved. The switching frequency of low-power DC-DC converters will rise to the megahertz level.

(2) Low output voltage technology: With the continuous development of semiconductor manufacturing technology, the operating voltage of microprocessors and portable electronic devices is getting lower and lower, which requires future DC-DC converters to provide low output voltage to meet the requirements of microprocessors and portable electronic devices.

The development of these technologies has put forward higher requirements for the design of power chip circuits. First, with the continuous increase in switching frequency, high requirements are placed on the performance of switching elements. At the same time, corresponding switching element driving circuits must be provided to ensure that the switching elements work normally at switching frequencies as high as megahertz. Secondly, for battery-powered portable electronic devices, the operating voltage of the circuit is low (taking lithium batteries as an example, the operating voltage is 2.5 to 3.6V), so the working voltage of the power chip is low. MOS tubes have very low on-resistance and low energy consumption. In the current popular high-efficiency DC-DC chips, MOS tubes are mostly used as power switches. However, due to the large parasitic capacitance of MOS tubes, the gate capacitance of NMOS switch tubes is generally as high as tens of pico-farads. This puts higher requirements on the design of high-frequency DC-DC converter switch tube drive circuits. In low-voltage ULSI design, there are many CMOS and BiCMOS logic circuits with bootstrap boost structure and drive circuits as large capacitive loads. These circuits can work normally under power supply conditions below 1V, and can work at a frequency of tens of megahertz or even hundreds of megahertz under load capacitance conditions of 1 to 2pF. This article uses the bootstrap boost circuit to design a drive circuit with large load capacitance driving capability suitable for low-voltage, high switching frequency boost DC-DC converters. The circuit is designed based on Samsung AHP615 BiCMOS process and verified by Hspice simulation. When the supply voltage is 1.5V and the load capacitance is 60pF, the operating frequency can reach above 5MHz.