Mosfet - what is it? Structural and technological features

In this article, we will talk about an element such as mosfet. What this, what properties it possesses, for what it is used in modern electronics, will be described below. You can come across two types of power transistors - MOSFET and IGBT. They are used in high-power pulse converters - inverters, power supplies. It is worth considering all the features of these elements.

Basic information

It should be noted that IGBT and mosfet transistors are capable of delivering very high power to the load. With all this, the device will be very small in size. The efficiency of transistors exceeds 95%. Mosfets and IGBT have one common feature - they have shutters isolated, the consequence of this - similar control parameters. The temperature coefficient is negative for these devices, which makes it possible to make transistors that are resistant to short circuit. To date, mosques with a normalized time of overload are produced by almost all firms.

Drivers for management

Since there is no current in the control circuit, in the static mode it is possible not to use standard circuits. It is more reasonable to use a special driver - the integrated circuit. Many companies produce devices that allow controlling single power transistors, as well as bridges and half-bridges (three-phase and two-phase). They can perform various auxiliary functions - to protect against current overload or short circuit, as well as from a large voltage drop in the control circuit of the mosfet. What is this chain, will be described in more detail below. It is worth noting that the voltage drop in the power transistor control circuit is a very dangerous phenomenon. Powerful mosfet can go into a different mode of operation (linear), resulting in failure. The crystal overheats and the transistor burns.

Short circuit mode

The main auxiliary driver function is protection against current overloads. It is necessary to carefully look at the operation of the power transistor in one of the modes - short circuit. Overcurrent may occur for any reason, but the most frequent is a short in the load or on the case. Therefore, it is necessary to properly manage the mospets.

Overloading occurs due to certain features of the circuit. A transient or a reverse-current recovery of a semiconductor diode from one of the transistor arms is possible. Elimination of such overloads is carried out by the circuit method. The trajectory formation chains (snubber) are used, the resistor is selected in the gate, the control circuit is isolated from the high-current bus and the voltage.

How does the transistor turn on in case of short circuit in the load

When a short-circuit occurs in the load, the current in the collector circuit is limited by a certain voltage in the gate, as well as by the slope of the characteristics of the transistor itself. In the supply circuit, there is a certain capacity, so the internal resistance of the source itself can not have any effect on the short-circuit current. As soon as the switching on occurs, the current increases smoothly in the transistor due to the fact that there is a parasitic inductance in the collector circuit. The same fact is the reason that there is a certain voltage failure.

False positives

After the transient is complete, the power transistor will be fully powered. And this will lead to the fact that a large power will be dissipated in a semiconductor crystal. Hence it can be concluded that the short-circuit mode must necessarily be interrupted after a certain period of time. It should be enough to prevent false triggering. As a rule, the time value lies in the interval 1 ... 10 μs. The characteristics of the transistor should be such that it easily withstands this overload.

Load impedance with transistor on

Similarly, with the case considered above, the current is limited by the characteristics of the transistor itself. It grows at a rate that is determined by the inductance (parasitic). Before this current reaches a constant steady-state value, the collector voltage increases. On the gate there is an increase in voltage due to the Miller effect.

The current in the collector increases, and it can significantly exceed the steady-state value. It is for this mode that not only is the channel mosfet switched off, but also the possibility of limiting the voltage is provided.

The voltage applied to the gate of the transistor depends directly on the steady-state short-circuit current. But with a decrease in the voltage at the gate of the semiconductor element, a rather interesting picture takes place. The saturation voltage increases and, as a consequence, the conduction loss increases. The stability of the transistor to a short circuit is closely related to the steepness of its characteristics.

Short-circuit current and gain

The higher the current in mosfets, the lower the saturation voltage. They are also capable of withstanding overload for a short time. On the other hand, semiconductors, which are more resistant to short circuit, have a very high saturation voltage. Losses are also very significant.

A larger maximum permissible value of the short-circuit current has a mosfet pioneer than a simple bipolar transistor. As a rule, it is ten times higher than the rated current (provided that the gate voltage is acceptable). Most manufacturers (European and Asian) produce transistors that withstand such loads, and are not damaged.

Overhead Overload Protection Driver

There are various methods for switching off elements during overload. With the help of drivers from different manufacturers, you can implement any protective functions, and the most effective. If an overload occurs, it is necessary to reduce the gate voltage. In this case, the recognition of the emergency mode increases in time.

Due to this, it is possible to exclude false alarms of the protection circuit. Here's how to test the mosfet: try changing the capacitor's capacitance value. If the reaction time for the short circuit changes, the whole circuit works correctly. The scheme uses several elements that have certain responsibilities. For example, connected to the driver output, the "ERR" -capacitor allows you to determine the time of the analysis of overloads.

Emergency operation mode

For this time interval, the current regulating circuit in the collector circuit is switched on. Due to this, the voltage across the gate of the semiconductor element is reduced. In the event that no overload stop occurs, the transistor turns off after 10 μs. Protection is disabled after the signal is removed from the input. Thanks to this, a trigger protection scheme is implemented.

When it is applied, it is necessary to pay attention to the time interval through which the switching of the mosfet transistor occurs. What is this inclusion and what are its features? Please note that this time should be longer than the thermal constant (temporary) of the semiconductor crystal, on the basis of which the transistor is made.

Disadvantages of the wiring diagram

The circuit uses resistors, which have high power, but they have very high inductance (parasitic, due to the use of some materials and technologies). And for the ideal functioning of the circuit, it is necessary that the capacitance be close to zero. Resistors used to measure the impulse current must comply with the above condition. In addition, the resistors lose tremendous power. And this affects the efficiency of the entire driver circuit of the upper arm.

But there are switching circuits that reduce power losses. The saturation voltage in any case depends on the collector current. Mosfet (what is it, considered in the article) this dependence demonstrates, it can be said, linear because the resistance of the channel (active) does not depend on the current on the drain of the transistor. But for powerful IGBT transistors this dependence is not linear, but it is easy to choose a voltage that will correspond to the required current protection.

Three-phase bridge driver

In such circuits, a resistor is also used to measure the current value. The protection current is determined by means of a voltage divider. IR2130 drivers are widely popular, which ensure stable operation of the circuit at a voltage of up to 600 Volts. The circuit includes a field-type transistor, which has a drain (it serves to indicate the presence of malfunctions). A mosfet on the board is installed using hard jumpers in high-quality insulation for these reasons. It has an amplifier that generates certain control and feedback signals. With the help of the driver, the time delay between the switching of the transistors of the lower and upper arms is formed in order to exclude the appearance of a through current.

As a rule, depending on the modification, the time is 0.2 ... 2 μs. In the IR2130 driver, which is used to implement the protection circuit, there is no function to limit the maximum value of the gate voltage at the moment of a short circuit. When designing a three-phase arm system, remember that the bridge is disconnected after 1 μs after the onset of a short circuit. Consequently, the current (especially when there is an active load) exceeds the value that was calculated. To reset the protection mode and return to the working mode, you must turn off the driver power supply or supply a voltage to its inputs.

Drivers of the lower arm

In order to control the transistors of the lower arm's mosfet, there are high-quality Motorola chips, for example, MC33153. This driver is special, as it can be successfully used for two types of protection (voltage and current). There is also a function that separates the two modes - overload and short circuit. There is a possibility of supplying some voltage (negative for control). This is useful for cases where it is necessary to control modules with high power and a sufficiently large value of the gate charge. The IGBT protection mode is disabled (these are the closest Mosfet analogs) after the supply voltage drops below the 11 volt mark.