This article will talk about an element such as mosfet. What it is, what properties it has, what it is used in modern electronics for, will be described below. You can meet 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 large power to the load. With all this, the device will be very small in size. The efficiency exceeds transistors at 95%. Mosfet and IGBT have one thing in common - they have isolated gates, the consequence of this is similar control parameters. The temperature coefficient is negative for these devices, which makes it possible to make such transistors that will be resistant to short circuits. To date, mosfets with a normalized value of the overload time are produced by almost all firms.
Drivers for management
Since there is no current in the control circuit, in static mode, you can not use standard circuits. It is wiser to use a special driver - an integrated circuit. Many companies produce devices that allow you to control 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 mosfet control circuit. What this chain will be described in more detail below. It is worth noting that a voltage drop in the control circuit of a power transistor is a very dangerous phenomenon. Powerful mosfets can go into another mode of operation (linear), as a result of which they will fail. The crystal overheats and the transistor burns out.
Short circuit mode
The main auxiliary function of the driver is protection against current overloads. You must carefully look at the operation of the power transistor in one of the modes - short circuit. Overcurrent can occur for any reason, but the most common are short circuits in the load or on the case. Therefore, the management of mosfets should be properly implemented.
Overloading occurs due to certain circuit features. A transient process or the occurrence of a reverse recovery current of a semiconductor diode of one of the arms of the transistor is possible. The elimination of such overloads occurs by the circuit method. The trajectory formation circuit (snubber) is used, the resistor in the gate is selected, the control circuit is isolated from the high current and voltage bus.
How does the transistor turn on during 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 steepness of the characteristics of the transistor itself. At the same time, there is some capacity in the power circuit, therefore, the internal resistance of the source itself cannot in any way affect its short circuit current. As soon as switching on occurs, the current builds up smoothly in the transistor due to the fact that there is a stray inductance in the collector circuit. The same fact is the reason that there is some kind of voltage drop.
False positives
After the transition process is completed, the supply voltage will be fully applied to the power transistor. And this will lead to the fact that a large power will be dissipated in a semiconductor crystal. From this we can conclude that the short circuit mode must be interrupted after a certain period of time. It should be enough to prevent a false positive. As a rule, the time value lies in the range of 1 ... 10 μs. The characteristics of the transistor must be such that it can easily withstand this overload.
Short circuit load when the transistor is on
Similarly to the case discussed above, the current is limited by the characteristics of the transistor itself. It grows at a speed that is determined by inductance (spurious). Before this current reaches a steady steady state value, an increase in collector voltage will begin. At the shutter there is an increase in voltage due to the Miller effect.
The current on the collector increases, and it can significantly exceed the steady-state value. It is for this mode that not only the channel mosfet is turned off, but also the possibility of limiting the voltage is laid down.
The steady-state short-circuit current depends on the voltage applied to the gate of the transistor. But with a decrease in the voltage at the gate of the semiconductor element, a rather interesting picture occurs. The saturation voltage increases and, as a result, the conductivity loss increases. Short-circuit transistor resistance is closely related to the steepness of its characteristics.
Short circuit current and gain
The higher the current factor for mosfets, the lower the saturation voltage. They are also able to withstand overloads for a short time. On the other hand, semiconductors that are more resistant to short circuits have a very high saturation voltage. Their losses are also very significant.
A larger maximum permissible value of the short circuit current has a pioneer mosfet than a simple bipolar transistor. As a rule, it is ten times higher than the rated current value (provided that the gate voltage is acceptable). Most manufacturers (European and Asian) produce transistors that withstand such loads, and are not damaged.
Upper Arm Overload Protection Driver
There are various methods for disabling items during overload. Using drivers from various manufacturers, you can implement any protective functions, and most efficiently. If overload occurs, the shutter voltage must be reduced. In this case, the recognition of the emergency mode increases in time.
Thanks to this, it is possible to eliminate false positives of the protection circuit. Here's how to check the mosfet: try changing the capacitor value. If the response time to short circuit changes, then the whole circuit works correctly. The scheme uses several elements that have certain responsibilities. For example, connected to the driver output, an “ERR” capacitor allows you to determine the time of congestion analysis.
Emergency operation
For this time period, the current stabilization circuit is switched on in the collector circuit. Due to this, the voltage across the gate of the semiconductor element decreases. In the event that the overload does not stop, the transistor turns off after 10 μs. Protection is disabled after the signal is removed from the input. Thanks to this, a trigger protection circuit is implemented.
When it is applied, it is necessary to pay attention to the period of time through which the mosfet transistor is turned on again. What is this inclusion and what are its features? Please note that this time should be longer than the thermal constant (time) of the semiconductor crystal, on the basis of which the transistor is made.
Disadvantages of the circuit
In the circuit, resistors are used, which have high power, but they have a very high inductance (spurious, due to the use of some materials and technologies). And for the circuit to function perfectly, it is necessary that the capacitance is close to zero. Resistors used to measure pulsed current must meet the above condition. In addition, resistors lose enormous power. And this affects the effectiveness of the entire upper arm driver circuit.
But there are switching circuits that reduce power loss. The saturation voltage in any case depends on the collector current. Mosfet (that this is considered in the article) demonstrates this dependence, it can be said linear, because the channel resistance (active) does not depend on the current at the drain of the transistor. But for powerful IGBT transistors, this dependence is not linear, but you can easily select the voltage that will correspond to the required protection current.
Three-phase bridge driver
Such circuits also use a resistor to measure the current value. The protection current is determined using a voltage divider. The IR2130 drivers are widely popular, which ensure stable operation of the circuit at voltages up to 600 volts. The circuit includes a field-effect transistor with a drain open (it serves to indicate the presence of malfunctions). A mosfet is installed on the board using hard jumpers in high-quality isolation for these reasons. It has an amplifier that produces a certain control and feedback signals. Using the driver, a delay in time is formed between the transistors of the lower and upper arms to exclude the appearance of 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 time of a short circuit. When developing a three-phase arm circuit, it is necessary to remember that the bridge is disconnected 1 μs after the start of a short circuit. Therefore, the current (especially in the presence of an active load) exceeds the value that was calculated. In order to reset the protection mode and return to the working mode, it is necessary to turn off the driver power supply or to supply a blocking voltage to its inputs.
Lower shoulder drivers
To control the lower arm mosfet transistors , there are high-quality chips from Motorola, 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. It is possible to supply some voltage (negative for control). This is useful for cases when it is necessary to control modules with high power and a sufficiently large value of the gate charge. The IGBT protection mode (these are the closest analogues of mosfets) is turned off after the supply voltage drops below 11 volts.