Gate driver

A gate driver ( MOSFET drivers, drivers or IGBT half-bridge drivers ) is referred to in the electronics, particularly power electronics, a discrete or integrated electronic circuit, which power switches, such as MOSFETs or IGBTs controls.

Further, a simple gate driver to be seen as a combination of a level shifter and amplifier.

• 2.2.1 driver circuit with bootstrapping
• 2.2.2 driver circuit with isolated power supply

Motivation

Often we read that transistors with an insulated gate electrode such as MOSFETs can be driven without power or at least de-energized, which is not correct though. In principle, such transistors, for example, bipolar transistors need as opposed due to their principle of only not constantly control current flowing, as long as you switch state should not be changed. However, the insulated gate electrode forms a capacitor in transistor ( gate capacitor ), which must be reloaded after each switching cycle of the transistor. Since a transistor requires a certain voltage at the gate to gate, this capacitor must be charged to a minimum of this tension. Conversely, this voltage must be turning off the transistor that are being dismantled, so the capacitor will be discharged.

When a transistor is switched, it does not go abruptly from non-conducting to conducting state (or vice versa) over, but passes depending on the charging voltage of the gate capacitance of a certain resistance range. Consequently, during the switching of current flow under a more or less large power is converted in the transistor, which heats it and may even destroy the worst case. It stands to reason therefore to configure the switching of the transistor as short as possible in order to keep the switching losses as low as possible.

Between charging current ( I), changing the charging voltage ( dU ) and recharging time (dt) is valid for a capacitor of capacitance C following relationship:

Since the required voltage swing are determined ( change in the gate voltage between on and off, USA) and the gate capacitance C of the transistor, thus the switching of the transistor DT is the smaller, the larger the current I, with which the gate controlled ( reloaded ) is. The amount of this charge-reversal is limited by the resistance and inductance of the gate current path. The source of the switching signals must be able to supply this discharge currents. The gate capacitance depends to a certain extent even from a range of applied gate voltage. For a concrete transistor manufacturers are therefore generally to the product of the gate capacitance C gate voltage U. This is the gate charge Q, which must be transported in the course of a shift in the gate into it or out of it. Typical values ​​of gate charge for power MOSFET are on the order of 100nC ( nanocoulombs ). Because of the already mentioned thermal losses in the transistor during the switching process (rapid switching on and off ) switching times on the order of microseconds, and are sought under particularly with periodic operation. Corresponding values ​​reach the discharge currents, which are required for driving the gate, under these conditions, well within the range of a few hundred milliamperes to the magnitude Amp. In the typical gate voltages of about 10 -15V outputs of several watts easily be required! Do large currents are switched at high frequencies (eg in DC choppers for large electric motors), you have several transistors often connected in parallel, the charging currents and the switching performance then multiply with the number of transistors.

The switching signals for transistors are usually generated by logic circuits or microcontrollers, which provide the signal to standard logic outputs available. Since these can usually only sustain currents in the tens of milliampere range, directly connected power transistors are switched relatively slow. Correspondingly high losses occurring during switching. At the same time, the gate capacitance of the transistor forms an electrical short circuit for the driving logic output in the switching moment. Without precautions, this can lead to an overload current side of the driver module that can destroy him due to the heating due to ohmic losses in the current path. To counteract this be the end use of custom driver circuits ( gate driver ) is used between the logic outputs and the power transistors.

Driver circuits

Control of individual transistors

To be able to switch individual power transistors fast, discrete electronic circuits or finished driver ICs offer. The following discrete driver circuits relate to the control of n- channel transistors. Similarly, the driver circuits can be employed by changing the reference potential for the p-channel transistors.

Simple drive circuit

The simplest form of a driver circuit composed of a bipolar transistor having a collector resistor as a pull -up resistor. No voltage is applied to the control input disables the bipolar transistor and so the gate of the output transistor is drawn by the resistor to the operating voltage of the driver circuit. Thus, the gate capacitance charges up across this resistor and the power transistor begins to conduct. Will be at the control input, a voltage is applied, the bipolar transistor shorts the gate of the power transistor, whereby the gate capacitance is discharged and the transistor begins to block.

The power transistor is therefore turned on via a resistor and switched off by short-circuiting the gate voltage. Since the discharge current through the bipolar transistor in this case is significantly higher than the charge current through the resistor, the transistor is turned off as quickly. This behavior may even be desirable in some circumstances because a too rapid of the power transistor has a high electromagnetic emission.

Logic gate driving circuit

As mentioned above provides a logic output only low output currents. Parallel connection logic gate whose output currents can be added to give a suitable for driving power transistors high output current can flow in total. Important in the parallel connection of logic gates is a steep edge on the input signal in order to achieve a nearly simultaneous overturning of all gates. To produce such a signal sure one of these gates can be connected in series to form the input signal. The logic gate, for example, be of the type HC4069 to achieve a higher driving voltage than 5V.

Push-pull driver circuit

To be able to deliver even higher output currents, the driver circuit can be implemented as push-pull output stage. So that the switching is not too small, and thus the electromagnetic emission is to be large, a resistor is inserted between the gate of the power transistor and the push-pull output stage. By the parallel connection of a resistor-diode combination, which reduces the total resistance for the switch, can be achieved in that the power transistor quickly - as a off. Fast switching reduces switching losses, slower shutdown reduces voltage spikes caused by parasitic inductances.

Driving a half bridge

For certain applications, it is not necessary to switch a load with a single power transistor, but a half-bridge. The simplest form of a half-bridge consisting of a combination of n -channel transistor and p- channel transistor. For each transistor, a driver circuit can now be used to drive the transistors in opposite directions.

Because p -channel transistors have generally poorer properties than n -channel transistors, only the half-bridge with n- channel transistors are established in the power electronics. For driving the high-side transistor, however, problems arise in that the potential of the control voltage at the non- gate, such as when using a p -channel transistor at this point to the positive potential of the supply voltage but to the sometimes rapidly varying the potential of the midpoint of the half-bridge relates. Specifically, the transistor would not be full by heading when the gate potential could be raised only up to the supply. It requires its own driver circuit.

Driver circuit with bootstrapping

, Must be made between the output of the half bridge ( the connection point of the two power transistors ) and the gate voltage to be applied to be able to turn on the upper transistor in an n -channel half-bridge. This can be done by means of a bootstrap circuit.

A voltage applied to the control input, the lower power transistor (slow) is turned on. Simultaneously, the gate voltage of the upper power transistor is short-circuited via the bipolar transistor and the low power transistor. Therefore at the output of the half bridge is connected to ground potential, thereby charging the capacitor through the diode. Is now connected to ground, the control input disables not only the lower power transistor, but also the bipolar transistor, whereby the gate capacitance of the upper power transistor is charging through the resistor from the first supply voltage. When the output voltage increases due to an inductive load or as the upper power transistor starts to conduct, this voltage swing across the capacitor continues planted, the diode blocks, and the potential for increasing the supply of the gate as desired by means of which the supply voltage.

Since the supply voltage is short-circuited if both output transistors conduct the same time, it is important that in each case a power transistor off before the other conducts. This is accomplished in this circuit with unequal switch-on and switch-off times of the power transistors.

It is with this driver circuit is not possible, the upper power transistor static switch, since the bootstrap capacitor loses its charge due to leakage currents. Before the upper power transistor comes out of saturation of the lower power transistor must be turned on again.

Driver circuit with isolated power supply

Another way, the upper power transistor of an n -channel half- bridge switch, and can even turn static, is the driver stage to provide electrically isolated, as by a shift converter or charge pump. For this purpose, there are driver ICs, which have the necessary circuitry for the most part already integrated.

Other driver circuits

In switching converters, it may be necessary to control the power transistors electrically isolated in order to maintain the electrical isolation of the switching converter. Via transformers ( Pulse Transformers ) can be transmitted with appropriate circuitry necessary to drive the control current of the power transistor. Thus, it is not necessary that the driving voltage is generated on the secondary side specially.

Generally there is a suitable integrated solution for any application. Especially with half-bridge driver chips there is a significant advantage over a discrete solution. That the supply voltage is not shorted momentarily when switching half-bridge driver chips generate some dead time ( lock time). This ensures that at no time both transistors are conducting.

Practice

Especially at high currents occur brownouts and voltage spikes on the ground connections. These voltage differences lead to differences in potential between the drive circuit and the power transistor, whereby the control voltage at the gate of the power transistor can be significantly higher than the supply voltage of the driver circuit. Due to high voltage on the gate of a power transistor could be destroyed. It is thus to ensure a good grounding to minimize these effects.