Principle of gate driver

What is a gate driver?

Briefly, a gate driver is a snubber circuit that amplifies low voltage or low current from a microcontroller or other source. In some cases, such as driving logic level transistors for digital signal transmission, using the microcontroller output does not compromise the efficiency, size, or thermal performance of the application. In high power applications, microcontroller outputs are generally not suitable for driving higher power transistors.

But why use a microcontroller to drive a power transistor? To better answer this question, let's consider a large application. Switching power supplies are at the heart of almost every modern electrical system. Any device plugged into a wall outlet can utilize a switching power supply for power factor correction and DC current rail generation. Automotive systems use switching power supplies to maintain systems such as batteries, motors, and chargers. The grid infrastructure requires efficient switching of the switching energy provided by the DC solar panels to transfer electrical energy to the DC storage system and the AC grid.

Due to the large number of topologies and increasing complexity in applications, modern switching power supplies typically use microcontrollers or other ASICs to coordinate their switching for high power transistor arrays to meet precise switching timing requirements. This can be a challenge because most microcontroller outputs are not optimized for driving power transistors.

High power transistors are almost completely different in characteristics from analog signal chains or other transistors in digital logic circuits. The breakdown voltage of power transistors can range from about 40 volts to 1,200 volts or more. Due to the need to achieve higher drain circuits and lower conduction losses, the drain-to-source resistance needs to be as low as a few tens of milliohms or less. The gate capacitance, which is inversely proportional to the drain-to-source resistance, typically exceeds 10,000 pF. The gate drive voltage and current requirements are highly dependent on the transistor structure and drain current rating, which is typically between 8 and 30 volts and between 1 and 5 amps. A high noise environment may even require a bipolar output drive.

Compared to signal chains or digital transistors with frequencies of tens or hundreds of megahertz, the upper limit of the frequency of conventional high-power transistors is only a few hundred kilohertz. With the advent of new technologies, it is possible to push the upper limit by an order of magnitude. This frequency limitation is due to increased gate capacitance and drive voltage requirements. The energy of the capacitor is equal to 1/2 times the capacitance and multiplied by the square of the voltage. The charge and discharge power dissipation of the gate capacitance is equal to the energy of the capacitor multiplied by twice the frequency - one charge, one discharge. A power transistor with a 15 nanofarad gate capacitance requires nearly half watts of power consumption at 200 kHz, 12 volt square wave drive conditions. For converters that can deliver 3 to 5 kW of power, the benefits of increased switching frequency, such as reduced magnet size and weight, are sometimes more valuable than the cost of a few watts of drive loss.

There is also a more difficult source of loss in the elements that determine the drive requirements of the transistor. During the charging and discharging of the gate capacitor, the switch will have a transition period between the fully open and fully closed states. At this time, a voltage will appear on the switch and a current will flow through the switch. Due to the high voltage and high current, such switching losses can result in considerable power consumption, sometimes tens of watts, and further efficiency degradation. Therefore, it is advantageous to shorten the duration of the transition period by charging and discharging the gate capacitance more quickly.

If the output voltage is even high enough to turn the transistor on, the low current signal provided by most microcontrollers is slow and cumbersome when driving high power transistors, and is extremely inefficient.

Now let's answer the question of what is a gate driver, a circuit that amplifies control signals from a microcontroller or other source to adapt it to the efficient and efficient operation of semiconductor switches.

There are many gate drivers that can operate with high bias voltages, such as those used in high power converters.