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Precision labs 시리즈: 절연 게이트 드라이버
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소개 - 절연 게이트 드라이버란 무엇입니까?
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Hello, and welcome to the TI Precision Lab discussing isolated gate drivers. In this video, we'll briefly introduce the concept of a gate driver and explain in detail what differentiates isolated and non-isolated gate drivers. We will also cover some of the key benefits of isolated gate drivers.
First, a brief introduction. What is a gate driver? Simply put, a gate driver is a buffer circuit used to amplify a low voltage or low current control signal from a microcontroller or other source.
In some cases, such as driving logic level transistors for digital signaling, the use of a microcontroller output does not harm the application efficiency, size, or thermal performance. In high power applications, a microcontroller output is not usually suitable for driving larger power transistors.
But why would a microcontroller be driving a power transistor? To better answer the question, let's consider the biggest application. Switching power supplies are at the heart of nearly every modern electrical system. Anything that plugs into wall outlets can take advantage of switching power supplies for power factor correction and DC rail generation.
Automotive systems use switching power supplies to maintain an increasingly complex ecosystem of batteries, motors, chargers, and so on. Grid infrastructure demands highly efficient switching power conversion from DC solar panels, directing power to both DC storage systems and the AC grid.
On account of the vast number of topologies and the growing complexity of applications, modern switching power supplies frequently use a microcontroller or other ASIC to orchestrate the carefully timed switching of an array of high power transistors. This can present challenges because most microcontroller outputs are not optimized for driving power transistors.
As you can probably guess, the characteristics of high power transistors are rarely comparable to the characteristics of other transistors found in analog signal chain or digital logic. Breakdown voltages of power transistors range from roughly 40 volts to an impressive 1,200 volts or more. The need for higher drain currents and lower conduction losses pushes drain source resistance to tens of milliohms or less.
Gate capacitance inversely related to drain source resistance commonly exceeds 10,000 picofarads. The gate drive voltage and current requirements depend significantly on the transistor construction and the drain current rating with common values between 8 to 30 volts and 1 to 5 amps. Noisy environments may even require bipolar output drive.
Compared to signal chain or digital transistors operating at tens or hundreds of megahertz, conventional high power transistors are limited to operation in the hundreds of kilohertz with emerging technologies that could potentially push this limit up by an order of magnitude. This frequency limitation is due to the increased gate capacitance and drive voltage requirements.
The energy of a capacitor is given by 1/2 times the capacitance times the voltage squared. The power consumed by charging and discharging the gate capacitance is the energy of the capacitor multiplied by twice the frequency-- once for charging, once for discharging.
A power transistor with 15 nanofarad gate capacitance driven at 200 kilohertz with a 12-volt square wave requires nearly half a watt. For a converter capable of 3 to 5 kilowatts of power transfer, the benefits of increasing switching frequency, such as size and weight reductions in the magnetics, are sometimes more valuable than the cost of a few watts of drive losses.
There is a more troublesome source of loss working to dictate the transistor drive requirements. As the gate capacitance is charged and discharged, there is a transition period between the switch's fully on and fully off states, where, simultaneously, a voltage appears across the switch, and a current flows through the switch. Because of the presence of both higher voltages and higher currents, considerable power dissipation, sometimes tens of watts, and further efficiency penalty results from these switching losses.
It is, therefore, advantageous to reduce the duration of the transition period by charging and discharging the gate capacitance quickly. The low current digital signaling provided by most microcontrollers is prohibitively slow and inefficient for driving high power transistors if the output voltage is even high enough to turn the transistor on.
So to answer our question, what is a gate driver, a gate driver is a circuit used to amplify a control signal from a microcontroller or other source to make it suitable for efficient and effective operation of semiconductor switches.
There are a number of gate drivers designed to operate while subjected to large bias voltages, such as those seen in high power converters. Broadly, these gate drivers fall into two categories-- non-isolated gate drivers and isolated gate drivers.
Most non-isolated gate drivers designed for high voltage operation are half-bridge drivers. Half-bridge drivers are designed to drive power transistors stacked together in a half-bridge configuration. They have two channels-- a low side and a high side.
The low side is a fairly straightforward buffer usually grounded to the same point as the control inputs. The high side, however, is deliberately designed with reference to the switch node of the half bridge, permitting the use of two n-channel MOSFETs or two IGBTs.
The switch node is expected to rapidly transition between a high voltage bus and a power ground, creating the cost-effective opportunity to use a bootstrap supply to power the high side from the same supply as the low side. To communicate whether an output should be high or low, it is necessary to include a high voltage level shifter with a small amount of leakage current, usually microamps or less.
This type of gate driver has several limitations. First, since it is all built on the same silicon die, it must operate within the process limits of silicon. The working voltage for most non-isolated gate drivers does not exceed 700 volts.
Second, the level shifter must withstand the stresses of high voltage operation and must communicate the output state in a highly noisy environment. Consequently, the level shifter usually adds some propagation delay to allow for adequate noise filtering. The low side driver is then matched to the longer delay of the high side driver.
Third, non-isolated gate drivers for high voltage operation are not flexible. Many complex topologies exist today, which require multiple outputs shifted above control common or outputs that can shift below control common. And increasingly common features seen in modern gate drivers is integrated isolation between input and output circuitry. These devices use one silicon die for control signals and another silicon die for output drive signals, physically separated by distance and insulating material.
Control signals can be transmitted across the isolation barrier in a number of ways, but unlike non-isolated gate drivers, the isolation barrier prevents the flow of any significant leakage current from one side of the barrier to the other. Since one input die can be isolated from multiple output dies, and output dies can be isolated from each other, an output common may drift freely from the input common or from another output common up to the limits of the isolation technology.
Unlike non-isolated gate drivers with inflexible level shifters and predetermined output roles, isolated gate driver outputs can be referenced to any node in a circuit and can be constructed as single-channel or dual-channel devices. Isolation technology limits far exceed the silicon process limitations of non-isolated gate drivers, offering greater than 5 kilovolts of barrier.
In addition to increased voltage limits and increased flexibility, isolated gate drivers can be designed for faster and more robust operation.
Isolation sees use for many reasons. Many applications call for isolated power supplies as a regulatory requirement, and isolated gate drivers can be used to simplify system construction. Sometimes the strength of the barrier can be leveraged for increased system resilience against surges, lightning strikes, and other anomalous events that threaten to damage the system.
In other cases, the flexibility of having an isolation barrier simplifies the design of topologies that would otherwise require signal transformers or level shifters, such as the inverting buck boost. Even in conventional half-bridge applications, where isolation is not strictly necessary, isolated gate drivers can outperform their non-isolated counterparts with superior propagation delay, higher drive strength, and greater robustness to high voltage transients.
Common topologies which make use of isolated gate drivers include traction inverters, motor drives, three-phase power factor correction circuits, and solar string inverters. These topologies all convert between AC and DC power, directly interfacing with both a high voltage DC bus and a three-phase system, such as a motor or the power grid.
In the next section, we'll take a look at these and other converter topologies in more detail with emphasis on why isolated gate drivers improve and simplify system design.
That concludes this video. Thank you for watching. Please try the quiz to check your understanding of this video's content.