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Design of Brushless DC motor - gate driver

2018/5/29 13:10:40

The first part: the design of the brushless gate driver

There is every reason to say that brushless DC motor is the coolest product in motor drives. You can get higher efficiency, power and torque, lower noise, electromagnetic interference (EMI) and vibration, longer battery and motor life, faster speed, better products, more surprises, fun and friends, a better look, and the worship of countless followers. This list has made me gradually fall into my hopes and dreams (see chart 1), so I can only say, "the results may be different."

Figure 1: brushless DC

The fun of Brushless DC motor drive is the algorithm. You can achieve sensor or sensorless monitoring, trapezoidal or sinusoidal control, field oriented control (FOC) or commutation. The options are as many as cooking eggs - but in fact, only ten are truly unique (the other means only a small change). But I'm not going to talk about that right now. I will discuss "step zero": design hardware for the motor drive system. In this step, you can leave at any time. Figure 2 shows me the impression of this phenomenon.

Figure 2: comfort decreases linearly with voltage level and analog content.

For the remaining six card readers, many brushless DC motor systems are designed to pursue high power and high efficiency, which means that the best way to implement is the micro controller (MCU) that uses a vertical MOSFET to control the gate drive. Before you test the best speed loop algorithm to control your motor, you simply connect the intelligence of MCU to the original current drive capability of MOSFET. The gate driver acts as a converter between the logical domain of MCU and the power domain of MOSFET and motor.

There are two architectures to implement this converter: discrete gate driver and integrated gate driver. There are many reasons to convince you to choose one. The split driver provides the highest power supply voltage and the best performance, but requires more components and lacks protection. Integrated drives provide more targeted solutions for motor drives, but will not provide you with the ultra high performance of the voltage support or the discrete gate drive. In addition to using three discrete gate drives on one chip, integrated drives such as DRV8320 can also provide additional functions, such as grid driven power, induction amplifiers, power devices, or integrated gate driven passive devices. Readers who have just skimmed the above paragraphs can see Table 1.

Table 1: discrete gate driver and integrated gate driver

The details in the table are more than the details of the paragraph

Second parts: Design of base brushless gate driver

I will create and display schematic and layout differences between discrete drives and integrated drives to test my ability to implement schematics and layout. Discrete and integrated gate drivers have their own advantages and disadvantages.

I will directly compare discrete and integrated gate drive architectures to demonstrate their board level differences.

The two key indicators for comparing schematics and layouts are the number of components and the size of solutions. The first metric is the number of components. This can be found relatively easily after the schematic diagram is completed. However, the estimation of solution size is more complicated. I often see the size of the solution dimensioning simply on the dimensions of the IC components. But I found this very inaccurate, because it did not consider the gap between external components, components and wiring on the circuit board.

I spent some time on local design software to create parallel schematic and layout for the discrete and integrated gate driver architecture for brushless DC motor drives. I chose TI a discrete gate driver and DRV8320 as my integrated gate driver. In addition, I used the standard QFN package of NexFET power MOSFET. Although this design uses the standard discrete FET, TI recently launched two vertically integrated half bridge power blocks that can be used for this application, saving more design space. This application makes me feel a lot of pressure on my poor circuit and layout skills, but I hope these pictures help those who want to compare the two brushless DC architectures.

Discrete gate drive

Integrated gate drive

Schematic diagram

Two-dimensional layout

Three-dimensional layout

Gate driver layout

Area: 54.54mm2

Grid drive supply layout

Area: 47.89mm2

Integrated to the gate drive

General Assembly

Integrated circuit: 4

Field effect tube (FET):6

Resistor: 20

Capacitor: 12

Diode: 6

Integrated circuit: 1

Field effect tube: 6

Resistor: 2

Capacitor: 5

Diode: 0

Table 1: discrete gate driver and integrated gate driver

This seemingly quick project has injected a lot of design and ideas. As you can guess from the above, in order to simplify browsing, I decided to create a double-layer circuit board without inner layer. But that means putting more ideas into the layout. Similarly, the gate drive setting components on the discrete gate drive and the IDRIVE pin components on the integrated gate drive need to be adjusted to obtain acceptable rise and drop times from the external FET. There are many minor adjustments in the layout so that the minimum size of the two solutions can be achieved.