MOSFET - Jatin

Owned by Jatin Mehta
Last updated: 2022-02-02
6 min read

MOSFET

What/Why transistors?

  • Become hotter the more current passes through it (usually protected using Resin case - black)

  • They have a recommended voltage and current in which they should be used

  • The layout of typical transistor:

  • Has two main functions:

    • Act as a switch - turn circuits on & off

    • Amplify Signals

How Transistors

  • Basically like a switch but u don't need to manually flip it, instead you can pass some voltage in the middle pin (Base) and then it will act as a on swtich and allow current to flow, else, it will act as an off switch

  • This is how the “amplification” part works. I.e you already have a circuit with a larger power supply but then you connect the transistor as the switch. You can supply a small amt. of voltage to the transistor and then the transistor lets the main circuit with more voltage flow.

  • Terminiology/Specs: Collector current vs base current and current gain

Types of Transistors:

Two main types of transistors:

  • MOS Field-Effect Transistors: For Larger Current (more than 1 amp)

    • Work by voltage

    • MOSFETs are field effect

  • Bipolar Junction Transistor- For smaller loads (<1 Amp)

    • Work by current

    • NPN vs PNP: Main difference is direction of current flow

What is a MOSFET?

  • It is a type of transistor

  • Stands for a metal-oxide-semiconductor field-effect transistor (MOSFET)

  • Commonly used today

  • The MOSFET consists of four terminals:

    • Drain (D)

    • Source (S)

    • Gate (G)

    • Body (B)

    • Drain and Source are like Emitter and Collector with Gate being the Base with Body being the resin or heat sink covering mosfet

Why MOSFET?

  • Scalability

  • Low turn-on current

  • High switching speeds

  • High off-state impedance

How MOSFET:

Defining Key MOSFET params:

  • VGS: Voltage between the gate and source terminals of the MOSFET

  • VDS: Voltage between the drain and source terminals of the MOSFET

  • Vth: The MOSFET threshold voltage

Operating Regions

The MOSFET has three basic regions of operation that may be defined with a few simple expressions.

Cutoff Region

When this is met:

In the cutoff region, the MOSFET acts like an OFF switch (allowing no voltage to flow through)

Linear Region

When this is met:

In the linear region, the MOSFET is partially ON and behaves similar to a variable resistor

Saturation Region

The MOSFET operates in the saturation region when the following conditions are satisfied:

In the saturation region, the MOSFET is fully ON

 

MOSFET Circuit

Capacitors and Capacitance

What Are Capacitors?

 

  • Store electric charge (like a battery) but they can charge and discharge much faster and store much less charge.

  • Acts like the storage tank where even if water is stopped for a certain amount of time, water will still flow out of the tank as it stores some water

  • Hence, it is used to smooth out interruptions in the supply of current (ex. switch being turned on/off)

  • Capacitors have 2 main specs: Capacitance and Voltage (aka the maximum voltage the capacitor can handle)

Capacitance is the ratio of the amount of electric charge stored on a conductor to a difference in electric potential. AKA the ability of a system to store an electric charge. (Measured in Farads - F , usually done in microfarads uF). More farads = it can store more charge.

Types of MOSFETS

  • N vs P channel has more to do with the material used which dictates electron flow and has some other useful info.

  • Enhancement Type: MORE COMMON; assumed by default unless datasheet says otherwise

    • OFF by default, voltage required to turn it ON

      • there is no conduction at zero voltage which implies it is closed or “OFF” by default as there is no existing channel. When the gate voltage is increased more than the source voltage, the charge carriers (holes) shifts away leaving behind the electrons and thus a wider channel is established.

  • Depletion Type:

    • Think of it as ON by default, voltage is required to turn it OFF

      • the channel is already established and it is evident that the conduction occurs even at zero voltage and it is open or “ON” by default.

Gate Driver

A gate driver is a power amplifier that accepts a low-power input from a controller IC and produces a high-current drive input for the gate of a high-power transistor such as an IGBT or power MOSFET.

Power MOSFET

  • Type of MOSFET

  • Designed for handling high power levels (still less than 200 V)

  • most common power semiconductor device in the world, due to its low gate drive power, fast switching speed, easy advanced paralleling capability, wide bandwidth, ruggedness, easy drive, simple biasing, ease of application, and ease of repair

Motor Controller

  • A motor controller is a device or group of devices that can coordinate in a predetermined manner the performance of an electric motor.

  • A motor controller might include a manual or automatic means for starting and stopping the motor, selecting forward or reverse rotation, selecting and regulating the speed, regulating or limiting the torque, and protecting against overloads and electrical faults.

  • Motor controllers may use electromechanical switching or may use power electronics devices to regulate the speed and direction of a motor. → In our case, the MOSFET does this

 

BLDC Motors

Brushless DC Motors were a new type of electrical motor, made possible due to a transistor switch. In this way, an electronic commutator is used to facilitate rotation instead of a mechanical commutator with brushes.

Advantages over Brushed DC Motors:

  • More efficient

  • Simpler mechanical design

  • More precise control

  • More reliable

  • Longer life span (brushes wear out as they are rubbing against commutators)

Disadvantages

  • Higher initial cost

  • More complicated motor controller

Structure Comparison:

Components of BLDC

  • Stator

    • The windings that creates a magnetic field when energized

  • Rotor

    • Mad up of permanent magnets

Two types of BLDC Motors

  • Inrunner motor - Rotor is internal, stator is on the outside of the motor. These have a more lightweight construction and a better rotational speed because of their smaller rotating diameter.

  • Outrunner motor - Rotor is external, so permanent magnets spin around the stator together with the motor’s case. These have a higher torque because of the longer arm and greater EMF applied to the rotor.

Phases

  • Multiple types of BLDC with different phases (most common is 3 phase)

3 Phase BLDC Motor winding types:

Rotor Positioning

Sensorless

Sensor

  • Hall Effect Sensor is very common

How Everything Connects

TODO

  • Motor Stuff

  • Beck EMF

  • H Bridge (x3) - Half bridge

    • Phase of commutating motor

  • VBUS

  • PWMs

    • 3 or 6 PWMs (either 1 per H bridge or 2 per H bridge - 1 high, 1 low)

  • Microcontroller (MC)

  • Why not directly PWM from MC to gates of MOSFETS?

    • You would fry the MOSFET as you are sending too much voltage

    • Thus Gate driver handles the PWM before it reaches the MOSFET

  • Calculate the precise position of motor (Sensorless approach):

    • Beck EMF

    • Current Sensing

  • Go on Digikey,

  • type gate driver, in stock, has datasheet,

  • everything greater than 24v for bootstap

  • Surface mount

  • n-channel MOSFET

  • ideally, supply more than 40 Amps