SMV ELECTRIC TUTORIALSAditya Kuroodi2016Relevant Course(s): EE115A. EE115B

MOTORS & PWM

Understanding Motors As a prequisite to understanding motors, keep in mind the following

ideas: AC vs DC Inductors and Capacitors in AC/DC MOSFETs Diodes Analog vs Digital

Analog and Digital Signals Analog signals are real-world signals (ex: battery voltage, radio

frequency, etc.) Analog signals have infinitely many values and continously vary over

time Main issue with analog is susceptibility to noise, low precision-to-cost

ratio Using analog also means large power loss due to continuity Digital signals only take on values from a finite set, often binary:

{0,1} Our goal is to use digital signals to control analog circuits (multiple

analog signals) Less precision loss Less power loss

Pulse Width Modulation (PWM) PWM is a method to vary the duty cycle of a square wave coming from

an MCU’s digital output in order to match an analog level We can use PWM to encode analog levels into a digital signal

PWM is a digital signal because at any given time, it’s either ON or OFF Note: at any given time we either provide FULL power supply voltage

or NONE PWM control depends on both duty cycle as well as modulating

frequency

Pulse Width Modulation (PWM) Suppose we try to control the voltage seen by

this lamp with a switch: If we close switch for 20ms, lamp will see 9V.

Then if we open switch for 20ms lamp sees 0V repeat this cycle 10 times per second

Now we have 50% duty cycle with 10Hz modulating frequency, and lamp will light up at half of max brightness (as if connected to 4.5V with no switch)

Note: If we repeat cycle slowly (on for 5 seconds, off for 5 seconds, etc.) the proper level would NOT occur

Need to insure that frequency is > load response time

Most PWMs operate at modulating frequencies in 10Khz-20MHz range

MOSFET as a Switch Suppose we want to turn a lamp (or LED)

on and off with a MOSFET Using N-Channel, we connect Source to

GND Load placed between voltage rail and

the MOSFET Input voltage pulses, either biases gate-

source to saturation or leaves transistor open

When gate voltage high, lamp is on Connect MCU digital output to gate of

MOSFET Send PWM signal of appropriate duty

cycle to adjust lamp brightness Note that PWM voltage level is only high

enough to toggle MOSFET, while lamp gets full VDD

NOTE: VDD >> Vin

MOSFET Power Switching Considerations Inductors, when quickly powered off, will generate huge voltage spike

in opposition to decreasing current V = L *di/dt Use FlyBack Diode (Snubber, Supression, Flywheel, etc.) to protect

circuitry (including the MOSFET!) Now current flows through diode, back through inductor and slowly

dies down from resistive losses

Capacitive loads will draw in large current when first connected to voltage rail (capacitors act like shorts initially)

Simply place resistor in series with whatever you want to protect to limit inrush current (an NTC thermistor better than static resistor) NTC thermistors start with high resistance, then lower resistance as they

heat up NOTE: by definition, motors are inductive loads!

PWM and Motor Control Basics We use PWM to control motors because the duty cycle percentages

matches nicely with motor speed Also, digital nature of PWM signal makes it more efficient to control

current to motor than linear methods (compare to a variable resistor) Motors are large inductive loads, so we often use diodes to protect

against voltage spikes Our PWM signal will connect to MOSFET gates, which act as heavy

duty switches to vary current to the motor Due to their lower RDS(ON) engineers tend to use mostly N-Channel

MOSFETS DC motor control often done using the Half-Bridge (H-Bridge)

topology

The H-Bridge Driver

Q1-Q4 are transistors (often MOSFETs) D1-D4 are Flyback diodes, often Schottky type Q1 and Q3 are high side FETs and Q2,Q4 are low

side The high side of bridge connected to power

supply, low side tied to ground Different combinations of opening/closing FETs Q1-

Q4 allow for different functionalities

Basic H-Bridge Application: Forward/Backward Closing Q1 and Q4 will power motor in one direction, and closing Q2 and

Q3 will reverse polarity and cause motor to run in other direction

Note that this connection will run motor at full speed. To get anything less, we need PWM control

The Danger of “Shoot-Through” Notice that you can short circuit power supply if you turn on the

wrong switches MOSFET shoot-through Considering the high currents you work with for motors, this is BAD

(fires)

Thus we’re left with only a few switch combinations that are safe

H-Bridge Component Selection Use desired features (high curremt, efficiency, etc.) of

load operation to guide component selection MOSFETs have RDS(ON) parameter when operated as

switches We want lower resistance so less power loss (less heat)

N-Channel RDS(ON) < P-Channel RDS(ON) How do you bias NMOS to turn on? Gate-to-Source must

be positively biased This makes it difficult to properly bias the high side FETs! Once you close high side switch, source and drain will be

at same level, that of power supply Need to maintain higher gate voltage somehow Charge Pump is a DC-DC converter using capacitors that

is often used for this application

Brushed vs Brushless DC MotorsBDC Stator, rotor, commutator, brushes

Current reversed mechanically with commutator

Easier to use, cheaper, good for short-lived and light applications

BLDC Stator, rotor, commutator control

circuit

Current through coils controlled with a circuit

Difficult to control, more efficient, better torque curve, long-lasting

BLDC Control Diagram

Gate Driver Block Diagram

Simplified Gate Driver Schematic