Transcript Week 7: Motor Control Basics

SMV ELECTRIC TUTORIALS
Aditya Kuroodi
2016
Relevant 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 gatesource 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 Motors
BDC
BLDC

Stator, rotor, commutator, brushes

Stator, rotor, commutator control
circuit

Current reversed mechanically
with commutator

Current through coils controlled
with a circuit

Easier to use, cheaper, good for
short-lived and light applications

Difficult to control, more efficient,
better torque curve, long-lasting
BLDC Control Diagram
Gate Driver Block Diagram
Simplified Gate Driver Schematic