1. SMV Electric Tutorials
Aditya Kuroodi
2016
Relevant Course(s): EE115A. EE115B

2. Motors & PWM

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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!

9. 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

10. 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

11. 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

12. 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

13. 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

14. Brushed vs Brushless DC Motors
BDC
BLDC
Stator, rotor, commutator, brushes
Current reversed mechanically with commutator
Easier to use, cheaper, good for short-lived and light applications
Stator, rotor, commutator control circuit
Current through coils controlled with a circuit
Difficult to control, more efficient, better torque curve, long-lasting

15. BLDC Control Diagram

16. Gate Driver Block Diagram

17. Simplified Gate Driver Schematic