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 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 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

BLDC Control Diagram

Gate Driver Block Diagram

Simplified Gate Driver Schematic