NATCAR SUMMER CLASS II 6 July 2004 Chung Hsieh

Topic of Discussion Power Supply Motor Drive Circuits Braking

Goals Identify general characteristic of voltage regulators Linear Switching Understand the operation of I and H bridge motor circuit

Power Supply Why do we need power supply? A need to step up/down DC voltage Need for constant DC voltage For any IC and circuits in general Need Noise free voltage

Parts of a Power Supply Voltage Regulator Step up or Step down Linear Voltage Regulator Switching Voltage Regulator Output Filter Suppress any noise from the Regulator

Some Voltage Regulator Terms Dropout voltage Voltage above desired output voltage in which the voltage regulator stops giving reliable output. Ground Pin Current Amount of current regulator sources, even in low load condition

Linear Regulator V D = Dropout Voltage I G = Ground Pin Current I L = Load Current

Linear Regulator The best choice for a specific application can be determined by evaluating the requirements such as: Maximum Load Current Type of Input Voltage Source (Battery or AC) Output Voltage Precision (Tolerance) Quiescent (Idling) Current Special Features (Shutdown Pin, Error Flag, etc.)

Switching Regulators Use AC induction to control Voltages. Input DC voltages is converted into AC square wave. The duty cycle and width of the square wave determines the output DC voltage. The duty cycle is controlled by pulse width modulation

Quick Lesson on PWM Take a square wave and vary the time it is in the “high” state

Back to Switching Regulator The AC signal is stepped down/up using transformers. Then Goes thru a rectifier and output a DC output. Switching Regulator are the only way you can step up a DC voltage, Linear regulator only steps down. But stepping up voltages comes at a price.

Types Of Switching Regulator Buck: used the reduce a DC voltage to a lower DC voltage. Boost: provides an output voltage that is higher than the input. Buck-Boost (invert): an output voltage is generated opposite in polarity to the input. Flyback: an output voltage that is less than or greater than the input can be generated, as well as multiple outputs. Push-Pull: A two-transistor converter that is especially efficient at low input voltages. Half-Bridge: A two-transistor converter used in many off-line applications. Full-Bridge: A four-transistor converter (usually used in off-line designs) that can generate the highest output power of all the types listed.

What Requirement Do We Have? Load Current Sum up all the current needed to drive your car, excluding the motor itself. Or decide on a “rational” upper bound on the amount of current needed. Input Voltage Source That’s easy, battery, 7.2V Ni-Cad Output Depends on your design of the car, typically 3.3V to 6V The others are really not as important in our design.

BREAK! Back in 10 minutes More Info on Voltage Regulators can be found at:

Motor Control The most basic unidirectional motor control is the I-bridge, sometimes called a half-bridge.

Half-Bridge The Transistor Q1 basically acts like an on/off switch. When V GS > V threshold (n-type) currents starts flowing across the MOS, and the motor as well. Diode D 1 is back EMF protection for other parts of the car. Diodes D 3 and D4 are reversed current protection for Q1. Diode D 2 is the reversed current protection for the microcontroller (probably not necessary, but placed just in case) R 2 is a pull down resistor for the gate of the MOS (without it, the MOS has a potential to run uncontrolled) L 1 and R 1 together is the model of the motor.

Full Bridge The half-bridge is simpler in design and can source a large current for the motor, but it is only unidirectional. The Full-Bridge (or H-Bridge) has 4 times as many transistors, but it capable of spinning the motor is both directions.

H-Bridge Structure This simplified model does not include all the component necessary for high-current operation, nor does it have the proper type of transistors at Q 1 and Q 2.

H-Bridge Operation (forward) Turning Q 1 and Q 4 on at the same time allows current to flow in the direction of the red line.

H-Bridge Operation (reversed) Same as forward operation, except Q 3 and Q 2 are on, thus the current takes a different path thru the motor, Spinning it backward.

Control of the H-Bridge Requires 4 outputs from the Microcontroller. Two on/off states for control of Q 3 and Q 4. Two PWM for control of Q 1 and Q 2. Also must realize the explanation given in the previous slides does not include all circuit elements necessary to bias, and safe guard the circuit from events such as over current and noise.

Other Methods for Braking Mechanical Breaking

Other Methods for Braking Using a small motor to drive the wheel in opposite direction. Can use two simple half-bridge and two PWM to control the Large drive motor and small break motor.

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