case robotics group summer internship report

CASE Robotics Group Project H-Bridge

CRG Summer Internship Program13 1

Abstract

Motors have been integral part of machines or any mechanical mechanism which is assisting

some kind of task. They have been run using some kind of electrical circuitry for their

movement and rotation is normally based on electromagnetic induction and coil mechanism

which could be controlled by an electrical circuit that is called Motor Driving Circuit. This

internship report is covering all the important aspects of design and fabrication of an intelligent

DC motor driving circuit that is known as H-Bridge. Report will discuss the basic working of H-

Bridge and the conceptual understanding of the working of components used in it. Then it talks

about design of a Smart H-Bridge circuit which has number of protections along-with a

microcontroller embed in it which assists all smart functions of this circuit. From design

procedures we move towards the fabrication of this circuit and in the end we have a product

which can assist DC motor speed and direction control with many nice features like

temperature, high current and voltage sensing.

Mirza Qutab Baig



Table of Contents

1. Introduction ------------------------------------------------------------------------------------ 3

2. H-Bridge Basics -------------------------------------------------------------------------------- 4

3. H-Bridge Types -------------------------------------------------------------------------------- 11

4. Project Requirements ------------------------------------------------------------------------ 18

5. Design Procedure ----------------------------------------------------------------------------- 19

6. On Board Microcontroller & Protections ------------------------------------------------- 20

7. Final Circuit ------------------------------------------------------------------------------------- 22

8. Conclusion -------------------------------------------------------------------------------------- 23



Introduction

This report discusses the design and implementation of a Smart Microcontroller Based H-Bridge

that is will be incorporated in control of DC Motor with high torque and RPM which would

consequently draw high current. The product will be comprise of many safety and monitoring

features which are High current protection, temperature sensors and Auto Shutdown Feature.

First we will go through very basic and integral component of H-Bridge that are transistors,

which will be taken through detailed literature review and then they will be selected on the

basis of low threshold, high current handling capacity and low heating effects. Then we will go

through several types of H-Bridges working techniques that are available and from those

techniques we will come up with most suitable technique for our product. Then we will be

coming to design stages. After designing and complete testing on simulation, we will come to

fabrication part and at the end we will obtain a product which will assist DC motor speed and

direction control along-with safety and monitoring features which are assisted by on board

microcontroller.



H-Bridge Basics:

Introduction to Transistors:

The transistor is a three layer semiconductor device consisting of either two n- and one p-type

layers of material or two p- and one n-type layers of material. The former is called npn

transistor while the latter is called pnp transistor.

Types of transistors with DC biasing:

(a) pnp; (b) npn.

We will find that the DC biasing is necessary to establish the proper region of operation for ac

amplification. The emitter layer is heavily doped, the base lightly doped and the collector only

lightly doped. The abbreviation bipolar transistor is often applied to this three terminal device.

The term bipolar reflects that holes and electrons participate in the injection process into

oppositely polarized material. If only one carrier is applied, it is called unipolar device.

Transistors Operation:

The basic operation of transistor is described using pnp transistor. The operation of npn

transistor is exactly the same if the roles played by the electron and holes are interchanged. In

the fig pnp transistor has been redrawn without base to collector bias. The depletion region has

reduced in width due to applied bias, resulting in a heavy flow of majority charge carriers from

p to n type material.



Lets us now remove the base to emitter bias in the next step. Consider that this is situation of

reverse biased diode. We know that flow of majority charge carriers is zero, resulting in a

minority charge carrier flow.

So one p-n junction of transistor is forward bias and one the other is reverse biased.

In the fig below both biasing potentials have applied to a pnp transistor, with resulting majority

and minority carrier flow indicated. As shown in fig a large number of majority charge carriers

diffuse across forward bias pn junction into n type material. Now we have to see that whether

all the majority charge carriers contribute to base current or pass directly into p type material.

We know from construction that n type is very thin and very low conductivity, a very small

number of these carriers will take this path of high resistance to the base terminal. The

magnitude of base current is order of microamperes as compared to milliamperes for the

emitter and collector currents. A large number of majority charge carriers diffuse into p

junction connected to the collector terminal. The reason for the relative ease for which majority

charge carriers can cross reverse bias junction is easily understood if we consider that for the



reverse biased diode the injected majority charge carriers will appear as minority charge

carriers in the n-type material. In other words, there has been injection of minority charge

carriers into n-type base region material. Combining this with the fact that all the minority

charge carriers in the depletion region will cross reverse bias junction of a diode accounts for

the flow indicated.

Applying Kirchoffs current law to the transistor in above fig as if it were a single node, we

obtain.

IE=IC+IB

The collector current, however is comprised of two components the majority and minority

carriers. The minority current component is called the leakage current and is given the symbol

Ico (Ic current with emitter terminal open). The collector current, therefore, is determined in

total by:

Ic=Ic majority + Ico minority

BJT as a Switch:

The basic element of logic circuits is the transistor switch. A schematic of such a switch is

shown. When the switch is open, Ic=0 and Vo=Vcc. When the switch is closed, Vo=0 and

Ic=Vcc/Rc.

In an electronic circuit, mechanical switches are not used. The switching action is performed by

a transistor with an input voltage switching the circuit. When Vi=0, BJT will be in cut-off, Ic=0,



and Vo=Vcc (open switch). When Vi is in high state, BJT can be in saturation with Vo=VCE=VSAT

=0.2V and Ic=(Vcc-VSAT)/Rc (closed switch). When Rc is replaced with a load, this circuit can

switch a load ON/OFF.

Junction Field Effect Transistors:

A transistor is a linear semiconductor device that controls current with the application of a

lower-power electrical signal. Transistors may be roughly grouped into two major

divisions: bipolar and field-effect. In the last section we studied bipolar transistors, which utilize

a small current to control a large current. In this section, we'll introduce the general concept of

the field-effect transistor -- a device utilizing a small voltage to control current -- and then focus

on one particular type: the junction field-effect transistor.

All field-effect transistors are unipolar rather than bipolar devices. That is, the main current

through them is comprised either of electrons through an N-type semiconductor or holes

through a P-type semiconductor. This becomes more evident when a physical diagram of the

device is seen:



In a junction field-effect transistor, or JFET, the controlled current passes from source to drain,

or from drain to source as the case may be. The controlling voltage is applied between the gate

and source. Note how the current does not have to cross through a PN junction on its way

between source and drain: the path (called a channel) is an uninterrupted block of

semiconductor material. In the image just shown, this channel is an N-type semiconductor. P-

type channel JFETs are also manufactured:

With no voltage applied between gate and source, the channel is a wide-open path for

electrons to flow. However, if a voltage is applied between gate and source of such polarity that

it reverse-biases the PN junction, the flow between source and drain connections becomes

limited, or regulated, just as it was for bipolar transistors with a set amount of base current.

Maximum gate-source voltage "pinches off" all current through source and drain, thus forcing

the JFET into cutoff mode. This behavior is due to the depletion region of the PN junction

expanding under the influence of a reverse-bias voltage, eventually occupying the entire width

of the channel if the voltage is great enough. This action may be likened to reducing the flow of

a liquid through a flexible hose by squeezing it: with enough force, the hose will be constricted

enough to completely block the flow.



Note how this operational behavior is exactly opposite of the bipolar junction transistor. Bipolar

transistors are normally-off devices: no current through the base, no current through the

collector or the emitter. JFETs, on the other hand, are normally-on devices: no voltage applied

to the gate allows maximum current through the source and drain. Also take note that the

amount of current allowed through a JFET is determined by a voltage signal rather than a

current signal as with bipolar transistors. In fact, with the gate-source PN junction reverse-

biased, there should be nearly zero current through the gate connection. For this reason, we

classify the JFET as a voltage-controlled device, and the bipolar transistor as a current-

controlled device.

Metal Oxide Semiconductor Field Effect Transistor:

A Metal Oxide Semiconductor Field Effect Transistor is a transistor used for amplifying or

switching electronic signals. The body of a MOSFET is usually connected to the source terminal

which makes it a three-terminal device similar to other Field Effect Transistors (FET). Field effect

transistors form a large family of switchable devices. Current flowing within field effect

transistors between the main electrodes (source and drain) is controlled by a voltage at the

gate electrode. The gate voltage opens or closes a conducting channel between source and

drain.

In the transferring current from one point to the other, only one type of charge carrier is

involved. For example, for N-channel MOSFETs, electrons are the only charge carriers involved

in current flow. So MOSFETs are unipolar components. Since essentially only a gate voltage is

a MOSFET can therefore be controlled without use of very little power. Both current and

voltage are required in order to regulate a bipolar transistor, hence power is required for

control.

MOSFETs used for low reverse voltages have excellent transmission properties. For high reverse

voltages, the transmission properties characterized by the drain source on resistance

RDSon worsen since the thickness and specific resistance of the silicon must be increased. Just

like for all other unipolar components, MOSFETs are also characterized by low switching losses.

MOSFETs as a Switch:

A MOSFET may be thought of as a variable resistor whose Drain-Source resistance (typically

Rds) is a function of the voltage difference on the Gate-Source pins. If there is no potential

difference between the Gate-Source, then the Drain-Source resistance is very high and may be

thought of as an open switch so no current may flow through the Drain-Source pins. When

there is a large Gate-Source potential difference, the Drain-Source resistance is very low and

may be thought of as a closed switch current may flow through the Drain-Source pins.



Drain-Source resistance:

Ideally we want Drain-Source resistance to be very high when no current is flowing, and very

low when current is flowing. The main issue using MOSFETs with micro controllers is that the

MOSFET may need 10-15 Gate-Source potential difference to get near its lowest Drain-Source

resistance, but the microcontroller may run on 5v or 3.3v. Some sort of MOSFET driver is

required.

Gate-Source capacitance:

There is also a capacitance on the Gate-Source pins which prevents the MOSFET from switching

states quickly. In order to quickly change voltage on internal capacitance, the MOSFET driver

needs to be high current. It needs to actively charge (source) and discharge (sink) the capacitor

too (for N channel).



H-Bridge Types:

1. Unipolar Single Quadrant Drive:

Which PWM technique is best for our motor control application?

It is not possible to have a meaningful discussion about different PWM techniques without first

understanding how energy transfer occurs in a motor. When studying energy transfer, we

typically refer to a two dimensional speed-torque diagram as shown below, where speed is the

x-axis, and torque is the y-axis.

If we multiply speed times torque, we get power. So area in the above diagram corresponds to

power. The blue areas are regions where the motor power is positive (i.e., the motor is

converting electrical power into mechanical power). But the red areas on the graph indicate

regions where the motor power is negative (i.e., the motor is converting mechanical power into

electrical power). This is where we can run into problems if we are not careful. If the motor is

generating electrical power, you need to have some place for that power to go. If your

application is an electric vehicle coasting down a hill, the generated electrical energy can be put

to good use by charging the batteries. But if you have an electric drive which needs to stop its

load abruptly, the resulting generator action can destroy the drive if there is no place to put the

energy.

Some PWM topologies will inherently prevent regeneration of energy back into your electrical

supply. In fact, some will only allow the motor to spin in one direction and generate torque in

that same direction. These drives as you might expect are called single-quadrant drives and



operate either in quadrant 1 or quadrant 3. A typical example of such a drive is shown below,

where a PWM signal is applied to a single transistor. The motor can only spin in one direction

and generate torque in that same direction. If you try to decelerate the motor (i.e., generate

negative braking torque) by lowering the duty cycle, the motor will simply coast down slowly.

This is because there is no way with this configuration to create negative current in the motor.

All of the kinetic energy in the spinning load is eventually dissipated as heat in the loads

friction, and none of it is converted back into electricity.

However, if we want the motor to go forward AND backward, we need to provide a way to

drive the motor current in the negative direction. We could achieve this with a separate

negative power supply, but this is usually cost prohibitive. A more common approach is to put

the motor in an H-Bridge.

In H-Bridge we have four transistors, each capable of being switched ON and OFF

independently. This allows for some rather interesting possibilities since we now have a

configuration capable of operating in all four quadrants. How we switch the transistors with

respect to each other will determine which power quadrants the motor will operate in.

2. Unipolar 2-Quadrant Drive:

We examined the single-quadrant PWM technique, which is a good fit for extremely cost

sensitive motor control applications where you want to control the motors speed by varying



the duty-cycle of a PWM signal. But the motor can only spin in one direction, and generate

torque in that same direction. We also introduced the H-Bridge as a springboard to

investigate other PWM topologies. Lets take a look at how to build a bi-directional speed

control power stage by using an H-Bridge. In particular, we will construct a 2-Quadrant

Drive since it can produce forward motion with positive torque (quadrant 1), or reverse motion

with negative torque (quadrant 3).

For Unipolar PWM operation in quadrant 1, Q1 is turned ON continuously while we apply a

PWM signal to Q4. When Q4 is switched ON, a current path is created from Vbus, through Q1,

through the motor, through Q4, and returning through ground. At the end of this PWM state,

Q4 is switched OFF. Since the motor winding has inductance, it will fight to keep the motor

current flowing in the same direction. An inductor protects its current, it effectively generate

whatever voltage is necessary to keep my current flowing. As a result, the inductor forces the

back-body diode of Q3 to conduct. But since Q1 is always ON, the motor current will return

through Q1, not the DC supply. When you think about it, you realize that since Q1 is ON

continuously, this circuit behaves exactly like the single quadrant drive discussed earlier with

one exceptionif we want the motor to spin in the other direction, simply turn Q3 ON all the

time and PWM Q2 instead. This results in quadrant 3 operation where the motor is running in

reverse, and generating negative torque.

Its interesting to note that in both quadrant one and quadrant three operation, the bus current

is either positive or zero, regardless of which direction the current is flowing in the motor! In

other words, this PWM technique cannot regenerate energy. The reason for this is because the

inductive flyback current is trapped in the top half of the H-Bridge, and never flows back into

the DC bus. This can either be an advantage or a disadvantage, depending on your application.

If we never have to worry about regenerated energy, then we dont have to add expense to our

design to deal with it. On the other hand, if you want to recover load energy, then this PWM

technique is not a good choice for you.



Another advantage of this technique is that it only requires one PWM signal at any given time.

This means you can potentially control more motors from one processor compared to some of

the other PWM topologies. Also, there is only one transistor that is switching at any given time,

so your switching losses are minimized. Finally, there is only one diode snap event per PWM

cycle (when Q4 turns ON again after the Q3 back-body diode has been conducting). So this

technique generates no more switching noise than the single-quadrant technique we discussed

earlier.

The main disadvantage with this technique is that even though you have four transistors, you

still cant operate in all four quadrants. Its like having a car with no brakes! If you want to slow

down, you have two options; lift your foot off of the accelerator and coast (lower the PWM

duty cycle), or suddenly throw the car in reverse (immediately transition from quadrant one to

quadrant three!). It results in super fast deceleration of the motor; it is usually not a good idea

since the resulting high currents will probably leave pieces of your drive lying all over the lab

bench!



3. Unipolar 4-Quadrant Drive:

So far we have studied two motor drive topologies that result in unipolar PWM voltage

waveforms on the motor, but are incapable of providing any braking for the motor in the event

you want to decelerate quickly. In this blog, lets look at a third unipolar PWM technique

that will provide motor braking by allowing energy regeneration back into your power supply.

We call this Unipolar 4-Quadrant PWMs.

The H-Bridge circuit that we will analyze is shown below:

The most significant change from the previous Unipolar topology is that we are now driving the

top and bottom transistors in a complementary pattern. In other words, whenever a bottom

transistor is turned OFF, the top transistor in the same leg is turned ON, and vice versa. Not

shown in the diagram is an implied dead-time from when one transistor turns OFF until its

complementary transistor turns ON. With most power FETs, this dead-time can be in the area

of about 100 nanoseconds to almost 1 uS. The quickest dead-time I have ever seen is on our

DRV-8312 device, which is 5 nS! Since dead-time causes distortion which is associated with the

current zero-crossings, a small dead-time of 5 nS results in almost NO distortion at all.

Returning to our circuit example above, lets analyze the condition where Fwd/Rev is set to 1

(forward motion). This means that Q1 will be ON continuously while Q3 and Q4 are PWMing in

a complementary fashion. Lets also assume that the PWM duty-cycle is high and the motor

loading is light, which means that the motor speed will also be high and the back-EMF polarity

is such that it is positive on the left side of the motor symbol.

Now lets abruptly lower the PWM duty cycle in an effort to decelerate the motor. With 2-

quadrant PWMs, whenever Q4 turns off, the inductive flyback current is captured in the top

half of the H-Bridge until it is extinguished. Once the flyback current is extinguished, the back-

EMF signal appears across the motor terminals. In this state, the H-Bridge looks like a high



impedance to the motor, and no current flows. But with this 4-quadrant topology, when Q4 is

OFF, Q3 turns ON, and we effectively short the motor terminals together. Since the motor is

spinning forward (back-EMF polarity is positive on the left side of the motor), this eventually

causes current to flow in the clockwise direction in the top half of the H-Bridge.

Now, this next point is important. When Q3 is now switched OFF, and Q4 is switched ON, the

inductor now looks for an alternate path to keep its current flowing in the same direction. And

what is that path? It turns out that the new path of least resistance is to flow in the reverse

direction through Q1, back through the DC power supply AS NEGATIVE CURRENT, and then

return in the reverse direction through Q4. If your DC bus has positive voltage and negative

current, then that means that it has negative power during that instance. This negative bus

current will charge up the bus capacitor to a higher voltage until the inductive flyback is

quenched, or the next switching state is applied.

You should be aware that the presence of negative bus current in and of itself does not imply

that we are regenerating. Momentary values of negative bus current are common, as energy

can slosh back and forth between the motor inductor and the bus capacitor within each PWM

cycle. To determine if regeneration is occurring, we must look at the average value of bus

current. If the average value is negative, then we have a long-term transference of energy from

the load back to your DC supply. If you follow the energy, you have kinetic energy (1/2 mass x

speed2) being converted to magnetic energy (1/2 L i2), and finally being stored in the bus

capacitor as 1/2 C v2.



4. Bipolar 4-Quadrant Drive:

Up to now we have investigated three different PWM techniques. Some could regenerate

energy back into the DC power supply, and some couldnt. But they all had one characteristic in

common: unipolar voltage waveforms. In other words, for any given PWM period, the motor

voltage waveform transitions between Vbus and ground or Vbus and ground. Now we will

investigate the claims of the bipolar PWM technique. For every PWM period, the motor

voltage waveform transitions between Vbus and Vbus, creating motor voltage waveform

amplitude that is twice that of unipolar PWMs. To do this, we will wire the H-Bridge up as

shown below:

How about the fact that there is no longer a forward/reverse signal? With bipolar PWMs,

forward and reverse information is encoded in the PWM signal itself. Assuming no load, PWM

values over 50 percent duty cycle because forward motion and values below 50 percent duty

cycle result in reverse motion.

The bipolar PWM technique is inherently a 4-quadrant technique. As long as the average

applied motor voltage is of the same polarity as the motors back-EMF voltage, and it is greater

in amplitude than the back-EMF, then the motor will operate in motoring mode. However, if

the average applied motor voltage is of the same polarity as the back-EMF, but its amplitude is

less than the back-EMF, then the motor will operate in generating mode.

Another advantage of the bipolar PWM technique is that it only requires one PWM signal from

your processor (two if the dead-time is generated within the PWM module itself). But perhaps

the biggest advantage of bipolar PWMs is the fact that motor current is always flowing through

the single shunt resistor, regardless of which state the PWM signal is in.



Project Requirements H Bridge Specifications:

We are supposed to design an H-Bridge with the following specifications observing all the IEEE

and IET standards.

1. 60V, 60A Dual H-Bridge

2. Operates on higher frequencies upto 32KHz

3. Unipolar H-Bridge

4. On board Microcontroller Master and Slave Configuration

5. Temperature Sensing

6. Current Sensing

7. Auto shutdown Feature

8. Back EMF Protection

9. Serial port data transmission

10. Self-Diagnostic Feature



Design Procedure:

MOSFET Selection:

MOSFET selection is very first thing that we are supposed to select in our design. We have many

options regarding selection of MOSFETs.

Logic Level FETs:

1. They have less threshold voltage than normal FETs.

2. Fast Switching.

3. Less On-State Resistance.

4. Suitable for high frequency and high power loads.

5. ESD Protection upto 5000V.

6. Elimination of Gate Driver IC

Power MOSFETs:

For high power load, On-state resistance should be low.

Motor control is one of typical applications of Power MOSFETs. They have same features like

Logic Level FETs. But there is one advantage of using power MOSFETs for high power bridges

that is its RS(on) is low than logic level FETs which allows maximum voltage drop across the

load. Switching behavior is most easily modeled and predicted by recognizing that the power

MOSFET is charge controlled. Hence, power MOSFETs are suitable for our design.

Hybrid/N-Channel/P-Channel:

Hybrid / N-Channel / P-Channel - Which one should we use?

Hybrid: N-Channel FETs + P-Channel FETs

Hybrid is used when we are getting +5V from MCU (Micro-controller).

In Hybrid Efficiency is directly proportional to load.

N-Channel FETs: For high loads which require high power.

On state resistance of N-Channel FETs is low so most of power is dissipated at load.

P-Channel FETs: Its on state resistance is high. So it is suitable for loads which require

low power.



On board Microcontroller & Protections:

The designed H-Bridge has been incorporated with many protections in order to avoid any kind

of damage to the bridge in case of harsh scenarios. All the protections are powered and

controlled by an on board Microcontroller.

Current Sensing

H-bridge has a current sensing circuit in which uses an op-amp to sense current. As the current

varies the potential on the bridge varies and when it crosses the threshold of op-amp, it

switches and sends a signal through which we determine that how much amount of current is

flowing in the circuit.

Excess Voltage Protection If voltage exceeds 60V or lowers below a particular limit (45V in the code), the microcontroller should

shut down the H-bridge to avoid any harmful consequences. For this, we have used a voltage divider

circuits which divides 60V such that across one resistor 0-5V is dropped which means that if the input is

60 volts, output voltage of the resistor is 5V.This is done because ADC of the microcontroller works in

the range of 0-5V.If the voltage becomes greater than 60 V, the ADC outputs the maximum value and

thus, microcontrollers shuts down the circuitry.

Temp Sensing

PIC 16F1946 has built-in Temperature Sensing Circuitry which measures the temperature of

controller and sends the signal to ADC which automatically converts this analog signal and

stores it in its register internally. We can check the value of register to know the temperature

and if temperature exceeds the safe limit then we shut the H-bridge. As this tells the



temperature of the microcontroller which is not desired in our case, we have used an external

temperature sensor LM35 which is connected to the MOSFETs so that in case of excessive

heating the controller is notified in time.

Auto Shut Down Feature The auto shut down feature has been incorporated in case of any undesirable situation. For example, if

our motors stall current or if our temperature sensor measures an increase in temperature that is

beyond limit or any low voltage is monitored, our H-Bridge automatically shuts off completely in order

to avoid any serious damage to our driver circuit or embedded system.

Back EMF Protection

For this purpose we have used four free-wheeling diodes to minimize the effect of back EMF in

our circuit. Whenever we switch the direction of current, a high voltage is induced which causes

the air to conduct and an arc is produced which is very dangerous for the circuit. We use a free-

wheeling diode which allows this back EMF to cycle through it and thus minimizing the effect.

Following diagram explains the protection.



Final Circuit

This is the final circuit of H-bridge. From the microcontroller we have a current sensor module through

which the current goes to the two transistors which define the direction of rotation. On top we have an

external current sensor LM35 to sense the external temperature of the circuit. On bottom right corner

we have current sensing circuitry. In between the transistors we have a Motor. We also have four 4

freewheeling diodes for protection. We will apply 60V and use a Buck IC to regulate the voltage up to

the desired value. The Pin Configuration of the Buck IC is as follows.



Conclusion

The project is a very good example of power electronics applications. This H-Bridge has been

used in many other applications. One of the most common applications other than DC motor

drive is Inverter circuit where it is being used with very same concept to generate Sine wave.

We have learnt about latest component available and being used in industry. We have tried to

comply with all the global standards of design and implementation in this project. We have

successfully gone through the proper schedule made for this internship that started from H-

Bridge basics days and going through component selection we made it to the hardware

fabrication stage in a fine manner. The project outcomes clearly depict hard work, dedication,

team effort, implementation of fine theoretical & practical understanding and a very huge

mentorship of CASE ROBOTICS GROUP.

case robotics group summer internship report

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