H-bridge

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H-bridge

S a x i o n U n i v e r s i t y o f A p p l i e d

S c i e n c e

3 / 1 3 / 2 0 1 1

G.M. Pasca and S. Petravicius

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Saxion University of Applied Sciences

Electrical Engineering course Third Year

A report about the work made in carrousel project “H-bridge”

H-bridge

Made by: Coordinators of project: Students: G.M. Pasca M. Kessner S. Petravicius R.A Josepa

March 2011

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Content

1) Introduction 4

2) Theory 5 2.1 Pulse width modulation 5 2.2 H-bridge 6

3) Components 8 3.1 PWM 8 3.2 Inverted signal 9 3.3 Optocoupler 10 3.4 Mosfet 11 3.5 Dead time control 11 3.6 Isolated DC/DC Converter 12 4) Circuit diagrams 13 5) Conclusion 16

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1) Introduction

The main aim of this experiment is to control a motor with a dc voltage signal by manipulating the duty cycle of the PWM signal in order to turn it left or right.The eficency of the circuit should be high.

This has to be done with a half H-bridge. The half H-bridge block diagram, divied in subsystems is presented below (Fig 1.1).

Another aim of the h-bridge carrousel project was to develop a printed circuit

board(PCB) for the “Driver” part, including the isolation.

Figure 1.1 H-bridge block diagram

PWM

Generator

Driver /

Isolation Bridge Load

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2) Theory

In this part of the report it will be explained the theory behind the H-bridge

design and control.

2.1) Pulse-width modulation (PWM)

Is a commonly used technique for controlling power to inertial electrical devices,

made practical by modern electronic power switches.

How PWM modulation works:

Pulse-width modulation (PWM), as it applies to motor control, is a way of delivering

energy through a succession of pulses rather than a continuously varying (analog)

signal. By increasing or decreasing pulse width, the controller regulates energy flow to

the motor shaft. The motor’s own inductance acts like a filter, storing energy during

the “on” cycle while releasing it at a rate corresponding to the input or reference

signal. In other words, energy flows into the load not so much the switching

frequency, but at the reference frequency. PWM is somewhat like pushing a

playground-style merry-go-round. The energy of each push is stored in the inertia of

the heavy platform, which accelerates gradually with harder, more frequent, or longer-

lasting pushes. The riders receive the kinetic energy in a very different manner than

how it’s applied.

A simple comparator (Fig 2.1) with a sawtooth carrier can turn a sinusoidal

command into a pulse-width modulated output (Fig 2.2). In general, the larger the

commands signal, the wider the pulse.

Output stays high as long as the command is greater than the carrier

Figure 2.1 PWM Generator

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The output of a PWM amplifier is either zero or tied to the supply voltage, holding

losses to a minimum. As the duty cycle (Fig 2.3) changes to deliver more or less

power, efficiency remains essentially constant.

2.2) H-bridge

An H bridge (Fig 2.4) is an electronic circuit which enables a voltage to be

applied across a load in either direction. These circuits are often used in robotics and

other applications to allow DC motors to run forwards and backwards. H bridges are

available as integrated circuits, or can be built from discrete components.

Figure 2.2 Pulse-width modulated output

Figure 2.3 PWM Control and duty cycle

Figure 2.4 H-bridge circuit

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The H-bridge arrangement is generally used to reverse the polarity of the

motor, but can also be used to 'brake' the motor, where the motor comes to a sudden

stop, as the motor's terminals are shorted, or to let the motor 'free run' to a stop, as

the motor is effectively disconnected from the circuit. The following table (Fig 2.5)

summarizes operation, with S1-S4 corresponding to the diagram above.

S1 S2 S3 S4 Result

1 0 0 1 Motor moves right

0 1 1 0 Motor moves left

0 0 0 0 Motor free runs

0 1 0 1 Motor brakes

1 0 1 0 Motor brakes

Figure 2.5 Switches operation options

Half H-bridge:

In half-H bridge switches S3 and S4 are replaced by two voltage sources. The

switches S1(Q1) and S2(Q2) are controlled by the PWM that we initially made and an

inverted PWM signal, here we offer the PWM signal to the mosfets which will be 50%

duty cycle the average value of the voltage across load 0 volts are. So this will stop

the load. If you have more than 50% duty cycle then the voltage across the load is

positive. Then the motor runs in certain direction. If the duty cycle it’s under 50% the

voltage across the load is negative and the motor turns the other direction. The duty

cycle is controlled via PWM and is used to adjust the motor clockwise and counter

clockwise.

Figure 2.6 Half H-bridge circuit

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3) Components

In this chapter there will be analysed the main components used in our circuit.

The main componets used are:

a) PWM power control chip: SG3524N

b) Inverter

c) Optocoupler

d) Dead time control

e) Mosftes

f) Isolated DC/DC Converter

3.1) PWM module : SG3524N

The SG2524 (Fig 3.1) incorporate all the functions required in the construction

of a regulating power supply, inverter, or switching regulator on a single chip. We

use the SG2524 to control the PWM signal. The frequency of the PWM can be

controlled with the help of a resistor (Rt) and capacitor(Ct) (Figure 3.2).

Figure 3.1 Functional Diagram SG3524N PWM controller

Figure 3.2 SG3524N PWM controller

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The formula used to calculate the frequency is:

Practical values of CT fall between 0.001 μF and 0.1 μF. Practical values of RT

fall between 1.8 kΩ and 100 kΩ. This results in a frequency range typically from

130 Hz to 722 kHz.

The PWM control circuit is present in figure (Fig 3.3) below:

3.2) Inverted signal

In order to control the H-bridge we need two PWM signals one inverted and

one non-inverted. For that we use a BC547 NPN as common-emmiter.

Inverters (NOT gates) are available on

logic ICs but if you only require one inverter it is

usually better to use this circuit(fig 3.40). The

output signal (voltage) is the inverse of the input

signal:

When the input is high (+Vs) the output is

low (0V).

When the input is low (0V) the output is

high (+Vs).

Figure 3.3 SG3524N PWM control circuit

Figure 3.4 SG3524N PWM control

circuit

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Any general purpose low power NPN transistor can be used. For general use RB =

10kohm and RC = 1kohm, then the inverter output can be connected to a device with

an input impedance (resistance) of at least 10kohm such as a logic IC or a 555 timer.

3.3) Optocoupler:6N135

These diode-transistor optocouplers (Fig 3.5) use an insulating layer between a LED and an integrated photo detector to provide electrical insulation between input and output. Separate connections for the photodiode bias and output-transistor collector increase the speed up to a hundred times that of a conventional phototransistor coupler by reducing the base- collector capacitance. For this project, we chose two optocouplers (Fig 3.6) one pin is for the normal signal and the other for the inverted signal. At the input Number two on the pin of the IC is placed a 750 Ω resistor in order to protect the diode against high current. At the output of the IC is placed a 4kΩ resistor for a better output signal at high frequency’s The output of the optocoupler is going at the gate of the mosfet.

Figure 3.5 Functional Diagram 6N135 Optocoupler

Figure 3.6 Circuit schematic

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3.4) Mosfet : IRF3205

In our circuit we use two IRF3205 Power MOSFETs (Fig 3.7) from International Rectifier, as a switches. These are used to control the half H-bridge. In order to protect the mosfet a 10 Ω resistance is used at the input gate of the mosfet. Advantages:

Ultra Low On-Resistance

175°C Operating Temperature

Fast Switching

3.5) Dead time control

The dead time is necessary to prevent the short circuit of the power supply in pulse width modulated (PWM) voltage inverters, this results in output voltage deviations. Although individually small, when accumulated over an operating cycle, the voltage deviations are sufficient to distort the applied PWM signal. The state of the art in motor control provides an adjustable voltage and frequency to the terminals of the motor through a pulse width modulated (PWM) voltage source inverter drive. As the power devices change switching states, a dead time exists. In order to control the dead time we implemented the following circuit (Fig 3.8):

Figure 3.7 Functional diagram

Figure 3.8 Dead time control circuit

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3.6) Isolated DC/DC Converter

Two Isolated DC/DC converters (NKE1215sc and NKE1212SC)(Fig3.9) are used in order to provide an high voltage at the mosfet gate. This voltage is used to for turning on and of the switches. The DC/DC isolator is used to protect the “driver” circuit against high voltage coming from the H-bridge load(E.G. Motor).

Figure 3.9 DC/DC converter

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4) Circuit diagrams

a) Final circuit

Figure 4.1 Complete circuit diagram

Figure 4.2 Final Circuit on bread board

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b) Driver Circuit

Figure 4.4 explains how the Driver board is connected with the rest of H-bridge components.

At PW input is connected the output of the PWM generator. There is no need for an inverter. The inverter is already implemented on the “Driver board” Its necessary to beimplement an Dead time control circuit before the PW input. It can be used any pin from PW input, because the pins are connected together.

Upper output it will be connected at the GATE pin of the MOSFET from the upper side, GND it will be connected at the SOURCE pin of the upper MOSFET.

LOWER output it will be connected at the GATE pin of the MOSFET from the lower side, GND it will be connected at the SOURCE pin of the lower MOSFET.

Figure 4.3 Schematic of the driver circuit

Figure 4.4 Driver Board Input and output

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c) Driver circuit in ultiboard

Figure 4.5 Schematic of the driver circuit in ultiboard

Figure 4.6 3d Schematic of the driver circuit front side

Figure 4.7 3d Schematic of the driver circuit in backside

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5) Conclusion

H-bridge is a nice subject of Carousel. Over a week, we gained more insight about the

performance which is how to control the motor of H-bridge. Most of parts are worked properly.

Beside that the theory is easy to understand but we still have some small problems during the

simulation. Our Mosfets got over heated in normal operation. That means there is a partial

short circuit between mosftes. This is due to the fact that we implement the dead time control

just for the non-inverted signal. To solve the problem is needed to be implemented a better

dead time control. But in reset everything is working proper.