Post on 22-Oct-2014
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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.