High freqvency pure sine wav 2

Chapter 4


The ferrite core transformer acts at very high frequency of 25 kHz to 500khz.
They have high magnetic permeability as well as low electrical conductivity, which helps
them to reduce their eddy current losses.
Due to this high frequency, the size of the transformer reduces because

furmula fyp

There are two types of ferrite core transformer

  •  Signal Transformer
  •  Power Transformer

Signal transformer are of very high frequencies and of small size while Power
transformers are of low frequencies and larger in size.
Ferrites are made of ceramic material and they are very delicate and break easily
if handled carelessly.
This ferrite core transformer is used to boost the dc voltage level through a very
fast frequency switching circuit, which is not possible with iron core. The circuit related
to this operation is very complex but by use of it we can be able to get pure sine wave

4.1 Air Gap And Flux Walking In The Ferrite Core

If a small DC signal is introduced in the input signal to a iron core transformer, it
will try to walk the flux into saturation. But due to high winding resistance, a IR drop is
created, which cancel out this DC component and avoid saturation.
In application of a ferrite core with push pull or bridge topology, the DC
component introduced in the positive half cycle is cancel out with the DC component in
the negative half cycle. So, there is no problem of saturation. But at some times it is
obvious that the switching periods of the MOSFETs and their Rds(on) mismatches,
which results in an unmatched DC components in positive and negative half cycle, so as a result a DC component persist. The magnetizing current of small size high frequency
ferrite core transformer is very low; as a result there is only a small IR drop which will
not compensate for the DC component introduced. So, it seems that ferrite core transformer although they are small in size and they operate on very fast frequency, but
they are easily satuarable and flux walking is very smooth.
According to the BH Curve characteristic, it is the slope of the BH curve which decides
that the material is easily satuarable or not. Steep slope mean it is easily satuarable, while
flat slope means it is not easily satuarable. We can also make an observation that the area
under the BH CURVE determine the hysteresis loss. Smaller area means less hysteresis
loss while larger area means small hysteresis loss.

BH curve characteractics fyp

From the above given BH characteristics it is obvious that ferrite core transformer
has less hysteresis loss as compared to iron core transformer. It was also discussed that
ferrite has less eddy current losses also. So it is now clear why to prefer ferrite core
transformer on iron core transformer but there is only one problem that it is saturated
The solution to this saturation problem is to introduce an air gap in ferrite core
transformer. The iron core transformer have inherently powdered iron air gap in them.
But in ferrite core we have to introduce it. The air gap reduces the slope of the
B/H loop, reducing permeability and inductance, and hence increasing the
Magnetizing current in the primary. Remember that magnetizing current flows in the
Primary — even if the secondary is open circuit. Small air gap will raise the magnetizing
current so that the IR drop in the circuit resistances will be able to offset the dc
asymmetry in the drive waveform. But the increased magnetizing current represents
increased energy in the mutual inductance which usually ends up in a clamp, increasing
circuit losses.


Chapter 5



After the charging stage of U.P.S, the next stage is DC to DC conversion stage, in which the 12 volt dc voltage from the terminal of the battery are converted to 360 volt dc voltage, which is then converter to 360 volt peak ac voltage in DC to AC stage. There are different methods used in this stage. Few of them are

  •  Boost Converter
  •  Buck Boost Converter
  •  Cuk Converter
  •  Half Bridge Topology
  •  Full Bridge Topology

The first three types are one switch power converter topologies and these are of low power ratting. The half bridge and full bridge regulators are two switch power converters and they are of high power ratting and these topologies are mostly used in high power ratting U.P.S.

5.1 Half Bridge Topology:

half bridge circuit dc to dc fyp

The push-pull converter drives the high frequency transformer with an AC-voltage, where the negative as well as the positive half swing transfers energy. In half bridge push pull regulator, c1 and c2 divides the voltage Vin equally across T1 and T2, hence the output voltage of the bridge is Vin/2 peak ac. The rectifier circuit at the secondary of the transformer may be of full wave type or bridge type. When high current is requirement, full wave type is used. When high voltage is requirement, bridge type is used.

5.2 Full Bridge Topology:

full bridge dc to dc fypThe voltage V1 at primary side can be +Vin, -Vin or zero. The polarity of V1 depends upon which diagonal transistors are on. If T1 and T4 are on, V1 is +Vin. If T2 and T3 are on, V1 is –Vin. If neither diagonal is on, V1 is zero. At secondary side ac voltage is rectified and smoothed by L and C combination. The output expression is

The turn ratio of the transformer should be greater than the voltage ratio. It is because to avoid different losses and there is also a reason because of the requirement of the introduction of a dead time between switching of the transistors. Hence there is a general criteria that:

The switching waveforms are shown in the figure on the next page. From the waveforms we can see that when T1 and T4 is on, V1 is + Vin, then there is introduced some dead time and at that time V1 is zero. When T2 and T3 are on, V1 is –Vin. V3 is rectified shape of the waveform at the secondary. I1 is the current at the primary side. Where I3 is the rectified continuous output current


timing waveform of full bridge fyp


The switching waveforms are shown in the figure on the next page. From the waveforms we can see that when T1 and T4 is on, V1 is + Vin, then there is introduced some dead time and at that time V1 is zero. When T2 and T3 are on, V1 is –Vin. V3 is rectified shape of the waveform at the secondary. I1 is the current at the primary side. Where I3 is the rectified continuous output current

timeng wavform of full bridge fyp



5.3 Transistor Selection:

The transistor at the primary side of the transformer should be of very high current ratting and its voltage ratting should be greater than 12 volt. The high current rating is required because we are making 700VA U.P.S and at primary the current should be

Iprim = 700/12 = about 50 amp

So we decided to select IRFZ44, whose current ratting is about 50 amp and its voltage ratting is 60 volt. Its Rds(on) is only 0.024ohm and it is readily available in the market.

5.4 PWM IC For The Gating Signals of Transistors:

There are many IC available in the market for PWM generation. We are using SG3524 regulating pulse width modulator in our project.

Pin configurationof SG3524 fyp

5.4.1 Advantages Of The I.C: It has many advantages like:

  •  Complete Pulse-Width Modulation (PWM) Power-Control Circuitry
  •  Uncommitted Outputs for Single-Ended or Push-Pull Applications
  •  Low Standby Current . . . 8 mA Typ
  •  Interchangeable With Industry Standard SG2524 and SG3524
  •  It includes an on-chip regulator, error amplifier, programmable oscillator, pulse-steering flip- flop, two uncommitted pass transistors, a high-gain comparator, and current-limiting and shutdown circuitry.
  •  SG3524 incorporate all the functions required in the construction of a regulating power supply, inverter, or switching regulator on a single chip.
  •  This ic was especially designed for switching regulators of either polarity, transformer-coupled dc-to-dc converters, transformerless voltage doublers, and polarity-converter applications employing fixed-frequency, pulse-width modulation (PWM) techniques.
  •  The pin 9 of the ic allow either single ended or push pull application.

The oscillator controls the frequency of the SG2524 and is programmed by RT and CT.

F= 1.30/(RT*CT) Hertz

Practical values of CT fall between 1 nF and 100 nF. Practical values of RT fall between 1.8K and 100K .This results in a frequency range typically from 130 Hz to 722 kHz.

The outputs may be applied in a push-pull configuration in which their frequency is one-half that of the base oscillator, or paralleled for single-ended applications in which the frequency is equal to that of the oscillator.

The output pulse of the oscillator is used as a blanking pulse at the output. This pulse duration is controlled by the value of CT as shown in Figure. If small values of CT are required, the oscillator output pulse duration can be maintained by applying a shunt capacitance from OSC OUT to ground

CT/Dead time characteristics(Datesheet SG3524) FYP

COMP which is the pin 9,is made available for compensation. Since most output filters introduce one or more additional poles at frequencies below 200 Hz, which is the pole of the uncompensated amplifier, introduction of a zero to cancel one of the output filter poles is desirable. This can be accomplished best with a series RC circuit from COMP to ground in the range of 50 kΩ and 0.001 μF. Other frequencies can be canceled By use of the formula f ≈ 1/RC.

There are a wide variety of output configurations possible when considering the application of the SG2524 as a voltage-regulator control circuit. They can be segregated into three basic categories:

  •  Capacitor-diode-coupled voltage multipliers
  •  Iductor-capacitor-implemented single-ended circuits
  •  Transformer-coupled circuits

We are using transformer coupled circuit application of this I.C

5.5 Gate Driver I.C Selection of MOSFET IRFZ44:

The gate driver IC used to drive the bridge circuit of IRFZ44 is HIP4081.It has an advantage that this single IC can drive the whole bridge and we do not need two separate I.C to drive the two arms of the bridge. It is 80 volt I.C, that can be easily used for our DC to DC stage.

Pin configuration OF Hip 4081

5.5.1 Advantages Of HIP4081

Independently Drives 4 N-Channel FET in Half Bridge or Full Bridge Configurations

  •  Bootstrap Supply Max Voltage to 95VDC
  •  User-Programmable Dead Time
  •  On-Chip Charge-Pump and Bootstrap Upper Bias Supplies
  •  DIS (Disable) Overrides Input Control
  •  Input Logic Thresholds Compatible with 5V to 15V Logic Levels
  •  Very Low Power Consumption.

timeng wavEform OF Hip4081 fyp

Circuit Hip 4081 fyp

5.5.2 Dead Time Adjustment:

The PWM inputs are the output pin 11 and 14 of the SG3524.The HDEL and LDEL are designed to be 250K ohm to introduce a dead time of about 100nf as shown in the below graph.

Dead time v/s HDEL/LDEL Risistence Datesheet Hip4081 fyp

5.5.3 Bootstrap Capacitor Design:

From the application notes of the HIP4081, we studied that the bootstrap capacitor must be greater than the ten times of the gate to source capacitance of the MOSFET used in the bridge.

So, from the datasheet of IRFZ44….

Qgs = 12.3 nC

Vgs(min)= 2volt

Cgs = Qgs/Vgs = 12.3/2 = 6.15 nF

So bootstrap capacitor > 61.5 nF

We used a value of 200 nF.

Bread Board circuit fyp

Input to Ferrite Core Transformer from Hip and bridge circuitry fyp

5.6 B-H Curve Plotting:

To find the flux B which cause the core saturation B-H curve is plotted. In order to plot the B-H curve of the transformer we first made a transformer of 11:110. The output of this transformer is checked by applying signal through the signal generator to theprimary and at secondary of the ferrite core almost 10 times the voltage is observed.

To plot the B-H curve signal from the signal generator is pass through the amplifier circuit and than it is applied at the primary of the transformer secondary of the transformer is connected to the integrator circuit. A small 1 ohm resistor is connected at the primary of the transformer . The circuit is shown below

B-H circuit (Mpa sfg-98,soft ferrite,A User Guide) fyp

The voltage across the resistor R1 and capacitor C is plotted on the scope.the voltage across R1 act as current flowing through it .it represent the H in the B-H curve where the voltage across the C represent B. when the X-Y button of scope is pressed B-H is observed shown


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