International Journal of Computer Trends and Technology (IJCTT) – volume 4 Issue 7–July 2013
Variable Output AC-DC Converter
Mr. Pavankumar.R.Patil
Dept. of Electrical & Electronics Engineering.
R.V.COLLEGE OF ENGINEERING
Bangalore, Karnataka, India.
Abstract: The design of an AC to DC converter, which takes line
supply as input and provides a regulated DC output voltage which
can be adjusted over a range of voltages is proposed in this project.
The main aim is to get a regulated output voltage for any change in
line supply. The line and load regulation is expected to be very less.
The cascaded connection of rectifier, converter transformer along
with output filter is used. To meet the specification the best suited
topology is forward converter topology. A two switch forward
converter is used to avoid transformer core saturation. To trigger
both the MOSFETs at a time through a single pulse transformer
the current mode control PWM IC UC3844 is used which avoids
using an extra current sensing circuit. The converter transformer
has one primary and three secondary windings. The three
secondary windings serves three purposes, one used provide power
supply to control circuit, other used for voltage feedback to control
circuit and the third to feed the load. The output voltage is fed back
to control circuit through an opto coupler which provides input to
output isolation. Two voltage regulators are provided one at power
circuit other at control circuit. Once the output voltage is set for a
particular value, the voltage regulator at power circuit senses the
variation in output voltage for any change in input and sends
proportional voltage to the control circuit this is sensed by PWM
IC through regulator at control circuit, then the IC will adjust the
duty ratio of the triggering pulse to MOSFETs which leads to
maintaining constant voltage at the output. This is achieved with a
regulation less than 1% and the required specifications are met.
Mr. M.N.Dinesh
Dept. of Electrical & Electronics Engineering
R.V.COLLEGE OF ENGINEERING
Bangalore, Karnataka, India.
extended design time and cost. There are basically four types of
regulators [1].
• Linear regulator
• Buck regulator
• Boost regulator
• Inverting regulator (also known as flyback or buck-boost)
II. 2 SWITCH FORWARD CONVERTER
The forward converter is an example of a buck converter. The
Forward converter without transformer core reset is shown in
fig.1. From the waveform we can observe the current builds up
at each switching cycle. It drives the core into saturation[2].
I.INTRODUCTION
The present day power supplies use many converter
topologies for DC-DC conversion. Each topology has both
common and unique properties, and the experienced designer
will choose the topology best suited for the intended application.
We will see that some topologies are best used for AC/DC
offline converters at lower output powers, whereas others will
be better at higher output powers. For applications where several
output voltages are required, some topologies will require less
number of components, while input or output ripple and noise
requirements will also be an important factor. Further, some
topologies have inherent limitations that require additional or
more complex circuitry, whereas the performance of others can
become difficult to analyze in some situations. A poor initial
choice can result in performance limitation and perhaps in
ISSN: 2231-2803
Fig.1. Without transformer core reset
That’s why we go for transformer core reset. There are many
techniques to achieve this. But here we use the 2switch forward
converter method which is more effective compared to the
others methods [3].
Here reset is achieved by resetting with RCD clamp. It is
shown in fig.2. This method has both advantage and
disadvantages. Advantages are Easy to implement. And Q1 peak
voltage is equal to Vin
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International Journal of Computer Trends and Technology (IJCTT) – volume 4 Issue 7–July 2013
Disadvantages are Additional power MOSFET (Q2) + high
side driver is needed. Two numbers of High voltage, low power
diodes (D3 & D4) are required.
Multiple output voltages are needed with relatively good
cross-regulation. Saturable reactor controllers are to be used as
auxiliary secondary side regulators. Applications where the
complexities of dual feedback loops and/or slope compensation
are to be avoided [4].
Fig.2. 2-switch Forward converter
III.CONTROL CIRCUIT
Voltage Mode Control
This was the approach used for the first switching regulator
designs and it served the industry well for many years. The basic
voltage mode configuration is shown in fig 3. The major
characteristics of this design are that there is a single voltage
feedback path with pulse width modulation performed by
comparing the voltage error signal with a constant ramp
waveform. Current limiting must be done separately. It has
many advantages. A single feedback loop is easier to analyze. A
large amplitude ramp waveform provides good noise margin for
a stable modulation process. A low impedance power output
provides better cross-regulation for multiple output supplies
And the disadvantages include, any change in line or load
must first be sensed as an output change and then corrected by
the feedback loop. This usually means slow response. The
output filter adds two poles to the control loop requiring either a
dominant pole low frequency roll-off at the error amplifier or an
added zero in the compensation. Compensation is further
complicated by the fact that the loop again varies with input
voltage.
Circuit Topology
For our application we choose voltage mode control because
there are wide input line and/or output load variations possible.
Particularly with low line-light load conditions where the
current ramp slope is too shallow for stable PWM operation.
High power and/or noisy applications where noise on the current
waveform would be difficult to control.
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Fig.3. voltage mode controller circuit
PWM IC UC3844
The two MOSFETs have to be triggered at the same time, for
this purpose we need a PWM signal generator. The PWM ICUC3844 is best suited for our application so the control circuit is
designed using the IC. The circuit in fig.4 shows the control
circuit which gets the voltage feedback from output and
generates the control signal for the MOSFET [5].
The UC3844 is a high performance fixed frequency current
mode controller and it is specifically designed for Off−Line and
DC−to−DC converter applications offering the designer a cost
effective solution with minimal external components. The
integrated circuits feature a temperature compensated reference,
an oscillator, current sensing comparator, high gain error
amplifier and a high current totem pole output ideally suited for
driving a power MOSFET. The other features are protective
features consisting of input and reference undervoltage lockouts
each with cycle−by−cycle current limiting, hysteresis, a
flip−flop which blanks the output off every other oscillator cycle
a latch for single pulse metering that allows output dead times to
be programmed for 50% to 70%.
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International Journal of Computer Trends and Technology (IJCTT) – volume 4 Issue 7–July 2013
Lmag= 11.3mH
IV.DESIGN
Power Circuit Calculations:
LC Output Filter
Crossover frequency (fc) selection
Specifications:
Input Voltage- 230+/-15% AC
Output Voltage- Variable in the range 50V-180V DC
Arbitrarily selected to 10 kHz. fc > 10 kHz requires noiseless
layout due to switching noise (difficult). Crossover at higher
frequency is not recommended
Output Current- 0-2A
Cout estimation
Output Power- 360W
If we consider a ΔVout = 250 mV the following equation can
be written on the basis of Cout, fc :[7].
Transformer
C= I(dt/dv)
Turns ratio calculation
dt= D/ fc
Vo = η*Vmin*Dmax*N
=125uf
N = 2.61
Where:
Where:
• fc crossover frequency
• Vout is the output voltage
Inductor
• η is the targeted efficiency
Vin = L(di/dt)
• Vmin is the min. input voltage
L = 12.5mH
• Dmax is the max duty cycle of UC3844
MOSFET
• N is the transformer turn ratio
Maximum duty cycle at high input line DCmin (Based on the
previous equation)
Peak primary current
Vo = η*Vmax*Dmin*N
Ip = (1.56*Po)/(Vdcmin)
Dmin = 0.31
= 4A
Vmax is the max. Input voltage [6].
Maximum off voltage stress
Magnetizing inductor value.
A minimal magnetizing current is needed to reverse the
voltage across the winding which is used for resetting properly
the core. (Enough energy must be stored so to charge the
capacitance)
( ILmag_pk = 10% Ip_pk)
Lmag=Vmin/10%Im/Ton
Lmag= (170*.5)/(.1*80000*.94)
ISSN: 2231-2803
With a 2-switch forward converter max voltage on power
MOSFET is limited to the input voltage.
Vm = 2.5*Vdcmax
= 500V
Usually a derating factor is applied on drain to source
breakdown voltage (BVDSS) equal to 15%. If we select a 500-V
power MOSFET type, the derated max voltage should be 425 V
(500 V x 0.85) [8].

IRF650pb has been selected

Part Number- IRF650
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International Journal of Computer Trends and Technology (IJCTT) – volume 4 Issue 7–July 2013

BVDSS-500V

RDS(on)-.27 ohm

ID-21
Secondary Diodes
D1 and D2 sustain same Peak Inverse
Voltage (PIV):
PIV= NV/(1-kD)
= 870V

Where kD is derating factor of the
diodes (40%).

PIV is high so IR720P diode can be
selected.
Photograph.2. Waveform generated at pin6 of IC UC3844
V. RESULTS
400W AC-DC variable output forward converter was
fabricated on general purpose printed circuit board as per the
design. Where control circuit was fabricated on separate general
purpose printed circuit board as shown in photograph 1.
The output waveforms of control circuit are captured at pin6
of IC UC3844, where time period was 12.5 microseconds which
shows that the frequency is 80 kHz. This frequency is set by
appropriately choosing the values of RT & CT at pin4. This is
shown in photograph 2.
Photograph.3. Gate to source waveform
Using this circuit we can vary output voltage from 50V to
180V smoothly. Hence we can get any output voltage between
50V to 180V. As an example the output waveforms for a
regulated output of 50V and 125V is shown in photographs 5
and 6 respectively.
Photograph.1. Prototype of Variable output AC-DC converter
Photograph.4. Output Voltage waveform (50V)
The waveforms captured here shows the gate to source pulse
has amplitude of 12V and a frequency of 80 kHz. Shown in
photograph 3.
ISSN: 2231-2803
Voltage division=5V
Probe multiplication factor=10
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International Journal of Computer Trends and Technology (IJCTT) – volume 4 Issue 7–July 2013
Amplitude=1 div,Output Voltage=5*10=50V
Table.2. Line regulation-Output voltage set to 150V
Input Voltage
Output Voltage
180V
150V
200V
150V
220V
150V
240V
150V
260V
150V
Photograph.5. Output Voltage waveform (125V)
Voltage division=5V
Load Regulation
Probe multiplication factor=10
Amplitude=2.5 div, Output Voltage=5*10*2.5=125V
The table 3. below shows line regulation for various values of
load current and adjusting the output voltage to 180.0V.
Table.3. Load regulation-Output voltage set to 180.0V
Line Regulation
The table below shows line regulation for various values of
input voltage and adjusting the output voltage to 180V and
150V.
Here input voltage is varied from 180V to 260V in steps of
20V by adjusting output voltage at 180V in Table 1. below. We
can observe the output voltage is constant 180V irrespective of
input voltage and the line regulation observed is 0.
And input voltage is varied from 180V to 260V in steps of
20V by adjusting output voltage at 150V in Table 2. below. We
can observe the output voltage is constant 150V irrespective of
input voltage and the line regulation observed is 0.
Table.1. Line regulation-Output voltage set to 180V
Input Voltage
Output Voltage
180V
180V
200V
180V
220V
180V
240V
180V
260V
180V
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Load current
Output Voltage
No load
180.0V
0.5A
179.8V
1.0A
179.5V
1.5A
179.3V
2.0A(Full load)
179.0V
Here input voltage is kept constant at 230V and output
voltage is set at 180.0V. From Table 3. we can observe the
output voltage is 180.0V at no load and is goes on decreasing as
the load increases. At full load of 2.0A the output voltage is
179.0V. By this we can observe that load regulation is less than
0.56%.
VI. CONCLUSION
A variable output AC-DC converter was fabricated on
general purpose printed circuit board as per the design.
For variation of input voltage from 170V to 270V, the
output can be regulated for any voltage between 50V to 180V as
per the design.
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International Journal of Computer Trends and Technology (IJCTT) – volume 4 Issue 7–July 2013
This converter was loaded up to 2A effectively.
The regulation observed was less than 0.56%.
REFERENCES
[1]. Abraham I. Pressman, Keith Billings, Taylor Morey, “Switching Power
Supply Design Third Edition”
[2]. Ned Mohan, Tore M. Undeland, William P.Robbins, Power electronics
converters, applications and design, John Wiley and sons, Third edition, 2003.
[3]. C. Basso, “Switch Mode Power Supplies: SPICE Simulations and
Practical Designs”, McGraw−Hill, 2008.
[4]. Muhammad H.Rashid, Power electronics: Circuits, Devices &
applications, Prentice Hall India, third edition, 2004.
[5]. Techser Power Solutions Pvt Ltd design documents
[6]. Philip T. Krein, ECE369—Power Electronics Laboratory. Urbana,
Illinois: University of Illinois, 1999,106.
[7]. Philip T. Krein, Elements of Power Electronics. New York: Oxford
University Press, 1998, 119-139,325-333
[8]. Robert W Ericson and Maksimovic, Fundamentals of power electronics,
Lucent technologies Inc, Second edition, 1999.
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