Closed
Loop
Flyback
Converter
Group 16
Ahlmahz Negash
Kim Wharton
Kami Zhong
Abstract
The closed loop flyback converter is a type of transformer isolated converter typically utilized in low power
switching applications. Through PSpice simulation, converter performance specifications were explored, and
time of prototyping minimized. The final design components include a MUR420 PIN diode, MTP 3055
MOSFET, SG3524 pulse width modulator, and an LM317 voltage regulator. A type 2 K-factor control model
was used to realize constant output voltage. The final cost of this converter is $20.89.
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Contents
Abstract .................................................................................................................................................. 1
Introduction ............................................................................................................................................ 3
Discussion ............................................................................................................................................... 3
Topology ............................................................................................................................................. 3
Theory of Operation ............................................................................................................................ 4
Design Specifications ........................................................................................................................... 6
Hardware Implementation .................................................................................................................. 9
Analysis of Final Results..................................................................................................................... 10
Conclusion............................................................................................................................................. 12
Appendix I ............................................................................................................................................. 15
Appendix II ........................................................................................................................................ 16
Appendix III ....................................................................................................................................... 17
References ............................................................................................................................................ 18
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Introduction
This design is a closed loop flyback converter, which is a transformer isolated converter. It is derived
from the buck-boost converter in that the inductor is split to form the transformer. The Flyback
converter applications are usually used in switch mode power supplies for devices such as cell phone
chargers. The ratio of the transformer turns can be used to adjust the output voltage range, which
effectively bucks or boosts the input voltage, making it useful in various output load applications.
Additional windings on the transformer can be cheap way of producing several different output voltage
levels.
Discussion
The following subsections explain, briefly, the basics of general flyback converter topology and
operation as well as a detailed description and analysis of our specific converter, including its operation
specifications, and hardware implementations.
Topology
The flyback converter requires minimal components and is very similar to a buck-boost converter. The
figure below shows the basic topology of an open loop flyback converter.
Figure 1. Basic Flyback Topology. Courtesy of “Flyback Converters for Dummies” by Ronald Dekker
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It should be noted that while the above diagram shows the secondary of the transformer having a
greater number of turns, this is not necessary. Often, a one to one turns-ratio is used such that the
transformer is simply an isolation transformer.
The switch (MOSFET) is located on the low side of the primary to eliminate “high side switching
problems” while the diode is connected to the low side of the secondary.
Theory of Operation
A flyback converter is a buck-boost converter with its inductor split to form a transformer. They are
commonly used in low power level applications, below 50 watts. A basic schematic of the converter is in
figure 1. The transformer winding often consists of a 1:1 turn ratio, however, using different turns ratio
on the transformer brings in more flexibility to optimize the design for duty cycle, stress, and efficiency.
One of the basic elements (without the control circuit) consists of a switching MOSFET, which in Figure 1
is represented by an ideal switch. At the moment t=0, the buffer capacitor charges to the output
voltage (Vout) and the current through the primary windings is zero. The MOSFET closes at t = 0, and
current starts to flow through the primary winding of the transformer. This will induce a voltage in the
secondary winding with reverse polarity as indicated by the dots on the transformer. The diode will be
in reverse bias with no current flowing through the secondary winding. The primary side current will
increase linearly inducing a voltage over to the secondary winding, causing the diode to block the
reverse voltage of Vin*n + Vout.
At the moment that the switch opens (seeFigure 2) the energy that is stored on the primary side of the
transformer induces a voltage at the secondary side. This voltage is high enough to forward bias the
diode. The voltage in the secondary winding will equal Vout plus the voltage drop across the diode.
While the switch is in the open position it has to block Vin + Vout/ #turns, where #turns is the number of
turns on the secondary side.
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Figure 2. Phase two, dumping the energy from the transformer into the buffer capacitor
(Source http://www.dos4ever.com/flyback)
This continues until all the energy stored in the transformer is dumped through the buffer capacitor. At
this point, the induced emf at the primary side will vanish. The parasitic capacitance of the switch will
charge to Vout + Vin. The primary side has now formed a series resonant tank with a charged capacitor.
This will cause a dampened oscillation as shown in figure 3.
Figure 3. Voltage over the switch during all three phases
(Source http://www.dos4ever.com/flyback)
Figure 3 shows the drain-source voltage during on and off phase of the converter. The voltage during
the close phase drops over the switch caused by the non-zero on resistance. The current will increase
linearly. This causes the voltage to drop over Ron linearly (point a in Figure 2). The switch opens at point
b and the secondary current starts to flow. The output voltage will transform over the primary winding.
The MOSFET will block Vin and all the energy will dump in the capacitor and the secondary current goes
to zero causing the induced emf at the primary to vanish. The drain and source capacitor is charged,
now connected in series with the inductance of the primary winding resulting in a dampened oscillation.
The switch will close again and the remaining energy in the LC tank will dissipate in the transistor.
A disadvantage of the flyback converters is the need for snubber to prevent voltage spikes. No
transformer is perfectly ideal. There are always magnetic field lines generated by the primary windings
that are not enclosed by the secondary windings. This cause leakage inductance usually modeled as a
small inductor in series with the primary winding of the transformer. The energy that is stored in the
transformer is dumped in the buffer capacitor, but this does not include the small amount of energy
stored as stray inductance. The opening and closing of the switch causes sharp voltage peak just as any
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