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AND8039 Datasheet, PDF (2/12 Pages) ON Semiconductor – The One-Transistor Forward Converter | |||
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AND8039/D
SWITCH
VOLTAGE VIN
RESET
SWITCH
CURRENT
MAGNETIZATION
CURRENT
Figure 2. Power Switch Waveforms
The output rectification and filter section works
identically to the buck converter. The voltage waveform of
secondary looks like an inverted primary winding waveform
except the zero voltage point is the input voltage point on the
primary waveform. The waveform goes positive when the
power switch is conducting. The output rectifier also
conducts during this time. This presents a unipolar, PWM
rectangular voltage signal to the inductor, just as found in a
typical buck converter. The catch diode then operates when
the power switch and the output rectifier are OFF.
Continuous current is then maintained through the output
filter inductor.
Design of the OneâTransistor Forward Converter
Please refer to the schematic in Figure 5 when
Component designations are mentioned.
Design Specifications:
Input Voltage Range: +140â+200 VDC
Output Voltage: +28 VDC
Output Current: 0.5 Aâ4.0 A
Max. Output Ripple Voltage: 30 mV
Predesign Estimates:
Output Power:
Pout(max) = (Vout)(Iout(max)) = 112 Watts
Peak Input Current:
Ipk â 2.8 Pout/Vin(min) = 2.24 Amps
Average Input Currents:
Iav(low) = Pout/eff(Vin(max)) = 0.66 Amps
Iav(hi) = Pout/eff(Vin(min)) = 0.94 Amps
Design of the Transformer
One begins with the transformer for every switching
power supply design. All of the needed parameters are now
known and it serves as the backbone for the remainder of the
design.
One must first select a core family that will house the
transformer. This is done first by reviewing various core
styles and their attributes. The most common offâline core
is the EâE core, for which there are several variations. The
standard EâE core is based upon the old 50â60 Hz
lamination core styles, which are very adequate for most
applications. There are some lowâprofile styles such as the
Philips EFD family which yields a very trim, low profile
appearance, but can cost slightly more for the basic
coreâbobbin sets. Selecting an approximate core size is done
by appreciating that first the core must have a sufficient core
crossectional area to contain the needed flux density to
transport the power from the primary to the secondary
winding(s). Secondly, there must be enough winding area to
contain the required turns of the needed wire gauges.
Thirdly, for offâline transformers, the core family must have
the ability to meet the minimum creepage and clearance
dimensions of the safety agencies after the transformer is
finished. To begin, one would use an equation like equation
1 which is an artificial quantity derived from the product of
the core crossectional area (Ac) times the winding area(Wa).
WaAc [ 0.7 (Pout Wd(pri) 108)ÅfB max (USA) (eq. 1A)
where: Wd(pri) is the average wire diameter needed to
carry the primary current in cm.
Bmax is the maximum operating flux density in
Gauss (Webers/cm2)
In the MKS system (Europe and the rest of the world)
WaAc [ 0.7 (Pout Wd(pri))ÅfB max
(eq. 1B)
where:
Wd(pri) is the average wire diameter needed to
carry the primary current in meters (m).
Bmax is the maximum operating flux density in
Teslas (Webers/m2)
The result is in cm4 (eq. 1A) or m4 (eq. 1B). The core
manufacturers usually provide the WaAc for each core size.
The core size can then be chosen and should be as large or
larger than this result. For offâline applications, of which
this is not, one should increase the result by about 20
percent to accommodate the added insulating tape needed
for an IECâqualified transformer. Also, a core and bobbin
set must be used that has sufficient creepage (distance over
a surface) and clearance (distance through air) dimensions.
For 110â220 VAC applications, this is 3.2 mm between
phases, and 8.0 mm between the input and output circuits.
This may be difficult determining the offâlineâsuitability
of a core and bobbin from its data sheet.
In oneâtransistor forward converters, the operating flux
density (Bmax) dictates how much magnetization energy,
which is not used, must be released by the core prior to the
next power switch conduction cycle. This is a point of
tradeoff, if Bmax is set too low, then there will be many turns
on the transformer, thus making the transformer larger than
it needs to be. Setting Bmax too high, makes the transformer
smaller, but increases the losses related to the core reset
function. A good point of compromise is to set Bmax at about
25 percent of Bsat at 100 kHz. This level should be reduced
by a factor of 0.04 per 100 kHz above this frequency. One
can then calculate the turns by:
Npri [ (Vin(nom) 108)Å4fB max Ac (US)
(eq. 2A)
where: Bmax is in Gauss (webers/cm2)
Ac is the core crossectional area in cm2
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