POWER SUPPLY TRANSFORMER AND INDUCTOR DESIGN
BASIC PRINCIPLES, THEORY, CALCULATION
Before discussing the calculation of magnetic components for switching power supplies, let me just quickly go over the basic concepts and definitions.
Transformer is a passive device which transfers alternating (AC) electric energy from one circuit into another through electromagnetic induction. It consists of a ferromagnetic core and two or more coils (windings).
A changing current in the primary winding creates an alternating magnetic field in the core. The core multiplies this field and couples most of the flux through the secondary windings. This in turn induces alternating voltage (electromotive force, or emf) in each of the secondary coil according to Faraday's law.
Power transformer in SMPS is used to change amplitude of highfrequency pulses by the turns ratio and to provide isolation between circuits.
Note that a transformer can't transfer a DC component of a pulse: in a steady state mode net
voltseconds across any winding should be zero, otherwise the core will soon saturate. DC output voltage can be obtained only by using rectifiers. Nevertheless, an average voltage across a real coil's terminals can be nonzero due to nonzero wire resistance. This DC offset can be used for
lossless sensing of an average current across an inductor or a transformer winding with unidirectional current: if you add an RC network parallel to the coil, the voltage across the capacitor will be proportional to the coil's average current. For better thermal stability the wire can be made of low TCR material, such as a copper alloy.
MAGNETICS DESIGNING
normally involves tradeoffs between size, cost and power losses. The main constraint in all cases (except for saturable inductors) is that peak magnetic flux density
B_{MAX} should not approach the core material's saturation flux value
B_{SAT}. Note that in higher frequencies, core loss rather than saturation can become the main limiting factor for B
_{MAX}. The flux change is a function of the applied
voltseconds and the core geometry. Excessive voltseconds applied to a coil cause core saturation. When it happens, the windings are effectively shorted out.
The table below provides the formulas for B
_{MAX} in a steady state operation as function of
N×Ac product, applied voltage and frequency for common voltage waveforms. Contrary to popular misconception, B
_{MAX} does not depend on the magnetic material properties or air gaps. It does not depend on the transferred power neither. That's why in theory the core size does not depend on the wattage. However, for efficiency and thermal reasons, we have to limit ohmic losses in the wires. That's why magnetics size increases with power. Most textbooks provide formulas for estimation of the core size based on the product of magnetic crosssection area by the window area available for the winding. Unfortunately, this method is not very helpful because these formulas are based on pretty much arbitrary selection of current density and on the assumption of a certain window utilization (fill) factor. In reality, depending on the insulation grade and application requirements, current density in the copper can be selected anywhere between 180 and 500 A/sq.cm. The fill factor likewise can be anywhere between 0.1 and 0.5 depending on the coil construction and insulation. As the result, if you don't have much experience in practical transformer design, you will need to go through several iterations.
Function 
Waveform 
B_{MAX}, gauss 
Sine wave 

Vrms×10^{8}/4.44N×Ac×F 
Square wave 

Vpk×10^{8}/4N×Ac×F 
Bipolar pulses with D=Ton/T=Ton×F
(0<D<0.5) 

Vpk×D×10^{8}/2N×Ac×F 
Unipolar pulses
with passive reset 

Br+Vpk×Ton×10^{8}/N×Ac 
In these and other equations: V  voltage
(volts), N  coil's turns, Ac  core's crosssectional area (sq.cm), F frequency (hertz), Br  remanence (gauss) 
Here is a quick simplified
transformer design procedure:
 Select the ferrite material based on your operating frequency;
 Find saturation flux B_{SAT} at maximum operating temperature from the datasheet and pick some derating, such as 75% or so. This will be you target maximum flux B_{MAX};
 Determine minimum required N×Ac product for the targeted B_{MAX} for the core excitation waveform(see the table to the left);
 Choose a core size based on similar designs and find its effective crosssectional area "Ac" from the datasheet;
 Find minimum required primary turns "N" as N×Ac product divided by "Ac";
 Calculate secondary turns for your output voltage based on a DC transfer function of the selected SMPS topology;
 Choose wire size based on your primary and secondary currents, operating frequency, and the available window.
When you get an actual prototype you can measure the hot spot temperature by placing a thermocouple under the coil and then adjust the design if necessary. S.A.Mulder proposed an empirical formula for thermal resistance of a wirewound transformer measured at the hot spot:
Rth≈(53...61)×Ve^{0.54} ^{o}C/watt, where Ve core volume in cubic centimeters [1990 Phillips App Note]. From this, a "rule of thumb"
hotspot temperature rise in Celsius vs. total losses P(watt) is:
T_{rise}≈50/sqrt(Ve)×P. Note that the above thermal relationship as well as most textbook procedures related to the magnetics thermal management are applicable to natural convection cooling. For applications with forced airflow or conduction cooling these procedures results in an overdesigned part because of an overstated temperature rise.
In general, ideal SMPS transformers need to transfer all energy instantaneously from one winding to another while storing no or little energy in the process. Some topologies do need certain amount of energy stored in magnetizing inductance for a proper operation. Conversely, a
power inductor is used in SMPS as an energy storage device. It accumulates energy in the magnetic field as current flows through it, and then transfers it into another circuit during the alternate part of the switching cycle. In power supplies, the inductors are also used for filtering out high frequency currents (in which case they are often called
chokes). In power inductors the current rather voltage is controlled. For such "
currentdriven" coils:
B=L×Ipk×10^{8}/N×Ac,
where L  inductance (in henry), Ipk  peak current in amps, B  flux in gauss. All units and formulas in this page are given in CGS (see CGS to SI
unit converter). Once you set B
_{MAX} and choose a core size, you can find from the above equation the number of turns for the desired inductance L:
N=L×Ipk×10^{8}/B_{MAX}×Ac.
Note that "L" is not constant. If the current keeps increasing, at some point B
_{MAX} will be approaching B
_{SAT} and "L" will start dropping. To prevent the magnetic material saturation at a required current, an air gap can be introduced. The length of a net discrete gap:
lg≈0.4×π×N×Ipk/B_{MAX}.
In practice, the gap may need to be selected slightly larger than the calculated value due to flux fringing. Combining the above equations for N and lg yields:
lg≈0.4×π×L×Ipk^{2}/(B_{MAX}×Ac).
The gap is used not only in inductors. It is also often introduced in transformers to increase working flux swing in singleended topologies, to store more energy in magnetizing inductance, and to stabilize the inductance value. For powder metal materials with a distributed gap and soft saturation curve, the calculation process may take several iterations. In short, you can first pick a powder core based on desired L×Ipk
^{2} by using manufacturer's charts. Then determine the turns
, where AL  specific inductance in mH/1000 turns (which is nH/turn) from the data sheet. Then find peak bias
H=0.4×π×N×Ipk/le (Oersted), determine the rolloff in percentage of initial permeability, and correct the turns for the desired L.
Below you will find more magnetics theory, transformer and inductor design information, tutorials, tools and free downloads.