开关电源磁性元件损耗计算
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Targeting Switcher Magnetics Core Loss Calculations
Feb 1, 2002 12:00 PM
By Clifford Jamerson, Consultant, Christiansburg, Va.
Magnetics product catalogs derive core loss vs.
frequency curves by measuring the core losses
that result from sinusoidal excitation at varying
frequencies and voltage amplitudes. The “B” in
the family of curves is the maximum flux either
side of the origin of the B-H curve. Thus, the total
swing in flux is twice that shown in the core loss
charts. The formulas for core loss in the catalogs
are empirical ones that give a best-fit to the
measured values.
Most application notes when estimating the core
loss of a magnetic component have a procedure
similar to:
1.Calculate the total flux swing using
Faraday's Law. If the voltage applied to
a transformer winding is constant during
a pulse, then the total flux swing is:
∆B=(V∆t×108)/NAe (1)
Where:
∆B=Total flux swing in gauss
V∆t=Volt-seconds in the pulse
N=Number of turns in winding
Ae=Cross-sectional area of core in cm2
2.Assume the total flux swing from (1) is
the same as that for a sinusoid with
same volt-seconds. Divide the total flux
swing by two and go to the core loss
curves at the specified switch frequency
to find the core loss per unit volume (or
unit weight), either in mW/cm3 or W/lb.
3.Multiply the core loss per unit volume×cm3, or W/lb×the core's weight.
The classical procedure is easy to use. However, for pulsed operation where the duty cycle is low, the
actual core loss will be higher than predicted by the classical procedure. For these pulsed applications,
you'll find a better procedure some experienced magnetic designers have used.
Core Loss
Core loss is proportional to the area enclosed inside the hysteresis curve. In reality, the actual width of
the hysteresis loop is influenced by the rate of change of flux, dB/dt, which has a nonlinear relationship
with frequency and flux amplitude. If the frequency is doubled and flux amplitude is held constant, then
the dB/dt is doubled. If the frequency is held constant and the flux swing is doubled, then the dB/dt is
also increased by a factor of 2. However, when we look at either the core loss curves or the best-fit
formula for any magnetic material, we see the core loss isn't directly proportional to the flux amplitude or
the frequency. Instead, the actual relationships are exponential.
Consider Magnetics “P” material as a typical example. For the frequency range of 100 kHz to 500 kHz, the
best-fit formula [1] is:
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