Learn More About MagAmp

The MagAmp Designer is a software program intended to assist design engineers in the selection of a Permalloy wound core for use as a magnetic amplifier (MagAmp).  In this application, the core is used as a saturating device to regulate secondary (or tertiary, etc.) outputs in a switched mode power supply (SMPS).  In this role, MagAmps can provide small, inexpensive control architectures that do not require the design of a dedicated regulator to control outputs other than the primary one.  An example circuit is detailed in Figure 1, with the MagAmp device labeled "SC."


The MagAmp operates by blocking the leading edge of voltage pulses coming from the secondary side of the power transformer (see waveforms in Figure 1).  Note that waveform e2 shows the MagAmp as having blocked the first 1µS of the secondary voltage (e1).  This implies (see waveform e3) that only 3/4 of the volt-second product will be allowed to continue to the output.  Therefore by selectively blocking more or less of the secondary side voltage waveform, the output voltage can be controlled (regulated) independently from the primary feedback loop.  The 'blocking ability' of the MagAmp is determined by a control winding (and current) controlled by the amplifier structure at A1.

Core Size Determination

The core size and material must chosen to meet the following criteria:

The core should saturate in a very predictable and very abrupt manner.  This criteria mandates a very square loop magnetic material.  If the core material saturates 'softly,' then regulation becomes more difficult.  This is because the amount of energy (volt-time product) allowed to pass on to the output becomes more and more difficult to judge as the core saturates more and more softly (in other words, the core will 'slur' into saturation, leaving some uncertainty as to how much energy passed along in the process).

The material losses should be relatively low.  This point relates to overall efficiency.  The lower the core losses, the better MagAmps become as an alternative to a full secondary regulator.

Core size must provide enough window area (large enough inside diameter) to accommodate the drive windings and should provide enough cross section (effective core area) to keep the turns count to a easonably small number.  Core size alludes to the core selection routine, which guarantees that the core suggested will be able to support the required volt-second product for regulation. 

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