# Calculating Flux density in a DC-DC converter coil

Hello Everyone.

I am working on a DC-DC converter design which uses a transformer with an additional inductive element on the secondary side. The coil charges a large capacitor.

Since there is an additional inductive element on the secondary side the inductive element builds up a field while the pulse is applied (30kHz 50% duty cycle). When the pulse ends the field collapses and produces another pulse of the same duration (30kHz 50% duty cycle) and polarity since a diode is in series with the coil and capacitor (see attachment)

My question is: How do I calculate the Flux density since there are (2) 30kHz 50% duty cycle pulses....Do I calculate it as if it were a single 100% duty cycle 30kHz pulse? Or does the second pulse not count since it is the field collapsing?

Thanks for the help! It is greatly appreciated.

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• core bsat.png
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anorlunda
Staff Emeritus
My question is: How do I calculate the Flux density since there are (2) 30kHz 50% duty cycle pulses....Do I calculate it as if it were a single 100% duty cycle 30kHz pulse? Or does the second pulse not count since it is the field collapsing?

The flux density varies with time. You can either solve the DC differential equations, LdI/dt, or you can approximate the square wave as a sin with the fundamental frequency and take the RMS.

Hello Everyone.

I am working on a DC-DC converter design which uses a transformer with an additional inductive element on the secondary side. The coil charges a large capacitor.

Since there is an additional inductive element on the secondary side the inductive element builds up a field while the pulse is applied (30kHz 50% duty cycle). When the pulse ends the field collapses and produces another pulse of the same duration (30kHz 50% duty cycle) and polarity since a diode is in series with the coil and capacitor (see attachment)

My question is: How do I calculate the Flux density since there are (2) 30kHz 50% duty cycle pulses....Do I calculate it as if it were a single 100% duty cycle 30kHz pulse? Or does the second pulse not count since it is the field collapsing?

Thanks for the help! It is greatly appreciated.

Without knowing anything about the core geometry and how the winding's are built this is impossible to answer.

Basically the process is come up with an equivalent reluctance circuit of your core based on the core geometry, then place the windings appropriately as MMF sources, then your primary is the driver (ie its providing the MMF) and then the reluctance circuit determines the total flux and how its distributed in the core, then you can start to see what your secondary and auxiliary winding are doing.

Primer:
https://en.wikipedia.org/wiki/Magnetic_circuit

Tom.G
Since there is an additional inductive element on the secondary side the inductive element builds up a field while the pulse is applied (30kHz 50% duty cycle). When the pulse ends the field collapses and produces another pulse of the same duration (30kHz 50% duty cycle) and polarity since a diode is in series with the coil and capacitor
First off, you have a series circuit of two secondaries and a diode. Whether the diode is between the those windings or after them make no difference.

Are you saying during the pulse that the top of the secondary is a given polarity (let's say "+" for ease of discussion) and after the pulse, during field collapse, that the right end of the 3rd coil is "+"?

If that is the case, the coils are connected in series opposing and the voltages will subtract from each other during the whole cycle.

If the secondaries are connected in series aiding, they are just a single secondary with the voltage being the sum from the two.

Here is a short slide slow that gives the rudiments (beginning information) of various converters.
http://www.ee.bgu.ac.il/~dcdc/slides/DC-DC part 4_Double .pdf