19.4.4 Design Example

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19.4.4 Design Example

• Select resonant tank elements to design a
  resonant inverter that meets the following
  requirements
• Switching frequency fs 100 kHz
• Input voltage Vg 160 V
• Inverter is capable of producing a peak open
  circuit output voltage of 400 V
• Inverter can produce a nominal output of 150 Vrms
  at 25 W

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Preliminary calculations

• The requirements imply that the inverter tank
  circuit have an open-circuit transfer function of

The required short-circuit current can be found
by solving the elliptical output characteristic
for Isc
hence
Use the requirements to evaluate the above
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Matched load

• Matched load therefore occurs at the operating
  point

Hence the tank should be designed such that its
output impedance is
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Solving for the tank elements

• Lets design an LCC tank network for this example

The impedances of the series and shunt branches
can be represented by the reactances
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Analysis in terms of Xs and Xp

• The transfer function is given by the voltage
  divider equation

The output impedance is given by the parallel
combination
Solve for Xs and Xp
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Evaluate tank element values

• The capacitance Cp should therefore be chosen as
  follows

The reactance of the series branch should be
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DiscussionChoice of series branch elements

• The series branch is comprised of two elements L
  and Cs, but there is only one design parameter
  Xs 733 ?. Hence, there is an additional degree
  of freedom, and one of the elements can be
  arbitrarily chosen.
• This occurs because the requirements are
  specified at only one operating frequency. Any
  choice of L and Cs, that satisfies Xs 733 ?
  will meet the requirements, but the behavior at
  switching frequencies other than 100 kHz will
  differ.
• Given a choice for Cs, L must be chosen according
  to

For example, Cs 3Cp 3.2 nF leads to L 1.96
µH
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Rcrit

• For the LCC tank network chosen, Rcrit is
  determined by the parameters of the output
  ellipse, i.e., by the specification of Vg, Voc,
  and Isc. Note that Zo? is equal to jXp. One can
  find the following expression for Rcrit

Since Zo0 and H ? are determined uniquely by the
operating point requirements, then Rcrit is also.
Other, more complex tank circuits may have more
degrees of freedom that allow Rcrit to be
independently chosen. Evaluation of the above
equation leads to Rcrit 1466 ?. Hence ZVS for
R lt 1466 ?, and the nominal operating point with
R 900 ? has ZVS.
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