Output Capacitance: an Important Part of Soft Switching Converter

Published:2011/8/29 1:29:00 Author:Phyllis From:SeekIC

The output capacitance is an important part of soft switching converter design. In order to maximize power density, the power conversion switching frequency is being increasingly used. Soft switching techniques like zero voltage switching (ZVS) are frequently used to extend the frequency further.

With the increase of switching frequency, the parasitic characteristics of power MOSFET are getting more and more important. Among all parasitic elements in ZVS topologies, an output capacitance is crucial parasitic parameter to set power converter design up as it determines how much inductance is required to provide ZVS conditions.

Traditional way is through rough assumptions to find out fixed value of output capacitance. However, this conventional equivalent output value is not helpful in reality. A new concept of output capacitance which gives equivalent stored energy at a working voltage of the power converter can optimize the power converter design.

In the soft switching topologies, energy stored in the leakage, inductor, and series inductance or magnetizing inductance of the transformer make zero voltage turn-on possible. The output capacitance of the switches is discharged by resonant action. To avoid additional power losses, the inductance should be precisely designed.

Under many circumstances, designers of switching power supplies would doubt about temperature coefficient of MOSFET capacitance since power MOSFET is usually operated at elevated temperature. Generally, MOSFET capacitance can be considered as constant over temperature. MOSFET is majority carrier device and so switching characteristics are basically determined by its capacitance. Factors that determine the MOSFET capacitance are not much changed by temperature such as depletion length, doping concentration, channel width, and permittivity of silicon. Also MOSFET switching characteristics like switching losses or on/off transition speed are also not changed much by temperature. Although the equivalent series gate resistance at gate would increase a little bit as temperature goes up, which slow down MOSFET switching at high temperature a little.

In many cases, output capacitance is measured with 1MHz frequency and 0V of Vgs. Actually, capacitance include gate-to-drain capacitance, gate-to-source capacitance, and drain-to-source capacitance. It is impossible to measure each capacitance separately. As a result, the gate-to-drain capacitance and drain-to-source capacitance together is described as output capacitance and it is measured by two paralleling capacitances.

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