Hello XM,
You are on the right track, but may need to make some corrections. To deal with the power loss, we have to consider two aspects.
The power loss of semiconductor switches mainly comes from two sources: the conduction loss resulting from the loss on the RDSON that is generated when ID goes through the body diode during dead time, and the switching loss which can be roughly classified as three components: the loss caused from current-voltage cross when the MOSFET switches between turn-on and cut-off, the loss on the parasitic capacitance during switching, and that caused by the trr time of the body diode in the MOSFET.
The above equations show the close relation between the switching loss and the switching frequency. In particular, the loss during dead time mainly depends on the switching frequency, since the dead time is usually fixed. In general, for a given output power, the higher the frequency the more switching loss portion dominance; the lower the frequency the more conduction loss portion dominance.
In a SR structure, the body diode is already turned on by freewheel current before the MOSFET turns on. Since the voltage drop on the body diode is usually less than 2V, the conduction loss is not significant in equation (1).
Equation (2) shows the effect on the switching caused by the parasitic capacitance (Coss) of the MOSFET. This Coss is the equivalent capacitance between drain and source, with a voltage equal to Vds applied. Thus, this portion of loss is proportional to the switching frequency and Vds. You can check if the loss in your design is mainly caused by Coss by paralleling a smaller capacitance between the drain and the source to make the two MOSFETs have almost the same Coss value, then analyze the results by using the equations given above. You’ll get an idea about why the efficiency improvement is not significant.
Finally, in a forward half-bridge structure, the switching loss contributed by trr is also significant since the secondary current is usually kept in continuous current mode. You can check if the loss in your design is caused by trr by paralleling a schottky diode on the SR MOSFET side and then observing whether the efficiency is improved notably. If the efficiency is improved significantly, you can select a MOSFET with shorter trr time for the SR MOSFET.
Now, go back to your question. For MOSFETs within the same series, the lower the RDSON, the larger the value of parasitic capacitance. For example, FDP047N10 from Fairchild Semiconductor has a RDSON of 4.7mohm and a Coss of 1500pF, whereas FDP100N10 has a RDSON of 10mohm, but its Coss is only 710pF. In other words, it is possible that the lower RDSON FDP047N10 may have a larger loss under high switching frequency due to its larger Coss. Other parameters, such as Qg, parasitic body diode in MOSFET, also contribute to the overall power loss, which compromises the efficiency improvement effect of lower Rds. So, apart from the RDSON, which should be as low are also possible, the parasitic characteristic is an important factor in MOSFET selection.
Hope you are satisfied with my explanation. Let’s go and have a cup of coffee.
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