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conduction and switching loss in switching converter

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dennislau

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Typically, the efficiency of the switching converter are varing with load current. At light loac current, the efficiency is poor. But at higher load current, the efficenecy is improving. Finally, when approaching the maximum load current, the efficency is getting poor again. Does anyone tell me the reason for this? And, could anyone tell me how to calculate the conduction and switching loss in the swithcing converters? And any references/paper suggested? Thanks.
 

VVV

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All the circuits need some power and will draw some current (quiescent current)even when the P/S does not produce any output current. Think of the oscillator, error amplifier, reference, etc. All these parts need some current to operate. This translates into power loss, which does not contribute anything to output power.

When the power supply is operating at light load, the quiescent currents begin to represent a significant portion of the total power, so the overall efficiency drops.

At high power, it is possible for the efficiency to drop (though not always), because losses do not vary linearly with the current. Take for example the output choke, whose resistcance will produce a power loss that is proportional with the square of the current. That will make the choke lose power non-linearly with output current. If the resistance is such that at the maximum output current these losses become a significant portion of the output power, then efficiency will drop.
A similar case is that of the MOSFET's, whose conduction losses vary with the square of the RMS current through them. Again, this is a non-linearity which can translate in decreased efficiency. Even the ESR of the caps will produce losses that increase with the square of the RMS currents through them and so are higher at higher output currents.

It is possible to design a P/S whose efficiency is at its peak at the maximum output power.
Typically, however, as a designer you will calculate these losses, based on the calculated currents/ voltages, and select adequate components, so as to meet the MINIMUM efficiency requirement at full power. (Note that even though the efficieny drops, it must still be above the specified minimum). Thus, this way you can compromise and choose a cheaper transistor or choke for instance (cost is also a design parameter), with a higher Rdson or DC resitance respectively, knowing it will result in lower efficency at maximum current, as long as you meet the minimum efficiency.

Conduction loss is done of course for each part by multiplying its resistance with the square of the RMS current through the device. Switching losses are a little more complicated, but essentially you integrate the power loss over the duration of the switching interval, assuming the current and voltage across the switching devices vary linearly. The switching interval is established by you, taking into account other factors, such as available drive current vs. input capacitances (for MOSFETs, for instance). Approximate formulas are available.

A good book is Abraham I. Pressman's "Switch-mode power suplies". You can also go to websites, such as ON Semi, National, Linear Tech, or TI and look for design tips.
On the TI website you can find the Unitrode Seminars, which contain many design equations. Try for instance SEM1200 and look at the 140W converter.
http://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=SEM401011&section=Overview
 

dennislau

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Thanks VVV.

As you said, the conduction loss and switching loss are non-linear which cause the efficiency drop non-linearly. I have one follow-up question, could I say, at light load current, the switching loss is dominant compared with the conduction losss. While at the high load current situation, the conduction loss is dominant in this case. And hence, there is an optimum point between these two cases. Am I correct? If yes, is it true for all different types of switching converters such as PWM and PFM mode?
 

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In principle, yes, that is the case.

There is one other factor that I did not mention: gate drive losses. This power is simply required to switch the MOSFET's input capacitance. For a regular PWM type, this is essentially constant, independent of output current, since the capacitance does not change that much.

Switching losses will decrease at light load, since the currents decrease, thus integrating power over the switching interval results in less power loss. Nevertheless, with PWM, swithing losses can represent a significant portion when the converter operates at light load.

These two factors led to the development of the PFM, where the converter decreases dramatically the effective operating frequency at light load, thus reducing both gate drive losses and switching losses. Coupled with a low quiescent current, this results is an rather dramatic efficiency improvement at light loads.
So, to answer your question, yes, it is generally true for all topologies, but by analyzing the source of losses, new techniques have been developped that make it less true. Take the PFM: most losses at light load are due to quiescent currents, no longer switching losses or gate drive losses.
 

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