Resonant converters do, though, have several disadvantages. Performance can be optimized at a single operating point, but not across a wide range of input-voltage and load-power variations. Also, the quasi-sinusoidal waveforms found in a resonant converter exhibit higher peak values than their equivalent rectangular waveforms. In addition, current can circulate through the tank elements even when the load is disconnected, leading to poor efficiency at light load. A similar switching technology is today often employed in low-power LED drivers in which a quasi-resonant or valley-switching topology is implemented. To start, current IQ1 ramps up until the desired energy level is charged in to coil L - see figure 5
. Then switch Q1 is turned off. When the switching transient is complete and the coil current equals zero, the drain voltage starts to oscillate around the input voltage level VDC. The amplitude equals V0. Circuitry connected to the Q1 drain pin senses when the voltage on the drain of the switch has reached its lowest value. The next cycle is then started. The effect of this topology is to reduce capacitive switching losses and electromagnetic emissions. On the other hand, a quasi-resonant converter has the same disadvantages as a resonant converter, as described above.
Figure 5: operation of a quasi-resonant, zero-voltage switching circuit.
Click on image to enlarge
EMC standards for conducted emissions generally set peak energy limits within the frequency band from 150kHz to 30MHz. Although carefully selected fixed-frequency modulation can be effective in spreading harmonic content, the disadvantage is that in some cases it might not provide sufficient attenuation of the fundamental. New research, however, suggests that modulating at a fixed frequency is not as effective in reducing the peak energy in the fundamental as modulating the carrier with a complex, random, or pseudo-random waveform. Perhaps the most important lesson to learn from experience is the importance of testing prototypes for EMC, before going to pre-production.
About the author:
Arnoldas Bagdonas is Field Application Engineer at Future Electronics (Lithuania) - www.futureelectronics.com
Courtesy of EETimes Europe
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Design archives: LEDs
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