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EETE NOVEMBER 2012

DESIGN & PRODUCTS POWER ELECTRONICS the tank voltage or current wave- coil current equals zero, the drain forms, thereby reducing or eliminat- voltage starts to oscillate around the ing some of the switching loss. This input voltage level Vdc -. The ampli- means that resonant converters can tude equals V0. circuitry connected operate at higher switching frequen- to the Q1 drain pin senses when the cies than comparable pwm con- voltage on the drain of the switch verters, leading to smaller inductor has reached its lowest value. The and capacitor values and costs. in next cycle is then started. The effect addition, zero-voltage switching re- Fig. 4: resonant converter circuit operating in a) zero- of this topology is to reduce capaci- duces converter-generated emi as voltage switching mode when fsw > fLC and a delay tive switching losses and electro- it has no current or voltage ripples between Q1 and Q2 commutation is introduced. magnetic emissions. on the other during switch commutation. Otherwise, the circuit operates in (b) hard switching mode. hand, a quasi-resonant converter Resonant converters do, though, has the same disadvantages as a have several disadvantages. perfor- resonant converter, as described mance can be optimised at a single above. operating point, but not across a emc standards for conducted wide range of input-voltage and emissions generally set peak ener- load-power variations. Also, the gy limits within the frequency band quasi-sinusoidal waveforms found from 150kHz to 30MHz. Although in a resonant converter exhibit high- carefully selected fixed-frequency er peak values than their equivalent modulation can be effective in rectangular waveforms. In addition, Fig. 5: operation of a quasi-resonant, zero-voltage spreading harmonic content, the current can circulate through the disadvantage is that in some cases switching circuit. tank elements even when the load it might not provide sufficient atten- is disconnected, leading to poor efficiency at light load. A similar uation of the fundamental. New research, however, suggests that switching technology is today often employed in low-power Led modulating at a fixed frequency is not as effective in reducing the drivers in which a quasi-resonant or valley-switching topology peak energy in the fundamental as modulating the carrier with is implemented. To start, current IQ1 ramps up until the desired a complex, random, or pseudo-random waveform. Perhaps the energy level is charged in to coil L - see figure 5. Then switch Q1 most important lesson to learn from experience is the importance is turned off. When the switching transient is complete and the of testing prototypes for EMC, before going to pre-production. 48 Electronic Engineering Times Europe November 2012 www.electronics-eetimes.com 190x136_EE_Times_Europe_neutr.indd 1 04.10.12 10:10


EETE NOVEMBER 2012
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