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#1

# Zero voltage switching

Actually what is meant by zero voltage switching i mean how it can be viewed? And what is exactly the ZVS condition and how it can be understood please help me with an example?
09-08-2013 11:22 PM
Top #2
Zero voltage switching means the power switches in a power electronic circuit is switched when the voltage on them is zero. That is obtained by (somehow) turning on the inverse diode across the switch firstly, which decreases the switch voltage to diode Vf level (almost zero but not actually zero) and then applying the gate pulses to the related switch. For example it can be viewed by observing the gate-source and the drain-source voltage of a mosfet at the same time. It is always obvious that the Vgs rises after the Vds voltage goes zero. Another practical way to observe may be checking the Vgs voltage. There will be no Miller-plateau in gate-source voltage waveform in a soft-switching mosfet.
09-08-2013 11:22 PM
Top #3
Look at any of the "green mode" controllers. The TI ones have a ZVS mode where the FET voltage resonates to zero, then switches. In reality, you don't have to resonate all the way to zero under all conditions. Why? Under light load, do you really care? It's under heavy load that you're looking to reduce losses the most.
09-08-2013 11:23 PM
Top #4
I'm all for helping people by answering their questions on forums like this one and I regularly do, but please people, don't ask us to spend our time doing this if you are not even willing to do a basic google/internet search on a topic like this that is very well documented by a wide variety of sources from semi mfgs to the IEEE to university papers, and even simply through internet searches. Please try to save the questions generally for specific topics that are not covered in easily gotten papers or books unless you've done at least a minimal search on your own to try to answer the questions yourselves and still need help with the answers, or if the answers in your research don't make sense to you.
09-08-2013 11:23 PM
Top #5
Jack, it may look strange but heaviest EMI occurs at light load, and switches get hot at light load, too. ZVS at light load is one of the biggest problems in power electronics.
09-08-2013 11:24 PM
Top #6
Kamran has made a lot of sense in his comment. Engineers, that have internet access should read the sources already available on the web. It should be all there for the basics. Now, if you want to know about devices that have the best anti-parallel diodes or what to do about it and secondary items,, then ask.
I do not want to stifle the inquisitiveness of young engineers. but some self-investigation should take place first. Dr. Ridley has some of the best power supply engineers in the world on this site. Believe it or not, they do work for a living.
09-08-2013 11:24 PM
Top #7
I think that it is good to start from Series Load Resonant converter (SRC) and Parallel Load Resonant converter (PRC). In these two topologies, the waveform shapes in resonant tanks are very closed to sine wave. ZVS or ZCS are very easy to be realized and viewed.
These two topologies are fundamentals of LLC. With deep understanding of SRC and PRC, it is easy to move to LLC.
09-08-2013 11:25 PM
Top #8
Perhaps someone could take a little time to explain why the LLC is more popular than the SRC and PRC for those not familiar with the technologies.
09-08-2013 11:25 PM
Top #9
If the OP is just looking to understand zero voltage switching, he can just consider a simple sync buck converter operating under load. When the top switch turns off, the inductor current slews the switch node capacitance down until the lower body diode turns on. At the end of the dead time, the low side gate turns on, but there is only a diode drop across the lower FET, so switching losses are very low. From there he can go on to explore resonant toplologies.
09-08-2013 11:26 PM
Top #10
SRC and PRC working conditions are related to many factors and have some limitations. I believe the limitations below are most important.

The output characteristic of SRC is close to current source. Wide range frequency modulation is needed to regulate SRC output voltage. Under light load or no load conditions, it is hard to control SRC output voltage.

The output characteristic of PRC is close to voltage source. Controlling the output voltage of PRC is easier than controlling that of SRC. But the PRC resonant current in the tank is quite high even at light load. This makes PRC conductive power loss pretty high and efficiency low at light load.

LLC has overcome the limitation of SRC and improved PRC the limitation above. I think that is one of the important reasons that LLC is more popular.
08-13-2015 11:36 PM
Top #11
Useful