Zero-current and Zero-voltage switching in resonant converters

[adnan012]

hi,

How Zero-voltage and Zero-current switching are achieved in resonant converters? what is the working principal?

 

[treez]

ZCS converter is like many DCM converters, where the fet current is zero at the point that the fet is switched on.

ZVS , is like in the Quasi resonant flyback where you have “valley detection”, so that you switch the fet on at the point where its voltage is zero and avoid Cds capacitive switching loss……however, its not usually practical to arrange the turns ratio such that vds goes right down to zero for the switch on instant.

True ZVS, where the fet is zero voltage switched at both the turn on and turn off transients is only something I’ve only ever seen in the LLC resonant converter and its variants.

Things like the phase shift full bridge (PSFB) have high turn off losses…….however, some people, put a cap across the PSFB FETs so that the fets get switched off before their voltage has risen up much. –Putting the cap in like that though, creates other ramifications which then need dealing with.


One of the most common “ZVS” converters is the “quasi resonant (QR) flyback”, however, as discussed above, its usually not practical to actually arrange for it to be truly ZVS’d.
In fact the QR flyback’s main advantage is not even usually mentioned in texts that discuss it…..that is, the QR flyback avoids the reverse recovery of the secondary diode at fet turn on. Plain DCM flybacks also avoid this, but without a QR controller, you are likely to end up too deeply in DCM mode and with overly high peak current.
So there you have it, the main advantage of a QR flyback, the lack of secondary diode reverse recovery without all of the associated dissipation issues.

I will send you simulation so you can see working principle…..basically in an LLC you get inductive current flowing, and then “re-direct” it so that it zero’s the voltage across the fet that you are just about to switch on

In an active clamp converter, it is also possible to arrange it to get ZVS of the FET, but again, rarely is it actually worth doing this, and you just use the active clamp for the fact that it resets the leakage inductance in a forward converter without necessarily needing a high side FET.
In the active clamp forward converter, you have a “reset FET”, which you switch on at certain times, so as to let the leakage inductor’s current “go back the other way”, …then when you’ve got it going back the other way, you cunningly switch the reset FET off, and then the leakage inductor current is forced to flow in a path that ends up discharging the cds capacitance of the main power fet…then when the vds of this fet is zero, you switch it on.

So that is pretty much it, in resonant converters you get an inductive current flowing, and of course, as you know, you cannot break an inductive current, so , just before you switch on the relevant fet, you re-direct the inductive current, (perhaps by switching off a fet in which it is flowing) and the inductive current then re-directs and then discharges the cds capacitance of the fet that you are just about to switch on…..to the point where the switch on happens when vds is zero….that is pretty much all there is to it with resonant converters.

…you use an inductive current to discharge the cds capacitance of the fet that you are just about to switch on….of course, since fets have a diode in them, what happens is the diode starts conducting and then there really is very low voltage held across the fet.