Harry Dellamano wrote...
Winfield Hill wrote ...
Would anyone care to elaborate, contrast, compare, and
expound on the relationship between the ZVS and ZVT
dc-dc converter concepts, approaches and technologies?
Etc., etc. [end, boki-style question.]
Hi Win,
ZVT is normally a fixed frequency switching PWM, with rise and fall times
controlled by LC networks to allow the switches to be changed with zero
volts across them. ZVS is normally variable frequency switching in which
one state controls input energy charging and the other state controls the
stored energy discharging to the output.
In HS physics class they told us that if you try to move energy from a
charged input cap to an uncharged output cap with a switch you would loose
half the energy, conserve charge and generate a lot of EMI. If you add an
inductor in series with the switch and open the switch at zero current,
all the energy will be transferred to the output cap with no EMI. Now this
switch is opened and closed at zero current to allow low losses but ZVS
normally employs ZCS.
boki style answer
harry
Thanks very much Harry, for your boki-style answer! I should have said
contrast and compare phase-shift ZVS with ZVT; both are fixed-frequency
approaches. Actually, Unitrode/TI in discussing the UCC3895 phase-shift
controller IC calls it ZVT switching, whereas LTC with their new LTC3722
calls it ZVS switching (or say ZCT and ZCS, as appropriate, natch).
In one IEEE paper* covering both ZVS and ZVT (few articles use both terms),
there was a distinction made with "soft switching," another widely-used,
under-defined term. Elsewhere a suggestion was made that FETs associated
with ZVS can and should have low-gate-current turnon (given that they're
really in ZCS mode), yet employ high-current hard-switching turnoff (given
that full current may be flowing at turnoff). Both these sources showed
capacitors added across the FETs, not as snubbers (no series resistors,
etc.), but as circuit elements. Are capacitors key soft-switching parts?
I'm interested in high-frequencies, e.g. 600kHz, and have been using the
FET's intrinsic Coss capacitance; although nonlinear, it is natural. :>)
Another issue is that at high currents we're not really talking resonance
with 1/4-cycle waveforms, but rather dV/dt = I/C, which results in faster
rail-rail transition times at high currents and pushes one toward adaptive
turnon delays. (It should be pointed out that in my present project design
an over 100:1 power-delivery range is involved. Design for full-power soft
switching and let the chips fall where they may at low power?)
More about role of soft-switching ... boki-style responses please!
* Maria D. Bellar, et. al., "A Review of Soft-Switched DC-AC Converters,"
IEEE Trans on Ind. Apps, 34 - 4, pg 847-860, July 1998.
Thanks,
- Win