Hi Graham,
Pooh said:
Terry Given wrote:
Hi again Terry.
I had been thinking simply along the lines of 'hard switching'.
Sounds to me like you're talking about something that uses some resonant
converter type thinking.
When you do a hard-switched full-bridge converter (FBC), you (should)
organise a small amount of deadtime between the upper and lower
gatedrives of one half of the bridge, to avoid shoot-through (both
switches on simultaneously, shorting out the DC bus). Often this is done
with asymmetric gatedrives, eg a diode across Rg for slow turn on, fast
turn off. Good PWM controllers allow you to set this dead time
I'm following you part of the way at least. The trick is to ensure that the
mosfet/igbt that's turning on does so at a low Vds / Vce it seems. That then
reduces turn-on losses.
A FBC driving an inductive load always (somebody will bite me for that
statement
turns on at zero-voltage: when the lower switch turns off,
mag current commutates to the diode of the upper switch. Then the upper
switch only sees a diode drop before it turns on - voila, ZVS. Alas the
lower switch gets a hiding at turn off. Often the leakage inductance
alone is enough to make this happen.
The idea then is to increase this dead time (say from 200ns to 1us), and
slap a big(ish) cap across each FET. The Lmag + Lleak then makes the
voltage on the lower FET slowly (thanks to the biggish cap) rise (LC
resonant circuit), so the lower FET has time to turn off while Vds is
held low by the cap. You can make it work with the FETs own capacitance,
but swamping it with an external cap is a very good idea in production,
as FET capacitance is not generally a controlled parameter.
Of course this resonant transition occurs even in a FBC (there is always
some L & C) but its usually way too fast to help turn-off. At very high
PWM rates (MHz), no external caps are necessary (see above caveat).
You'd have to control leakage inductance pretty carefully to do this surely ?
yep. or add an external L. Leakage is controlled entirely by winding
geometry, so if you can make that constant, leakage will be too.
Often simply changing the winding topology is sufficient, but beware
proximity effect - for example moving from a sandwiched winding to one
atop the other will significantly increase leakage (4x or so, IIRC
leakage is proportional to the square of peak MMF), BUT proximity effect
may get a whole lot worse (the number of effective layers just doubled;
with a sandwiched winding 0.5S-P-0.5S the effective number of layers in
the Primary is half the actual number). At high power its more efficient
to design an optimal, low-leakage transformer, and throw in a little
series L. Usually its quite small.
on proximity & skin effect:
proximity effect = skin effect, but caused by currents in adjacent
conductors.
With skin effect only (say a 1-turn winding on a choke) if you increase
wire size, DCR goes down. But the ac-dc resistance ratio goes up, and
the total resistance remains constant (ignoring the optimum point, which
isnt a great deal better). OTOH more copper = better heat conduction, so
dT will drop a bit.
With proximity effect its very non-linear, and strongly depends on how
many layers in the winding. I once re-engineered a 1500W 400V - 24V
dc-dc transformer. It had 12 layers of 0.6mm Cu foil, running at 100kHz
(delta = 0.2mm) so the foil was three skin depths thick. The windings
were interleaved, so the effective number of layers is 6. Using Snelling
fig. 11.14 the Fr (ac-dc resistance) ratio is about 80 - i.e. the AC
resistance is 80x the DC resistance! These transformers set their
windings on fire (the bobbins didnt burn, but were totally destroyed).
I dropped the Cu foil down to 0.1mm, ie DCR increase 6x. But Fr drops
from 300 to 1.3 (told you it was non-linear), so AC resistance = 7.8 x
original DCR, compared to 80x - in other words the AC copper losses went
down by a factor of 80/7.8 = 10x. The >> 200C temperature rise at full
load dropped to around 45C. And all the doubting thomases who had said
"rubbish" to my solution were forced to eat a decent size piece of
humble pie. Being the modest chap I am, I gloated mercilessly.
You could reduce turn-off losses if the load current decayed during the power
transfer cycle too I guess.
reductio ad absurdum: remove the load completely, turn-off losses will
plummet
There's a practical example I have that may be doing this. It has an LC in series
with the TX. I thought it might be a fully resonant converter but testing it on
the bench suggested not. I was hoping to extract the 1 page of 5 from a pdf
schematic to illustrate. I can probably do this toimorrow. Would welcome your
comments.
Regds, Graham
will be interested to look.
Cheers
Terry
