J
Joerg
- Jan 1, 1970
- 0
Klaus said:I think the issue of oscillation using paralling MOSFETs is relevant
when you operate in the linear mode of the MOSFET. When you drive it
hard on, then there should be no problem (the internal of a trench
MOSFET is parallel devices, isn't it?)
Yes. However, they can send off brief bursts of oscillation during the
transition. I had that at a client even though they used only switcher
modules from reputable manufacturers. I did pre-compliance tests and
mods there. After being happy with the margins I told them to line up
the EMC lab session. There, they failed. Turns out they had changed
manufacturers on the switcher and the unit sent to the lab had that new
type in there. This was why they failed.
The new switcher was letting off 260MHz "birdies" which was the highest
I've ever seen on a switch mode supply. Almost total silence on the
analyzer and then a tall "forest" around that frequency. A shield over
that switcher and some ferrite fixed it. I couldn't improve the switcher
design itself because it would have voided its TUEV/UL cert.
In a current design we are using 8 small MOSFETs in parallel instead
of one big MOSFET. The losses are lower even though the resulting
RDSon is the same, since the MOSFETS cool to the PCB (so there is less
thermal resistance) and there is no hotspot on the PCB (there was
before since the PCB has significant thermal resistance). Since the
hotspots are gone, the individual MOSFET runs cooler. Remember RDSon
is about 2 times larges for a MOSFET running hot). Lastly, we can
avoid the heatsink, since the MOSFETs are distributed on the PCB to
allow the entire PCB to act as a heatsink.
That is smart. Often the cost of seven additional FETs that are smaller
is lower than the heat sink because labor is involved. Did you get away
without gate resistors? No birdies on transitions?