Re: Full Bridge SMPS mosfet selection criteria...
- From: legg <legg@xxxxxxxxxxxxxxx>
- Date: Sat, 25 Apr 2009 02:46:07 -0400
On Fri, 24 Apr 2009 20:21:57 -0500, "mook johnson" <mook@xxxxxxxx>
wrote:
When selecting a Mosfet for a new full bridge (or half bridge) design, what
criteria do you look for? Assume cost is not an issue and your looking for
someting in a to-220 package.
VDS rating is fairly obvious.
How much do you overrate the current capacity of the device for reliable
design?
What about the CISS, CRSS, and COSS?
in my application I have high voltage (300 - 600Vdc) but low power. The
mosfet currently in use (IRFBG30) don't follow the gate signal at low output
power levels on a full bridge design. When the gate turns off the voltage
does not fall immediately and ramps linearly down not quite to half way
before other side turns on.
I'm assuming this is the output capacitance of the msofets that are holding
the volage after the channel is turned off and a fet with lower capacitance
would be better.
This design is dreadfully inefficient in this mode. Its pulling 60mA at
500V (30W) to deliver 10W of power.
When the load is increased to 40W the fet signals clean up nicely and
efficiency goes up in the 80% range.
I have checked the output inductor and it is not going discontinuous at the
10W load.
any thoughts? Lower Coss Fets or something else?
Oh magnetizing inductance of the primary is 20mH.
Be carefull where you measure currents for power calculations, as
errors are easy to introduce in pulse circuits, with varying waveforms
of mixed frequency components.
Losses that accumulate to 20W in a small converter should be easy to
locate, simply through temperature rises. This is particularly true if
the power is being lost in the switches alone.
If this is just a DC transformer circuit, without pwm, a higher
magnetizing current can ease switching at lower power levels. Any
switching assistance that reduces the drain voltage to half Vcc or
less, is doing it's job.
Using a fet rated above 800V in a 600V circuit is probably overkill.
An IRFBE20 might work better here.
IGBTs might be suitable if the frequency isn't too high and
freewheeling diodes are present. Off-line ballasts for flourescent
lamps (to 20W) still use bipolar parts...
RL
.
- References:
- Full Bridge SMPS mosfet selection criteria...
- From: mook johnson
- Full Bridge SMPS mosfet selection criteria...
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