Re: Calculating the drain I - SSTC

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "S.Gaeta" <sgtporky-at-prodigy-dot-net> 

Hi Terry,

Thanks again for the calculations. I am building the circuit on vector
board, and last night I thought it was rediculous that I used 16 AWG
stranded wire to go from the H bridge to my terminal block which will
connect to the 10 AWG primary (it's about a 3" run). I was going to double
up on the wire, but I guess with 4 amps, it's not really going to be a
problem the way it is.

A kind of bizzare thing happened this afternoon. I built the 4420/4429 part
of the driver, and tested it out by feeding a 200 KHz, 5V square wave from a
function generator into it. I was very pleased to see that I phased all the
driver transformer windings correctly, and the MOSFETs will switch when they
are supposed to. There was no voltage applied to the drains of the MOSFETs.
I noticed that the drivers were only running very slighty warm. I was
noticing that I was getting a slight overlap on the two wavefoms because the
rising, and falling edges were slightly sloped on the output of the driver
transformer, but not on the input of the driver ICs. This really troubled me
as this slight cross conduction will cause big problems later when I power
the MOSFETs. I was trying to think about how I would be able to alter the
duty cyle but, my thoughts were interrupted when my waveforms suddenly
turned to crap because one of the drivers stopped working. I was powering
the 4 chips with 15 volts DC, and they were only drawing a total of about
200mA. Now that shouldn't have happened! The only thing I can figure is that
maybe they didn't like impedance of my signal generator. Wait a minute....
At one point I was varying the frequency all over the place just to see what
would happen, and if the cross conduction would go away (it didn't).
Something in the circuit would whistle when I hit 11 KHz (Yea, who switces
at that frequency!). Sometimes I get a little too brazen, and foolish when I
play with the little low power stuff.

All the best,
Sue

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Saturday, December 27, 2003 3:02 PM
Subject: Re: Calculating the drain I - SSTC


 > Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>
 >
 > Hi Sue,
 >
 > The #10 wire is 0.1019 inches in diameter.  The skin depth is 0.00553
 > inches.  So, we can find the conductive area and the resistance.
 >
 > area = pi x r^2
 > outside = 3.14159 x (0.1019 / 2)^2 = 0.008155 square inches
 > inside = 3.14159 x ((0.1019 / 2) - 0.00553)^2 = 0.006481 square inches
 >
 > 0.008155 - 0.006481 = 0.0016739 square inches of conductive area.  That is
 > the same area as #17 wire (.0016089) which has a resistance of  0.00516
 > ohms per foot.  At 4 amps, every foot of wire will dissipate
 >
 > 0.00516 x 4^2 = 0.08256 watts
 >
 > It is really not that exact, but it should be sort of close.
 >
 > So if I have not messed up anywhere, your wire should stay cold and have
 > only tiny loss.
 >
 > Cheers,
 >
 >          Terry