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Re: SCR based coil



Other problems would be
The forward gain (hFE) of power transistors is pretty low (A 2n3055 is only
15, although, that probably isn't a great choice in this application (Vceo
of 60V)). Driving it hard enough to keep it switched on, particularly as
the current comes up, would be quite a challenge.

Bipolar transistors (like the horizontal output ones, for instance), don't
switch all that quickly. So they do dissipate quite a bit of heat in HF
switching service. Say the switch time is on the order of 1 uSec (typical
for a big power transistor). That is 10% of the cycle time for a 100 kHz
carrier (and, you have to switch twice). So, for 20% of your cycle, you
will be dissipating a fair amount of energy.

BTW: The forward voltage drop between collector and emitter is less than
the base-emitter drop. A typical Vce on might be 0.1 V, which for a 15A Ic
would only be 1.5W.

The inappropriateness of the BPT for this application is why motor inverter
manufacturers and switching power supply manufacturers (both of who make
lots of Class D amplifiers using switching devices) have almost entirely
gone to power FET's or IGBT's (more the latter for high power.) As a result
the price on these devices has dropped dramatically. Check out the web site
for firms like International Rectifier for info on these devices

----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: SCR based coil
> Date: Friday, January 29, 1999 12:04 PM
> 
> Original Poster: "Steve Young" <youngs-at-konnections-dot-com> 
> 
> Reinhard,
> 
> I'm no expert on transistor power dissapation, but I am quite sure the
> transistor will only dissapate approximately the collector-emitter
voltage
> DROP times the current.  Thus, if one uses a Class D circuit (essentially
> either completely on or completely off), then the dissapation will only
be
> a couple of volts times the current during the "on" state.  More power
will
> be dissapated during the times the transistor is transitioning from one
> state to another, but this time could be kept as small as possible (fast
> rise/decay times on the rectangular pulse drive).
> 
> --Steve
> 
> ----------
> > From: Tesla List <tesla-at-pupman-dot-com>
> > To: tesla-at-pupman-dot-com
> > Subject: Re: SCR based coil
> > Date: Wednesday, January 27, 1999 10:38 PM
> > 
> > Original Poster: RWB355-at-aol-dot-com 
> > 
> > Hello Nick, 
> >  
> > You wrote:
> > "I wondered if anyone has thought about using big bipolars for the
> >  drive. The transistors used for scan coil drive circuits in TV are
rated
> a
> > 15A
> >  at 450V and cost 0.89 each.  They are therefore perfect for use in
big
> banks
> >  to drive a tesla."
> > SNIP
> > 
> > The transistor ratings you mention above are OR values not AND values.
In
> > other words, the transistors are able to handle either a MAXIMUM of 15A
> or a
> > MAXIMUM of 450V, but not both at the same time. This depends on the
power
> > disapation rating of your transistor. For example if it has a 150W
> rating,
> > then you could switch 15A (max), but only at 10V, etc. 
> > 
> > For a transistor that could switch 450V AND 15A at the same time, it
> would
> > have to be able to handle 6750 watts. That would be a VERY BIG
> transistor.
> > 
> > Plus transistors are slow devices if you compare them to a SCR, etc. A
> > transistor acts like a variable resistor, whereas a SCT acts more like
a
> > switch. Using a big trigger pulse on an SCR will turn on the complete
die
> > (semiconductor surface). This fact plus the fact that an SCR acts more
> like a
> > switch lets these devices handle very high ratings, eventhough the
SCRs,
> etc
> > are not very large (size-wise).
> > 
> > coiler greets from germany,
> > Reinhard
> > 
> > 
> 
>