Re: SCR based coil
I really have to jump in and say something here:
> Original Poster: "Jim Lux" <jimlux-at-jpl.nasa.gov>
> 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.
That gain is only valid for certain values of collector/emitter
current. Gain usually falls off as the current approaches the device
maximum. Check the characteristic curves out.
> 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.
Not only that, but you have to derate the device as die temperature
rises beyond 25C.
> 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.
Not necessarily true. Vce(sat) climbs as collector current does. It
can easily exceed a volt near the maximum device current.
> 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
> > DROP times the current. Thus, if one uses a Class D circuit (essentially
> > either completely on or completely off), then the dissapation will only
> > a couple of volts times the current during the "on" state. More power
> > 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
At 15A collector current, that amounts to 30W e.g.
> > ----------
> > > 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
> > a
> > > 15A
> > > at 450V and cost u0.89 each. They are therefore perfect for use in
> > banks
> > > to drive a tesla."
> > > SNIP
> > >
> > > The transistor ratings you mention above are OR values not AND values.
> > > 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
> > > 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
> > > switch. Using a big trigger pulse on an SCR will turn on the complete
> > > (semiconductor surface). This fact plus the fact that an SCR acts more
> > like a
> > > switch lets these devices handle very high ratings, eventhough the
> > etc
> > > are not very large (size-wise).
> > >
> > > coiler greets from germany,
> > > Reinhard
Unfortunately, SCRs have di/dt limitations to prevent hot spotting of
the die. I have some Westcode devices rated at 2kA+ and 2.5kV. Even
with rather impressive looking di/dt ratings, these devices could
only replace the gap in a coil running hundreds of amps in the
primary at very low frequencies in the 10 - 20kHz region.