Re: bridged ignition coils

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

Original poster: "Jolyon Vater Cox by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>

Isn't there a way of using an H-bridge as a current interruptor, powered
from a low-voltage high current supply, if some "dead-time" is allowed for-
i.e. a there is a period between positive and negative excursions when all
the transistors are turned off?

I understand that in commercial full-bridge inverters encorporating PWM and
dead-time control
the switching devices are normally shunted with anti-parallel diodes to
"clamp" induced voltages to the power rail during the "off" period - a
practice apparently not appropriate in the suggested application / wouldn't
"snubber" capacitors across all the bridge transitors (to absorb the
high-voltage transients and to "tune" the ignition coils) be more applicable
here?

Jolyon




  ----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Friday, May 02, 2003 7:25 PM
Subject: Re: bridged ignition coils


 > Original poster: "Stephen Conner by way of Terry Fritz
<teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>
 >
 > At 10:51 02/05/03 -0600, you wrote:
 > >Original poster: "Jolyon Vater Cox by way of Terry Fritz
 > ><teslalist-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>
 > >
 > >
 > >----- Original Message -----
 > >From: "Tesla list" <tesla-at-pupman-dot-com>
 > >To: <tesla-at-pupman-dot-com>
 > >Sent: Friday, May 02, 2003 12:06 AM
 > >Subject: Re: bridged ignition coils
 > >
 > >
 > > > Original poster: "Stephen Conner by way of Terry Fritz
 > ><teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>
 > > >
 > > > At 07:34 01/05/03 -0600, you wrote:
 > > > >Original poster: "Jolyon Vater Cox by way of Terry Fritz
 > > > ><teslalist-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>
 > > > >
 > > >
 > > > I don't know about the drive circuit though. You need hundreds of
volts
 > > > going into the primary.
 > >
 > >Aren't the hundreds of volts induced in the primary as a result of the
rapid
 > >interruption of the primary current?
 >
 > Yes
 >
 > >Therefore, if the rate of change in the primary current was sufficiently
 > >fast wouldn't the above requirement be fulfilled, and  wouldn't a bipolar
 > >square wave,with a fast rise and fall times, be sufficient to do this?
 >
 > No, assuming that you're driving with an H-bridge like you suggested. The
 > H-bridge never interrupts the current, it just changes the voltage. As you
 > know, for any inductor, V=L*di/dt. With the H-bridge circuit (and the
 > capacitor discharge one) you're enforcing V and the coil determines di/dt.
 > The rise/fall time of the voltage waveform doesn't really come into the
 > equation.
 >
 > With a flyback circuit, that builds up a high DC current and then
 > interrupts it by opening a switch, you're forcing di/dt, and the coil
 > determines V.
 >
 > The upshot is that capacitor discharge and H-bridge drivers need more
 > supply volts than a flyback to generate a given di/dt, in fact about L^2
 > more ;) The classic flyback circuit runs happily off 12 volts, whereas the
 > others need at least 300 for the same spark.
 >
 > Steve C.
 >
 >
 >
 >
 >