Re: bridged ignition coils

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

Original poster: "Stephen Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>

At 07:03 03/05/03 -0600, you wrote:
>Original poster: "Jolyon Vater Cox by way of Terry Fritz 
><teslalist-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>
>
>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?

Yep I suppose that would work... but read on


>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?

Again there's no reason in theory why this wouldn't work. In practice, 
MOSFET transistors have a built-in antiparallel diode, and IGBT/bipolar 
transistors are destroyed if the reverse voltage (emitter more positive 
than collector for an NPN transistor) goes above 5-10V. So a high-voltage 
diode in series with the collector/drain is what's needed here. Now if you 
analyse a bridge circuit made in this way you'll find that you just have 
four instances of the ordinary flyback driver type circuit.

This might have some advantage over a single flyback driver because it 
fires the coil with first one polarity and then the other. In a HEI coil 
where the magnetic circuit is all iron (I assume HEIs have no airgap) that 
might avoid saturation and give you more power. In SMPS applications, you 
can shove about 4x as much power through a ferrite core transformer with a 
bridge driver as you could with a flyback driver using the same core.

But in a normal oil filled can-type coil, there's a huge airgap, so 
nothing's going to saturate. The bridge probably would give no advantage 
over the single transistor circuit.

Steve C.