Re: TRIAC dV/dT and fluoroescent lamps

From: Larry Brasfield (donotspam_larry_brasfield_at_hotmail.com)
Date: 03/16/05 

Date: Wed, 16 Mar 2005 12:11:50 -0800

"Adam. Seychell" <invald@invalid.com> wrote in message
 news:42380e96$0$29450$afc38c87@news.optusnet.com.au...
> I'm building an electronic fluorescent lamp circuit with the basic lamp circuitry shown below. The input frequency is typically
> 100kHz during normal operation. The starting sequence begins by turning on the TRIAC , and setting the frequency so about 200mA
> flows through the filaments. Once heated the circuit is powered off for about 1ms, and the TRIAC is then allowed to turn off.
> Power is turned back on at the LC resonant frequency thereby producing high voltage across the lamp to start the arc. Once started
> the lamp reduces impedance and the circuit behaves like conventional inductor ballast.
>
>
> o---2.0mH--. ,-----.--------.
> | | | |
> |---| | |
> | | | |
> | | | |
> 180V | | | |
> square wave | | | 1.0nF |
> input | | --- |
> | | --- |
> | | | T2|
> | | | ----- TRIAC
> | | | /\ \/
> | | | -----
> |---| | T1|\___G
> | | | |
> o----------' '-----o--------`
> |
> GND
>
>
> I'm concerned about reaching maximum off state dV/dT ratings of the TRIAC and causing it false trigger during lamp operation or
> starting. The device I have tested is a BT134 with a rated off state dV/dT of 250V/uS (type). Strangely the TRIAC does not seem to
> trigger when exceeding the rated dV/dT.

Between the rating being conservative, device variation,
and your choice of temperature at which to do this test,
there is no need for surprise as this result.

> For example, if I temporary remove the lamp so the circuit effectively becomes an unloaded series LC then the voltage across the
> TRIAC was measured to be 700Vpk at 140kHz. This corresponds to a dV/dT of over 600V/uS across the TRIAC, yet still did not turn
> on. However, the TRIAC will sometimes trigger when the circuit is powered instantly, but if the 140kHz voltage is ramped to 700Vpk
> over a few milliseconds, then the TRIAC never triggers. Is there more to the story than just an off state dV/dT rating ?

Yes, there certainly is. That rating likely applies
during "commutation", when the carrier distribution
differs from what occurs during your testing.

By the way, (if I were you), I would be sure to take
a look at the dI/dt of your circuit. You might find
you are abusing the poor triacs.

And be sure to look at several datasheets unless
it's a hobby project or your employer craves to
get into vanishing-sole-source pickles. Several
manufactures make such a part and they are not
likely to have the same specs or margins. 

-- 
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
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