Re: capacitive load on triacs

In article <d4eu7601t62@xxxxxxxxxxxxxxx>,
Winfield Hill <hill_a@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
[...]
> A bit misleading in the OP's case. The di/dt issue addresses the
> case of repetitive low-gate di/dt drive while expecting fast triac
> current rise. This involves conduction-spreading, an issue that's
> reduced with more aggressive gate drive.

In crowbar applications, the worst capacitive load case, the gate drive
really should have a rise time under 100nS. You want to hit the part with
nearly the makers limit for gate current.

[...]
> junction-temp spec for the BT139. As with the dI/dt issue, adding
> series inductance is sometimes helpful because the part's available
> thermal mass increases by the square root of the increased duration
> of the heat pulse.

Also, the inductance decreases the energy that has to be eaten in the SCR.
A small impedance prevents the current from rising to a huge number until
after the voltage on the SCR is decreased. The inductor doesn't even have
to remain an inductor at the full current. An inductor that is lossy to
high frequencies and saturates as the full current seems to work fairly
well to keep an SCR alive. I've used a 0.25 inch toroid of 3F3(I think
from memeory) material with a few turns of #14 wire. The current waveform
looked something like this:

^ Way up off screen
! on the scope
............*......
............*......
............*......
............*......
............*......
............*......
..........**.......
.......***.........
...****............
**................

I think what happens is that the lossiness of the inductor lets an initial
current flow because the loss makes it look like a parallel RL circuit.
This initial current gives the SCR a chance to have its morning coffee
before it needs to do any real work.

--
--
kensmith@xxxxxxxxx forging knowledge

.

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