Derivate Signal which only gives a 0 1 signal changes my results

Hello,

I have a little problem with my model. We are creating a Marx Generator on Simulink. When I close my switches (spark gaps) with a clock everything is working and I get my wished results. But, I'd like that the sparks fires when the derivate (I use discrete derivate) of the increasing capacitor voltage is lower than a setted value. Because, In my opinion, fireing the sparks with a clock is not really related to the practice. When I change the clock to a discrete derivate it changes my results. Even if the discrete derivate only produces 1 or 0 as a signal to close my switches. I can't explain my self how or why this happen. Could someone explain me how I can fix that?

Please take a look at my pictures.

Kind regards

Roman Vogt