How can I increase inductive spike voltage?

Question

I would like to know how I can increase the maximum voltage of inductive spikes.

I’m investigating their use in electrolysis - the bigger the spike the better. To create the spikes, I’m using an IRF840 MOSFET and a 40mH coil as in the circuit shown below.

As I understand it, the peak voltage of the spike will depend on the coil inductance and also di/dt which will I assume will be a function of how fast the FET shuts off.

My coil is fixed with an inductance of 40mH but I have six of them and yet, if I string them together to make an inductance of 240mH, the peak voltage remains roughly the same, maybe because the total resistance of the coils is now about 10ohms compared to 1.5ohms for a single coil.

So the other option would seem to be to encourage the FET to shut off faster and for that, based on another post in this forum, I thought one can use a ‘pull-down’ transistor as in the following diagram.

Are there any other circuit modifications I could make to increase di/dt, including perhaps a different FET for the job? At the moment the peak voltage is a bit over 220V and I'm hoping it can be made over twice that.

Answer 1

In fact the voltage at the drain of the MOSFET will begin to "ring" at the resonant frequency of the capacitance and the inductance. The capacitance comes from the MOSFET drain capacitance in parallel with the distributed capacitance of the coil.

The peak will be higher if the capacitance is lower, the on time is very brief and the resulting current peak very high.

In practice, a boost of more than 10:1 or 20:1 is difficult to achieve (and unsupported by most controller chips- the 'on' time becomes very brief), so a transformer (or tapped inductor) makes more sense than trying to use your circuit.

Or, if you want to avoid using that kind of inductive component, you can use two stages to easily get higher ratios.

Answer 2

"Bigger" depends on how to define this.

Voltage peak, V x t ,[volt-seconds], or power = V²/R = V²(f,t) / (real{Z(f,t)})

There are a few basic rules to electronics.

• All insulators = dielectrics
• this includes the interwinding capacitance of large inductors and the dilectric medium (Water has a Dk=80, Air =1) so bigger chokes is not always better.

Here's a simple experiment.

Use a rasp file as an intermittent switch with a nail slide and a decent air coil with a spark plug and close the electrical circuit with the series loop current using a tap off the larger coil which is open to the spark plug as a step up transformer. 1:10 to 1:100 using a good low ESR battery. Avoid shocks and battery heat. The low C of Air and low L of an air coil will give a very high f resonance yet a very fast rise time and voltage . 2~3kV/mm for a 3mm spark plug = 6~9kV

• i.e. they all have capacitance and a relative value of Dk compared to air which is close to the vacuum reference. But with a low impedance 12V LED String with reverse diode protection, you can power it from a D Cell temporarily this way. Just remember polarity , you make the low side intermittent to generate a positive spike when current is released from L.

Vp = LdI/dt and dV/dt= I/C which combine give the resonant frequency and impedance ( look up)

• meaning they have a Capacitance per unit area of the electrodes volume which increases with reduced gap, as does the breakdown voltage in kV/mm. Hydrogen also has a lower explosive limit of 4% with some trigger voltage.

• the lower the FET RdsOn the larger the Coss during transition , although this has a geometric and packaging variation as well as Voltage rating so examining 50k different varieties of FETs with a spreadsheet from D-K can find the best for T=Ron x Coss time constant not given in datasheets.

• There are many basic ways to chemically separate atoms in molecules such as electrolysis and it can can be done with DC, AC and HF pulses.

I am by no means an expert on Electrolyser design expert but you can learn what has worked for certain parameters of size and efficiency then match or try to improve it with electrode materials and methods.

With a free registration, you can read how H2 Electrolyzers work. https://www.researchgate.net/publication/341293476_Proton_Exchange_Membrane_Electrolyzer_Modeling_for_Power_Electronics_Control_A_Short_Review#fullTextFileContent

• unless you are trying something different.

You can use a MOSFET Buck regulator or DC

There is a more than a quarter century of books and research papers on hydrogen cells yet this has not stopped Engineers and Chemists to keep improving the efficacy and safety of this next generation fuel cell.

If it was this simple, it would have been perfected long ago.

But if your goal is simpler, it's not that hard, just refine your specs. (not given) and Re-google ehhm, I mean research. ;}