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20th March 2017, 17:49 #1
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Calculating current in pulsed/square wave DC circuits
I would just like some one to confirm whether or not 1/(2PifC) and 2PifL of close enough approximations for impendence with a DC square wave.
Or, if this is not the case, what are the correct formula
I.E. So that I can then use Ohm's law to calculate the current....at least roughly.

20th March 2017, 17:49

20th March 2017, 18:04 #2
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Re: Calculating current in pulsed/square wave DC circuits
Hi,
Both formulas are for pure sine waveform without DC compnent.
The problem with capacitors:
At the (considered infinite) high rise rate at the edges of a square wave you theoretically get infinite current.
But this is not possible.
There will always be something that limits the current. Maybe stray inductance, ESR, or other series impedance.
**
The problem with inductance.
You say DC. Thereore I assume it is not symmetric (only zero and positive, but not positive and negative).
If so, then the current is determined by the DC resistance of the inductance. The coil inductance becomes irrelevant after a short time.
The current may become very high.
In both cases you can´t determine the currents.
Even if you can give all electrical values to be able to calculate the current...
* you should say which current you are interested in.
* average, ripple pp, RMS including DC, RMS excluding DC..
Klaus

20th March 2017, 18:04

20th March 2017, 22:24 #3
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Re: Calculating current in pulsed/square wave DC circuits
Hi,
This doesn't answer the question, more related to the thread title, I think you can calculate the load current from the duty cycle for squarewaves, e.g. momentaneous current multiplied by the duty cycle "on" time.

20th March 2017, 22:24

20th March 2017, 23:13 #4
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Re: Calculating current in pulsed/square wave DC circuits
You can solve your question by 2 methods:
1) If the circuit is not too complicated, solve the steady state current in the time domain... and there you have it.
2) If the circuit is more complicated and linear (linearized if it is not.. in order to be able to apply Superposition Theorem), then, simply solve the relevant circuits in the sinusoidal steady state and 1 circuit in DC steady state and there you have it. The current is simply the sum of those other currents in those circuits because of the Superposition Theorem.

20th March 2017, 23:41 #5
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Re: Calculating current in pulsed/square wave DC circuits
I will be more specific. It concerns the following schematic. I want to know, at least approximately, if the peak current flowing through C6 and the UCC373 chips. Because, for convenience reasons, I am using a single UCC27425 that has both noninverting and inverting on the once chip. However these have a maximum current rating of 4.5A rather than 9A.
It has been rather surprising to me how bloody hard I have found it to find any SPECIFICS on this sort of thing, while searching the net. And even the answers in here don't seem to give me any SPECIFICS that will enable me to gain a decent understanding of how to perform this sort of calculation.
One detailed worked example with this specific schematic, with some explanations about each step in the calculation would be GREATLY appreciated and will set me up for all similar circuits in the future. Assuming that the supply voltage is 12V (as per the spec) and the duty cycle of U5 is 90%.
My problem is that I am not formally trained in electronics and there are significant gaps in my knowledge. So just a general description of how to do the calculation, as in here so far, is not enough.
The author of this schematic must have done some calculations to find the right variety of gate driver to use, as opposed to just trying different chips until he found one that didn't blow up.Last edited by boylesg; 21st March 2017 at 00:09.

20th March 2017, 23:41

21st March 2017, 01:53 #6
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Re: Calculating current in pulsed/square wave DC circuits
Oh and the primary coil of the GDT or gate drive transformer has an inductance of 0.12 mH.
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