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Electronics

Test on transformer

Photo of a transformer for switching power supplyI had fun playing a very informative little game with a mains filtering choke which is widely used in the design of a switching power supply, having 2 strictly identical windings.
This is placed upstream of the power supply, just on arrival of the 230V ~ mains voltage, it is only used to clean the mains network from the high parasitic frequencies generated by the switching power supply ( they often operate at frequencies between 40 KHz and 70 KHz ).


The manipulation consists of this:

A function generator delivers a square signal with a period of 75µs, ie f = 13.3 KHz with a duty cycle of 9.33 % in pulse mode, duration of the positive pulse of 7µs on the 1st winding.
The CH2 channel of an oscilloscope is connected to this same winding.
Channel CH1 is connected to the second winding ( see photo below ).

The time base is set to 10µs / div, and the sweep delay is engaged (2 µs / div)

DSC 12729 [320x204px]

 

 

 

 

 

 

 

What should you see on the oscilloscope?
Look carefully at the image that will follow, the effect highlighted here is the basic principle of switching power supplies:

oscillogram

So some details : I deliberately chose a frequency close to 15 KHz rather than around 70 KHz ( frequency commonly used in this type of power supply ... ), because the signal was clearer to view and thus facilitate the understanding, quite simply.
On the photo of the oscillogram, I added the markers in order to understand the conditions under which I carried out this playful manipulation.

In this photo, I used the "sweep delay" function to expand the signal to better visualize it.
oscillogramYour trained eye will not have failed to notice that I only have 3½ squares for the positive impulses, and that it was enough simply to change the caliber of the time base to 2 µs / div to see this impulse even better. .. Yes this is true, but I set my trigger to fire on a rising edge, so my pulse would have started from the far left edge of the screen. To better visualize my 2 signals (CH1 / CH2), I therefore used the pre-selected scanning delay of 2 µs, and shifted all of my 2 signals by 2 squares to the right. Thus we visualize the phenomenon perfectly!

But what exactly is this phenomenon, why does the CH1 signal look like this?
Here is the simplified explanation: an inductor or coil subjected to a DC voltage and therefore a direct current behaves when it is powered up ( rising edge of the CH2 pulse ) like an energy accumulator, and therefore has an FCEM when this voltage disappears.
During the rising edge, the current stored by the choke is at its maximum, and it therefore accumulates energy with an energy peak ( a ~ 130 mV) then a damping until full charge ( I recall for the record that the peak-to-peak voltage of CH2 delivered by my generator is -40 dB, or 200 mv c/c ) .
During the falling edge, there is no longer any voltage or current flowing in the primary of the choke, and the latter instantly restores this energy by reversing the direction of the current, always with an energy peak ( b ~ 130 mV), and with this same damping because the secondary of the choke is suddenly found at the 0V potential.

The following photo shows the energy peaks well:
oscillogram

In fact this is not quite correct, because my x10 probe has an input impedance of 10 MΩ / 25 pF + the input impedance of the oscilloscope which is 1 MΩ / 25 pF.
This is not always negligible ( especially at high frequency ... )

This is exactly the type of behavior that we are looking for in a switching power supply.
I hope that this first manipulation interested you, its goal is obviously the sharing of knowledge.
It can be done by anyone, provided you are equipped with at least an oscilloscope and a function generator capable of delivering impulses.
As usual, I remain open to your questions insofar as I am able to answer you, do not hesitate to add comments if necessary ...

 

To go a little further ... 

another manipulation. on a switching power supply operating in real conditions

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