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Current Transformer in ZCS inverter circuit question

anotherbrick

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hello dear forum,

I am trying to make an ultrasonic horn half bridge inverter
I want that inverter operate in ZCS mode

I want use UC3867 ZCS IC
I tried to use 1:1000 CT with shottky diode bridge with 10 K resistor load for driving zero pin of the IC

however when I observe the output current both with Shunt resistor and CT output
I get a phase difference

I fear with this phase difference ZCS IC wont work properly
becouse MOSFET firing timing wont be correct

Is there a way for overcoming this problem or is my fear unnecessary ?

thank you

PS. the osci picture without ZCS IC in operation ( MOS firing from uController )
CTrafo.JPG
 
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mtwieg

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You should post a more complete schematic. It's unclear what a "ultrasonic horn half bridge inverter" is.

As for the current transformer, it's not surprising you see a lot of phase shift. 10kohm is probably much too high of a burden resistor. The burden resistor should have a resistance much lower than the CT's secondary impedance at your frequency of operation.
 

Easy peasy

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Use a 200:1 CT,
--- Updated ---

and make sure the burden R is suited to the V.uS of the CT ....
 

FvM

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Why do you assume that the shunt measurement shows correct current phase? More likely the resistor inductance gives an incorrectly leading measurement, unless you have a precision shunt with verified phase error.
 

mtwieg

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Looks like the frequency is just around 20kHz. Any general purpose shunt resistor should be fine, as long as it's not wirewound.
 

Easy peasy

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L / R should be small for phase accuracy, 10k is too big ...
--- Updated ---

also 10k / 1000 = 10 ohms on the other side - not too good ...
 

FvM

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also 10k / 1000 = 10 ohms on the other side - not too good ...
Impedanced are transformed by squared windings ratio. 10k -> 10m
Regarding shunt measurement, I agree that it can work at 20 kHz. But without knowing the resistance value and probe geometry, I keep my doubts.
 

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Hmmmm, L/R still too low for low phase shift ....
--- Updated ---

Hmmm, say we have 1k, 1mA, 1V on the 1000T side, on the 1T side we have 1A, 1mV, 0.001 ohms - yes this is 1000^2 for impedance - 10k seems high even for a 1000:1 CT ...
 
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anotherbrick

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hello dear forum members thank for answers

according to your advice I took 100 Ohm for burden res and amplified the CT output with high freq opamp
and I rectified after - now there is no big phase shift

however my problem is not solved completely

I planned to use a UC3867 ZCS IC for driving my half bridge inverter
I set the min freq of UC3867 9 KHz and I left error amp pins open and I connected my rectified CT output to the Zero pin of UC3867
first I drove the mosfets with STM32 uC at 20 KHz ( I didnot drive MOSfets with UC3867 )

I was expecting that UC3867 IC MOS outputs would be modulated by Zero pin and
these outputs would be synchronized with the CT output

however this didnot happen and following photo of osci.
UC3867 didnot synchronize with CT and still produce MOS outputs at its own min freq ( not 20 KHz )

my question; how can I make UC3867 IC work for bringing my inverter to ZCS mode automatically ?

sorry this is my best english, hope you understand what I mean

CTrafo 4.jpg
 
Last edited:

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