I'm using a current transformer (CS4200V-01L) to measure the current in a half-bridge. I'm facing an issue where the signal out of the current transformer is asymmetric, almost as if the current transformer is DC biased. This is the rectified signal, measured across R42:

Current waveform

Below is the relevant circuit extract. The signal is rectified, fed to the burden resistor (R42), low pass filtered and fed into a microcontroller in the range 0 to 3.3V.


How come the signal is asymmetric? I figured that the current transformer operated as a floating current source - my simulation seems to think it is too.

enter image description here

Obviously there's something else going on here. I thought the transformer may be saturated but the current doesn't look to be higher than 20A and the transformer has a maximum sensed current of 35A. Also it does the same thing at 10A, and I don't think that would explain this problem anyway.

Any input would be appreciated.


Bad assumptions

  1. It is not saturated. It is poorly driven by a dual switch so that the currents are unequal. Show your drive details or else double the Vgs.
  2. A current transformer never saturate from excess current, rather only excess voltage where the typical threshold is -10% inductance
  3. The rated voltage is 1V out using max current input of 35A for \$R_T=5.7\Omega\$.
    • instead you used 40 Ohm Load with sense Voltage on 1K+0.1uF AC coupled, which although not significantly different as you indicated , has exceed the Tesla or weber/m of 1Vsec. /35A.pri.
  4. The fundamental reason of asymmetry is due to your rectifier source not being symmetrical with 2 identical FETs yet different Vgs voltages (apparently) resulting in different RdsOn and compared to 40 Ohms transformed down to primary side presented a significant “Kirchhoff resistance” of I*RdsOn drop in the high side FET.
  5. You assumed we know where your Nodes captured are located
  6. You assumed you had a symmetrical linear circuit
  • \$\begingroup\$ I'm not sure I fully understand - are you suggesting that the transformer is saturated from excess voltage? Would you expect the problem to improve if I lowered the value of R42? On 4, I apologize if I was unclear, the first trace is taken across R42 and the simulated trace is across R5. \$\endgroup\$
    – intecher
    Jul 12 '19 at 18:18
  • \$\begingroup\$ Understand anything yet? \$\endgroup\$ Jul 13 '19 at 6:39
  • \$\begingroup\$ Your drive Peak V is asymmetrical due to RdsOn due to different Vgs not shown and thus load regulation error ( loading) is asymmetric. Got it?? The saturation will begin if you raise the input current not due to current, but due to voltAge on secondary since you have too high a load R for this transformer as specified in the datasheet. Capiche? \$\endgroup\$ Jul 14 '19 at 12:41
  • \$\begingroup\$ I have increased the Vgs from 12 to 15V which has solved the asymmetric drive and hence the issue. I have also resolved separate the transformer loading issue. \$\endgroup\$
    – intecher
    Jul 17 '19 at 16:11
  • \$\begingroup\$ Glad you followed, my advice \$\endgroup\$ Jul 17 '19 at 16:12

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