The thing that sticks out to me is that the CT swings negative than positive, whereas I expect the opposite. Could the CT be wired backwards?1. Input current 0.522A Yellow - measured across Rcs (20R); green and red at QA and QD gates.
2. Input current 0.517A Yellow - measured across Rr (20K)
View attachment 170798View attachment 170799
1. Input current 1.016A Yellow - measured across Rcs (20R); green and red at QA and QD gates.
2. Input current 1.012A Yellow - measured across Rr (20K)
View attachment 170800View attachment 170801
If I understood you correctly, as I remove 20K resistor I am not limiting the reset voltage anymore? So if the problem is saturation this idea should work for me. Reset voltage will develop at stray capacitance of CT and if my diode is the tough one, it should survive it. Am I got you right?Something is limiting the reset on the CT, for starters remove the 20k resistor, make sure the CT diode is 100V - ideally a 1A schottky, it needs to be very fast ( Trr < 30nS )
As I mentioned above, I set dead time at 500 ns for now, so it should be enough?If we need to reset in 200nS say, the reset volts need to be at least 4uS/0.2uS x 2.6V = 52V for the whole 200nS ( the capacitance of the CT wdg may inhibit this - requiring a longer reset time ).
Yes I checked transformer using pulse generator and scope, transfer ratio is 50:1 @ 200 kHz. I also double checked the diode.We can possibly assume the CT is damaged, i.e. nearly shorted turn due to flash-over from being run open ckt.
and/or the CT diode may be damaged / inappropriate.
Main transformer will work @ 350VDC at primary as nominal value. Now it is less than 200 volts, so I don't think this is problem for current sensor.Just looking at the ETD54, 25T, 400V (?) 292mm^2 Ae best case, gives 110mT each way for flux
at 125kHz, this will give rise to some core heating - unless you have some very good ferrite, e.g. 3C96 -> 4.3 watts min core heating.
Thank you so much for all these materials, it will be useful for me in the future!Yes, as pointed out above by Easy Peasy you are using unipolar CST and you have no reset...you need diode/zener reset on the secondary, to give you the reset volts that Easy Peasy describes.
The attached gives some theory on CSTs
I picked the unidirectional solution because I saw it in the datasheet and it should work fine for my PCMC purposes. I know, there's a lot of methods to sense the current but I want to understand the unidirectional solution first.You are best not using the “Unidirectional” type of CST, which is also shown in the schematic, because the current in the branch between the input capacitor and the primary FETs is NOT unidirectional, but it is bi-directional, and even worse, it is non-symetrical. (-the current is not unidirectional because of the “back-flowing” leakage inductance current.)
I don't have a gap in ETD54 core, I have shim inductor 30uH in series with main transformer.I also hope that you have a gap in your transformer core (ETD54)......its required for PSFB's...because they are lovers of magnetising current....which they slosh back and forth and get zero voltage switch on.
My Vin is 350VDC nominal and 380VDC max. I am running from a TDK Lambda PSU for now, and then I will be using PFC circuit. Thank you for a useful information once again!You dont say your vin....but you have caps in series at the input...so are you running off a rectified three phase output?....if so, there is some good knowledge on that here, and on the PSFB
In my opinion crucial disadvantage of this "AC" solution is that CT in series with main transformer is not detecting a shot through currents in one of the legs.CT's can be successfully used in series with the main Tx, and in the opinion of some PE engineers - work better than in the DC feed - mainly due to AC flux swing and that the power stage can go to 180 out of phase....
Yes, please see my picture in previous post. For any coil, volt*seconds ON = volt.seconds OFF and this mean that areas under the curve must be equal. As I have a dead time 500 ns, I must provide a sufficient amplitude of voltage on the secondary of my CT during this 500ns reset time (to make V*s equal). A 600V diode can handle up to 600V during this reset time so V*s can be huge. It is the way i see things. Zener could clamp this voltage to ensure diode will not be damaged during this reset time. This is what I meant.To ensure a transformer is reset, you need , for any coil, volt*seconds ON = volt.seconds OFF.
I am not sure what you meant by saying you have a 600V diode involved in reset of your unipolar CST? (or have i got you wrong?..if so, apologies)....the docs i sent you show the diode/zener reset method being used.
The thing that sticks out to me is that the CT swings negative than positive, whereas I expect the opposite. Could the CT be wired backwards?1. Input current 0.522A Yellow - measured across Rcs (20R); green and red at QA and QD gates.
2. Input current 0.517A Yellow - measured across Rr (20K)
View attachment 170798View attachment 170799
1. Input current 1.016A Yellow - measured across Rcs (20R); green and red at QA and QD gates.
2. Input current 1.012A Yellow - measured across Rr (20K)
View attachment 170800View attachment 170801
Hello, mtwieg. Strange, It is seems fine to me. CT swings positive first and negative then. Am I wrong?The thing that sticks out to me is that the CT swings negative than positive, whereas I expect the opposite. Could the CT be wired backwards?
Yes, im sorry, if i didn't make it clear. QA and QD have no phase shift (diagonal pair). Tx exposed by maximum duty cycle - D clamp.There appears to be no phase shift .... ?
I will repeat my measurements on Tx this time, sorry for confusing you. I thought gate signals on QA and QD will be enough.try showing the volts across the Tx, CH1 - CH2 .... you won't see much without some actual phase shift.
Yes, you are totally right, my mistake is to put 20k instead a 2k resistor. Will change it.R72 helps to demagnetize the CT. Recommendation is R72=100*R74 as i remember. why you put 20k?
I took this values from DS. DS says this filter has a pole @ 482 kHz. Seems fine to me.is the R73,C62 time constant ok for used freq?
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