Current sense transformer to regulate current

[treez]

Hello,
We are dong a 7.4Kw LLC converter battery charger. (Battery voltage is 300vdc to 410vdc) We will regulate the output current by regulating the filtered output of a current sense transformer in series with the LLC secondary. (LTspice sim and pdf schematic attached)

Why is it that none of our competitors is doing this?

 

[moffy]

The only problem I can see is that the charging current for the 165uF filter capacitor, will be mixed in with, what I assume, is the actual load current. Could be hard to distinguish i.e. look noisy.

 

[treez]

thanks, you mean inrush? as you know , that is only at startup, so that will not be a problem. The average of the secondary coil current = the load current, hence filtering the signal at output of CST gives a representative signal of the load current. The "noisy" nature that you speak of will be dealt with by the rc filter shown in #1

 

[chuckey]

As the capacitor ages, its ESR will rise, so its ripple current will fall. So if you are regulating the total current the battery current will rise.
Frank

 

[moffy]

thanks, you mean inrush? as you know , that is only at startup, so that will not be a problem. The average of the secondary coil current = the load current, hence filtering the signal at output of CST gives a representative signal of the load current. The "noisy" nature that you speak of will be dealt with by the rc filter shown in #1

The current I = C(dv/dt) which we all know, but it means that the cap current is a differential of cap voltage, that makes it very peaky and noisy. If you have to filter heavily to remove this then you introduce stability issues into the control loop. Better to use a less noisy current and therefore less filtering/more stability.

 

[FvM]

Looks like a valid option if you don't need high accuracy. Rectifier reverse recovery charge is obviously a limitation.

Why is it that none of our competitors is doing this?

Means you didn't yet notice a secondary AC current measurement in a battery charger. How can you be sure that nobody uses it?

 

[treez]

Rectifier reverse recovery charge is obviously a limitation.

thanks, as you know, this is an LLC converter so there is very very little reverse recovery of the secondary diodes.

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If you have to filter heavily to remove this then you introduce stability issues into the control loop

.
Yes, that's right, that's why we will not filter it too heavily. In any case, if we put a sense resistor after the output cap of the LLC stage , then that is in itself effectively a heavily filtered signal, as the output cap will have filtered it.

The point is, that this is a battery charger, the battery is an enormous capacitance, much bigger than the output capacitance of the LLC stage, so a good bit of ripple could go into the battery, meaning that putting a sense resistor after the output capacitor is going to be a quite noisy place too….admittedly this could be filtered, but that just brings back the problems that moffy explained about filtering and instability