Because of their increase in efficiency, compared to just using a current sense resistor, especially in high current SMPS's, current transformers are in my opinion the best way to go. However, I still have two questions which I haven't been able to find an answer for, one is theoretical and the other more practical.
Considering the principle of a transfer, and the fact that most current transformers have 1 primary winding and anywhere between 40-200 secondary windings, how come when you use a current transformer to sense the current through the primary winding of an offline SMPS, there isn't an insanely high voltage in the secondary winding, causing the insulation to break down? For example, say I'm building an offline half bridge converter (AC in the Netherlands is a stable 230-240VAC), and I'm trying to measure the input current (estimated to be 1 ampere) flowing through the primary winding with a 1:50 current transformer. That primary current would be stepped down by the current transformer to 20mA on it's secondary winding, but shouldn't that also step up the voltage of 240VAC to 12,000VAC on it's secondary winding? Clearly that isn't the case, otherwise the insolation of secondary winding would break down instantly, arc over, and kill the whole SMPS. So how come a current transformer does step down current, but not step up voltage by that same ratio?
I was also wondering whether there is any kind of logic or formula to calculate or estimate the minimum size of a current transformer for a certain throughput? For the normal output transformer of SMPS's there are tables and formulas to calculate how much power it can handle, but that's much less clear for current transformers. I would have thought that in theory you could use pretty small ferrite toroids, as long as they have a high permeability. They only have to handle a small amount of power, and not the full power of the winding they are trying to sense the current of, correct? Or am I wrong?
Considering the principle of a transfer, and the fact that most current transformers have 1 primary winding and anywhere between 40-200 secondary windings, how come when you use a current transformer to sense the current through the primary winding of an offline SMPS, there isn't an insanely high voltage in the secondary winding, causing the insulation to break down? For example, say I'm building an offline half bridge converter (AC in the Netherlands is a stable 230-240VAC), and I'm trying to measure the input current (estimated to be 1 ampere) flowing through the primary winding with a 1:50 current transformer. That primary current would be stepped down by the current transformer to 20mA on it's secondary winding, but shouldn't that also step up the voltage of 240VAC to 12,000VAC on it's secondary winding? Clearly that isn't the case, otherwise the insolation of secondary winding would break down instantly, arc over, and kill the whole SMPS. So how come a current transformer does step down current, but not step up voltage by that same ratio?
I was also wondering whether there is any kind of logic or formula to calculate or estimate the minimum size of a current transformer for a certain throughput? For the normal output transformer of SMPS's there are tables and formulas to calculate how much power it can handle, but that's much less clear for current transformers. I would have thought that in theory you could use pretty small ferrite toroids, as long as they have a high permeability. They only have to handle a small amount of power, and not the full power of the winding they are trying to sense the current of, correct? Or am I wrong?
