What kind of transformer is this ? (EEVBlog #1119)

[rf-design]

Hi,
It took us a while to design this transformer, so we will keep it as a puzzle for you to delve into.
The transformer is just for power isolation. We use the optical fibers for communication.
However, the key requirements for the transformer are:
1. It has to have as low capacitance as possible. As several have pointed out, a 500V 8ns edge will go through a capacitor like butter if the capacitance is too high.
2. That it be as balanced as possible - this is so that the common mode output voltage, which could get into the circuit being measured, is minimized.
3. That it be reasonably efficient. We have to put 3W across the gap.
The former is 3D printed, and yes the shape is important, as is the material. The toroid is a fairly standard one.

The driving system is also pretty important, and you can use your imaginations on that 
cheers, Bart

Let me contribute a little bit to the now common puzzle:

1. Common-Mode-Impedance
As somewhere below indicated the isolated scope have a much lower isolated-common to chassis-common impedance. So the DUT iso-common connection should be able to drive the iso-common mode impedance without disturbing this voltage in relation to the iso-probe tip voltage. For medium to high power bridge application the effect could be minor but lower power application could be also affected by the stated but not numbered iso-common mode impedance.
From the local probe-tip common to iso-gnd on the board it is the common-mode inductance of the cable. Typical the probe cables have only resistive center but metalic outer shell. The resistive lead allow a flat higher frequency response with typical 1M scope loads. The common-mode inductance resonate with iso-capacitance. This is in part the stray from the built and the coupling capacitance of the iso-power transformer. So resonate frequency could be within the measurement frequency band and the currents could be still significant if you meet the frequency. One solution is to make also the coaxial-shell resistive to damp the common-mode impedance Q.

2. Symmetric Isolation Power Transformer
Figuring out the equivalent circuit diagram there are 4 stray inductance. 4 coupling capacitance and one coupling inductance. The driving voltage should be as symmetric as possible to minimize dynamic charge injection. This a very important property for low voltage measurements where dynamic charge could disturb uV measurement which are still DC but get shifted by nonlinear high impedances. In Bench-DMM this property is well understood. For iso-scope application not only the PWM should be 50% but also the wave-shape, or rise/fall, of the driving should by balanced. In effect the balance should be on the charge level. So adjusting modulation amplitude to counteract four-coupling capacitance mismatches in the actual iso-transformer is a common procedure in special HV measurement builtups.

3. Iso-Power Transformer Intermodulation
Depending on the stray-cap ratio of the transformer to the board part of the iso-common-mode current will flow through the alternating bridge parts on the driving and rectifying side. This create an intermodulation voltage which is seen also in perfect charge-balance. This could be disturbing of this power-drive signal is seen in the scope-signal an will be more problematic for higher iso-common source impedance from the DUT.

Overall the product is a very reasonable approach for 200MHz measurements with low impedance common-mode drive. At higher bandwidth it is difficult to get the GS/s ADC's to the isolation side. Tek and R&S play the analog-signal isolation game. Fluke is doing analog opto/magnetic. Also if the common-mode terminal gets high impedance the impact could be higher than the measurement value.

There is up to know no golden and cheap solution but we are a step further. The Remaining is my personal puzzle.

Thanks Bart for yours and your group contribution.
BR Reiner