How bad is my low power fly-back Transformer?

I am designing a DC-DC converter for impedance matching of my piezoelectric energy harvester with flyback converter. The switching frequency of the dc-dc converter is about 1kHz. The voltage is around 20V and current is about 2mA.

I made a transformer using EPCOS EFD 20/10/17 B66417U0160K187 core with air gap. It has one primary and two output windings. One output (secondary-1) winding is used to power the wireless sensor. Second output (secondary-2) is used for oscillator (switching for MOSFET) and start-up circuit.

I took the data using the Agilent Impedance Analyzer. In the attached figure, the input impedance, inductance are measured from 100Hz to 4kHz.
Then new lines were plot by shorting both the output winding. Equivalent circuit values are also shown.

I want to know, can anyone told me how bad I wound my transformer? Is it possible to measure leakage inductance from these graphs?


Kind Regards
Hamid
Hanyang University
Korea

1. Why is the switching frequency so low?
2. Is the difference between the input and output voltage large? If not, then fly-back topology should be avoided.
3. Why is there an air-gap? Fly-back transformers must have an excellent coupling. The gap will reduce the coupling dramatically.

To comment on whether the leakage inductance is in a normal range or not, the turn ratio must be known. To give a rough idea, for a turn ratio of 1:20 and magnetization inductance of 50uH, 350nH to 250nH of leakage is a "normal" value.

The primary input inductance is 15mH, having 323 turns, DC resistance of 28-ohm using 0.1mm dia wire of AWG-38. I need to do impedance matching with Piezoelectric material, which requires this inductance value.

All the papers i read on Piezoelectric energy harvesting, they use inductors or fly-back transformer as coupled inductor for impedance matching and voltage level conversion. The coupled inductor use gap to store energy. Piezoelectric material requires high value of resistance for impedance matching and have low output power (around 1mA with 10V, i connected many in parallel to get desired voltage and current values).

I have three winding in transformer, I couldn't find any simple technique/practical method to calculate the leakage inductance and efficiency of transformer. One loss in my transformer is due to high dc resistance of 28-ohm, i can reduce it though.

Switching frequency is low to reduce the power consumption low.