Measurment of primary inductance and leakage inductance using Oscilloscope

Primary inductance is a simple matter:

Monitor the bulk capacitor voltage while also monitoring the drain current using a current probe. Using the equation V = L*di/dt we can measure the change in current over a period of time to find the in circuit inductance. So the equation becomes V = L (i1 - i2)/(t1 - t2) then solving for L, L = V*(t1 - t2)/(i1 - i2).

Measure di using the current waveform (pick any two points during the on time of the switch on time note their values and the times, i1 and t1, and then i2 and t1). Now you know the in circuit inductance. Depending upon what voltage, frequency and waveform used to measure the inductance the in circuit inductance may vary some from what is measured using an LCR meter.

Measuring leakage is more complex. One way to approximate it is by using the discontinuous ring portion of the drain voltage and the leakage spike. Operate the supply at low line to be sure your clamp circuit is not conducting and at a load to be sure the supply operates discontinuously.

We know that 2*pi*f = 1/sqrt(L*C). Therefore measure the frequency of the discontinuous ring, this is your f. You have previously found Lp, the primary inductance so now you can solve for C. This is your interwinding capacitance.

Now using the leakage spike portion of the drain voltage, again measure the frequency but this time you will assume the inter-winding capacitance is the same and solve for L to find the leakage inductance. Llkg.

I know that this is a long and complicated explanation so if you have any questions please feel free to ask.

-Wesley