Pulse transformers and V.dt ratings

Your insight and comment would be much appreciated.

Edit: my question does not come through clearly enough, and the waveform pic seems to be confusing as it is not actual but an illustration of a waveform for a saturated transformer, similar to what I had observed. I will grab some actual waveforms and add them to this post.

To be clear about my question: Am I correct in calculating the observed V.us as 190 and if so then why does the transformer exhibit a V.us of 190 when it has a rating of 15.

• Specifically, what is confusing you? Your waveform picture is not 50% duty cycle and that confuses me. Oct 9, 2016 at 13:42
• The waveform is an illustration of what I'm observing and interpreting as saturation in the transformer, not an actual capture, and this effect startsbeing observable on a scope at say 53kHz for 50% DC, and at lower frequencies for lower or higher DC. Specifically I am asking why does the transformer with a Vus rating of 15 work at an apparent rating of 190. Oct 9, 2016 at 13:48
• Looking at your waveform picture I visually estimate that beyond 3 us, it is saturating therefore I conclude 30 volt.us and certainly not 190. Oct 9, 2016 at 15:02
• The transformer does not see a flat 10V because of R4 10 ohms (and to a lesser extent C4). So the V*us rating required needs to be calculated different (I don't know any simple formula offhand). Oct 9, 2016 at 23:27

Am I correct in calculating the observed V.us as 190

No, you're not, I think. For simplicity, let's consider 'worst case' with 50% duty cycle (in fact it is not worst, all duty cycle are equivalent, but let me not go to such details at the moment).

First, transformer coil (because of C4 Cap) gets only AC component, so it magnetizes from -V.us to +V.us which effectively doubles available span.

Second, at 50% duty cycle C4 charged to the half of the driver supply (i.e. to 5V DC). So, primary winding gets only 5V amplitude).

Third, saturation should be prevented only while half of the clock period, because while the other half transformer magnetizes to the opposite polarity.

Hence transformer should have 0.5*5V*10us=25 (not 200) V.us product to avoid saturation. Datasheet states for Minimum V.us, so concrete sample may have value, greater than 15.

So, those are reasons, which in my opinion may explain why you find it works well at 53kHz.

In first switching pulse, considering capacitor discharged, transformer will see 10V at starting pulse; at 50kHz, 50% DC, one pulse have 10us, V*time product is 10*10=100Vus; this may lead to saturation at least in first few cycles. After "stabilization" flux excursion from 1 to 3 quadrant, Vus product decrease to about 5*10 V*us. Maybe 15Vus stated in datasheet have a high margin and work even at 50Vus.