The key word here is "normalised". Whatever value Rds(on) is at 25C is taken as unity and the plot shows how this changes with temperature. So a graphed value of 1.1 would indicate a resistance of about 15mOhm x 1.1 =3D 16.5mOhm. If the actual value for that device is 10mOhm, then you could expect it to increase to 11mOhm. The data sheet should give you thermal conductivity values that will let you estimate the die temperature but a heatsink temp of 34C or so would normally indicate that there is plenty of headroom. Another thing to check would be the drain peak voltage. Make sure the snubbers are working properly and can handle the power levels reqyuired. If the snubbers go o/c then the peak voltages could be punching through the MOSFET & destroying it. Putting addional R-C snubbers wouldn't do any harm. The MOSFETs are 100V types so you should be OK here provided things work as expected. Also, the snubbers seem to be low values. 4n7 & 100ohms could use quite a lot of power (Although maybe not too bad on a 12V supply). Not sure about the return to the source of the MOSFET either, across the transformer to the plus line would be more conventional?? On the plus side, the snubber current is going to pass through the transformer and so assist the output. Maybe. It may also contribute to the ringing you're getting. Certainly it will show up as a component of the soure resistor current. Anyway, It sounds to me like the inductor is saturating. Can you try a larger core? RP On 27 June 2016 at 19:21, Manu Abraham wrote: > Thank you guys for the info. > > I watched the video. The probe issue according to the video, exhibits > anomalies by around 400Mhz. I am nowhere near that frequency at > 120kHz. The next thing that dawned upon me was to adjust the trimmer > on the probe. but that didn't seem to make much or any effect at all. > Maybe it did make a bit change, but it was not visible enough for me. > The scope probe came with a spring clip, that could be placed on to the > barrel. Tried looking at the Gate waveform, but no obvious change at all. > > With a bit of impatience, not seeing the other end, wanted to give it a r= un > for a while. > > With a 23W load at the output, ran it for a while at 15% dutycycle withou= t > the feedback loop. With a thermometer on the heatsink. Starting from 31C, > the heatsink warmed up to 42C. It ran well for 10min. The current drawn > was climbing, but became steady at around 3.63A. The current drawn was > ramping up and down, but it was hovering around the same values. The > output voltage was slightly lesser than what I expected it to be (~ 280V. > At some point it fell as low as 230V as well.). > > Retried the setup with 20% dutycycle, in the same state. The current was > climbing up slowly from 3.6A. Some 3-4mins down the lane, the current > started climbing up wildly, reached about 9.5A (heard a tick sound). > MOSFET's blew, fuse (10A) blew. The temperature on the heatsink was > starting to climb from 33C to 34C. But I presume, the die temperature was > much higher and hence the result ? The Drain voltage at that time was > about 11.8V, IIRC. > At 20% duty cycle, the out voltage remained nearly moreover the expected > value of 300V. > > Anyway, even with an ample heatsink, need to have a proper and good > feedback loop if the converter is to behave. > > > One thing that I do not understand, the vendor specifies a Rds=3D 15m > But, then they specify in Fig 11: that the normalized ON resistance vs > temperature, Rds @ 25C is 1Ohms. Agreed that Rds is temperature > dependant, but what cant fathom is that if the Rds is nowhere near 15m > in those charts. Wonder how they made up that figure. ?! > > So, given that Rds=3D1.2Ohms. No wonder why the MOSFET blew up. > > Back to measuring drain current again. > > > On Sun, Jun 26, 2016 at 6:36 PM, Denny Esterline w= rote: >> Make sure >>> that all probe leads are as short as possible and that there is >>> minimal loop area between the earth lead and the signal lead at the >>> probe. >> >> >> ^^^^^ this. >> And "short as possible" does not mean "clip lead from the probe to groun= d". >> >> [looking for reference picture...] >> There's a relevant picture on this thread: >> http://electronics.stackexchange.com/questions/221292/tip-barrel-test-of= -oscilloscope >> >> Actually, the whole thread is kinda relevant... >> >> -Denny >> >> P.S. there's a black hole link on that page for the interested. >> https://youtu.be/2vzvWUqUtb8 >> Leads to a series of talks by "Bob Pease and friends" that just swallowe= d >> me for a couple hours. >> This specific video at about 8:00 is talking about this specific scope >> probe problem. >> -- >> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .