Discrete MOSFET vs MOSFET Module (Need Help) - Page 1
Forum section: electronics / projects

author: Glenn0010
author_rank: Regular Contributor
timestamp: May 22, 2018, 05:39:55 pm
content: Hi all so I need some expert advice!I have made 2 different BLDC Motor Controllers they have the exact same functionality and code except for one difference. In one I am using discrete MOSFETs and in the other I am using a MOSFET module the MTI85W100GC.I have tested them under the same conditions. 50V 20A phase current. I have probed over the high side mosfet using a ground spring to measure my switching transients both under the exact same conditions in the exact same way.But there is a massive difference in the resultsHere is the first one which is 'correct'An here are the results every thing is within reasonHere is the second version and I am probing across the two points circled in red.And here is the nasty resultThe transient on turn off which is typically the worst case, is better than that of the first version.However, As can be seen there is a HUGE negative transient on turn on which makes 0 sense to me. I do not know why this is. Could it be that since I am probing over the GND Trace of the module it is emitting some noise into my measurement or could it be something else. I think if that negative transient was really present the magic smoke would have come outI also probed from ground to phase (i.e. lowside FET) therefore over no traces, and got a clean waveform with no negative transient so that is also pointing to the fact that since I am probing over the ground trace it may be emitting some noiseHope someone can help me on this because I am stumped!!!Cheers

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 22, 2018, 06:11:23 pm
content: Probe from GND to GND and show the waveform. Yes, in the same location.Tim

author: Marco
author_rank: Super Contributor
timestamp: May 22, 2018, 06:52:00 pm
content: You could try putting a ferrite on the probe cable close to the probe. The floating DUT's ground plane is moving 50V relative to earth during switching because of the way the probe is connected, its parasitic capacitance to earth is being charged/discharged through the ground lead.

author: Glenn0010
author_rank: Regular Contributor
timestamp: May 22, 2018, 07:35:12 pm
content: Quote from: T3sl4co1l on May 22, 2018, 06:11:23 pmDo you mean from phase to gnd (that would be lowside)? I have done that but unfortunately the scope did not seem to save my data. What I had ws a pretty large overshoot of around 88V and an undershoot of -2V but nothing anywhere near the -40VQuote from: blueskull on May 22, 2018, 06:28:38 pmThe part number of the discrete MOSFET is IPB039N10N3G. They are very similar to that of the ModuleQuote from: Marco on May 22, 2018, 06:52:00 pmI am guessing you mean a ferirtee bead ground the ground spring? If I am honest I have never worked with them and do not really know their effects. Also I do not have any available. If I use one could it deistort my signal in a way that I would not get the true reading during MOSFET turn off for example?

author: Marco
author_rank: Super Contributor
timestamp: May 22, 2018, 07:44:53 pm
content: No, I mean taking a clip on ferrite and winding the probe cable through it one or two times, putting it close to the probe.

author: Glenn0010
author_rank: Regular Contributor
timestamp: May 22, 2018, 07:58:19 pm
content: I'll see if I can find one and give it a shot

author: Glenn0010
author_rank: Regular Contributor
timestamp: May 22, 2018, 08:00:44 pm
content: I have a dead time setting of around 333ns.And rise and fall time of around 250ns.So basically the difference in the body diode could be an issue. But wouldn't the -40V transient kill the fets?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2018, 05:13:24 am
content: Quote from: blueskull on May 22, 2018, 07:45:44 pmMore like 2-3 times? Where did you see 1300?Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2018, 05:15:08 am
content: Quote from: Glenn0010 on May 22, 2018, 08:00:44 pmNo, 50V supply + 40V spike = 90V, rating is 100V, probable actual breakdown 110-120V.The 40V spike does not actually appear across a MOSFET diode, it does however appear across the ample stray inductances in the circuit, which you are probing across.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2018, 06:14:25 am
content: Quote from: blueskull on May 23, 2018, 05:40:03 amOh yeah the diode...Let's see...... the IPB's data is unreasonable, as it's measured at 100 A/us. Peak recovery current is not given. The MTI's is at 1500 A/us, so the charge is much larger. t_rr is similar, though we should expect the MTI performs more poorly at the same conditions.Tim

author: Glenn0010
author_rank: Regular Contributor
timestamp: May 23, 2018, 04:13:12 pm
content: So what would be the best course of action to correct such an issue? would you try to reduce the dead time or add a better body diode in parallel with each fet?The previous values of 240ns rise and fall times were for the discrete fetsFor this Module the rise time is aroun 140ns and fall time is around 170nsI have some magnified view for you guys with the time values maybe it is helpfull:Turn OffTurn on:Some nasty ringing in there