Phase shifted Zvs full bridge power supply questions - Page 1
Forum section: electronics / projects

author: Prabhat3647
author_rank: Newbie
timestamp: March 02, 2021, 05:31:36 pm
content: Hello all,This is my first post, kindly correct me if I make some mistake. I am currently 2 year engineering student and working on a phase shifted power supply as part of a project in college. I shall give a brief description of the project.It is 24 V, 15 A, SMPS which in this case is controlled by a microcontroller(dSPIC33CK32MP202), though I am not a fan of such digital control systems, it has showed its advantages in another system of a high power battery charger. This microcontroller is pretty advance as in it can generate precise, synchronized PWM signals and offer many other settings. However in this case it has been configured to generate phase shifted PWM signals depending on the PID control algorithm which takes the output voltage(24Volt from the output) and makes changes to the phase between the two legs of the converter so to ensure constant 24volt at the output. The good thing is that the microcontroller is working perfectly as intended( I have checked the phase also changes and adjustable dead time).The main problem here is that the person( prior to me who was working on this project, later had to give up because of the failures) had made the gate driver section based on the bootstrapping driver UCC27712 which for some reason, the high side signal was collapsing, resulting in heating FETs and also latter blowing drivers.So as instructed by my professor, I switched to the pulse transformer method( I had some experience here since I built a tesla coil before with the same method and also took part in the tesla coil communities where I learnt lot about pulse transformer method of gate driving.Now I was also able to make the gate driver circuity and verified on an oscilloscope that the gate pulses are right. However I see that when no load is connected to the FETs, there is still some current flowing which makes me think if that's the shoot through current( please tell what else could be the cause of current of around 30-40 mA at an input voltage of 30v Dc, imagine what it will be after rectified 320V DC!!).I also tried to add deadtime but then no effect was observed. I am operating a frequency of 100kHz, so a deadtime of no more than 1.5 us will be acceptable, as told by my professor). I tried deadtime all the way from 20 ns to 1 us but all i get is increasing shout through current.Now my main question to all is that, whether that 30-40mA is shoot through current or something else?Also is deadtime even possible by pulse transformer driving method, since a transformer cannot pass a dc current . Also during the deadtime period, I observed that the gates ring a bit during the deadtime period which mostly turns them on.Just for information, I see no issue with the microcontroller generating deadtime on the oscilloscope.I am attaching the complete schematics which even include PFC stage but for now I am not concerned about that, I will see about the PFC later since that was also giving error.With reference to the schematic, I have bypassed the opto isolators and buffer stage( TLP2250 opto isolator and 74LS125 buffer). I am directly giving signals to my two TC4427 gate driver which is then fed two separate pulse transformers and then to the FETs)Thank you.

author: julian1
author_rank: Frequent Contributor
timestamp: March 02, 2021, 07:02:56 pm
content: Maybe DC current flowing through the GDT? Does the TC4427 drive the GDT directly, or is there a blocking capacitor?

author: strawberry
author_rank: Super Contributor
timestamp: March 02, 2021, 09:38:41 pm
content: place current shunt between DClink cap and FET bridge to observe current waveform abnormalities/shoot through with oscilloscopegate transformer leakage inductance ?gate driver wrong bootstrap diode ?...

author: Circlotron
author_rank: Super Contributor
timestamp: March 02, 2021, 09:53:33 pm
content: Unrelated observation - in the PFC section the current sense resistor should be grounded at the bottom, not the top.

author: Prabhat3647
author_rank: Newbie
timestamp: March 03, 2021, 03:31:06 am
content: Quote from: julian1 on March 02, 2021, 07:02:56 pmI have used a 2.2uF dc link capacitor. So this option is ruled out.Quote from: Circlotron on March 02, 2021, 09:53:33 pmVery true, if you observe there is a similar problem with the current sense resistor on the output side of the transformer. But for now I am only worried about the no load current which is flowing through the MOSFET.Quote from: strawberry on March 02, 2021, 09:38:41 pmThis is a nice idea I shall try that. I doubt leakage inductance since I have used that handmade pulse transformer on a suitable core in a tesla coil before and worked properly. Had the leakage inductance been more there would some issues at the gate waveform also but I see a nice waveform with slight spikes. Also I have no bootstrap diode since I am using GDT based driving.