AC/DC TIG Welder - Inverter design PCB - Page 1
Forum section: electronics / projects

author: TasosVL
author_rank: Contributor
timestamp: September 05, 2021, 11:35:24 pm
content: Hello,Coming back from my old posthttps://www.eevblog.com/forum/projects/acdc-tig-welder-design-inverter-topology/msg3529274/#msg3529274, i am re-designing the main inverter PCB which is a two switch foward converter taking the 310V DC rectified mains and switching them at 40KHz. The IGBTs will be the STGW30NC60WD and the reset diodes D8 D7 will be the MUR1560.The current sensing transormer is shown on the schematic L1 and L2.The output to the main transformer is PRIM+ and PRIM-.An external capacitor bank of 2000uF is connected to the CAP+ and CAP-.Two relays are used to bypass the NTCs. The output of the bridge rectifier is connected to DC+ and DC-.Also i attached the final LTspice shematic and simulation plots of the main inverter IGBT current and voltage as well as the output welding current on AC modeI need some suggestions to improve the layout of the PCB, i am limited to one layer as this is a homemade board.

author: strawberry
author_rank: Super Contributor
timestamp: September 06, 2021, 07:17:46 am
content: film capacitor close to shorten switching loop inductance. ~2uFuse separate wire/connection for relay gate drive and current sense.twist gate and current sense pairs.

author: TasosVL
author_rank: Contributor
timestamp: September 06, 2021, 08:23:53 am
content: Do you mean to add a 2uF capacitor on the DC rail on the board?Yes i plan to use twisted pair for the gates and current sense transformer.

author: strawberry
author_rank: Super Contributor
timestamp: September 06, 2021, 06:25:04 pm
content: like thishttp://danyk.cz/svar_en.htmlhttps://www.wima.de/en/our-product-range/dc-link-capacitors/dcl-mkp-4/other option is to slow down switching therefore increasing loses

author: TasosVL
author_rank: Contributor
timestamp: September 06, 2021, 07:15:24 pm
content: Yes the main inverter is designed around that site you mentioned.I have now included a film cap on the PCB 2.2uf 400VDC.Do you think the clearances on the traces are ok?

author: strawberry
author_rank: Super Contributor
timestamp: September 06, 2021, 10:01:22 pm
content: PCB Toolkit

author: TasosVL
author_rank: Contributor
timestamp: September 18, 2021, 10:00:23 pm
content: Ok so i have made the main inverter PCB and reinforced all the cooper tracks with 1.5mm cooper wire and lots of solder in order to minimize the stray inductances. All the components are mounted on a heat sink using mica on the IGBTs and diodes. The bridge rectifier is also mounted on the same heatsink. The 2000uF capacitor bank is connected to the white connector on the board.I did some testing and measurments using a variac and gradually stepping up the voltage. The output inverter was on DC mode so no switching on the output side, only the 2 of the 4 output diodes are working (you can see more in the simulation shematic).The PWM controller chip and gate driving circuits are not shown here (schematic on the images below)The main inverter IGBTs are the HGTG20N60B3DGate rise and fall time ~280nSGate voltage is 20Volts (the 18 zener could not clamp it down enough?)C-E oscilloscope measurments using my DIY 200X probe on the images belowThe IGBTs failed in 30seconds of continious operation with the output shorted @20Amps. The breaker tripped and the 3A fuse in the variac didnt blow. The IGBTs had all their leads shorted with no visible damage. I didnt measure the heatsink temperature but when touching should be around 60 Celcius near the IGBTs. Also the top of the IGBTs seem to have similar temperature to the heatsink.Given the output transformer is 1:5 ratio the average IGBT current should be less than 2Amps.Could this be due to the high gate voltage or a thermal runaway? I need some general advice what to check-measure for this kind of circuits in order to have a reliable operation and more data for the next time the IGBTs go bad