BIGTREETECH SKR V1.4 Turbo freewheeling diodes - Page 1
Forum section: electronics / projects

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: June 02, 2020, 04:42:22 pm
content: not sure as to whether it really fits into the projects category of the forum, as it is more or less 3D printing related.i recently acquired a BIGTREETECH SKR V1.4 Turbo board to play with MARLIN 2.0 firmware and (hopefully) later implant it into an UM2+ just for the lulz. while studying the board's documentation, i noticed that the heater- and fan-control outputs do indeed incorporate some beefy 40 V MOSFETs, but are lacking of any protection against inductive loads, aka freewheeling diodes. shame on you, bigtreetech!to overcome this potentially hazardous issue (think of a blown heater MOSFET and the board no longer able to turn the heater off), i prophylactically equipped the four outputs around Q1, Q2, Q3 and FAN0 with some schottky diodes that i had in my stash.each of them is bodged with their anode to the respective drain and their cathodes all routed to +24 DC after(!) the 20 A fuse.maybe this post will help anybody who is planning to use this board and save him/her some blown MOSFETs!famous last words: use this mod at your own risk.

author: thm_w
author_rank: Super Contributor
timestamp: June 02, 2020, 10:54:10 pm
content: Good to know as I ordered SKR1.4.But do you really expect heater or fan to have much inductance? Mosfet can handle some avalanche energy.With the heater maybe, as some beds use a spiral pattern.Mosfet 1:https://datasheet.lcsc.com/szlcsc/WSK220N04_C148446.pdfMosfet 2:https://datasheet.lcsc.com/szlcsc/WSF3085_C148428.pdfSchematic here:https://github.com/bigtreetech/BIGTREETECH-SKR-V1.3/blob/master/BTT%20SKR%20V1.4/Hardware/BTT%20SKR%20V1.4-SCH.pdfIf you're running a powerful bed I would highly recommend external mosfet, they state 140W maximum rating. Not sure about heater, but they are generally under 50W anyway.

author: langwadt
author_rank: Super Contributor
timestamp: June 02, 2020, 11:12:36 pm
content: Quote from: thm_w on June 02, 2020, 10:54:10 pmthose transistors have an avalanche rating and the on resistance is only a few milliOhm so 140W@24V would only be a few 100 mW dissipation

author: thm_w
author_rank: Super Contributor
timestamp: June 02, 2020, 11:28:06 pm
content: Quote from: langwadt on June 02, 2020, 11:12:36 pmYes, the external FET recommendation is not for heat dissipation, its for connection or connector issues.Worst case 12V/140W = 12A. Which the screw terminal can probably handle, when everything is connected up properly. If it works its way loose over time, solder cracks, etc. the connector will burn up and ruin your board.With an external FET board they will have ring style screw terminals, which are good for 20A+.Maybe its not an issue these days but it used to be fairly common to see burned boards.

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: June 03, 2020, 03:50:21 am
content: Quote from: thm_w on June 02, 2020, 10:54:10 pmno, i actually don't expect that much of inductance from the heaters. in fact, i would expect them to be somewhere in the 10 µH or less ballpark. still have to measure it, though. so, yeah, with the avalanche rating given in the 220N04 datasheet, i'd probably be fine w/o the freewheeling diode. however...what i did not fully understand from the datasheet was the footnote of the EAS rating: it looked to me like a single pulse rating rather than a repetitive rating. as i'm going to use the heaters in PID mode, also using a quite high PWM frequency rating (approx. 60 Hz, #define SOFT_PWM_SCALE 3), i am not sure as to whether the MOSFET would handle these repetitive pulses well without a diode.so, better safe than sorry.in case you are asking why i'm using these PWM settings:1. bang-bang mode really hits the 24 VDC supply (Meanwell RSP320-24), even more so on the original Ultimaker power supply, and adds to skin banding effects on the prints.2. low frequency PWM (#define SOFT_PWM_SCALE 0) already does away with that, but then you have a maximum annoying flicker on the LEDs of the case lighting3. at #define SOFT_PWM_SCALE 3 i get pretty smooth skins and almost no flickering now.

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: June 03, 2020, 07:41:30 am
content: Quote from: langwadt on June 02, 2020, 11:12:36 pmagreed. but then, have a closer look at the schematics of the BTT SKR board: unfortunately, BTT decided to drive the MOSFETs with just 5 V (instead of, say, 10 V or 15 V), giving away a lot of the low-Rdson benefits that the MOSFET could deliver. that's obviously the reason why BTT spec'd the board's power to just 140 W. which is, btw, the rating for a 12 VDC supply, because the spec calls out for a bed heater having at least 1 Ohm resistance. this gives you (with a heavily degraded Rdson of approx. 10 mOhms or more when hot) approx. 1.5 W of power dissipation in the MOSFET. pretty much the limit on a pcb as cramped as the SKR Turbo. just to be clear: i'm not complaining, just trying to figure out how they arrived at the power specs.at 24 VDC, you would get 280 W allowable heater power for the same implied current ( R = 2 Ohms), while the bed heater resistance of the UM2+ is approx. 3.5 Ohms. so i think i'm safe in using the on-board MOSFET.