Optimal position of current sense transformer in Buck converter - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: November 26, 2017, 05:59:30 pm
content: Hello,Do you agree that in the attached schematic, the current sense transformer primary is not in the optimal place.Rather, it should be upstream of the FET….as it stands, it is in the switching node, which due to its rapid dv/dt, will make the signal from its secondary more noisy.Do you agree?

author: jbb
author_rank: Super Contributor
timestamp: November 26, 2017, 07:31:59 pm
content: I think you’re right that it would be better placed at the source of M1. Not sure how much difference it will make, though, because one end of the secondary is grounded and the second is filtered.I’m interested in the choice of cascaded buck. It looks like your worst condition is 400V in 100 V out; a normal buck should work fine for that ratio. Possible devices: 650V CoolMOS switch, 600V SiC diode, careful layout.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 26, 2017, 08:40:36 pm
content: I don't see why this needs to be cascaded...Tim

author: ocset
author_rank: Super Contributor
timestamp: November 27, 2017, 05:57:37 am
content: Thanks, the cascade is incidental in this case.Its actually a cct for a downhole app, 185degc ambient.The cascading means one controller can be used for when the original cct was 400v to 24v and the cascade is needed.It means the upstream fet can be higher rdson and not suffer high current.The downstream fet can be lower rdson becauase its lower voltage.If a single stage were used, the overall losses would be worse because of the non availability of high voltage, low rdson fets.Theres also an element of heat spreading going on...ie less dissipation in a single fet than if a single fet stage were used.

author: jbb
author_rank: Super Contributor
timestamp: November 27, 2017, 06:27:56 am
content: Ah, so perhaps the spec is actually 24 - 100V output :-)

author: jbb
author_rank: Super Contributor
timestamp: December 02, 2017, 09:30:30 pm
content: Sorry I was a bit slow on this one.Quote from: treez on November 27, 2017, 05:57:37 amMy brain just went 'click.' An SiC MOSFET + SiC Schottky diode may actually do better than you think. If you have a talk to the SiC suppliers (Wolfspeed/Cree, Rohm, Infineon, ST, Microsemi, Semikron) you might be able to get some bare die chips rated for 175 deg C or more.These can be integrated into a custom power module on aDirect Bonded Copper (DBC) substrate(perhaps by the supplier themselves, or someone like Vincotech). A DBC is like a simple PCB (no vias!!) where the top side has some components on (sintered down + gold bond wires), the core is a thermally conductive ceramic like Al2O3 or AlN (better thermal conductivity) and the bottom side is a flat copper sheet for attachment to a heatsink.The joy of a DBC is that you can make an isolated base plate module (good for integration) with low thermal resistance (good for thermals) and some simple circuitry (good for integration). The DBC power module could include a small decoupling cap, the MOSFET, diode, and sense resistor (on the low side to sense diode current as Blueskull suggested). It might even be possible to integrate small magnetics, I don't know.It will be expensive to set up, and require more than a few phone calls, but could be rewarding.Also, be wary of any ferrite in the design. It's performance probably goes to crap at these temperatures.An example of DBC:

author: jbb
author_rank: Super Contributor
timestamp: December 03, 2017, 08:18:08 am
content: Ooh, PEEK 3D printer...Also, what are the chances of being able to integrate a wee toroidal transformer on a DBC substrate?“You get into ITAR land.” Do not go into ITAR land if you can help it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 03, 2017, 08:44:58 am
content: Seems more likely you could surface mount a transformer and solder or bond that down, or use a horizontal ferrite bar and wrap bondwires around that.Would anyone actually build it that way? Maybe, who knows... I mean, anything is possible, but at what price is the question.Tim