SiC vs Si vs Si Schottky Vs. SBR - Page 1
Forum section: electronics / projects

author: Hiemal
author_rank: Regular Contributor
timestamp: February 12, 2023, 02:52:50 pm
content: Alright, so there's not a whole lot of information I can find online about what scenarios each diode excels at, at least, in the SiC diode category.Si diodes are cheap. They have fairly high V-drops, and have reverse recovery that causes increased EMI and switching losses.Si Schottkies have low V-drops, no reverse recovery due to no minority charge carriers, and consequently, reduced EMI and lower/no switching losses. But, they have higher reverse current leakage, and are typically limited to under 200V. Inexpensive.SBR diodes are kind of a weird mix of a schottky and a silicon diode; some datasheets list reverse recovery times, while others do not. Seem to go up to 400v, a bit higher than schottkies. Also share schottky V-drop, and reverse current leakage issues.SiC diodes are rated up to 1.2kV and higher. V-drop is similar to a regular Si diode, but they don't have any reverse recovery time. Typically more expensive.What sort of situations would you use SiC vs. Si, for example? Like. I.e. Would you try to use SiC diodes in a hard switched step up converter, at around 120V @ 8A, or would Si diodes be okay? What would the major drawbacks be? Cost? EMI? Heatsinking?I know it's kind of a generalized question so I do appreciate the time anyone takes to respond and give thoughtful answers. Most of the papers and stuff I've read about SiC diodes mention their upsides but don't really elaborate on specifics, like, how much improvement there REALLY is vs. standard diodes. Just that they're "better".Thanks so much.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 12, 2023, 09:11:39 pm
content: Schottky are almost always better in SMPS application. Now that they're available at essentially any voltage, there's little reason not to use them!About SBR (Super Barrier, or SuperJunction something or other, I forget which all technologies apply to them), they're quite good; they do have some recovery, but they're also independent of current and temperature; it's a weird hybrid, but an effective extension into the higher voltage range that ordinary schottky (up to 40-80V or so) don't cover.Even better still, synchronous rectification, usually courtesy a suitable SR controller. These usually work best with specific designed-for applications, so may not be very general, but they still account for a lot of units!)Si PN diodes are better in surge and pulsed application, including mains rectification (inrush and mains surge) and snubbing (fast PN). There are some other edge cases where PN is better than schottky, due to the lower capacitance (snubbers often included).SiC schottky is especially poor with surge, due to the high internal resistance; a typical use-case is SiC in a PFC boost converter, with a Si PN bypassing it (i.e. from FWB output to main filter cap) during inrush/surge.Mind, these are commercial temperature examples; everything changes at high temp, where schottky leak a ton and PN have increased recovery losses. In that case, you'll need to consider more carefully what the balance of power dissipation (or rectification efficiency in general) should be, and how much and where you can sink that heat; and you'll probably need special packages to do it (e.g. metal-ceramic MIL spec things; ceramic PCB or hybrid instead of resin-based). SiC of course shines at high temperatures, but Si itself isn't right out even at 200°C.Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: February 12, 2023, 09:27:37 pm
content: The forward voltage of higher voltage schottky diodes goes up quite a bit. Part of the current also flows through a parasitic parallel PN diode. So there can also be some reverse recovery, especially after use with rather high current and with the higher voltage types.There are 100 / 200 V schottky diodes, but they are not that great anymore.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 13, 2023, 01:36:13 am
content: Yes, another contraindication for pulsed use.Indeed, the guard ring PN diode carries much of the current under surge conditions; this is why they're not used for ESD purposes*.Also a part of the reason why SiC are so relatively poor at surge: when the Vf-If plot includes surge conditions, you see a double-kinked curve, which is the PN guard ring acting in parallel above 3V or so (being the typical Vf for SiC PN). While for say 1-2.5V the curve is fairly sharp and flat, being just the schottky + ESR curve.*By themselves. You never see schottky diodes in ESD arrays for example. Lots of people (myself included) do have a habit of using BAT54S for ESD protection, but that's more like: you get a low clamping voltage for signal purposeswithESD protection included. PN diodes (BAV99 say, or take your pick of ESD clamp diode array parts) have about the same peak clamping voltage under surge, if you don't need the low Vf at signal currents (~mA).Tim

author: Circlotron
author_rank: Super Contributor
timestamp: February 13, 2023, 05:08:28 am
content: Quote from: Hiemal on February 12, 2023, 02:52:50 pmAbout 2001 I did a heap of work on a 240VAC continuous mode PFC boost converter for a 3kW psu. DC buss voltage was 450V. Because we used a RHRG3060 fast recovery diode we needed to take care of reverse recovery issues. One downside of this was the added length of pcb tracks and additional component legs meant Vds overshoot on the main mosfet was hard to keep under control at full load and low AC in. Just after the project was finished (of course) SiC Schottkys became available. Tried using one of these on the PSU just to see, and things were ridiculously easy by comparison. I likened it to when IGBTs took over from force commutated SCRs in 3-phase variable frequency induction motor drives.