Looking for low current SiC or GaN Schottky diode - Page 1
Forum section: electronics / projects

author: JohnG
author_rank: Frequent Contributor
timestamp: June 17, 2020, 03:04:00 pm
content: Folks,I'm looking for a low current, ideally < 1 A, Schottky diode with a voltage rating of at least 500V, and negligible reverse recovery. By negligible, I mean Qrr < 10*Qds, where the latter is the depletion capacitive charge.I want a small diode because I am trying to minimize both size and capacitance. I have used the Cree/Wolfspeed C3D1P7060Q in the past with very good results, and aside from voltage drop, it was pretty close to ideal. It had no noticeable reverse recovery, and I have run it in resonant rectifiers at over 30 MHz with excellent results. Unfortunately, this part is not recommended for new designs, and I don't know of anything else.I have never found a silicon diode that is even close, and I suspect that there are not any. I be happy to be proven wrong, though, because I really want a small 500V+ diode with no RR.Any help appreciated.Thanks,John

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 17, 2020, 05:14:36 pm
content: Small SiC schottky are boutique. I've seen one SMA or SMB sized part which was $10/ea or something like that.I guess there's no point when you can get 200mA Si diodes with fast recovery (10s ns) in SOT-23 and up. Not quite the voltage rating, but easy enough to stack.I remember seeing GaAs diodes some years ago, but good luck finding them anymore I think. Might've been IXYS RF, which is dead now...GaN diodes don't exist as far as I know, but you can sync rectify with a MOSFET, at even lower voltage drop.Tim

author: JohnG
author_rank: Frequent Contributor
timestamp: June 17, 2020, 10:32:40 pm
content: The Cree diodes I listed are $0.43, so while not cheap, I would not consider them boutique. Maybe the demand was too small?Even 10 ns trr is too long. The problem with trr and Qrr is that they are highly temperature dependent and difficult to match over temperature. Cds, while non-linear, hardly varies with temperature at all. Most (not all) data sheets do not even bother providing anything close to enough info on reverse recovery, anyways. The Cree diode was being used for a lab application, but also for a demo board. For the former (a desat detection circuit), the lack of Qrr makes it easy to compensate for diode characteristics. However, I already have on hand all I will need for at least one lifetime since it's a lab fixture.The demo board cannot have any non-commercial parts, and I will probably just skip the diode because it is something of a bandaid. It just means extra work for me to figure out a workaround. Blueskull, I appreciate the offer, but I have access to experimental parts. Unfortunately, they are not viable for the demo board because any parts I use must be in commercial production, reasonably obtainable, and my budget for these demos is unlikely to increase.I was really hoping I had missed something in my search for a replacement, but maybe it's just not going to happen anytime soon.Thanks,John

author: jbb
author_rank: Super Contributor
timestamp: June 17, 2020, 10:44:59 pm
content: Maybe you could get a GaN FET, tie gate to source, and accept the terrible forward drop (2 or 3V) required for the gate-drain voltage to bias the channel?EPC list this as their ‘freewheeling’ mode and encourage people to use synchronous rectification.

author: JohnG
author_rank: Frequent Contributor
timestamp: June 18, 2020, 01:44:28 pm
content: So far, EPC devices only go to 200V for commercially available devices, and the only 600V+ commercial GaN or SiC devices are large compared to what I need. I use this diode for a few things, namely a bandaid as an anti-parallel diode for large GaN FETs when the deadtime is a lot larger than optimal for what would otherwise be a synchronous rectifier. Many controllers designed for Si MOSFETs have long minimum deadtimes (> 20 ns) which can compromise efficiency. Like I said, a bandaid for something that can be fixed with better controller (or better code if you are using a microcontroller).Another use is in tiny little offline boost converters that you want cheap. That Cree/Wolfspeed diode was much cheaper than going to any other solution, and the performance of the converter much better than when a silicon diode was used. Reverse recovery losses in silicon junction diodes can be a huge, often dominant source of loss, even in ultrafast diodes.They also can work as a very nice bootstrap charging diode. Sure, a silicon diode can work here as well, but the SiC diode runs cooler. By itself, it's not a huge deal, but when it is sitting next to a bunch of hot FETs and gate drivers running at 500 kHz to 1 MHz, it helps.Also had good luck using them in desat detection circuits. Genesic made an even better one for this (lower current rating so smaller), but when the stock ran out, there were no more to be had. I see now that GeneSiC makes a comparable part to the Cree/Wolfspeed, but it's in a bigger, higher inductance package. And, my last experience with them still lingers...John