Transistion periods of MOSFETs - Page 1
Forum section: electronics / projects

author: Shocker
author_rank: Contributor
timestamp: December 15, 2015, 07:02:03 pm
content: Hi,My gut feeling said to me that as the Vds of a MOSFET reduces, so would the transition periods and propagation delays between switching both hard on and hard off - and vice-versa. Originally, i was looking at MOSFETs with around 150Vds but found that the switching periods were too slow. I decided to back off on the Vds specification because insanely fast transition periods are more important that Vds. I'm looking for as close to or better than 1ns transition periods as possible. I'm also looking for tiny propagation delays too. I would have profound love for a device that can switch hard on and off again within ~3ns. So I'm now looking at MOSFETs with a Vds of around 30-40V and i don't see any real improvement.I've been looking at the smallest of devices (Eg. SOT23) because my application only requires little current, and i fully understand that the parasitics of large device packages will be a limiting factor to the transition periods.Even though the 30V devices are only switching a fraction of the the Vds of the 150V devices, I assume there is a reason that the transition periods haven't shortened? What's the likely cause?As a ball park figure, i'm looking at what the datasheets are stating that their devices can switch at. I know that these are subjective because the test conditions vary between devices (I am comparing devices with 'similar' test conditions) but i don't know how to compare them relatively quickly using any other method. Is there a good estimation at their transition periods using their capacitance or charge values?Thanks.

author: Fungus
author_rank: Super Contributor
timestamp: December 15, 2015, 07:21:39 pm
content: Quote from: Shocker on December 15, 2015, 07:02:03 pmNope.Quote from: Shocker on December 15, 2015, 07:02:03 pmMOSFET gates have a capacitance. Driving a MOSFET gate is like charging/discharging a capacitor.To switch them rapidly you need more amps from whatever is driving the gate.(Or get a MOSFET with lower gate capacitance)

author: DanielS
author_rank: Frequent Contributor
timestamp: December 15, 2015, 07:37:53 pm
content: Quote from: Fungus on December 15, 2015, 07:21:39 pmMostly that, due to how tricky delivering high current gets when you are aiming for 1ns edges.

author: dmills
author_rank: Super Contributor
timestamp: December 15, 2015, 07:42:03 pm
content: Reducing the actual operating voltage does however reduce the amount of Miller nastyness you have to deal with.Qg is the number you probably care about to a first approximation, and I would suggest that an RF LDMOS or even enhancement mode GaN part (EPC make some reasonable examples) are the only things that will come close for those sorts of speed targets.Layout will be 'interesting' as even a few nC of charge in a ns means amps of gate current and kA/us di/dt (Which means that **ANY** inductance will have you.What problem are you actually trying to solve with this improbable mosfet?Regards, Dan.

author: Fungus
author_rank: Super Contributor
timestamp: December 15, 2015, 08:20:59 pm
content: Quote from: DanielS on December 15, 2015, 07:37:53 pmIt might even be worth getting a specialgate driverIC. That's what they're for.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 15, 2015, 08:30:10 pm
content: At this point, you should probably tell a bit about your application...It's starting to sound more like a miniature synchrotron kicker, than just another switching supply.Tim

author: rx8pilot
author_rank: Super Contributor
timestamp: December 15, 2015, 09:12:25 pm
content: As the others said....the speed of switching is determined by how fast you can charge the gate.Given the ability toe charge gate stays the same, reducing the gate capacitance will speed things up. Given the gate capacitance stays the same, driving it with more current will speed things up. The faster you go, the more the PCB parasitics play. Trace inductance that is tiny can create spikes that will disrupt the circuit or damage the gate. It is a delicate balance of the MOSFET characteristics, the gate driver, and the PCB layout if you are hoping for high-speed. If high current is also part of the plan, you have to manage that as well - super fast rising/falling edges make a mess that is hard to control.

author: DanielS
author_rank: Frequent Contributor
timestamp: December 15, 2015, 11:27:16 pm
content: Quote from: Fungus on December 15, 2015, 08:20:59 pmHow many off-the-shelf MOSFET drivers have strong enough drive to switch a FET in 1ns? I did a quick search and the fastest driver I have found has a rise time of 4ns, which is already slower than OP's desired total cycle time. They also have 15-25ns propagation delay, which may not fit OP's definition of "tiny propagation delay."Supply bypassing and board layout will be challenges too, not just finding the driver IC itself.

author: nuno
author_rank: Frequent Contributor
timestamp: December 16, 2015, 01:08:50 am
content: Your limitation may be your layout inductance. If it's just the "gate capacitance", a stronger driver fixes it. But then, you may reach a point where the gate driving layout inductance doesn't let you go faster, in particular the inductance at the driver-source layout segment (let's call it Zs). This inductance provides feedback, limiting your speed; as the Vgs current raises on Zs, voltage drops at it, raising Vs and thus lowering Vgs, holding back the charging current for "Cgs" and making it charge slower.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 16, 2015, 03:55:07 am
content: You can eliminate Miller inductance with four terminal transistors, and still have gate drive loop inductance.Tim

author: Marco
author_rank: Super Contributor
timestamp: December 16, 2015, 09:21:42 am
content: Quote from: Fungus on December 15, 2015, 07:21:39 pmIt's not really an useful abstraction for switching behavior, which takes place on (part of) the miller plateau. Dropping Vds creates gate currents due to Cgd and it also increases Cgd at the same time, so what happens is ... uhm yeah.