Gate driver rise/fall time in BLDC application - Page 1
Forum section: electronics / projects

author: Tin Duc Vo
author_rank: Contributor
timestamp: May 12, 2016, 04:16:58 pm
content: Hey everyone,I'm working on a brushless DC motor driver right now. I am planning to use an off-the-shelf MOSFET driver to drive my MOSFETS, but need clarification on the rise and fall times of the driver. To my understanding, faster rise/fall times put more strain on the MOSFET driver by causing there to be a higher current. If one requires a faster rise/fall time, then there will be more current to the driver. My concern is the effect of the rise/fall time. In my application, the switching will happen at 20kHz. A 100ns rise/fall time results in 10Mhz switching, which is much faster than the 20kHz, so it seems it would work. However, I would like to know what happens if I increase or decrease this fall time. For example, what would happen if I increased the rise/fall time to something that would barely be able to switch at 20kHz? Thank you.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 12, 2016, 05:02:57 pm
content: The driver's rated rise time occurs under the conditions specified. Usually a nominal load like 1nF and no series resistor.Typical drivers switch in the 20ns range. If you need slower switching, increase the series resistance between driver and gate pin.What happens at very slow risetimes? Depends on load current, but the general idea is excessive switching losses. Rise time less than 1/20th cycle time (i.e., 20kHz is 50us, so 2.5us) is generally not practical.Rise time more than 1/5th cycle time looks more like a class C amplifier: you're better off doing it linear instead. This is part of why linear RF amplifiers are predominant over 10MHz.Tim

author: nuno
author_rank: Frequent Contributor
timestamp: May 12, 2016, 08:53:11 pm
content: In general, switching losses increase with longer switching times.But peak switching transients increase with shorter switching times, and there's more propensity to ringing and such.So you need to find a (your) balance.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 12, 2016, 09:30:30 pm
content: Input voltage matters when you are choosing rise/fall times, because you want to limit dV/dt (voltage change rate) to avoid excessive switching noise.If you are inputting hundreds of volts for a big motor, you may want to use rise/fall times about a microsecond or two; then, if you want to keep switching losses down, you need to keep frequency down so that edges don't happen that often.My rule of thumb is - spend 1%-2% of time in switching. Do your math!If this is working at maybe 10 volts, you can switch considerably faster. With modern mosfets, 20ns is considered fast, but on the other hand, it's normal to switch at 500 kHz today. 20ns rise + 20 ns fall would be 2% time spent in switching at 500 kHz, which is OK, but not excellent, from the switching loss viewpoint.But your 20 kHz is a really easy requirement using low voltages and modern MOSFETs and integrated drivers - you don't want to switch in 20 ns, as you can switch considerably slower andstillget very high efficiency. Aim for that 1% spent in switching.PCB layout, bypass capacitors and snubbers are still extremely important, even though slowing down switching helps hugely.BTW, use a MOSFET driver specified for good peak current (i.e., maybe 1-2A for a 100W motor controller?) and fast operation (i.e., not much above, say, 50 ns) even if you don't need all that speed. Limit speed by adding gate resistors. Experiment.

author: uncle_bob
author_rank: Unknown
timestamp: May 12, 2016, 11:48:49 pm
content: HiIn most designs, you look at this the other way:What can you do to reduce the power lost in the main MOSFET? Trying to optimize the FET driver power is generally a bit far down the list. When the big MOSFET is switching, you can have both voltage and current present at the same time. MOSFETS are happy (low power dissipated) with voltage across them and zero current or with current through them and zero voltage between drain and source.Bob

author: nuno
author_rank: Frequent Contributor
timestamp: May 13, 2016, 01:34:23 am
content: What's your system voltage and motor peak current, Tin?

author: Siwastaja
author_rank: Super Contributor
timestamp: May 13, 2016, 06:14:48 am
content: MOSFET gate loss and MOSFET driver power loss neither depend on rise/fall time.