Could use some help on reducing transistor turn off ringing in boost converter - Page 1
Forum section: electronics / projects

author: Fusion916
author_rank: Regular Contributor
timestamp: February 15, 2018, 05:53:34 am
content: So I designed a 12V->25V boost converter using an TL1454A running at around 40khz with a maximum output of about 100W. At first I had a problem with mosfet turnoff causing large ringing on the PWM output going to the mosfet. I have mostly eliminated this ringing by increasing the series resistor to the gate drive but at the cost of having a pretty shitty PWM slope.Here are some captures of the PWM output at 25W, 261 ohm series gate resistor, as you can see the slope is bad because of this:Here is the current ramp of the inductor at each cycle (around 15-16A peak):As you can see there is still a bit of ringing at transistor turnoff, but it was much worse with a smaller series gate resistor (I started off at about 10ohms). I do have a snubber at the mosfet output. Here is an approximate schematic of the output stage (lots of series capacitors instead of the 1 capacitor shown in the real circuit):Any suggestions on changing the snubber values, or any other circuit modifications I can make while decreasing the size of the series resistor/increasing the slope of the PWM output?

author: Benta
author_rank: Super Contributor
timestamp: February 15, 2018, 10:32:34 am
content: You won't be able to get rid of the ringing completely. Some of it is from interwinding capacitance in the inductor (LC resonance) and that can't really be avoided.

author: amspire
author_rank: Super Contributor
timestamp: February 15, 2018, 11:03:36 am
content: Tu get a fast turn off, you need a high current mosfet driver, and that TL1454A can only manage 40mA. You have made it worse by adding the gate resistor, but the truth was probably you were exceeding the absolute maximum current on the TL1454 before then - perhaps dragging that output above the TL1454A power rails.Find the mosfet with the best gate-drain capacitance that will fit even if it does not look as impressive as another mosfet with very low Rds. If you have a mosfet with a 500pF gate-drain capacitance and you want a 100ns turn off, then dV/dT = 25/100ns = 250,000,000 V/s.That means you need a gate current = 500pF x dV/dT = 125mA.You mosfet is currently still biased on as it is meant to be turning off, and that means it is an active device that can easily oscillate. Properly driven, you will should have less ringing.

author: Fusion916
author_rank: Regular Contributor
timestamp: February 15, 2018, 08:20:39 pm
content: Thanks for your suggestions. I ordered some different mosfets with lower input capacitance and gate charge, going to try that.Is there anyway I get get away with also lowering the gate resistor and modifying the snubber?

author: jbb
author_rank: Super Contributor
timestamp: February 15, 2018, 08:52:12 pm
content: Like blueskull said, layout is important. It is very likely possible to drive the MOSFET faster and reduce the ringing. Could you show us the layout?

author: amspire
author_rank: Super Contributor
timestamp: February 15, 2018, 09:55:22 pm
content: Quote from: Fusion916 on February 15, 2018, 08:20:39 pmYou only need a lot of the extra parts you have added because you were not turning off the mosfet properly - you were leaving it on as a high powered amplifier during time the voltage was rising. When you turn the mosfet off properly, you may still need some kind of snubbing but it may be much simpler. With a good layout, you probably do not need a snubber at all.You are also wasting a lot of energy as heat in the mosfet.The basic problem is that controller chip only has 40ma drive available. If you look at your mosfet and it has 500pF gate to drain capacitance, then you can work out that you cannot allow the rise time to be faster then (C x dV)/I = 0.3us. If you have a 16A peak in the inductor, then you can work out what capacitance the circuit needs to stop the rise time being faster then 0.3us.C = I x dT/dV = 16 x 0.3us/25 = 0.2uF. You would need to put a 0.2uF capacitor across the mosfet which would be ridiculous.Unless you can find a mosfet with an extremely low reverse capacitance, you will have to add a mosfet driver that can sink several amps between the controller IC and the gate.That gate resistor is not a solution - when the mosfet is properly driven, it is definitely not wanted.

author: rx8pilot
author_rank: Super Contributor
timestamp: February 15, 2018, 10:03:11 pm
content: Before you get on a mission to change a bunch of things....take a look at your measurement technique. Probe technique is critical to get a sense of the magnitude of your challenge.

author: Fusion916
author_rank: Regular Contributor
timestamp: February 15, 2018, 10:37:42 pm
content: Quote from: amspire on February 15, 2018, 09:55:22 pmYes you are right, I needed to pay more attention to this design aspect (transistor drive/gate capacitance). I'm also going to take a closer look at the layout tonight and also order some gate drivers as a backup option. At least this is a learning experience.

author: amspire
author_rank: Super Contributor
timestamp: February 15, 2018, 10:38:15 pm
content: Quote from: amspire on February 15, 2018, 09:55:22 pmThinking a bit more about this, definitely just go ahead and get a mosfet gate driver IC. 40mA gate drive is just not enough for a 16A inductive load on the mosfet.Once you get the gate driver, you can use any mosfet you want and your circuit will be much more efficient.The only other alternative is to find a different controller IC with a much higher gate drive current rating.

author: amspire
author_rank: Super Contributor
timestamp: February 15, 2018, 10:47:37 pm
content: Quote from: Fusion916 on February 15, 2018, 10:37:42 pmYou need the minimum distance and resistance between the inductor and the diode, and also between the diode and the output capacitor. You may need to add some smaller capacitor across the output cap to handle the transients better.The ground connections have to be substantial and very short between the mosfet source and the capacitor ground. With a good layout, a good fast diode and a properly driven mosfet, I cannot see why you would need snubbers. A low Rds mosfet should be so cool, it does not need a heatsink.

author: Fusion916
author_rank: Regular Contributor
timestamp: February 15, 2018, 11:46:42 pm
content: Thanks for all of the help. I appreciate it.