Driving low Impedance coil at high frequency with N-channel MOSFET - Page 1
Forum section: electronics / projects

author: sonnytiger
author_rank: Frequent Contributor
timestamp: March 29, 2013, 02:40:40 am
content: I want to drive an 8 turn coil made of solid copper about 5cm across using an N-channel MOSFET (the attached datasheet). I will be driving it at a frequency high enough not to cause huge currents to flow through the coil (less than 60A) but low enough not to cause excessive loss across the MOSFET (less than ~500kHz). I will need an RC circuit across the coil to snub spikes (capacitor of ? farads and a resistor of ? ohms). I will probably use a 555 timer to vary the frequency to the MOSFET, so i would need to hook a smaller semiconductor to the output of the 555 in order to switch a higher voltage to turn on the MOSFET properly. The inductance and resistance of the coil is going to be quite small (the gauge is fairly large ~2.5mm across) so I would imagine the frequency would have to be rather high, hopefully below ~500kHz. The purpose of this is just for experimentation, maybe some inductive heating or something like that. So with that aside anything to add to the ideas said above? What would an appropriate drive frequency range be and what would acceptable resistor and capacitor values be for snubbing purposes?Edit: Forgot the datasheet sorry

author: c4757p
author_rank: Super Contributor
timestamp: March 29, 2013, 03:10:03 am
content: I'll let someone else answer the actual questions - somebody with more experience than I have with this stuff could be more helpful. I will just throw out a couple things I don't know if you thought of:1) It can take some effort to efficiently drive a big MOSFET at a high frequency. That MOSFET has a typical gate charge of 43 nC, so it is going to take a gate drive current of around 8.6 mA RMS per 100 kHz, with significant peaks depending on rise time. Pay attention to the gate driver and decoupling.2) 100 kHz is at the upper end of the NE555's frequency range, and 500 kHz is right out. Try something like the ICM7555 for high frequencies. Alternately, an SMPS controller like UC3843 can be used for this. It also can output a gate drive current of up to 1A, giving a (slightly optimistic - OK, completely ideal) rise time of 43ns. This will also let you independently vary frequency (up to 500 kHz) and duty cycle.3) I don't know what "less than 60A" is, but despite the current rating of that MOSFET, you might want to parallel a couple of them to share the current. Keep in mind that this compounds #1. Also, they will need heat sinks - 28.8W at full current, much more if you don't have a fast rise time.4) Is this just a coil, or does it have a core? Core material could make a huge difference in inductance. You should measure it.Quote from: sonnytiger on March 29, 2013, 02:40:40 amNE555 is good up to 16V, ICM7555 to 18V. Why do you need to do that?

author: sonnytiger
author_rank: Frequent Contributor
timestamp: March 29, 2013, 05:30:02 am
content: I forgot to mention it was air core, i only have one MOSFET of the type I will be using, and 555 because I also have one of those lying around.

author: sonnytiger
author_rank: Frequent Contributor
timestamp: March 29, 2013, 05:41:10 am
content: I am driving it from a 24V battery (convenient high current source) so will probably power the 555 timer circuitry with an external 5V power supply because it is handy.

author: Kremmen
author_rank: Super Contributor
timestamp: March 29, 2013, 06:42:28 am
content: Quote from: sonnytiger on March 29, 2013, 02:40:40 amWhat you are attempting is entirely doable, but if your approach is to grab what happens to lie around, solder them together and hope for the best, i predict disappointment.Points, some of which have been made already:- Forget the 555 and use a proper driver. Plenty of those around and they won't break the piggybank.- Spend some time googling e.g. "MOSFET gate drive" or something similar. The gate drive circuit will be a key element of success or failure. You will be doing hard switching on a relatively difficult load so you need to get this right. Grossly overdriving the gate will induce heavy ringing in the tens to hundreds of MHz causing all kind of unpleasantness. This is one intro paper that might be useful reading:http://www.ti.com/lit/ml/slup169/slup169.pdf-The MOSFET datasheet is uncomfortably brief, lacking all interesting graphs. However, it does state the Vgs(th) as 2.5V max. I would want to use significantly higher drive voltage, on the order or 10V to ensure full turn-on at these high currents approaching the device capability. (you say less than 60A - how much less).-Take precautions to handle the inductive kickback from the coil. You need to limit the spike to something the switch can tolerate or else. At 500 kHz and significant currents your freewheel diode should be low loss rapid recovery. Schottky diodes come to mind.-Pay attention to board layout and trace widths. A 500 kHz medium power transmitter may meet a less than enthusiastic reception in the neighborhood, not to mention that it won't work properly or at all if you make gross blunders.

author: KJDS
author_rank: Super Contributor
timestamp: March 29, 2013, 08:25:35 am
content: I predict smoke, and a curse or two, followed by rapid disappointment.Whilst there's nothing very difficult with switching high currents quickly, it would make a lot of sense to start out low, with a resistive load and learn about how to drive gates properly. Then add a big lump of gate capacitance to a small FET to simulate the the big FET input but on a 10p part and see what that does to the driver, and to the output. Make the load more inductive and see what happens, but get a diode on it first to cope with the switching spikes. That way you will learn along the way, rather than just seeing smoke from your one big device and learning nothing more than sprinting before you can walk isn't always good.

author: sonnytiger
author_rank: Frequent Contributor
timestamp: March 29, 2013, 01:14:32 pm
content: OK well it seems this is more difficult then I though, would explain why I had problems in the past. People seem to be misunderstanding, I would like to drive it under 100kHz, I just don't know what frequency would result in the very small inductance (it has an air core about 3cm in diameter, I changed the dimensions, and only 13 turns) limiting the rise time so that it does not go above 60A at 50% duty cycle. It seems the biggest thing is using a different chip, people say PWM drivers, like for switch mode power supplies?Edit: another question, what is the importance of a gate drive IC? What dos it do differently? Okay I read the short Wikipedia and think I understand now the significance though the article is probably missing some important stuff.Edit 2: achmed99, yes that is what I was planning on doing.

author: sonnytiger
author_rank: Frequent Contributor
timestamp: March 29, 2013, 04:59:39 pm
content: Thanks kremmen that paper is making me understand MOSFET'S much better, I now understand that it conductance depends on the charge in the gate, and since it stores charge it has capacitance and this capacitance must be charged up. The faster the better to allow for less losses. The fog has lifted!

author: Marco
author_rank: Super Contributor
timestamp: March 29, 2013, 05:05:21 pm
content: Quote from: sonnytiger on March 29, 2013, 02:40:40 amUnless you make the circuit resonant, they are not spikes ... the resistor has to dissipate all the energy you put into the inductor when the switch is on.So basically you have to make the circuit resonant ...