Regulator for heatbed (3d-printer) - Page 1
Forum section: electronics / projects

author: LarsT
author_rank: Contributor
timestamp: November 26, 2017, 06:00:41 pm
content: I've made an attempt at making a pwm-regulator for heater plate using a 555 timer and an N-channel mosfet (30N06L), but whenever I turn it on the mosfet quickly warms up and within half a minute it's close to 150C, even with a small heatsink attached, and I simply don't understand why. The timer is set up as astable with a frequency of approx 10Hz with a duty cycle from 5-95%. I've looked at the gate- pulse with an oscilloscope and it shows vertical edges up to 11.4V, no problems there. The heatbed is approx 1.4ohm when cold.From what I can find from the datasheet on the mosfet it should be able to handle 8amps with 11V at gate just fine. What am I missing here?

author: Kleinstein
author_rank: Super Contributor
timestamp: November 26, 2017, 07:31:31 pm
content: It might be a good idea to have a series resistor (e.g. 20-100 Ohms) in the gate line. With some bad luck the MOSFET might oscillate at a very high frequency without the resistor.

author: LarsT
author_rank: Contributor
timestamp: November 26, 2017, 07:58:20 pm
content: I know, but I don't think this is causing the problem. It seems the pulses are fine, and no oscillation present.

author: georges80
author_rank: Frequent Contributor
timestamp: November 26, 2017, 11:44:02 pm
content: Quick & rough calc:8A and 0.030 Vds means if you have 100% duty cycle (on all the time) then:8A * 8A * 0.030 ohms = 2W dissipation in the FET.How 'small' is your small heatsink...cheers,george.

author: LarsT
author_rank: Contributor
timestamp: November 27, 2017, 08:11:56 am
content: Quote from: georges80 on November 26, 2017, 11:44:02 pmI don't have a datasheet on the heatsink, but it's aluminum and the size of a stamp. I will try to find a bigger heatsink.I did some rough calculations before building the circuit and got 2.2W of dissipated power (Rds=0.035, 8A). The mosfet's datasheet specifies a dissipation capacity of 62.5C/W, which should correspond to (175-25)/62.5 = 2.4W.That's probably too close even with a (small) heatsink attached. Perhaps a relay would be a better choice for this? I might even ditch the 555 timer and use a microcontroller with temperature sensing and pid-regulation.

author: mikerj
author_rank: Super Contributor
timestamp: November 28, 2017, 12:02:59 pm
content: Quote from: LarsT on November 26, 2017, 07:58:20 pmHave you actually measured the rise/fall times of the gate drive? These need to be fast to minimise power dissipation in the MOSFET.

author: richard.cs
author_rank: Super Contributor
timestamp: November 28, 2017, 01:06:27 pm
content: It needs a heatsink for long-term reliability, at 2 W the Rth gives a junction temperature of 150 C which might be in-spec but it's won't have a long or happy life. As mikerj says you also need to check that you are switching fast enough to avoid much higher power losses than you expect. Once you've done that you either give it a large enough heatsink to keep the temperature acceptable with your calculated losses (I would aim for 30 C/W or better) or you could change to a different MOSFET with a lower on resistance.

author: Unknown
author_rank: Unknown
timestamp: November 28, 2017, 01:40:46 pm
content: The easiest fix is to use a better mosfet. You can get jellybean devices with Rds(on) an order of magnitude lower than the one you have. Yours will be dissipating approx 2W at 100% duty cycle. The junction to ambient thermal resistance is quoted as 62.5oC/W so 125oC temp rise above ambient with no heatsink. You also need to be aware that as the temp goes up, so does the Rds(on) and according to the data sheet on that device, by the time it reaches 150oC it has doubled so you now have almost 4W dissipation.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 28, 2017, 01:53:52 pm
content: It is not clear to me why something with any kind of thermal mass needs a frequency as fast as 10 Hz? Is that common for 3D printer heatbeds?

author: thm_w
author_rank: Super Contributor
timestamp: November 29, 2017, 01:18:12 am
content: Quote from: Richard Crowley on November 28, 2017, 01:53:52 pmMarlin is using ~8Hz if you use PID bed control. Frequency <1kHz shouldn't really be an issue unless you are using relays though, right?For relay controlled beds, you can enable slower modulation (~0.1Hz).

author: LarsT
author_rank: Contributor
timestamp: November 29, 2017, 10:19:32 am
content: Quote from: mikerj on November 28, 2017, 12:02:59 pmThe regulator runs at 10Hz and looking at the graph in "millisecond resolution" and I can't recall seeing any slope at all. I can meaure it to be sure, but I'm pretty confident that's not the cause, at least not for such a low frequency.

author: LarsT
author_rank: Contributor
timestamp: November 29, 2017, 10:29:46 am
content: Quote from: Richard Crowley on November 28, 2017, 01:53:52 pmI was aiming for 1Hz and 10uF felt just about right when picking parts and then I stuck with it.

author: LarsT
author_rank: Contributor
timestamp: November 29, 2017, 10:39:40 am
content: Quote from: Z80 on November 28, 2017, 01:40:46 pmI forgot to take the temperature rise into account, but it makes sense now.Looking around a little I see the mosfets FDP8870 and IRLB8743 recommended several places. Both mosfets have Rds below 4mOhm. I guess they're both better choices.

author: LarsT
author_rank: Contributor
timestamp: December 01, 2017, 08:26:59 pm
content: How about using two (or more) mosfets in parallell?