What is a better solution to drive a IRF3205 with an NE555? - Page 1
Forum section: electronics / beginners

author: Chriss
author_rank: Frequent Contributor
timestamp: February 22, 2018, 10:14:55 pm
content: Hi!I'm interested to drive an IRF3205 with an NE555 with a 30KHz pwm signal.I read a lot about a FET driver ic, push-pull totem pole driver etc.I think the most badly option is to drive a FET directly from the output pin of the NE555.So, I decide to make thinks better because in the future this FET should drive a fan which willconsume a current around 10A when it is full on.Actually the whole think should work on 12v.My question is:Should I just drive the mentioned IRF3205 connect direstly through a resistor on the NE555 pin3 (output)or is a better way to use a totem pole driver with two transistors like on the attached picture?Please don's take care about the NE555 configuration, this is not configured to run in pwm mode I know,just pay attention to the output circuit pls.Thank you very much for any suggestion.My best regards.Thanks.

author: Paul Price
author_rank: Super Contributor
timestamp: February 22, 2018, 10:34:50 pm
content: With a 12V supply and 32KHz, the 555 timer output would be just fine to drive the MOSFET. The MOSFET would be almost completely fully on when the rising edge of the 555 output is >5V and completely on when the output reaches 11+V.As far as turn off, the fall time of the 555 output would even be faster than the turn on time and quickly turn off the FET.

author: Chriss
author_rank: Frequent Contributor
timestamp: February 22, 2018, 10:36:28 pm
content: So, you vote to use the FET just direct to the NE555 output?

author: Paul Price
author_rank: Super Contributor
timestamp: February 22, 2018, 10:42:27 pm
content: With a 20/22 ohm resistor in series.

author: Chriss
author_rank: Frequent Contributor
timestamp: February 22, 2018, 10:45:20 pm
content: Yea, of course.Thank you.

author: ogden
author_rank: Super Contributor
timestamp: February 22, 2018, 10:57:45 pm
content: Quote from: Paul Price on February 22, 2018, 10:42:27 pmThat 100A MOSFET have beefy gate, but hopefully push-pull 200mA capability of ne555 will be enough for 30KHz. It shall be. Thou I would check gate voltage waveform with scope.

author: rs20
author_rank: Super Contributor
timestamp: February 22, 2018, 11:00:28 pm
content: I'm not so sure -- doing a (admittedly very simplistic liberal calculation) assuming:-- Switiching time = IRF3205 total gate charge / NE555 output current-- MOSFET dissipates 10 amps * 12 volts while it is switchingGives a dissipation of 2.8W. While I realise that my calculation isvastlyoverestimating in many regards*, I'd like to know if there were any calculations behind your statements Paul? Even if it turns out to be true that this particular combination of parameters is OK, it might be more educational to explain where the cutoff lies.In any case, a more optimized choice of transistor can reduce dissipation in the FET -- just picking the lowest on-resistance is a bad idea because low on-resistance comes with high gate charges --> long switich times --> higher switching losses. If you really want to have some fun, you can download digikey's transistor catalogue and use spreadsheet formulae to find the best transistor for your application, perfectly trading off switching losses vs resistive losses.(Also, the decision pedantically depends on how willing the OP is to trade off wasted power in heat dissipation vs solution size vs BOM cost.)* 2.8W would require heatsinking, but given that the real figuremightbe about 10 times smaller (0.28W), I'dguessthat the MOSFET would not need a heatsink and just get a tiny bit warm.

author: Paul Price
author_rank: Super Contributor
timestamp: February 22, 2018, 11:00:49 pm
content: You're welcome!You might be mindful that the fan you are driving could create inductive turn-off spikes that could exceed the 55V rating of the MOSFET. In this case a protective zener diode or a snubber resistor-cap combo might be needed.Fans that have internal transistor commutating for motor drive will not produce spikes.RS20: I would expect MOSFET switching loss to be closer to the .28W level. Turn on time I would guess to be in the low microseconds.Having a slower turn-off just using a 555 as a drivers means less inductive spike amplitude, if any.I don't know what fan you are using, but I would try out this fan driver first with a small heat sink and check dissipation with use of your "digital" temperature sensor(quick touch with a finger!).

author: Paul Price
author_rank: Super Contributor
timestamp: February 22, 2018, 11:37:55 pm
content: Note: The FET spec sheet shows 62Amp 101nS turn on time with 10V gate drive voltage with 4.5 ohm series gate resistor, but this is with a high current, fast-risetime gate drive voltage. The more-limited 555 high state output drive current will give a turn-on time >101nS but you are only needing to achieve ~10Amps. An oscilloscope will quickly show how quick the FET is turning on and how big are any inductive spikes.

author: Chriss
author_rank: Frequent Contributor
timestamp: February 22, 2018, 11:47:21 pm
content: Ok people here are all the stuff with measuring etc.I put in series a 10 Ohm resistor with the FET Gate pin.Made measuring with my scope and here are the results, I think it is really good.I have only one/two questions.Please pay attention on the pictures where are spikes and ripples on the first channel the yellow one.When I connected to the FET a light buld 12v/25w I got spikes on the D pin of the FAT.When I connected the small fan from the PC which is 12v and Imax=0.15A.I think something is to do to avoid that kind of signal distortion.Btw. the FET is cool even if the light bulb is attached and dimmed up and down.No heat is sensing on the FET.Here are the pictures and my schematic how I made it:

author: ogden
author_rank: Super Contributor
timestamp: February 22, 2018, 11:54:49 pm
content: Gate pin waveform shows ne555 is excellent driver for that MOSFET, at least for given frequency. PC fans can act like inductive load, that ringing/overshoot seems clamped (inside fan) and safe for mosfet. Congrats!

author: Paul Price
author_rank: Super Contributor
timestamp: February 22, 2018, 11:55:45 pm
content: The gate drive waveform looks excellent with the 555. The only complicaton of the output distorted waveform would be very nearby AM radio interference. A snubber (.1uf series with 22ohm) D-S should clean this up.To be sure, you need to test this circuit with the real 10-amp fan as a load.It would be nice to post a 1us/div scope scope pictures of the gate voltage and drain voltage to clearly see both gate and turn on/off rise and fall times.

author: ArdWar
author_rank: Frequent Contributor
timestamp: February 22, 2018, 11:57:50 pm
content: Of course with 0.1A the losses are insignificant.The low switching frequency also helps mitigate the weak drive. Though, at 10A load and 200mA gate drive the MOSFET switching losses might be about 4 watts, and additional 1 watt from Rds(on). That might be fine, or not, depends on how your MOSFET is heatsinked.

author: Chriss
author_rank: Frequent Contributor
timestamp: February 23, 2018, 12:14:45 am
content: Here are the pic's about the heat sink on the FET.It is a small one but I hope it will hold, if not I can put a bigger one if needed...The FET is cool like a dead man after 48h.I think the whole think is set up correctly what the FET configurations means.Am I correct?I will check later with the R/C components connected as you described toshut down the ringing if it is possible.Are there any suggestion to modify in any direction maybe to make think better but not over complicated?Thank's

author: Paul Price
author_rank: Super Contributor
timestamp: February 23, 2018, 12:17:27 am
content: Rds(on) is spec'd at .008-ohm at 107Amps, but at 10-amps, when operating at 100% duty cycle only, then Pd <=.8W due to Rds(on).A small heatsink may be required. Yours looks much bigger than might be needed operating at room temperature, but the size is dependent on air flow, air flow temperature and operating max. ambient temperature.You must view the output switching waveform with the actual fan, unless you are controlling 100 .1A fans in parallel. In the case of the ringing I see here the distorted waveform would not cause any interference problems unless the fan controller is extremely close to some sensitive device. No need to add additional components.

author: ArdWar
author_rank: Frequent Contributor
timestamp: February 23, 2018, 12:32:21 am
content: That should be more than enough, It's not that hard to get 15K/W Rth(j-a).By the way, for hard switching application MOSFET turn-on/turn-off losses are almost always higher than conduction losses. Also Rds(on)usually is specified at 25oC, it'll be higher when the die gets hotter. Not that much, but keeps in mind if you don't have much thermal headroom.

author: Paul Price
author_rank: Super Contributor
timestamp: February 23, 2018, 12:43:49 am
content: Keep in mind that the wire lead length to the fan to be controlled will create some inductance, and any inductance will create ringing and spikes on turn-off. If long wires are involved (>5cm) then you might need a snubber to reduce RFI/spike voltage.The slower turnon/turnoff times when driving the FET with the 555 ensure you will have less RFI and spike amplitudes.

author: Chriss
author_rank: Frequent Contributor
timestamp: February 23, 2018, 08:49:07 am
content: Paul:Can you pls tel me how you got the Pd<=.8w so quickly?Is there a diagram in the datasheet what I can use for a quicker reading of this parameter?Or I have always calculating it.The wire is from the fan and the FAT is not longer than 15cm.My best regards.

author: Chriss
author_rank: Frequent Contributor
timestamp: February 23, 2018, 08:34:12 pm
content: Ok, here my result from today when I connected the real fan to my pwm controller.Ifan= 10-12AUfan=12VThe FAT is totally cool on a minimum duty and a maximum duty too.Also when I was testing on a longer period of time approximately 10min on a duty of 30% on time.FAT still cool.But I have some problem with the spikes which killed my D3 on my schematic in my previous post above.D3 is the dieode between the D-S of the FAT.The spikes are very high, as you can see on a duty of 50% the spikes rise almost to 40V.When I gone higher duty around 80% the spikes was over 100v and killed the D3.Any idea how to prevent this to happening and hot to eliminate the spikes?I know, I used a general purpose rectifier diode, but at the moment I don't have any schottky diode.I also know does the diode was killed because the diode is not rated for that high voltages....Would a schottky do the job?Check the picture about the scoping.Thank'sMy best regards.

author: rs20
author_rank: Super Contributor
timestamp: February 23, 2018, 09:30:05 pm
content: Did you install the diode at all? Did it make proper contact? The whole point of the diode is that it clamps the voltage spikes -- if the voltage is reaching 100V, then the diode has alreadyfailedto do its job; whether it subsequently gets damaged by that voltage spike is of little interest.Now if the diode is getting damaged by excesscurrent, then failing to act as a diode, then the symptom of that failure would be the voltage spiking to 100V. Given that you're putting 12A through it, but the diode is only rated for 1A average continuous / 30A non-repetitive, it could be excess current that's the problem (but I'm not sure about this). I'd be wanting to try a diode that's rated for 10A.There may also be details about the switching performance of diodes thatmightbe relevant here, but I've never heard of diodes getting damaged by excessively fast transition.

author: Chriss
author_rank: Frequent Contributor
timestamp: February 23, 2018, 09:50:23 pm
content: I think the problem is the specification of my diode what I use.The diode should handle minimum the current of the motor or maybe more.I think the spikes are drive the diode in a short because an over voltage generated by the spikes,and so the FET is actually delivering a big amount of current to the diode but for a short of period of time.After several min the diode reach his maximums and crack.But lets see what other peoples also say.UPDATEDAs I wrote.I found a fast switching power schottky diode in a PC PSU,this is a MBR1645DT and replaced my old 1N4001 with this one.Now is everything fine and cool.Here is my signal with the MBR1645DT:I think this signal and this pwm controller succeeded.What you think?My best regards.

author: Damianos
author_rank: Frequent Contributor
timestamp: February 24, 2018, 08:31:08 pm
content: So, what you have to do now is to test it in the entire range of PWM duty cycle.Then it is ready to start production!-------------------------------------------------------------------------------------Some points on your conclusions:The spikes are produced due to to the "leakage" inductance of the motor (I'm not sure if the term is correct). These are in the foreword direction of the diode, so it is (was) not the overvoltage that break the diode but the overcurrent. The reason that there were spikes with the diode there was that the conventional rectifier diodes "need" some more time to start conducting.Yes, the diode must be specified for the maximum current of the motor. This because when the motor just "tries" to start rotating or is stalled is a pure inductor, especially with low duty cycle.

author: Chriss
author_rank: Frequent Contributor
timestamp: February 24, 2018, 09:55:22 pm
content: Damianos:I was smiling until I read your part of post "Then it is ready to start production!"Maybe it is a good idea, I don't know, never thought about it.I tested it in the whole range of pwm in this meaner, I let the fan running for 30min continuouslyand changed time by time the speed.Everything was fine, the fet and the new schottky diode was also cool all the time.Is there any other way to make better tests?Thanks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 25, 2018, 06:17:30 am
content: Oh hey, those schematic symbols I drew so many years ago.Tim

author: Chriss
author_rank: Frequent Contributor
timestamp: February 25, 2018, 12:36:26 pm
content: Paul Price:Sorry I don't sent the requested scoping picture from your replay #11Here I share the 1uS/div scoping on the G and D.The D is fully opened after 200nS compared to the signal of the NE555.The output voltage rise to 9.2V on the NE555 after 200nS.This measurement was taken on a full load of the FAN under the max power.What are you think?

author: Chriss
author_rank: Frequent Contributor
timestamp: February 26, 2018, 08:32:44 pm
content: Quote from: T3sl4co1l on February 25, 2018, 06:17:30 amHi!If you have practical experience with this schematic can you pls check my scope diagram from my Reply #24 andwhat you think? is it good enough for using it or should I maybe tune a bit more the G-D signal?Thanks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 27, 2018, 01:41:27 am
content: What about it? There's a bit of turn-off drain ringing, a modest R+C damper would help with that (maybe 22R + 1n). Turn on is slow and smooth. Doesn't look like it would be a problem at the switching frequency. If the transistor is getting too hot for your liking, use a bigger heatsink, or consider improving drive speed (or, consider getting a smaller and faster transistor).If you expect highly inductive loads, the R+C might be increased, or a clamp diode or TVS added, to clamp excessive voltage (better to divert peak voltage somewhere, even if it's burned in a TVS, than to make the MOSFET handle it).I don't recommend the complementary emitter follower, as the 555 output does not swing full +V/GND as it is, and the follower drops 0.7V above that. Discrete alternatives are more complex (a rail-to-rail amplifying stage, then a follower), and integrated alternatives you probably don't have on hand (e.g., TC4420 gate drive IC).Tim

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 27, 2018, 04:37:56 am
content: Quote from: Chriss on February 23, 2018, 12:14:45 amat what current are you testing? IRF3205 is a 80A mosfet, 8mOhm Rdson. if you are testing the 25W lightbulb @ 12V, thats 2A current. P = I2R = 4 * 8e-3 = 32mW on the mosfet, thats nothing. real test should be near mosfet rating or half of it to be safe ie 40A, but for your sake its 10A max if i see it correctly. at 10A, the mosfet will dissipate 0.8W fully on, assume half of it with zero loss switching 50% PWM, there's where if you want to sense some heat and decide if your gate drive is efficient or not. the push pull in the OP you posted should be the better and easy solution if you find any issue with 555 directly driving the fet. other beffy IC should be better but i think will be overkill for your 120W application. energy loss can rise up due to overkill driver (fast switching) i think. ymmv.

author: Chriss
author_rank: Frequent Contributor
timestamp: February 27, 2018, 10:41:55 am
content: T3sl4co1l:Thanks for your help mate! I really appreciate it.At the moment I have a 16A schottky dioede parallel with the DC motor,the ringings are still there but much lower. Like you see on my last scoping.I will try to put the R+C dumper as you mentioned and see how much will that help.That R+C dumper I should put on the D and GND or between the +12v and GND?Mechatrommer:I'm testing the circuit around 10-12A current.Everithyng is cool even on the 10, 50, 90% duty too.Can I say does this circuit is actually ok for a current of 10A?I assume the max current what could be delivered through this circuit is 16A because of the rating of the schottky diode.Of course with a heta sink on the diode.A m I right?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 27, 2018, 11:38:18 am
content: Oh, no wonder then -- the diode belongs at the source of the transient: the switch. Put that thing right down on the board, beside the transistor, and beside a +V to GND capacitor. All gone!Tim

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 27, 2018, 12:08:09 pm
content: Quote from: Chriss on February 27, 2018, 10:41:55 amthen the switching loss is not much concern.Quote from: Chriss on February 27, 2018, 10:41:55 amfrom your description, it should. you just have to concern about the transient and back EMF because mosfets are sensitive to over voltage, energy doesnt have to be much to toast a mosfet. i would say guard the mosfet as much as you can afford with transient diodes and snubber.Quote from: Chriss on February 27, 2018, 10:41:55 ami'm not sure which diode but if its for transient or voltage reversal protection and a big diode, 16A usually a continuous DC current when diode operating in forward mode, in transient mode, the diode usually rated in hundreds of ampere of surge current. your circuit using IRF3205 should be able to draw near the spec current of 80A (at 100degC) it just a matter of your cooling system. i would say 5-10W of mosfet dissipation for cheapish (fan) cooling system, thats around 35A @ 8mOhm Rdson. thats in DC, maybe you can get by with 70A in 50% PWM not really sure havent dont thatyou have to work out the SOA chart and derating factor as well... ymmv.and btw another thing is, also check your driver (555) it may not be happy with 1.2A of gate current sourcing and draining, but it may.. ymmv.

author: Chriss
author_rank: Frequent Contributor
timestamp: February 27, 2018, 09:11:28 pm
content: Here I'm againI played with the RC snubber but without success.But when I put the schottky diode beside the FET and not on beside the motor here is the result:The type of schottky diode is all the time MBR1645DT.Pic1 is the scoping when the diode was on the motor andPic 2 is the scoping when I put it near as I can to the FET.What do you think, is this switching off ringing acceptable when the pwm signal is on 50% duty andthe load is 10A DC fan motor?Everything is still cool, no matter if the duty is 10% or 90%.Tested over 30min continuously.

author: David Hess
author_rank: Super Contributor
timestamp: February 28, 2018, 01:48:49 am
content: Current through the diode is initially close to the drain current so it needs to be sized for that; the leakage inductance is trying to maintain the current.I would add a big decoupling capacitor across the series connected switch and load to minimize the loop area of the high frequency AC. That may be contributing to the flyback you are seeing.The output current from the 555 is limiting switching speed which is likely an advantage.Some DC brushless fans do not like being PWMed like this leading to early failure. Old motherboards which do this have simple LC output filters and a diode and operate like an open loop buck regulator.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 28, 2018, 03:20:58 am
content: Yes, much better.Tim

author: Chriss
author_rank: Frequent Contributor
timestamp: February 28, 2018, 09:55:28 am
content: Hi David!Quote from: David Hess on February 28, 2018, 01:48:49 amHow big should this cap be? any suggestion?Thanks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 28, 2018, 10:22:25 am
content: Offhand, a 1000uF electrolytic capacitor rated for at least supply voltage, should do.Tim

author: David Hess
author_rank: Super Contributor
timestamp: February 28, 2018, 03:49:58 pm
content: This is one of those cases where a film or ceramic capacitor is worse than a solid tantalum or aluminum electrolytic because of dampening considerations. (1) The rule of thumb is usually something like 47 to 100 microfarads of aluminum electrolytic capacitance per amp which is consistent with T3sl4co1l's 1000 microfarad suggestion. (2) A solid tantalum part would be expensive even though it could have lower capacitance and failure due to surge current becomes a reliability issue. A 100 microfarad or larger polymer aluminum electrolytic capacitor might be suitable but I would want to test that with an oscilloscope. The test I use is to increase the capacitance with a worse case load until diminishing returns of reducing power supply bounce are reached and then doubling that value but maybe someone can suggest a more scientific approach; I'm used to dealing with poorly defined loads so it is just simpler this way.(1) This does not mean that an additional large ceramic or film high frequency decoupling capacitor should not be used because it usually should. But the large aluminum electrolytic capacitor is still required for dampening.(2) This is the same rule for simple regulators and why small bulk decoupling capacitors are located where power enters a board. High performance power supplies may use a small ceramic or film output capacitor with a low value series resistor and careful frequency compensation in order to improve performance but none of this is required here and the brute force approach of a relatively large aluminum electrolytic capacitor is better.

author: Chriss
author_rank: Frequent Contributor
timestamp: February 28, 2018, 03:53:34 pm
content: David:Can you pls post some sort of schematic with this cap in circuit?Just to be sure I'm understand correct everything, because the language differences pls.Thanks.

author: David Hess
author_rank: Super Contributor
timestamp: February 28, 2018, 04:39:13 pm
content: It is not that complicated. It needs to go between the source of the MOSFET close to the MOSFET and the Vcc connection to the load close to the load like at the cathode of the diode.

author: Chriss
author_rank: Frequent Contributor
timestamp: February 28, 2018, 06:27:34 pm
content: I put the cap as mentioned but nothing significant was happen.Also tried with other values of cap but nothing.The ringing peek voltage is 14.5v at 50% duty on a motor load of 10A.Is this peek actually possible to reduce or complete eliminate?The datasheet say V(BR)DSS 55v min.Is my controller actually good enough with this setup and this scope diagram to handle my fan or whateverwith a current of 10A?Should I in real reduce this ringing?Thanks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 28, 2018, 06:50:30 pm
content: The ringing is small, it won't damage anything.It could still cause interference, who knows.If you have any ferrite beads handy, you can try one on the drain pin, or in series with the diode.Tim

author: Chriss
author_rank: Frequent Contributor
timestamp: February 28, 2018, 07:34:30 pm
content: Ok.Just for the science and fun, I will find a ferrite bead.Actually I saw somewhere I think in PC PSU. I will hunt down and give a try.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 28, 2018, 11:45:35 pm
content: Increased ringing i suspect...