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226  Using Arduino / General Electronics / Re: Inrush current protection using MOSFETs on: March 19, 2013, 01:54:26 pm
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Sorry, I was thinking about schematics like this: http://circuit-zone.com/index.php?electronic_project=438

No worries, I thought something was up....

Perhaps I've misinterpreted the app note entirely, so let's start from basics.

VGG is not a connection from another device such as an NPN transistor, but it's simply the power supply ground? Your wording seems to imply VGG is controlled by another device not pictured.

Figure 3 that uses an N MOSFET on the high side, I think I understand - as of course - when the circuit is powered up, VDD = 12V (for arguments sake) and so the gate is charged through CGD' and RGD, slowly - which prevents inrush.

Is that correct? Further, I didn't think you should/could ever use NMOS on the high side?

Only when we've sorted these issues, then let's move onto the PMOS application of Figure 1  smiley

Cheers!
227  Using Arduino / General Electronics / Re: Inrush current protection using MOSFETs on: March 19, 2013, 10:04:16 am
When I say defeats the point entirely, I am referring to the fact that the circuits presented are there to switch the load on slowly, whilst Figure 1 does not seem to do this.

So yes, we're talking about Figure 1, for sure.

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For example with a resistor from Gate to Source to keep the Gate at 12V.
If the Load is switched on slowly, the Gate should be slowly lowered from 12V to a lower voltage. Perhaps with a NPN-transistor with its collector to the Gate.

Figure 1 does not have a gate-source resistor, nor any NPN transistor for this purpose.

RGD and CGD' are there to provide the slow ramp of gate voltage - like I said in my original post, I understand how these passives are doing exactly that in Figure 3, but not in Figure 1. As Figure 1 defaults to on, i.e. does not prevent inrush current, hence - defeats the point, as the point is to prevent inrush current.
228  Using Arduino / General Electronics / Re: Inrush current protection using MOSFETs on: March 19, 2013, 09:24:13 am
Thanks for your reply.

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In Figure 1, the 12V is the Source pin of the P-channel MOSFET.
If the Gate is low, the P-channel MOSFET is switched on.
If the Drain is 12V, the Load is powered. But the Gate is still low and the Source is still 12V, keeping it switched on.
To slowly switch the load on, the Gate voltage has to be lowered slowly from 12V to a lower voltage.
Does that answer your question ?

I'm afraid not, no - the whole purpose of the solutions proposed in the app note are to ramp the MOSFETs on slowly so as to prevent high inrush currents.

If as you say, the gate is at 0 volts (because it's connected to the 0v rail), and you apply power to the circuit, i.e the source becomes 12V - then VGS is -12V, and the MOSFET is switched fully on, which then allows a large inrush current to flow immediately.

Hence, defeats the point entirely.
229  Using Arduino / General Electronics / Inrush current protection using MOSFETs on: March 19, 2013, 07:34:12 am
Hi all,

So - to prolong the life of all the caps on a design I'm working on, and to prevent tripping of anything, I've been looking at inrush protection using MOSFETs. Now, Motorola have a nice app note, AN1542 about this exact issue. http://www.bonavolta.ch/hobby/files/MotorolaAN1542.pdf

However, there's a couple of little caveats that I don't quite understand.

Figure 3 on page 8 uses an NMOS FET to control inrush current by charging the gate slowly, i.e. limiting how fast gate voltage rises. Of course, with N FETs, VGS must be positive for the device to turn on. So the schematic makes sense.

However, a question - I thought one should never use NMOS in a high side application, only ever in the ground return?

Figure 1 however, also on page 8, uses PMOS to do the same job. However - I can't figure it out, P FETs require VGS to be negative for the device to turn on. So, question - when you first apply power to the circuit, the source is say 12V, and the gate is at 0V, so VGS is negative - and the FET is switched fully on, defeating the point entirely?

Am I wrong?
230  Using Arduino / General Electronics / Re: Dielectric fluid filled PCB enclosure for heat dissipation on: March 16, 2013, 10:37:55 am
I'm using the suppressors to deal with flyback when switching inductive loads, as per my other thread you've helped in - hence the power dissipation.

The MOSFETs have an on resistance of only a few mOhms, and are only switching ~5A at the 10s of kHz.

The IGBTs are driving inductive loads, at currents of ~10A up to around 75Hz. The h-bridge is a VNH2SP30, so no - no bipolars here thankfully! Stepper chip uses MOSFETs in the bridges too.

The problem will arise in the future I foresee, with up to 20 high side switches potentially, so this thread was more to get a feel to see if the solution was viable re cooling.

Cheers!
231  Using Arduino / General Electronics / Re: Calculating power dissipation in a tranzorb for inductive flyback clamp on: March 16, 2013, 09:04:03 am
BTW you can get power mosfets that are rated at greater than 5mJ repetitive avalanche energy, so your tranzorb is theoretically redundant if you use one of those. But including the tranzorb is kinder to the mosfet.

I had looked at these yes, but decided to stick with a transient suppression diode. Thanks for your input!
232  Using Arduino / General Electronics / Re: Dielectric fluid filled PCB enclosure for heat dissipation on: March 16, 2013, 06:26:24 am
Thanks for the replies...

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I still think the first priority should be to see if the power consumption of the circuit can be reduced.

This is of course my first priority, at the moment I'm not dissipating anywhere near that power - but a future version will include the possibility for multiple high side, high current power switches which will up the dissipation.

(...just because I've just done the calculation...) as an example, on the board are 8 transient suppressors dissipating up to ~400mW each, so we're at 3.2W.

Now include the 12V regulator, 5V regulator, 3.3V regulator, 8 MOSFETs driving not insignificant currents, 4 IGBTs that are self clamping, h-bridge IC, stepper motor IC all tightly packed into a 100 x 80mm double sided PCB.

When I get a moment at some point today, I'll do a full calculation of the worst case power dissipation and report back.
233  Using Arduino / General Electronics / Re: Dielectric fluid filled PCB enclosure for heat dissipation on: March 15, 2013, 04:52:03 pm
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And lastly any dielectric fluid will increase stray capacitances around the board, this may affect some things.

This is something I haven't thought of... hmm.
234  Using Arduino / Microcontrollers / Re: Port type to use, output compare or normal digital? on: March 15, 2013, 04:27:08 pm
That's perfect! That opens up the potential for spare PWM channels!
235  Using Arduino / Microcontrollers / Re: Port type to use, output compare or normal digital? on: March 15, 2013, 04:08:50 pm
So, sorry to revive a somewhat old thread - but after circumventing this problem, I am now at the stage in my project where I'm about to finalise board layout and start routing - clearly pin assignment is unavoidable now.

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Yes, the timers have a lot of options...

So many options, that I'm struggling to get my head around them all.

It's obvious to me that the output compare pins are the ones used to generate a PWM waveform, using the output compare modulator with the timers.

I have 8 external devices that I want to switch on, and switch off with the highest level of time precision offered by the device, and so will use the timers for this as opposed to actual 'software'. Also an IC on the board with a pin that I must pull high / low with time criticality in mind.

I also need 8 PWM pins. So, I'm using the analog comparator for something else, so that knocks off pin 5 - OC3A/AIN1. So I'm left with 14 OC pins, and 17 'needs'.

The question is, the 9 time critical needs - I don't want to feed these a PWM signal, I just want to switch a pin high, or low at specific times. Can I not just use any pin for this, and do this within an ISR generated by the timers?
236  Using Arduino / General Electronics / Re: Calculating power dissipation in a tranzorb for inductive flyback clamp on: March 15, 2013, 03:48:13 pm
Ah, of course - that makes sense. Thanks!
237  Using Arduino / General Electronics / Calculating power dissipation in a tranzorb for inductive flyback clamp on: March 15, 2013, 10:49:53 am
Hi guys,

I'm using a bidirectional tranzorb to protect the drain of a MOSFET being used to switch an inductive load - so when the inductive flyback exceeds the breakdown voltage of the tranzorb, the pulse is conducted back to battery positive.

The device is from Vishay's range - http://www.vishay.com/docs/88301/15ke.pdf

Calculating the energy stored in the inductor is no problem, I've got 5mJ stored. Now, to calculate the heat dissipation in the tranzorb, I seem to recall reading/hearing it would be the energy stored by the inductor multiplied by the switching frequency, that is - P = E * F.

Would this be correct, if so - I'm not sure what to type into Google to read more about this, things like 'power dissipation switching frequency' seem to come up short. If correct, in my worst case situation I'm dissipating 415mW. Of course, some of the energy is dissipated in the wires, traces etc - but predominantly the tranzorb I presume.

Any thoughts? Cheers!
238  Using Arduino / Microcontrollers / Re: External flash with Atmega2560 on: March 13, 2013, 02:36:24 pm
Retroplayer, thanks for the info - as mentioned I'm going with an SD solution for the hardware and code simplicity - but good to know none the less.
239  Using Arduino / General Electronics / Re: Dielectric fluid filled PCB enclosure for heat dissipation on: March 13, 2013, 12:21:54 pm
Thanks for all the replys, regarding Cray 2;

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The cards were packed right on top of each other, so the resulting stack was only about 3 inches high. With this sort of density there was no way any conventional air-cooled system would work; there was too little room for air to flow between the ICs. Instead the system would be immersed in a tank of a new inert liquid from 3M, Fluorinert.

That pretty much sums up how my PCBs will be stacked, albiet one two as of now.

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However we were not dealing with such high powers as tens of watts. I think the problem you have is that with such high powers most low viscosity electrical inert fluids will boil.

My rough estimate of tens of watts is for a future design, that really - will likely be 20 watts absolutely max, but distributed from a much larger number of transistors all over the boards whose sum reaches 20 watts. I guess only experimentation will tell.

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I know a lot of overclockers have put their PC motherboards into pure mineral oil.  It's non-conductive and carries heat better than air, to my knowledge.  I doubt it has thermal conductive properties equal to proper heatsinking, though.

To go this approach, you need to guarantee a few things.
- All components are sealed.  ICs are good.  Caps must be airtight.  Switches and potentiometers would cause problems if the oil were allowed to create a barrier between wiper/contact and element.
- The case itself is sealed.  Obviously.
- The mineral oil is really pure, and stays that way.  With liquid flowing around, you can't have metal shavings or dust hanging out in there.

Yes, a mineral oil will have superior thermal conductivity & capacity compared to air, but inferior to say copper.

Your point regarding switches and pots is a good one, my current design uses a few jumpers for hardware setup - but then again, the next revision is likely to use analog mux's to sort this problem. As for caps, are standard electrolytic caps airtight - I figured all caps would be?
240  Using Arduino / General Electronics / Dielectric fluid filled PCB enclosure for heat dissipation on: March 12, 2013, 04:48:00 pm
    Hi all,

    I've been working on a personal project for months now, and am getting to the point of designing my own enclosure - which I will be casting and machining from aluminium which will have a number of fins to dissipate heat.

    Now, to fit my PCB into the space specification that I have set - I need to pack all my components in very tightly indeed, so there will be minimal area on the PCB to dissipate the heat.

    I could of course design an aluminium/copper insert that slots into the enclosure with the PCB, that contacts 'heat pads' on the PCB, and conducts the heat away into the enclosure body - but that of course takes up valuable real estate.

    What I propose to do instead, when the PCB slots nicely into it's enclosure - there will be ~1-2mm airgap from the board & components to the inside of the enclosure. I would like to then fill this gap with a dielectric fluid that would conduct heat from the components to the enclosure itself, which of course then dissipates heat to the environment.

    We're not talking huge amounts of heat here by any means, but up to tens of watts maximum.

    So, a couple of questions really;

  • Can anybody recommend, know or, or have experience of a specific dielectric fluid with a high thermal conductivity?
  • Does anybody know if perhaps such a fluid would interact with typical components, ICs, passives, the boards FR4 material etc?

Many thanks in advance[/list]
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