Unfortunately, the requirement is worse than that - it's 10 years shelf-life AND 25 years operational.
However, I accept the principal in what you suggest and we do have items on the aircraft that have scheduled maintenance, though we avoid it like the plague if we can.
Part of the problem is that we can't get at some of these caps to replace them once the unit is fully assembled, due to a partial encapsulation which is applied for mechanical reasons. This effectively means we would have to bin the whole PCB, which is rather expensive.
With regard to failures in use, things are not as bad as they first seem. We have comprehensive BIT, (Built In Test), that monitors the circuit at all times and shuts down motor drive in the event of a problem. If the fault took out the circuit breaker, the loss of power to the equipment would have exactly the same effect as a shutdown by BIT.
Under these circumstances, whilst the pilot would have lost some functionality, he would still have control and would be able to get home without undue dificulty.
The supply to our equipment is nominally 28V DC, derived from AC generators and 3-phase rectification, so, whilst it is subject to variations, it is essentially clean of any high frequency noise.
The PWM takes place inside our equipment and comprises a three-phase motor winding, with each phase connected to a totem-pole power mosfet drive, connected across the 28V. The windings are therefore switched between 28V and 0V, with current drawn from and then dumped back into the 28V supply.
The array of tantalum capacitors is there to source/sink the PWM currents and limit voltage ripple on the 28V to an acceptable level. A series inductor between our internal 28V and the aircraft 28V then limits the exported current ripple to a prescribed figure.
We could have gone for linear control of the motor, (there are days when I wish we had!), but we would have simply traded all these problems for that of getting rid of all the heat generated.
Quite - I've had difficulty getting this point through to a number of people.
I have posted a follow-up to this thread stating that we are going to use MLCCs. However, since then, a colleague from a different site has told me that he has experienced a lot of reliability problems with multi-layer ceramics, caused by defects in manufacture that can lead to combustion during use.
I'm currently digging around on google trying to get more info on this
- I certainly don't want to trade one unreliable component for another.
If anyone has any specific knowledge/experience of MLCCs, I would be most grateful for some feedback.
Our PWM frequency is nominally 80kHz, and this is a reasonable compromise between switching losses and supply capacitance requirements for our application.
I've re-visited my design notes and, with the SFE tants, the I*esr product dominates capacitor ripple voltage, being some five times larger than the dV/dt component. I've since been looking at MLCCs, which have a quoted esr of 2m-ohms, and using these for the supply reservoir, I could drop the total capacitance to around 100uF and still meet the ripple voltage requirement.
However, there are doubts regarding the reliability of MLCCs, so I'm reviewing things at the moment.
A couple of people had suggested using cap multipliers, but unfortunately, as John Larkin points out, active parts don't store energy.
Perhaps this wasn't clear from my earlier post: Many capacitor failures can be traced to solder process and mechanical stresses. For example, MLCCs don't usually fail due to current or voltage stress alone, instead, they fail after being damaged by mechanical stresses resulting from soldering, board handling, and temperature cycling. Many methods have been devised to minimize such effects, including these:
Use the smallest possible package sizes and parallel multiple small caps.
Take steps to minimize PCB flexing for SMT components and/or use packages with leads.
Avoid encapsulation, particularly with hard encapsulating materials.
Avoid all processing steps that cause mechanical stresses, e.g., lead bending, handling with pliers, dispensing loose parts into bins, etc.
Another poster mentioned snubbing. Perhaps you already replied to that, but it's worth mentioning that a combination of snubbing and shunt filtering (parallel caps with no series resistor) can be much more effective than shunt filtering alone.
Well, since the pcb is encapsulated, when the tantalums and MLCC fail catastrophically, you have to replace the pcb anyway.
It looks like the only capacitor type you have left is solid polymer.
If you use solid polymer, you only need to check the ripple voltage during scheduled maintenance. If the voltage looks like it will go out of spec between now and the next scheduled maintenance cycle, junk the pcb. Otherwise log the ripple value and put it back in service.
If the pcb has been on the shelf for 10 years, obviously you want to test it on the bench before installing it in the a/c. Include the ripple voltage as part of the checkout procedure.
Since the degradation in ripple voltage is gradual, this will give plenty of advance warning of a system failure. This is infinitely better than an unexpected and catastrophic failure at night, close to the ground, in the rain.
One of the biggest problems in flying is handling multiple failures. Pilots who can easily cope with one or two problems may become saturated with one more failure and crash the plane.
Anything you can do to avoid an unexpected catastrophic failure may end up saving the plane and the pilot.
Look into polymer tantalums. Super-low esr and no ignition mechanism. If you're esr limited, the polymers would allow you to go with less uF and higher voltage ratings, for more reliability.
Your original post was clear enough and I took it to mean that MLCC failures could occur due to misshandling at some stage during assembly.
However, someone has since told me that many failures occur in MLCCs due to problems in the manufacturing process - e.g. cavities and loss of metalisation - but that the manufacturers point to handling problems as a smoke-screen for these failures. That's what I've been Googling to find more info on. Problems that can be prevented by appropriate build control is one thing, but I've no wish to swap one unreliable component for another.
With regards to snubbing, I considered fitting snubbers early in the design and decided we didn't need them. I need to do some digging to check my reasoning.
Since we're dealing with what's essentially a current source here, all a snubber would do is slow down the rate of change of current in the caps. Are you saying that's an issue?
I've seen standard MLCs burn up just like tantalums. Same mechanism. High pulse current. You can even do it with plastic film caps if you try hard enough !
Yes, here's more confirmation of the failure mechanism of multi-layer ceramics, and a possible solution using Metallized Plastic capacitors (MLP):
-------------------------------------------------------------------- "Optimizing Output Filters Using Multilayer Polymer Capacitors in High Power Density Low-Voltage Converters"
Bruce Carsten ITW Paktron Lynchburg, Virginia
Surface-mounted MLC capacitors have found some use at frequencies of 1MHz, where their impedance can be less than that of electrolytics. Price has been something of a limiting factor in commercial applications, but the major technological problem has been the construction of semi-stable capacitors larger than luF, which can be surface-mounted on printed circuit boards. Even if they survive the stress of soldering, temperature cycles tend to crack larger ceramic capacitors due to the coefficient of thermal expansion mismatch between the capacitor and substrate.
Metallized Plastic capacitors have similar electrical properties to MLCs without the differential expansion problem, but until recently, they have been larger than MLCs of the same capacity. This has changed with thinner dielectrics and new construction techniques; they are now volumetrically competitive with ceramic capacitors and will become more so in the future.
[...]
MLP "Plastic" Capacitors. These are the lowest cost capacitors of the low HF impedance capacitor types. No expensive raw materials are used, so costs can be expected to hold or decline with improvements in film manufacturing technology.
The self-healing capability of metallized plastic capacitors make them quite reliable, with relatively graceful and typically gradual failure with over-voltage.
[I once successfully (and somewhat dramatically) demonstrated the robustness of this self-healing property to a skeptical fellow engineer by cutting the corner off of a capacitor with a hacksaw, and then driving a nail through a drilled hole in the middle. About 25 percent of the capacity was lost, but it would still withstand rated voltage.]
ESR is very low, typically 3 milliohms at 1MHz for a 10uF, 50V capacitor, and can be lowered further if needed with heavier metallization (which will have minimal effect on capacitor volume). RMS current capability is correspondingly high, on the order of 1A/uF. Voltage and current ratings apply to 85 degrees C, where ESR is at a broad minimum with polyester dielectric; operating temperatures to 125 degrees C are possible with derating.
Well, if parasitics in the caps led to high dI/dt causing excessive voltage, I could see the problem, but that's not the case here - I've done extensive measurement using a 100MHz DSO during the high current events and there are no troublesome voltage spikes on the caps and voltage remains comfortably within rated operation throughout.
(We have an array of 14x470nF sm ceramics in parallel with the tants to help deal with the HF content and this seems to work well in that respect).
If there's another problem mechanism in tantalums relating to high dI/dt, I'm very interested to hear about it.
Re snubbing: I was assuming that the main purpose of the caps was to reduce conducted and radiated EMI, much of which is presumably in the form of ringing waveforms. Damping with critical value can result in less EMI overall with less capacitance required. Paul Mathews
I've been looking at these but I've not been able to find anything with a higher working voltage than 25V, (searching not helped by the fact that Google keeps hanging up whilst trying to access data for no apparent reason).
I'm coming round to the view that we might be best using solid polimer aluminiums, with a ripple-voltage monitor linked into BIT. They quote a life of more than 10 years, but when you look at the curves of projected life against temperature and RH, things look much more optimistic - greater than 100 years at 75C and 45%RH. (How true these forecasts are is another matter, but I won't be around to worry about it).
Having gone all around the houses, I've come to the conclusion that solid polymers are the way to go, with a ripple-voltage monitor linked into BIT.
As you say, they forecast very long life under more benign conditions than the +125C/85% RH extreme - in practice, I reckon they would meet both the shelf-life and operational life requirements in our environment.
The main source of EMC emissions in this design is the bi-directional, rectangular supply current waveform and that's what the tantalums are there to deal with. Yes, there are also EMC issues relating to the PWM motor drive waveforms and we had to introduce specific measures to deal with this, in order to meet our EMC emmissions spec.
However, we don't fit snubbers to the PWM outputs as these would require a peak pulse current rating of 20A at a PRF of nominally
1) Kemet high temperature wet slug Tantalum caps; 125V@85C is highest voltage i could find, de-rated to 62V@200C - values 1.7uF to 82uF.
2) BC high temperature Aluminum caps; 200V@125C is highest voltage i could find, "useable" to 150C - values 2.2uF to 100uF.
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