Inductorless DC-DC converters for space application

[p1ayaone1]

I am designing a small satellite which uses a magnetometer and some coils (which interact with Earth's magnetic field) to provide attitude control.

The strength of the geomagnetic field is 30 uT to 60 uT (according to Wolfram Alpha), and the magnetometer has a resolution of 0.015 uT.

The satellite observes the CubeSat specification, which means it is basically a 10-cm cube. It is really small.

Now I need to power this thing. What I really need is voltage regulators with the highest possible efficiency. Also, because of the magnetometer, I don't want to have inductors in the power system, because they would be "always on".

The desired output voltage is 3.3V with 150 mA absolute minimum, but I would much rather be closer to 200 mA. (It should be able to go lower than 150 mA, but that figure is the minimum maximum. ) I will probably also put a 5 to 8 V bus, but my main concern is the 3V3 bus because that's where all my electronics are powered.

The input voltage is two Li-Ion cells. These could be in series (5.5 Vmin to 8.2 Vmax) or in parallel (2.75 Vmin to 4.1 Vmax). I'm ambivalent wrt series/parallel connection, as long as the regulator is sufficiently efficient.

Linear Regulator
OK, fine, it would *work* but this is an efficiency-critical application, and I don't want to lose all that extra voltage as heat.

DC-DC Converter
Buck/boost, Sepic all use inductors. No good.

Charge Pump
There are a tiny few that support variable input voltage, but the dominant functionality is voltage inversion and voltage doubling. Also, current ratings are quite low. I suspect the solution lies in this class of DC-DC converters.

Anybody got any neat ideas?

 

[gnurf]

Use a DC-DC converter with a shielded toroid-wound inductor. With your low current setup I'm guessing you'll be fine. What reasons do you have to believe otherwise?

 

[skeptic2]

How about a pulse width modulated switch between the higher voltage batteries to the 3.3 V supply. The supply voltage would be compared with a reference and when the voltage drops below a preset threshold, the comparator would turn on the switch which would allow the supply capacitors to charge. When the capacitors charge up to the upper voltage threshold the comparator would turn off the switch. The biggest disadvantage would be ripple on the 3.3 V supply. That could be mitigated by setting the upper and lower supply voltage thresholds close together or reducing the hysteresis of the comparator. Additional filtering could be done with resistors and capacitors at some loss of efficiency. I'm picturing the switch operating in the range of 10s of kilohertz to make it easier to filter.

How do you get rid of the heat from the electronics?

 

[p1ayaone1]

gnurf, I know that toroid-wound inductors will have EMI (albeit quite low) unless the toroid is exactly 360 degrees. And shielding only goes so far. The question would them become: what is the B at a particular distance from the inductor at a particular current? Any idea how to estimate that? In any case, the value should be less than 0.015 uT.

skeptic2, the idea you proposed is similar to the charge pump, and I've seen some switching in the MHz range. The problem is that without the comparator, the charge pump gives only integer multiples of the input voltage. I'll search for an integrated solution.

The question of heat is interesting as well. Since we are designing for space, a fan is obviously useless. Hot components will get heatsinks, but I really have no idea how effective those will be with no atmosphere.

The way I understand it, is that space (rather, the few air molecules up there at 800 km) is extremely hot. But, since the molecules are so far apart from one another, the perceived sensation is one of cold. Thermodynamics in space should perhaps be reserved for another thread...

 

[berkeman]

I am designing a small satellite which uses a magnetometer and some coils (which interact with Earth's magnetic field) to provide attitude control.

The strength of the geomagnetic field is 30 uT to 60 uT (according to Wolfram Alpha), and the magnetometer has a resolution of 0.015 uT.

The satellite observes the CubeSat specification, which means it is basically a 10-cm cube. It is really small.

Now I need to power this thing. What I really need is voltage regulators with the highest possible efficiency. Also, because of the magnetometer, I don't want to have inductors in the power system, because they would be "always on".

The desired output voltage is 3.3V with 150 mA absolute minimum, but I would much rather be closer to 200 mA. (It should be able to go lower than 150 mA, but that figure is the minimum maximum. ) I will probably also put a 5 to 8 V bus, but my main concern is the 3V3 bus because that's where all my electronics are powered.

The input voltage is two Li-Ion cells. These could be in series (5.5 Vmin to 8.2 Vmax) or in parallel (2.75 Vmin to 4.1 Vmax). I'm ambivalent wrt series/parallel connection, as long as the regulator is sufficiently efficient.

Linear Regulator
OK, fine, it would *work* but this is an efficiency-critical application, and I don't want to lose all that extra voltage as heat.

DC-DC Converter
Buck/boost, Sepic all use inductors. No good.

Charge Pump
There are a tiny few that support variable input voltage, but the dominant functionality is voltage inversion and voltage doubling. Also, current ratings are quite low. I suspect the solution lies in this class of DC-DC converters.

Anybody got any neat ideas?

Hard problem. Can you just use the parallel connected batteries and not bother with any voltage regulation? What is the input voltage range of your 3V3 circuitry?

 

[p1ayaone1]

berkeman, there is a good deal of 3V3 circuitry (an MCU, a camera, an SD card, temperature sensors, the magnetometer itself) so I would prefer not to run those directly from the battery, but your point is well-taken.

Additionally, the Tx, Rx, and attitude coils will all run on a higher-voltage bus (at least 5V, perhaps up to 12V) to that would require boost circuitry (read: inductors) as well.

 

[p1ayaone1]

A hackish solution would be two of these in parallel, since the current rating to too low.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LTC1751EMS8-3.3%23PBF-ND

I'm sure a more elegant solution is out there!

 

[berkeman]

berkeman, there is a good deal of 3V3 circuitry (an MCU, a camera, an SD card, temperature sensors, the magnetometer itself) so I would prefer not to run those directly from the battery, but your point is well-taken.

Additionally, the Tx, Rx, and attitude coils will all run on a higher-voltage bus (at least 5V, perhaps up to 12V) to that would require boost circuitry (read: inductors) as well.

Fair enough. Then I would put the batteries in series, and use a buck topology DC-DC, with a toroidal inductor, with a mu-metal shield around it.

BTW, when you mentioned full-circumferential winding of the toroid, that is for immunity to external B-field interference, not for minimizing external leakage B-field from the toroid itself (AFAIK).

Also, remember that with mu-metal shields, they need to be anealed after they are formed, so you can't just bend up a shield out of a sheet of mu-metal and have it work. I've had good luck with this company:

http://www.magnetic-shield.com/

They do offer kits for prototyping shields, and can help you with annealing prototype shields.

 

[berkeman]

A hackish solution would be two of these in parallel, since the current rating to too low.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LTC1751EMS8-3.3%23PBF-ND

I'm sure a more elegant solution is out there!

But those are doublers, right?

 

[berkeman]

BTW, I don't think that a switched cap solution for lowering the source voltage will be very efficient. Maybe no more efficient than just using a linear regulator.

Remember the classic question about where the energy goes when you connect a charged capacitor to an uncharged capacitor through some resistance, and then take the limit as that resistance goes to zero... It's a classic problem that shows that you lose energy when you transfer charge to lower the voltage with capacitors...

 

[p1ayaone1]

nope they are proper regulators. The only problem is the output current is too small!

As for mu-metal, would you wrap the inductors in that then have it treated, or could I build a "box" for the whole circuit, except the magnetometer and attitude coils, and then have it treated?

(Note that the MCU algorithm will prevent the magnetometer from measuring while the coils are being fired.)

 

[p1ayaone1]

and you're right about efficiency. Page 4 of the datasheet has the efficiency of the 3V3 regulator vs. output current for different input voltages. It is 50% efficient when Vin = Vout, and the efficiency is greater when Vin < Vout (that ESR is conducting for less time)

 

[berkeman]

nope they are proper regulators. The only problem is the output current is too small!

I'll have to look into them. Are you familiar with the classic capacitor energy question that I mentioned? Take two caps of equal value C, charge one to Vi, connect them in parallel to make a new lower voltage Vf. What is the energy content of the one charged cap C at Vi? What is Vf? What is the total final energy of the two caps at Vf? Where did the other energy go?

As for mu-metal, would you wrap the inductors in that then have it treated, or could I build a "box" for the whole circuit, except the magnetometer and attitude coils, and then have it treated?

I'm not sure of the best shape shield to use to shield a toroid -- maybe contact Magnetic Shield Corp (or browse their website) to find out what other folks use. Probably something like two 5-sided boxes that interleave along the edges when put together...?

(Note that the MCU algorithm will prevent the magnetometer from measuring while the coils are being fired.)

What does that mean? The DC-DC will be running somewhere in the 50kHz to 150kHz range. How could you interleave magnetometer measurements with that? Do you briefly shut off the DC-DC to make the measurement quickly, while capacitors hold the 3V3 and other rails to minimum droop? If so, do you even need a shield at all?

 

[p1ayaone1]

First point:
I can't say I know which equation you're talking about... I know the energy in a cap is the product of the charge stored and the voltage over 2. (1 coulomb at 1 volt inside a cap has 0.5 joules of energy)

I understand that all caps have an ESR which dissipates power during conduction, causing energy loss in the system. Ceramic caps can be used to mitigate this loss.

Second point:
The question is whether to focus on shilding the inductors themselves, or the whole circuit? Overall weight is also a consideration.

Third point:
That comment was pre-emptive and a bit unclear on my part. I was not referring to the DC-DC coils (what I've been calling inductors), but the attitude coils (which are admittedly inductors themselves, but they will be designed and hand-made to provide the highest possible radiated B)

I was half-expecting someone to say: "well, if you're so concerned about EMI, how can you use coils to change the satellite's attitude?" What I mean is that the coils (one on each axis) will be used to correct the position of the satellite by inducing a magnetic field of some strength, but those fields will interfere with the magnetometer (also on three axes). So I will take a reading of Bx, By, and Bz, and fire the attitude coils (say Lx, Ly, and Lz after the reading is complete.

I want *lots of* B from the attitude coils because I don't need to run them all the time, but I want as *little* B as possible from the DC-DC converter(s) becuase they need to run always, including them the magnetometer is running.

 

[p1ayaone1]

the B from the attitude coils will interact with the Earth's field, and generate a tiny little torque.

 

[berkeman]

I would just shield the toroidal inductor on the buck DC-DC.

On the capacitor switching question, it goes like this:

Start with one cap C charged to Vi.

Connect it in parallel with another cap (initiall discharged) of the same value C. This reduces the voltage from Vi to Vf = Vi/2, because the total charge stored has to be conserved, but you have twice the capacitance in the final configuration. And since Q = CV, doubling the capacitance leaves you with half the voltage.

But the initial energy stored was Ei = 1/2 C*Vi^2, and the final energy is half that (because the capacitance doubled, but the voltage term gets squared in the energy calculation).

You've lost half of your energy by switching the charge from one cap onto the parallel combination of two caps. Where did the energy go? If you model the setup as having a resistor between the two caps when they are connected, you can show that the energy is dissipated in the resistor as the 2nd cap is charged up to Vf. But you can also show that the energy is lost, even as you take the value of the resistor to be vanishingly small.

So I'm not sure you will be able to make an efficient version of a buck DC-DC using only switched caps. I could be wrong -- maybe there is some trick I'm missing...

 

[p1ayaone1]

Wow. half the energy is lost!

Ei = (C*Vi^2)/2

Doubling the capacitance, and noting that Vf = Vi/2

Ef = (2C*Vf^2)/2
Ef = (2C*(Vi/2)^2)/2
Ef = C*(Vi/2)^2
Ef = (C*Vi^2)/4 = Ei/2

-QED-

So back to that part a few posts back, how do you suppose they get very nearly 90% efficiency for the 5V regulator (LTC1751-5) when Vin = 2.7V and Vout = 5V?
(It's on page 5 of the datenblatt)

 

[berkeman]

Boosting with the caps is different, I think. You're pushing the charge up onto a storage cap, where it is held with the diode as the pump cap comes back down to fill with charge. I guess I should go through the math to figure out what's different, in case it shows some topology trick where the buck down in voltage could be made more efficient too.

 

[p1ayaone1]

I guess I should go through the math...

As you wish, but it's not something I really need. I'm more interested in terminal characteristics than IC implementation...

 

[RocketSci5KN]

http://focus.ti.com/docs/prod/folders/print/tps60501.html

 

[berkeman]

http://focus.ti.com/docs/prod/folders/print/tps60501.html

Fascinating! Thanks RocketSci. So with different topologies of flying caps, you can make pretty high efficiency for some voltage down-conversion ratios. Cool.

And that looks just like what the OP is asking for.

 

[p1ayaone1]

Physics Forums is the greatest!

Application circuit is simple and small, and meets all design requirements. Will hit up TI for some free samples and see if I can get this circuit working.

Thanks all!

 

[gnurf]

Additionally, the Tx, Rx, and attitude coils will all run on a higher-voltage bus (at least 5V, perhaps up to 12V) [...]

The TI step-down device has an input voltage of 1.8V to 6.5V, so, to avoid exceeding this limit, the two liion cells must be connected in parallel (with a voltage of < 4.2V). What then with the transceiver and attitude coils?

 

[p1ayaone1]

The TI step-down device has an input voltage of 1.8V to 6.5V, so, to avoid exceeding this limit, the two liion cells must be connected in parallel (with a voltage of < 4.2V). What then with the transceiver and attitude coils?

Good question. I will use the MAX1680 or something similar (gotta love free samples from TI) to provide a regulated 5V supply. Doubling that further is not out of the question, we'll have to see how the Tx/Rx perform.

http://datasheets.maxim-ic.com/en/ds/MAX1680-MAX1681.pdf

For the record, this satellite will not be launched. Because of this, the Tx/Rx are intentionally underpowered to save precious $$$. The rest of the system will hopefully be declared spaceworthy once it passes environmental, electrical, and functional testing.

 

[Carl Pugh]

? Use a super capacitor and a switching power supply.
Turn the magnetometer off when the super capacitor is charging.

 

[p1ayaone1]

? Use a super capacitor and a switching power supply.
Turn the magnetometer off when the super capacitor is charging.

I don't really know anything about supercapacitors... What is the advantage over Li-Ion?

 

[skeptic2]

On the capacitor switching question, it goes like this:

Start with one cap C charged to Vi.

Connect it in parallel with another cap (initiall discharged) of the same value C. This reduces the voltage from Vi to Vf = Vi/2, because the total charge stored has to be conserved, but you have twice the capacitance in the final configuration. And since Q = CV, doubling the capacitance leaves you with half the voltage.

But the initial energy stored was Ei = 1/2 C*Vi^2, and the final energy is half that (because the capacitance doubled, but the voltage term gets squared in the energy calculation).

You've lost half of your energy by switching the charge from one cap onto the parallel combination of two caps. Where did the energy go? If you model the setup as having a resistor between the two caps when they are connected, you can show that the energy is dissipated in the resistor as the 2nd cap is charged up to Vf. But you can also show that the energy is lost, even as you take the value of the resistor to be vanishingly small.
So I'm not sure you will be able to make an efficient version of a buck DC-DC using only switched caps. I could be wrong -- maybe there is some trick I'm missing...

Yes, Berkeman, I was familiar with this problem and what you say is true, if the two caps are the same size. But what if the source cap is twice the size of the load cap?

Given:
C1 = 2 F
C2 = 1 F
Vi = 1 V
Qi = 0.5*C2*Vi^2 = 0.5 Q
Vf = C1*Vi/(C1+C2) = 0.667 V
Qf = 0.5*(C1+C2)*Vf^2 = 0.667 Q
Eff = 66.67%
Clearly the 50% efficiency is only true if both capacitors are the same size.

You didn’t say how large your battery is so I’m guessing somewhere around 2000 mAh. That is equivalent in energy to 7.2 kF. If C2 is 100 uF then the ratio between the two capacitors is 72 MF. This time we have:
Vf = 72MF*Vi*(72 MF + 100uF) = ~1.0 V
Qf > 0.9999999 Q
Eff > 0.9999999%

Granted, we haven’t taken into account the power consumed by the comparator and switch but if care is taken choosing your components, a switched capacitor has the potential to be more efficient than the 80 – 90 % of the two referenced ICs.

 

[sophiecentaur]

If the quantity you are measuring with your magnetometers is very slowly varying then why should a small amount of emi, at many tens of kHz, be a problem? Why would your measuring algorithm not be able to filter that out? Using an 'intelligent' DC converter, you would need to have no inherent energy losses. A crude voltage doubler followed by a regulator seems a bit low tech.

@berkeman - this is a well known 'question for the student'. The only way to connect the two capacitors without energy loss is to involve an inductor during the switching process and, with a switch, grab a time when the resonance gives you no magnetic energy again. 1. Connect a 'suitable' inductor in parallel across C1; volts will start to fall as current flows into L.
2. When the volts have dropped to 1/√2 (I think it would be), change over from C1 to C2.
3. Wait until the resonance in the L C2 circuit has brought the volts on C2 back up to 1/√2.
4. Disconnect L.
This gives you two capacitors, both with the same polarity of voltage and with half the original energy in each.

 

[Carl Pugh]

playaone1,
Supercapacitors can be be used at any voltage below their rated voltage.
Li-Ion batteries have a limited useful voltage range.

Google supercapacitor and go to the wikipedia website for an explanation on how supercapacitors work. wikipedia say, supercapacitors typically have 1000 of times higher capacitance than electrolytic capacitors.

? Could the magnetometer be off for long periods of time?
If the magnetometer was off, this would save enough power that a series regulator could be used.

 

[skeptic2]

Sophiecentaur, there is something I don't understand about your example. Perhaps you can walk me through it and explain the part where I'm getting lost.

Assume a 1 F capacitor, C1, charged to 1 V containing 1Q.
1. Connect a 'suitable' inductor in parallel across C1; volts will start to fall as current flows into L.
2. When the volts have dropped to 1/√2 (I think it would be), change over from C1 to C2.
4. Wait until the resonance in the L C2 circuit has brought the volts on C2 back up to 1/√2.
5. Disconnect L.
This gives you two capacitors, both with the same polarity of voltage and with half the original energy in each.

It seems that in order for each capacitor to have half the original energy, each capacitor must be charged to √2/2. I think this is what your example does. That means that each capacitor must contain C*V coulombs or 0.7 Q for a total of 1.4 Q.

Where did the extra 0.4 Q come from?

 

[sophiecentaur]

It comes from the fact that current flowed around the circuit, through the inductor. In this case, the 'charge conservation' law doesn't apply - it doesn't need to because it's not a total charge involved but an 'imbalance' of charge. The only respect in which the conservation of charge applied is in the fact that the whole circuit starts off neutral and ends neutral.
(I must say, you had me there for a moment )
What is conserved, however, is Energy - because (ignoring resistance) none is dissipated.

 

[sophiecentaur]

And why is everyone so inordinately chuffed about super capacitors? They have their uses and are eminently suited to many applications but their one huge disadvantage is surely that their voltage is so variable. Not so much of a problem is the circuits they feed have been designed to cope with it but it makes a lot of designs a lot more complex. It seems to me that batteries are doing pretty well at the moment - with increasing capacities and fast charging times - for most applications.

 

[skeptic2]

I'm sorry but your explanation is a little vague. I wonder if you could either go into more detail or point me to a reference. I have searched for and looked at references but haven't found anything that addresses this point.

It seems to me that it must be the charge that is conserved as it is in the traditional two capacitor problem. That problem is a paradox only when zero resistance is assumed. When resistance is added, the lost energy is clearly dissipated in the resistance. What happens in your example when a small amount of resistance is added? On the other hand, charge represents a quantity of electrons, something that to me would be a great deal harder to create or lose.

 

[sophiecentaur]

VAGUE ?!
The cheek of it. (I thought it was more than adequate and not too bad as I was making it up as I went along)
Nevertheless, it seems right enough - helped by the fact that the answer produced the conservation of energy - quite a good case for the prosecution M'lud.

In the 'two capacitor problem' there is no 'alternative' current path to the wires joining the two capacitors. What is being shared is the imbalanced charge as no charge can flow anywhere else. I would agree that, as no 'discharging' can take place then the imbalanced charge must not change: a kind of charge conservation, if you like but not in the 'particle Physics' sense of the phrase.
Connecting an inductor across the terminals of C1 allows the charge on C1 to be any value from +CV to -CV and back up to +CV, depending on which part of the oscillation cycle you disconnect the L. You would agree that is what happens in an LC circuit? Where is your requirement for 'conservation of charge' during that process? Energy is not lost, of course, because you have a current flowing in the inductor so the Electrical Potential Energy is transferred to Magnetic Energy.
I repeat - no electrons are ever lost or gained by the circuit; they are just re-arranged about it according to the permitted rules. There may be more or fewer excess electrons on one plate but that number is totally balanced by a change in the number of electrons on the other plate.

 

[mheslep]

A problem with using any kind of a switching circuit as a solution here is that for a given inductance the B field is proportional to the current I, and current pulses/spikes intrinsic to a switcher are many multiples of the steady state current through a linear converter. And, it is not just the switching magnetics that have inductance: capacitor-diode-resistor leads, circuit traces, all of it does; therefore everything that conducts switching currents may be radiating. The switching currents may also couple into everything else connected to the battery power supply - an undesirable effect that can be attenuated many orders of magnitude by prudent design but perhaps not sufficiently in this case.

Therefore I'd be disinclined to use a switcher unless the application measurements can be made relatively quickly while the switcher is actively disabled/blanked as berkeman suggested earlier:

Do you briefly shut off the DC-DC to make the measurement quickly, while capacitors hold the 3V3 and other rails to minimum droop? If so, do you even need a shield at all?

I've seen disable / blanking circuits like that used before; you can generally oversize your output capacitor to holdup for whatever time you require. I suspect it will cost you little to design in blanking, and if later during integration you find your magnetometer is not impacted by your converter running continuously, you can just forget about syncing up the blanking circuit.

 

[sophiecentaur]

Doesn't this all boil down to the range of signal frequencies in which you are interested and the frequency range of interfering signals? If you are looking for a low frequency / DC signal, then you would filter accordingly and I can't see how the HF switching waveform could not be filtered out (I am, of course, referring to analogue filtering - which is not always how it's done these days). Turning your charging circuit on and off could actually be introducing components into your (now modulated) signal which could be as much of a worry as the presence of high frequency switching components, which may well be a long way out of band and, hence, easily filtered.
It would have to depend on the actual requirements.
I just had another thought. What is the likely level of other interference sources on the satellite? There will be a specified maximum and you can be sure that one of the passengers will be producing something near that value. That could be far more of worry than you own PSU.

 

[mheslep]

It's not clear that anything is 'easily filtered' with the usual discrete component filters relative to the sensitivity of these instruments - 0.015 uT.

 

[sophiecentaur]

"Easily filtered". OK - a bit glib there, perhaps.

But your situation is by no means unique and there are many good engineering solutions to the problem of producing suitable 'clean' supplies of power. I can see how a chemical cell, unconnected to any charging circuit, has its attractions. However, if your signal are really as embarrassingly small as you think, then you are in the realms of SNR - or S/Interference Ratio. You quote sensitivity in T but, also the frequency response is highly relevant here. If the magnetometers are low frequency devices then why would you expect significant response at 100kHz or more (you could choose a much higher switching frequency if you wanted)?
If your numbers regarding bandwidths, linearity, sensitivity etc. all lead you to conclude that a high frequency DC - DC converter is not viable then fair enough. However, the low frequency ('on/off') solution may involve other problems. What other sources of RFI are likely to be on the satellite or is it all your own territory, after it's been launched?

This autonomous satellite system needs a lot of expertise poured into it if it is to be successful and, presumably, you are getting ideas from other directions than this forum. I have been involved with loads of projects in which there have been aspects that weren't adequately addressed, initially. That's why I am making these comments. I hope you have a lot more to draw from than the few interested contributors to this thread - who's provenance (including mine) is unknown. It does bother me that so many posts refer to Wikipedia as the ultimate fount of knowledge. It's a great start but, for a very expensive project like this one, you shouldn't risk relying on any of it, on its own. This may be at the cutting edge of Engineering and you can't leave anything to chance.

 

[p1ayaone1]

This autonomous satellite system needs a lot of expertise poured into it if it is to be successful and, presumably, you are getting ideas from other directions than this forum. I have been involved with loads of projects in which there have been aspects that weren't adequately addressed, initially. That's why I am making these comments.

...

This may be at the cutting edge of Engineering and you can't leave anything to chance.

You are correct to the nth degree. This is a pilot project at my school, 6 students plus an experienced space engineer as a supervisor. If we can come up with a viable design, it will be passed to the next generation who will use it as a baseline for Concordia's entry in the Canadian Satellite Design Challenge (http://www.geocentrix.ca/index.php?option=com_content&view=article&id=2&Itemid=2 ).

So far we've had a little something for all walks of engineering. Mechanical will need to come up with a viable chassis that meets tight specification (0.1mm), thermal performance and that will not fall apart under intense vibration. (They call the environmental test "Shake 'n Bake").

Parts of the design include H-Bridges implemented with BJTs (for all the analog junkies out there) controlled by a combinational logic circuit (ibid. for digital).

Of course the communications system is its own little bag of monkeys - how much BW can we get, what modulation is most efficient and most reliable, how are the data packets constructed, how many channels, what bands give enough range but don't sap up the entire power budget, and so on.

Programming? That'll be fun. Ground station application with 2D and 3D visualization? Absolutely! A CMOS camera connected to an Atmega? All in a day's work.

And as all readers of this thread will know, there is the famous power system. The design is yet to be approved, but it looks like we'll go with the BQ2057 as a battery charger, the TPS6050 for the 3V3 bus, and the TPS6013 for the 5V bus (all from the wonderful folks at Texas Instruments' free samples program). The MO has been simply to find inductorless circuits whose input and output voltages are within margin, and whose current throughputs are high enough.

Will the design be perfect? No. But we will be within scope (and budget hopefully). And it turns out that this type of design is totally ADDICTIVE! I'd love to share more of this experience with you PFers over the next 5~6 months if you are interested.

Next up is sourcing parts, breadboarding, and code-writing, and after that is PCB and mechanical assembly.

Attached to this post is the power subsystem schematic, which in a way you have all designed.

 

[sophiecentaur]

Sounds a fantastic project to be involved in. I wish you the very best of luck.

 

[skeptic2]

Will the design be perfect? No. But we will be within scope (and budget hopefully). And it turns out that this type of design is totally ADDICTIVE! I'd love to share more of this experience with you PFers over the next 5~6 months if you are interested.

Are you kidding? We're here because we're addicted too. Of course we'd like to hear about your design as it is developed and particularly the reasons behind the decisions you make. For instance, what were your reasons behind deciding to go with the BQ2057 charger? It is a linear instead of a switched charger isn't it? The temperature sensor was a particularly good decision for something designed to operate in space.

 

[p1ayaone1]

The temperature sensor was a particularly good decision for something designed to operate in space.

It does have that nice feature, but it's disabled (the TS (temp-sensing) pin has been set to Vcc/2).

The reason is this: Ok, the battery is hot. Now what? Switch it off, and the whole satellite dies. Leave it on, and the battery burns out, and the whole satellite dies.

There is also a possibility that things get really hot because of the sun, not the battery. I wouldn't want a false-positive to switch off the battery charger.

There is a safety feature not shown on this schematic: there are several analog temperature sensors strategically located throughout the satellite. There is one on the battery, one each on the Tx and Rx, and the rest on the chassis walls. The MCU will poll those sensors, and if, say, the battery gets hot, the entire satellite will go into "sleep" mode (stop drawing current) while things even out.

 

[gnurf]

You said something about a Cubesat in an earlier post. Assuming the five rows of four 6.7V sources represent the solar cells on five sides of a cube, what happens when one or more of those rows (i.e. sides of a cube) generate no power because they receive no sunlight while others do?

Solar cell equivalent circuit: no irradiance ==> no current from the source, and you're left with a small resistance in series with a diode to ground (ignoring the shunt resistor).

Also, with your current solar cell configuration, if a single cell fails short (I'm not sure if a short-circuit is a common failure mode for a solar cell, but still..) your entire solar array dies.

The solution to both problems is to use protection diodes, e.g. one for each side.

 

[p1ayaone1]

Your assumption (5 sides x 4 modules) is correct, I will update the schematic to make it more clear.

Originally I had a large number of 0.5V cells in series, and I had bypassed those with diodes. I though that non-illuminated PV cells act like open circuits.

The design was modified to parallel connections, since each module (consisting of several cells) supplies enough voltage to drive the charging circuit and regulators. Given the initial assumption that dark cells equal open circuits, it was an easy to connect every cell in parallel.

If in fact that diode exists in the EC, each module would need a series diode in its branch on the parallel circuit, otherwise the dark cells would short out the whole circuit.

I will study the EC of the PV cells further, and if this is all true, I will simply add a diode to each module, allowing current only to flow out. Shottkey might be wise since reverse voltage won't be too high, and forward voltage drop is minimal.

This being a cube shape, it is obviously impossible that all cells are illuminated at the same time. The design does need to handle some cells generating no power. The power budget (i.e. required average and instantaneous current) takes this into account.

 

[gnurf]

It does have that nice feature, but it's disabled (the TS (temp-sensing) pin has been set to Vcc/2).

The reason is this: Ok, the battery is hot. Now what? Switch it off, and the whole satellite dies. Leave it on, and the battery burns out, and the whole satellite dies.

I don't get this. If the battery charger senses an over-temperature condition, won't it simply stop charging? What do you mean when you way the whole satellite dies? Temp out of bounds --> stop charging --> run satellite on solar cells and/or discharging batteries. No?

There is also a possibility that things get really hot because of the sun, not the battery. I wouldn't want a false-positive to switch off the battery charger.

Some liion cells have internal temp sensors which you can access via a third (or forth) terminal on the battery. Regardless, you must of course make sure that your temperature sensor returns the information you intended (the bat temp), so it's up to you to place the sensor in such a way that it does in fact measure the battery temp as accurately as possible (sorry if that was obvious).

I actually think the biggest problem for your battery might not be heat, but the very cold temperatures it will be exposed to during the typical half-hour eclipse in a low-earth orbit. Throw your liion cells in the freezer and see how they react (i.e. compare the discharge curves in different temperatures). I see now that both your regulators have pretty good head-room relative to a potential detrimental temperature effect on the battery voltage, so this might not be an issue.

 

[p1ayaone1]

https://www.physicsforums.com/showthread.php?t=345524&highlight=lithium-ion

A quick read over this thread seems to indicate that charging LiIon in extreme cold might be dangerous, but I suppose that's not a problem.

I could of course throw the circuit in the freezer, but maybe I'll just wait till February and leave it outside overnight. Not quite space-cold, but nearly. I'm sure I'll get a few nights of -20C soon enough.

 

[skeptic2]

Can your experienced space engineer give you an idea of how cold the satellite is likely to get. Nearly half of the satellite's sky will be filled by the Earth at an average temperature of around 12 C, plus the fact that the satellite will be generating some heat, so it seems to me it will take a long time, much longer than 1/2 hour, for the satellite to cool off.

 

[p1ayaone1]

Nearly half of the satellite's sky will be filled by the Earth at an average temperature of around 12 C

Sorry, I don't understand. What is 12 C?

The actual temp of the satellite will depend on how much solar radiation is absorbed/reflected. There is no ambient temperature in space afaik.

 

[p1ayaone1]

Thank's not to say the temp is 0K, but temperature is a property of matter, of which there is very little floating around at ~800km.

 

[skeptic2]

Sorry, I don't understand. What is 12 C?

The actual temp of the satellite will depend on how much solar radiation is absorbed/reflected. There is no ambient temperature in space afaik.

I should have said 12 degrees Celsius. Agreed, without matter it's difficult to get rid of heat. It can be said that the temperature of space is average temperature of the radiation from all sides. Since the Earth is about 55 deg Fahrenheit, and if the satellite were not rotating, the side facing the Earth would not cool down much below that. The other side of the satellite, though receiving very little thermal radiation, also would not be able to radiate its own heat very rapidly.