Hello,
I need a DC-DC boost converter to drive a solar water pump during no-sun periods. Efficiency is a major concern to conserve energy. We pump for cattle in a no power lines area.

The source: a 210 Amp @ 48 VDC battery bank, that can be recharged by an efficient little truck alternator.
The load: I need 90 VDC @ 1 Amp. Current must be limited somehow to 1 Amp, to avoid the load overcharging the DC-DC converter.

I have a reasonable electronic handling, and a couple of old Tektronics 2215 running still fine to do testing.
I am thinking on a switching boost converter with 555 or similar driving MOSFETS. I have IRL2910PBF MFets rated @ 100 Volt 48 Amp.

Any ideas to point me to an efficient design ?
Thanks very much, and Happy New Year.
Horacio

 

You need about 100W. That's higher than is typically done with a simple boost. You could do either a flyback , half bridge, or push-pull forward converter. With relatively low input voltages (under 50V) the push pull is attractive and gives best efficiency.

 

Thanks for your guide,

I have a little experience with push pull design in 50 Hz in the range of 20 Watts, controlled by PIC with 5 ms off, 5 ms up, 5ms off and 5 ms down. But that is to produce AC.

In this DC case, would you think on a conversion in 25 KHz with a ferrite transformer then rectify with fast diodes ?

Or other kind of conversion ?
Any primer design idea ?

Thanks again and best wishes.

Horacio

 

A freq of anywhere between 25k - 50kHz should be pretty easy to do. You'd need FETs rated for about 200V or more, but current rating of maybe 5A. You would need some ultra fast recovery diodes rated for about 400V/ 1A or more.

 

Thanks for yor answer,

I will start with 25 KHz and buy diodes in the 500 nSec recovery range.
Half a cycle at 25 KHz is 20 microSec so 500 nSec diodes should be fast enough. Right ?

For the ferrite transformer design and winding, can you suggest a web calculator, if any ? Or some reading to help in this transformer design ?

Best regards,
Horacio

 

100W booster is still doable.
The input current only is some 2 Amps.

However, 100V rating MOSFETs are not enough.
They may work for a while, since high voltage spikes are not so much powerful.

But in general, any SMPS circuit should have a rating at least 2x the highest voltage. For the capacitors you can get pretty much to the limit, if you use low-voltage MOSFETs, you will them burning out after a while, or if they are only slightly overloaded.

A healthy SMPS can be overloaded 3x or 5x for a short time without any damage. A SMPS with MOSFETs on the margin will burn out very easily (if you don't limit the current somehow).

 

100W booster is still doable.
The input current only is some 2 Amps.

Average current, but peak will be more like 6A.

Here's why a push-pull is the best choice, from the OP:

Efficiency is a major concern to conserve energy.

 

A healthy SMPS can be overloaded 3x or 5x for a short time without any damage.

No, not really.

A SMPS with MOSFETs on the margin will burn out very easily (if you don't limit the current somehow).

Any SMPS will blow if the current limiting is not fast enough. In some designs we used two "limiters", a very fast uncompensated loop and a slower compensated loop to hold constant current mode. Later we went to current mode control which limits current on cycle-by-cycle which is much better.

 

No, not really.

Depends. If it is designed for lamps, it must deliver a very large cold-start current. So you can not just take a MOSFET that is capable of 1.25x the current. You must choose a suitable MOSFET with a large enough case, and also take care the ratings.

And what if you want to supply motors, or just any kind of load? You need to be generous with the transistor.

Any SMPS will blow if the current limiting is not fast enough. In some designs we used two "limiters", a very fast uncompensated loop and a slower compensated loop to hold constant current mode. Later we went to current mode control which limits current on cycle-by-cycle which is much better.

Think of the cables, the inductors resistance, and the voltage source resistance. This all adds up to some internal resistance.

It is not so easy to design the cooling system for permanent high currents, but to choose a large transistor with current carrying reserves makes the circuit more robust. If you just short it with a thin cable, the transistor will not pop up immediately, but the cable will burn out instead.

In a permanent circuit, a fuse is used.

Yes also the current can be monitored, as well cooler temperature. Any commercial circuit would monitor current, temperature, as well it would have a fuse/electronic fuse.

Even a big power transistor only costs some Euro, no matter for a one-off project.

 

The DC pump motor that runs at 90v will (should) also run fine at 48v, but at about half speed.

Try connecting it direct to 48v and if it runs ok that would be problem solved, provided you don't mind the pump delivering about half of the water volume per hour.

 

The DC pump motor that runs at 90v will (should) also run fine at 48v, but at about half speed.

Try connecting it direct to 48v and if it runs ok that would be problem solved, provided you don't mind the pump delivering about half of the water volume per hour.

Thanks for all the answers,

Not really. The pump has an internal device of MPPT technology to work optimally with solar panels. It is a superb european design from Grundfos.

When sun shines...
I connect directly the pump to the panels and MPPT logic does the rest.

When no sun shines...

I am looking for a solution to pump from batteries. A converter to make it work near 90 VDC, that is the point in which the pump has its best overall DC energy to Hidraulic energy efficiency conversion.

If I connect directly to 48 VDC from batteries, the MPPT logic inside the pump will see a very low internal impedance DC source and will rate itself to maximum flow. Max flow of my pump is 15% over the max well capacity. This fires the "dry condition protection of the pump". Which I dont want to. I dont want to use a security mechanism for ordinary operation. I dont want to do hundreds of starts per day, when I am sure I can provide electronically the energy to make smoothly work my pump a low speed very near the max flow point of my well. That is the goal.

I hope to have been clear in the description. I am just a civil engineer, wishing to learn some electronics.

So...

I need to build for no sun days, a push pull converter or a boost converter.

I can do both, it will be a nice challenge, with your help.

Any examples to start ?

Best regards and wishes,
Horacio

 

Thanks for supplying the important pump information.

You have two problems;
1. A 90 watt step up DC-DC converter suitable for motor starting and reliable for continuous use is NOT a beginner SMPS project. It would be a very difficult task for someone who does not have a lot of SMPS experience.
2. The fact that the pump has internal MPPT closed loop control makes life even more difficult, and compounds with problem number 1. The MPPT is a closed loop regulation system and will likely try to fight your DC-DC converter closed loop voltage regulation, and now you need to also have in depth info or do expert testing on the actual modes of operation of the MPPT system.

One possibility would be to totally remove the pump MPPT controller, and then you only have problem 1 to deal with and can spend your time designing and building a good DC-DC regulated voltage converter with motor start capability.

Have you found out the actual motor specs? If the MPPT expects 90v for peak power operation, it is likely a lower voltage motor using a DC-DC stepdown MPPT converter. It would be silly to build a step up converter to then go through a step down converter!

The motor itself might be a common 36v or 24v type, which will be much better interfaced by a 48v Vin 90W DC-DC step down converter.

 

Thank you THE RB for your time,

I perfectly understand what you mean, but, if I remove the MPPT logic from the pump, I loose at least two things:

1. The superb performance of the MPPT controller that I am getting now on about 200 sunny days / year

2. The pump warranty

BTW, I dont know if by physical constraints it can actually be done. Water sealing without the MPPT unit ? Other constraints ?

When you say that the converter must face a "start engine" I (Amp) requirement, is not so. The pump has a superb built in electronic soft start logic.
When I put an ammeter and start the pump, I notice:

1. One or two 500 mA pulses that correspond with the alignement of the rotor the the best position for starting.
2. One minute of 700 - 800 mA running with very low flow
3. Five minutes increasing the flow, and the Amps taken, up to the moment the pump arrives at "regime"

So I will not be afraid of high starting currents for the converter design.

One favor please: can you point me to a free drawing soft to document the circuitry I am working on ?

What scheme do you think needs less skill for a 100 Watt project: The push pull converter or the step up converter ?

Many thanks for your cooperation,
Horacio

 

You can get one of these 12V to 35V converters from eBay.

All you need to do is to exchange these components:

-Both capacitors
-MOSFET
-Flyback diode

Maybe better heatsink for the MOSFET as well.

Be careful when you unsolder the MOSFET, double-sided PCB.

I have actually done such a modification.

You need to change the adjustable resistor as well (towards higher value).

90V is doable with this circuit + modifications. The original MOSFET will destroy at about 80 volts.

For a one-off converter, this is the cheapest and easiest solution.

I also doubt the supply circuit for the chip on the PCB can withstand 48V. You need to supply extra 12V to the chip maybe.

 

...
When you say that the converter must face a "start engine" I (Amp) requirement, is not so. The pump has a superb built in electronic soft start logic.
When I put an ammeter and start the pump, I notice:

1. One or two 500 mA pulses that correspond with the alignement of the rotor the the best position for starting.
2. One minute of 700 - 800 mA running with very low flow
3. Five minutes increasing the flow, and the Amps taken, up to the moment the pump arrives at "regime"
...

Great! That's more good info. I assume at final amps it is at 1A as per your original post?

Also, what is your running voltage when doing the above test and is it a fixed regulated voltage for the entire test period.

Also how did you get the 90v figure for running the pump? A MPPT driver can work with any input voltage withing a certain range.

If it was my project, I would attach a regulated variable DC supply and test the running voltage and current of the motor and driver as the next important step. Assuming the MPPT is a step down type you might find a DC voltage below 90v that gives best efficiency, or if efficiencies are fairly constant you can find a DC voltage best suited for you to supply.

(you said previously);

... If I connect directly to 48 VDC from batteries, the MPPT logic inside the pump will see a very low internal impedance DC source and will rate itself to maximum flow. Max flow of my pump is 15% over the max well capacity. This fires the "dry condition protection of the pump". Which I dont want to. I dont want to use a security mechanism for ordinary operation. I dont want to do hundreds of starts per day, when I am sure I can provide electronically the energy to make smoothly work my pump a low speed very near the max flow point of my well. That is the goal. ...

Maybe this is a blessing. If connecting a 48v DC unlimited amps supply makes the pump run 15% too fast, then we know the motor voltage is below 48v, and the MPPT is simply doing it's job of trying to squeeze maximum power from the 48v ??amps input.

I think all you need to do is to provide a very simple linear current limiting circuit between your 48v battery bank and the MPPT driver. The MPPT driver will then squeeze the most motor power it can based on the available supply voltage and current. So all you need to do is tweak the current limit setting and bingo you have a pump motor operating at the exact power you desire, no DC-DC converter needed.

How much current did the device draw from your 48v supply when it was running 15% too fast? Exact volts and current there would be good info.

You can make a linear current limiter by a couple of transistors and a couple of resistors. One transistor needs tobe larger and on a heatsink, but that depends on the current and voltage.

Actually, I'm thinking if the 48v battery bank remains fixed in voltage you can get exactly the same result from just putting a power resistor between the battery bank and the MPPT driver!

 

Thanks RB,

For your time, patience in study, and brain ...

I went this afternoon to my place "en el campo" and played around with tests, making V vs A notes.

I restarted all over using even a tractor 24 Volt lamp as limiter, and quasi got the same conclusion you did.

No Dc Dc converting, operating directly from 48 VDC, and wasting in the pass BJT the excess heat.

Unfortunately I do not have a DC regulated power supply in these voltages, but I will choose a DC BJT limiting circuit and solve my problem. BJT's are natural current limiters, so I will easily do a current limiter. Even been only a civil engineer. Why not ?

The sun, instead of directly heat my terrain, will be collected by my panels, loaded in my batteries, and then, perhaps 60 % will go to the pump and be converted in hydraulic energy, and 40 % will heat my BTJ pass transistor. Anyway it is still for free. Agree ?

Thanks again for your time, and have an excellent 2013!

I will come back with the selected DC Current Limiter before building it, to have your opinion and help.

Sincerely,
Horacio

 

Hello,

I will try the CKT attached, I will go to BA next week and buy the transistors. I will build Page 3 Figure 4 device.

What do you think ?
Seems to be straightforward.

Thanks again for all your help,

Horacio

 

Can you please post schematics as a GIF or PNG? Many people won't open private PDFs due to the possibility of java viruses/trojans etc.

As for a circuit for a two transistor constant current limiter, this (circuit on the left) is the typical way to do it (top transistor needs to be a larger one with a heatsink).

the LEDs would be where your load (MPPT device) goes.

Of course, if you want a high efficiency SMPS constant current limiter that is almost as easy, but we do need to know the exact voltage current you will be supplying the MPPT device with, ie 40v 2A etc.

 

The gif format ckt is attached,

I will do it in two steps:

S1 A low efficiency BJT pass limiter, then acurately fix V and A requirements of the MTTP load,

S2 When optimum V and A will be known, I will ask your help to buid a more efficient switching limiter to replace the pass BJT.

Thanks,
Horacio

 

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