No Transformer Pure Sine Wave Inverter

[Boyntonstu]

This project is for back up power during electrical outages.

I have been able to obtain and renew 100 Ah LA batteries.

I use a capacitor charger and I can charge a battery string up to the peak AC voltage.

I would like to invert the output of a series string of 13-14 100 Ah batteries to PSW 1500 Watts 120 VAC.

I found this circuit but I am not sure if it is a Pure Sine Wave output.

**broken link removed**

IGBT vs MOSFET issues also to be considered.

Advice most appreciated.

 

[dick_freebird]

Too crude to be anything but a square wave inverter, and
the DC balance (should = 0) depends on matching of all the
cells in the stack (as well as ideal 50% duty cycle). This will
sooner or later become untrue, that they all have equal charge
and voltage; I'd bet on sooner.

Interesting if your main goal is minimalism, but that many
batteries would probably buy you a ready built inverter and
one or more fat 12V deep cycle batteries. Do yourself a BOM cost
analysis before you get excited. Getting a long series string of
batteries to charge evenly would want some significant circuit
design (charge control per) in its own right.

 

[Boyntonstu]

Too crude to be anything but a square wave inverter, and
the DC balance (should = 0) depends on matching of all the
cells in the stack (as well as ideal 50% duty cycle). This will
sooner or later become untrue, that they all have equal charge
and voltage; I'd bet on sooner.

Interesting if your main goal is minimalism, but that many
batteries would probably buy you a ready built inverter and
one or more fat 12V deep cycle batteries. Do yourself a BOM cost
analysis before you get excited. Getting a long series string of
batteries to charge evenly would want some significant circuit
design (charge control per) in its own right.

1> The deep cycle batteries are free to me.

2> The capacitor and bridge rectifier is a current pump and it is used to recharge electric cars.

https://www.youtube.com/watch?v=-i6XeCSlVc8&feature=c4-overview-vl&list=PLR2BB9KjyNfUqyJ9NOjnERFYVxtIHFjMQ

3> I received this advice for a 240 VDC system but no circuit diagram or output description:

High Power Inverter Suggestions

Hi,
You came to the right guy. This is what I do for a living, I design power inverters in the 100 watt up to 10KW power level. In this case you have two good choices for H-Bridge switches: 1) Power MOSFETs, and 2) IGBTs. Either would work well, and for the MOSFETs, you may need to parallel a few for each leg of the H-Bridge whereas for IGBTs, usually one will do for each of the 4 switches. You would not use a D/A converter as such, you would use PWM (Pulse-width modulation) from an analog chip or from a DSP. However, a 240 VDC source is not enough for transformerless operation. You need 240V*Sqrt[2] = 339V and that really isn't enough to compensate for drops in the circuit. You really need about 375 VDC to make a 240 VAC Sine wave from a PWM circuit. You'll also need an output LC filter to remove the switching frequency and the best choice is usually about 20 KHz to 25 KHz. This type of design requires the best breadboarding technique. Actually, I wouldn't make it without a PC Board for at least the most critical parts of the circuit, which are the gate drive and the power H-Bridge layout.

Gate drive, for either a MOSFET or IGBT are both very similar and it requires the proper drive circuit. You can't just drive it from the logic or DSP output. If you reallly are set on building this, then I highly recommend the IR2110 half-bridge gate driver. For a full H-Bridge, you'll need two of them and a 12V to 15V DC supply to run the chips. It's critical to keep the layout as short as possible and you must use a series gate drive resistor between each Drive output from the 2110's and the gates that they drive (that's 4 equal valued resistors of between 10 Ohms and 50 Ohms each). These help prevent ringing in the drain (FET) or Collector (IGBT) circuit. Selecting the proper MOSFETs or IGBTs is also critical. Use a breakdown voltage of at least 500 VDC to 600 VDC. THe battery power supply must be well bypassed as close to each 1/2-bridge section as possible between the upper fet drain and the lower fet source or its equivalent for an IGBT. You'll need a fairly high value low esr aluminum electrolytic cap in parallel with a couple of polypropylene film caps of at least 500 VDC each. One at about 2.2uF and another at about 0.47uF, both in parallel with a 10,000uF to 20,000uF, 500VDC Low ESR aluminum elytic cap for bulk storage. The impedance coming out of the batteries is way too high to use directly with the H-Bridge without all the bypassing I just discussed above.

Don't forget safety! It is critical to keep in mind that the potentials you'll be working with are very lethal and won't give you a second chance if you put your body across the 375VDC supply. Always use a fuse in series with the battery positive. For a 20A RMS output, your battery drain will be in the area of about 20ADC or a bit above, so use a 25A to 30A fast-blow fuse, or better yet, a superfast blow semiconductor type fuse. This may save you a lot of money on blown FETs and drivers, and may save your life as well. Be sure to put test points on your breadboard or PWB for easy hookup of scopes and DVMs. This is also a good safety practice.

Be sure and read the manufacturer's datasheets completely for the FETs, IGBTs, Gate Driver ICs, etc and do not exceed their ratings. In fact, make sure you have at least a 20% derating below the breakdown voltage and below the max current ratings, the derating should be more like 50%.

In summary: I have about a month's experience using a capacitor charger at 12 - 24 VDC and it works great.

The charger is simple, cheap, self voltage adjusting, and one can adjust the current output by adding/removing capacitors.

(I use free 370 VAC air conditioner capacitors that have been swapped out for 440 VAC units)