5000 W (5kW) inverter project

[phaedrus]

Hi all,
I just need some validation of a concept.
I need to build a 5kW inverter,input 72V (12Vx6 battery) and 2 outputs of 230V single phase.These two outputs will be seriesed to give an output of approx. 415V,which will be fed to a motor VFD.The transformer has an auxiliary coil of 12V which is being used for the feedback.
The primary of the transformer is a center tapped winding,which will be used in a push pull configuration.The transformer was already built and working with a circuit which is defective.
The application only needs to run for about 60 seconds during a power outtage. It will be a rudimentary square wave (modified sine wave) inverter,i.e. square wave with a deadband.
My query is,is it possible to use a SMPS IC like SG3524/5 or TL494/594 running at 50Hz mains frequency.I think there are a few push pull DIY circuits out there,but they are for much lesser power ratings.
Would like to have some advice from the gurus out here.
Cheers!

 

[Silvio]

Hello Phaedrus, From what I can understand you already have a transformer. I don't know if its an iron core or not.

Regarding the chip I think the SG3524 is more adequate for your need as this has also a current sense amplifier built in and also shutdown circuitry.
You did not say what are you going to use as switching transistors or Mosfets or IGBTs. I do not know the current source and sink capability of the sg3524 but you can look out at several data sheets and I am sure you will find out.

It all depends on what you are driving and what base current or gate current you are switching, I guess that the chip alone does not have enough power and a current amplifier has to be added to it. This chip can also regulate the output voltage via an opto coupler if needed.

If in the other case you need a dead time then you can use the SG3525 which this can be set as desired, this chip lacks the current sense amplifier and have to be adopted by other means if needed.
Regarding the power output is a question of switching transistors or other devices that can handle the amperes needed to switch and cater for the load needed.

Regards Silvio

 

[phaedrus]

Hello Silvio,
Thanks for your kind reply.
At this point,I was only checking if the topology is suitable.
Yes it is an iron core transformer.
I would be mostly going with a SG3525,since it has both dead band and soft start,which the other ICs dont seem to have.One other concern is that except for the ON Semi datasheet all other datasheets specify frequency down to 120Hz only.I will be using it at 100Hz.Do you think this will be an issue?
The original circuit uses 4 IGBTs(STGW30NC120HD) in parallel for each leg of the txfer,so I will be using the same.If needed I would use a driver IC (TC4427) for driving the gates.
Can you suggest any circuit for current protection,since SG3525,does not have a second error amplifier?
I have read your summary on snubber design and gate resistor selection.That, and your youtube video on fake MOSFETs , was very useful!
Thanks again!

 

[Silvio]

@Phaedrus
Usually for low voltage input push pull topology is used, in high voltage input however other topologies are used due to that push pull needs to have a double primary winding. My experience with inverters however is that you have to be careful with spikes generated during switch off of the switchers and a suitable snubber has to be adapted on each switching transistor array otherwise very high voltage spikes will be reflected in the output. ( I burnt my multimeter once) I guess you already have this from the previous circuitry.
As far as the switching frequency well all I can say is the the SG3524 goes down to 100Hz with no problems. I never tried the SG3525 for this low frequency though.

For current protection I think a current sense trafo hooked with the output feeding the shutdown pin will do the job for you. IF this will latch or auto resets its your option

regards Silvio