1kW inverter - boost converter vs. transformer

[TauLepton]

I am designing a 1kW 12VDC - 120VAC pure sine-wave inverter for a school project. My question is about the method of stepping up the voltage. Other topics on this board all recommend using a transformer, but 1kW power transformers on digi-key.com are all $150 or more. At the same time, a 1kW inverter costs $97 at wal-mart.

My question is this: Should I use a boost converter or a transformer-based conversion to step-up the voltage? If using a transformer, should it be done on the DC or AC side of the inverter? Most importantly, why?

Thanks for your time.

 

[abraham]

If you use a transformer it should be used on the AC side of the inverter. Because transformer only changes amplitude of ac voltage.

You can first convert the 12VDC to ac signal using an inverter. If you use a common full bridge inverter the AC output of the inverter is less than the DC input. Therefore you will get AC waveform with amplitude less than 12V. Then you can apply this to a step up transformer to get 120V AC output.

 

[karesz]

Hi,
for projects with unexperienced peoples on line voltages: I would ONLYan ISOLATED source select!!

These is practically a conventional transformer=bulky + expensive, but practically disturbance less!

Trafo in the switched voltage converter as isolating element; is practically an Flyback architecture, their are smaller, cheaper & similar on the efficiency, but generate some EMC/EMI... I would select these way!

If you select the switched converter solution, is in my opinion a plus trafo as "Secondary side" element irrelevant, but maybe other specialists are on other meaning?
Why; if its on the AC side(Output) of the switcher= you have line frequency transformer to apply, same as a normal isolations trafo=bulky + expensive & why did you have maked a plus switcher into the system? Their can be isolated too!
K.

 

[TauLepton]

Karesz, it sounds like you are recommending that I use a flyback converter to step up the DC voltage. Is this correct? I thought the Flyback design was not suitable for 1-kW applications.

 

[karesz]

Hi TauLepton,
Im sorry_my mistake, I wished to wirte (of course) "Push-Pull" Converter!
Btw some design aids...
h**p://books.google.ch/books?id=Lecr2EjohMQC&pg=PA71&lpg=PA71&dq=Push%E2%80%93pull+converter+power+limit&source=bl&ots=VkLfr6IO8n&sig=OD2qcNVX0gVMqtPoKvafCfBQ6Ow&hl=de&ei=jbCJS4ueLMSk4QbAjZ2mDw&sa=X&oi=book_result&ct=result&resnum=8&ved=0CDYQ6AEwBw#v=onepage&q=Push%E2%80%93pull%20converter%20power%20limit&f=false

w*w.dxportal.com/stats/content/362909-1000-watt-12V-DC-to-220-DC-inverter-circuit.html
w*w.aaroncake.net/circuits/inverter.asp

K.

 

[karesz]

Hi,
Maybe it helps you a little bit too:
"i need 12V DC to 220V AC inverter circuit"
K.

 

[xu.hob]

There have some transformers /inductors/ferrite core .

 

[TauLepton]

Ok, I still have some questions:

karesz, you said that if I used a push-pull converter it would provide adequate isolation without the need for a bulky power transformer. I don't see how this works with my PWM switching transistors: they need a way to generate a voltage Vgs to turn on, which won't happen if the output side is electrically isolated from the gates of the transistors.

My other question is about selecting transformers. I am very new to this area. What specifications do I need to look for? My understanding is that for higher frequencies (e.g. 10 kHz instead of 60Hz) a smaller transformer is acceptable. How do I know what will work for the application?

 

[karesz]

Hi,
I assumed, that you will switch it with some higher frequency as 60Hz, as your 2. question suggest it too; maybe at 10KHz or more...
Anyway, your switcher are on the primary side!
the push-pull transformer (becouse called: transfomer) has a secondary coil too & these is FULL ISOLATING from your primary.
I would say, you need applications literature to read... Internet hase lot of them.
K.

 

[TauLepton]

I am not talking about the switching for the DC-DC conversion. I am referring to the switching for the DC-AC conversion, which I am doing by applying a PWM signal to some power mosfets connected as an H-bridge. This happens after the dc-dc conversion. That is, the top and bottom of the H-bridge are connected to my isolated high-voltage output from the push-pull converter. If the PWM generating chips are powered by 12V, the gates of the transistors will be isolated relative to the drain and source and they will not switch correctly. Are you saying I should use the push-pull converter, then convert the voltage down again to power the other electronics? I know that would work, but it seems like there must be a less complex way...