Question on boost converter

[kamelot]

Hey all,
I'm new to the forums and my first post here. Hope I can get some help here.
I am building a 12v inverter for CFL. The output is 300v. I successfully made the circuit with push-pull topology and SG3525. Now I want to know, what are the disadvantages if I use boost converter. Will there be peak current problems? What will the efficiency be like? Is it practical or is it unreasonable? I hope someone can help me and thank you everyone.

 

[RCinFLA]

I assume you are producing 300 vdc to chop and drive an inductor to CFL.

The difference between the push-pull converter and a boost converter is push-pull tranformer is a power converter where primary is tightly coupled to secondary winding. Energy is not stored in core of push pull transformer. For a boost converter the primary charges the inductor which is flushed to secondary side during discharge.

It's all a matter for core loss in comparing the two. Generally, a push-pull transformer can be physically smaller core then a flyback boost transformer core. A good design for both will have comparable efficiency.

 

[FvM]

Ideally, a CFL inverter should have a symmetrical (50% duty cycle) waveform, for equal wear of electrodes and symmetrical intensity distribution. It's most easily achieved with a push-pull inverter topology. A CFL inverter should have something like a constant current characteristic for current stabilization. This is usually achieved by designing the transformer with some leak inductance, otherwise an additional ballast choke would be needed.

 

[kamelot]

Hi all,
The CFL already has a ballast that enables it to run from 220VAC at line frequency. I'm just going to drive it using a 12v battery. Producing 220VAC is not worth the hassle here, so I though of converting to 300vdc as the ballast has a bridge rectifier at the beginning (in the CFL). Is there any problem associated here with using boost?

 

[RCinFLA]

Depends on the design. There is typically some 100 Hz ripple on the filter cap which lowers the average DC level delivered to the high frequency chopper circuit. The associated high frequency ballast inductor value is modified to account for the average DC level delivered.

Worse case you may end up driving the CFL bulb harder since your average DC level is higher. 300 vdc is probably about the right voltage as the 220vac mains will deliver about 310 volt peak with some ripple sag. Your DVM voltmeter should read average DC voltage and you should match that.

 

[rvionics]

Boosting from 12V to 300V by conventional Boost may lower your conversion efficiency as you will be using an 500V FET for Boost switch (getting lower RDs on in high voltage is costly & there is an practical limit), but in isolated pushpull you still can use low voltage FET such as 40V or 60V depending on your design, leakage, layout etc.