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TPS5420 input capacitor sizeing - with Graetz bridge rectifier

Miklos Lammel50528
Prodigy 60 points
Other Parts Discussed in Thread: TPS5420, TPS54260, TPS5430

Hi!

 

I planning to use the TPS5420 and I have a question regarding this IC. 

Facts:

As voltage input I have a 230V/22V toroid transformer, and a Graetz-bridge rectifier. It means that my input voltage is 22V*sqrt(2)=31,1V peak.

Usually for the linear voltage regulators the circuit needs to have big capacitors after the brigde rectifier and before the regulator, to have a possibly constant voltage on the regulator input. However the TPS5420 datasheet ( I know it's DC-DC buck converter) says that it only needs 10uF filter capacitor on the input side ( even when the output is 2A).

Is this assumeing, that the input voltage is already constant? So do I need a bulk capacitor after my graezt brige rectifier? And how big( what are the calculations for it)? 

This 10 uF filter capacitor should be (according to datasheet) a high quality ceramic X7R type capacitor and at least 35V type (beacause my input voltage = 31,1V), but rather 50V type. I have searched a lot and found that theese capacitors are really expensive if availably at all. Is it really that necessary? Or can I use a normal low ESR electrolyt capacitor? Are there any calculations on that, how the ESR of the input capacitor affects the circuit performance? What would be the consequence if a higher ESR capacitor was used? Blow up?

How big bulk capacitor do I need?

I also would like to run some simulations on this circuit, and for this I need some starting point. Is there a Simulink, Pspice, or any other simulation model available for this TPS5420?

Any books, links ideas are welcome.

 

Regards,

Miklos

  • 0
    TI__Guru**** 191445 points

    The TPS5420 data sheet provdides some information on input capacitor selection.  I can give you some gneral guidelines as well.  The 10 uF minimum of input bypass capacitance assumes a point of load application where the input voltage source is located close by the converter.  In that case, the input voltage is a steady and constant voltage and teh output capacitance of the input voltage circuit can be considered as the bukl capacitance.  I am not familiar with teh Graetz bridge, but I can guess that the output voltage will have a fair amount of ac ripple.  I would recommend that you whatever capactiance on that out put that you deem typical.  The input capacitance to the TPS54260 is determined by teh amount of ripple voltage you can tolerate being injected back on teh input voltage line.  You also have to choose the capacitors to meet the ripple current requirement.  We assume the ac ripple current is 1/2 of the output dc current.  To decrease  cost you can use 2 x 4.7 uF rather than a single 10 uF.  Make sure you place the ceramic input bypass as close as possible to the Vin and GND pins of the device.  You may also want to use some bulk capacitance in this application.  The value is not too critical.  I usually use something in teh range of 100-470 uF AL electrolytic.  I like the lower ESR types (160-600 mohm).  There is no model available for the TPS5420, however there is a TI-Tina model available for the TPS5430.  These devices are similar and you should be able to use the TPS5430 model for your purposes.