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PI filter for HV DC PSU after rectifier / before regul , components values ?

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Maxoverdrive:
Hello

I work on DC powered sensors I need clean and low ripple DC voltage ( low current ) before my regulator

I have a 60 V transformer with a bridge rectifier to 85V DC
I have a 75 v Transformer with a bridge rectifier to 99 V DC
I have a 110 V transformer with a bridge rectifier to 156 V DC

I need to filter after the rectifiers , PI filters looks a nice solution with one cap / one inductance / one one cap
/
What king of easy to find component values I can use ? my load current is under 500 mA
I have make searches on the net but there are a lot of value and most looks ''quick and dirty '' solutions up to gambling

Thanks in advance

MO

jkostb:
This question can only be answered if you first specify the ripple and how much it needs to be attenuated in order to meet your requirements. Also you need to know the ripple frequency (switching frequency). Then you can calculate the required corner frequency of your LC filter and calculate L and C. Based on the know L, frequency and max current you can start looking for inductor.

Maxoverdrive:
@JK

I plan to set the PI filter at the output of the rectifier so that 50 Hz AC ''only'' the purpose is to produce a perfect DC ''as possible '' to avoid to feed the thrysistor stage with ''crap'' .
Some circuit have just a 470 uF capacitor
The purpose of the PSU are to feed a sensor , the current requested by the sensor itself  is low in the range of 20 mA , I have selected a transformer 220 / x V big enought to deliver 1 Amp safe and also because they was already available .

I want to avoid to make multiples trials and built a stack of PCB

Regards

MO

bob91343:
A rule of thumb might be that the reactance of the filter capacitor needs to be less than one tenth of the load resistance at the ripple frequency.  This is just a ballpark estimate; you need to do some serious calculating and experimenting to find appropriate values.  The more you filter, the lower the ripple, and the higher the rectifier ripple current and the slower the power supply will change voltage (such as turning on and off).  And more expensive and larger.

Use of an inductor will reduce the requirements of the capacitors somewhat, as well as of the rectifiers, as long as there is no resonance at ripple frequency between the L and C.  But it also increases cost and size and weight.

Many have scratched their heads trying to invent a formula for this.  As far back as F.E.Terman's books there are some serious attempts at computing the values, and the texts are worth reading.

Maxoverdrive:
@Bob

Thanks for your explanation . I have now a better understanding of the problem .

I have a doubt , when I purchase a ready to use DC PSU from a big brand manufacturer and also $$$$ I am not sure to get the best performances as my load is unknow for the the manufacturer ?

I continue to work on the subject to set an acceptable solution

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