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

[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

[bob91343]

If you don't know the load then you can't specify a power supply.  There is no power supply that can meet all possible demands.  So before spending money, decide why you want the unit and what you want it to do.

Some proof of that is my inventory of power supplies.  I have many of them, some of which I use often and others I never use.  Some are overkill for the application.  Even with that I often find I need to cobble up something to do a particular job when the appropriate supply isn't available.  Everything from lighting a lamp to indicate that my garage door is closed to a multiple output unit for digital experiementation to one for a radio amateur transceiver, and so on.

[julian1]

Have you seen this approach?

EEVBlog #1116 - How to Remove Power Supply Ripple

[JohanH]

If you are after filtering 50Hz AC ripple in a PI filter. Then big and bulky DC chokes do wonders like in the old days with tube power supplies. They will reduce both voltage and current ripple to a tenth or more compared to a resistor PI filter. These are becoming rare nowadays, but for instance Hammond still have a line of these (D.C. Reactors or filter chokes 153-159 Series and 195-196 Series). Play with Duncan's PSU Designer http://www.duncanamps.com/psud2/. It will calculate and show everything you need for a PI filter.