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Input Filter desing for 1KW SMPS

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adnan012

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Hi,

I want to design input filter for a 1kWat full-bridge converter. SMPS consists of a PFC unit working at 100KHz and a full bridge phase shift convert. I need high order line filter. Id there any tool available for filters?

Regards
 

I have done simulations of the same sort of LC input filter. As I try to find optimum values, it usually works well if I start with the same L & C values in my converter.
When you think about it, the input filter is doing a similar job (although in reverse) as the output and smoothing stage. You want the input filter to allow a smooth current waveform going in, and a choppy current waveform going out.

Then I reduce their values a step at a time, until the capacitor charge varies by 10 or 20 percent, and the inductor current varies by 10 or 20 percent. You can set whatever requirements you wish, of course.
 
Thanks for reply.

How to estimate "input impedance of the switching power supply" as mentioned in "Input Filter Design for Switching Power Supplies" document.

Regards
 

Hi,

No idea. The SMPS circuit in question could be reduced to/viewed as a 3-pin IC of Vin, Vout, and gnd? You know that, we don't.

What's the current and voltage going into the SMPS? - Can we roughly extrapolate from there with Ohm's Law? Also, will the impedance be a shifting, dynamic figure due to a wide/changing Vin so better to calculate Vin min and Vin max in combination with Iout min and Iout max? No idea, just thoughts.

Not sure if Input Impedance Measurement and Calculator can help or if it is for an unrelated kind of input impedance. The input impedance calculator is almost halfway down.
 
Hi,

the input impedance will vary with load, thus it´s no fixed value.

Maybe a good starting point is to calculate with full load.
With full load the highest possible input impedance is: Z = U * U / P

Klaus
 
Thanks for reply.

I am facing a strange issue with the power supply. The supply works well with isolated resistivity load up to 800Watts. The supply is designed for FM radio station. The Transmitter output power is roughly 100 Watt. Transmitter DC - ive terminal is shorted to the body and to earth. The original supply for this transmitter do not have -ive DC terminal shorted to earth.

During testing, ac rectifier bridge -ive terminal (in PFC stage ) was blown in the following condition

1) 220V ac cord was plugged into to the power supply.
2) Supply was swtiched off . (the on off switch breaks one of the two AC lines)
3) when DC output connector of the supply was touched to the Transmitter DC input connector , ac rectifier bridge -ive terminal (in PFC stage ) destroyed.

The main power supply has an auxiliary 14 V AC/DC converter to power PFC ic and DC/DC converter IC. The auxiliary supply -ive terminal is connected to the Rectifier bridge -ive
terminal. I think (current flows from bridge rectifier - ive to the auxiliary supply -ive ).

At this stage i am not using any Y capacitors at any stage. The secondary (28VDC side) is fully isolated from the Primary side.

Kindly help me to find out the issue.

Regards
 

Hi,

A bit late with more input Z stuff, I see. I could not comment on the bridge blowing, sorry. Is it possibly related to an inrush current issue? Admittedly a stupid question if you say the AC is off when this happens. Maybe post a schematic otherwise it's like asking someone to find a specific street in an unfamiliar town with no map.

To add to things pertinent to the thread title, and to limp feebly behind Klaus's succinct summary, here are four further links related to calculating input impedance. The first shows ways how to calculate input and output impedance (doubtful very implementable with a resistor for 800W, maybe...); the second is an EDA thread on this very subject and seems a good summary too; the third is another website's thread related to this and may be of interest as it mentions the TI app note (...sometimes someone else's question/thread indirectly answers one's own); the fourth is a Vicor app note about input source impedance and effect on SMPS performance.

Measuring Input and Output Impedance
Input impedance of an SMPS?
SMPS input impedance as function of frequency
Input Source Impedance and Its Effects on DC-DC Converter Performance and Characteristics
 

Input impedance of a converter should greater than the output impedance of the filter and filter input impedance should match to source impedance. Impedance of various stage will decide the stability of power converter. Normally source impedance should be 50Ω
The input impedance of a converter can be measured from the ratio of the input voltage of the supply and the current into the terminals Z in = Vin / I in

EMI Filter.png
 
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How to estimate "input impedance of the switching power supply" as mentioned in "Input Filter Design for Switching Power Supplies" document.
Post #8 last link is the correct way, if the converter is regulated.
 

I think (current flows from bridge rectifier - ive to the auxiliary supply -ive ).

Your post #7 sounds as though you might have two bridge rectifiers sharing a power supply. Or, they are arranged in series somehow. The design may look good on paper but there is a frequent mistake which is easy to overlook. Namely, a point in the cycle when full supply current finds a path through two diodes, one diode in each bridge rectifier. It creates a short circuit.

2) Supply was swtiched off . (the on off switch breaks one of the two AC lines)

If you had components fry in this situation then it's likely your filter capacitors held their charge after power was shut off, then found a discharge path when you contacted something to something. To prevent accidents, a common strategy is to install bleeder resistors across large capacitors, so they discharge when power is shut off.

The above suggestions are a couple of obvious general possibilities.
 
A simple drawing of the real ckt will help you to find the blowing bridge problem, the fact that the chassis is live will not help

Mains filters for converters are very tricky to do well, every LC resonance must be evaluated...
 
As it says in snva538 by ti.com, the corner frequency of the input filter should be much less than the feedback loop crossover frequency of the smps.
So with a PFC input….you have to be mindful that the loop crossover is only 10Hz or so.
Two possible conclusions:
1. PFC can't work with any kind of input filter.
2. The quoted design rule can't be applied for PFC.

Real live suggests to go with the second.
 

Yes i must admit it would be some heck of a filter that could conform to that. It says that in non-pfc types this rule is due to noise, and that the smps may be too noise sensitive if the rule is not followed. I guess at the pfc frequencies, that noise argument isnt relevant.
 

We usually have LC low pass filters for PFC, cut off frequency 1/10 to 1/5 of PFC switching frequency.
 
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