Power inductors for an Smps

[Yazoo]

I am trying to find compatible power inductors used in an Smps which puts out 200 volts. The originals are made by Sumida, 100 uH with a Dcr of 0.16 ohms, max current 1.7 amps and Srf 5.7 MHz. The second is 1 mH, DCr of 1.7 ohms, max current 0.53 amps and Srf of 1.7 mHz. I have to be honest, apart from the current and inductance ratings, I don't understand the significance of the other parameters. What should I look for for the best matches? The Sumida reference numbers are RCH110NP-101 and RCH110NP-102.

Thanks in advance.

[iccaros]

more information please, do you have a schematic you are building to?

Many of the common SMPS whcih go from 9v to 200v use a 100 - 300 uh, current is based on what you need as that will have to pass through it.  If you look up SMPS on the board you will find a lot of examples. 

[Cliff Schecht]

When you are designing an SMPS, the inductor choice is part of the design process. Typically what I do is calculate the peak current that the inductor will see and what the on and off times are under maximum and minimum load conditions. Knowing this and the input/output voltage, you can calculate the Volt-seconds/Amp which literally equates to inductance in Henries. If you keep the units correct and calculate a microsecond based (V*s)/A then your inductance will be in uH (microhenries).

The actual steps are detailed in the document below for various SMPS topologies. Have you decided whether the inductor current is going to be continuous or discontinuous? Have you chosen a style of feedback control (voltage-mode, current-mode). How about a max switching speed or the duty cycle limit? These are just some of the factors you will need to learn to choose based on your design criteria. Give this a look through, it's the method I use to design switchers when the datasheets are lacking! http://www.ti.com/lit/ml/slup071/slup071.pdf

[Cliff Schecht]

Also the self-resonant frequency is typically chosen to be much higher than the switching frequency. After self-resonance the inductance will actually start to look capacitive and will not work correctly in your system.

[Yazoo]

This is the schematic: http://milas.spb.ru/~kmg/files/projects/200vdcdc/200dcdcSch.pdf

It's to power the FET version of the ENGL 530 that KMG designed and built

I'm afraid the theoretical side of electronics is largely beyond me. I can trace through a schematic and get a basic understanding of what is going on but once the formulas start appearing I head for a darkened room to lie down.

[iccaros]

now seeing this I think I understand your question, if you were to buy  a different part you would want

100uh 1.5a or better, the more amps the bigger the unit in size
and the 1h .5a or better. I doubt your amp or pedal will use more than 500ma for B+

[Cliff Schecht]

You want to make sure the inductor is rated high enough to handle the PEAK current that it will see. I think the 1.5A minimum is a good starting point for a 100uH inductor that will be seeing 500mA RMS current.

[Yazoo]

Thanks for the helpful guidance. The only other thing I'm still confused about is the Self Resonant Frequency.

Murata has a FAQ which states"The inductor's self-resonant frequency is the frequency where the inductor and stray capacitor (among coil windings) resonates At higher frequency than this, the inductor looks electrically capacitive rather than inductive, and is expressed as a minimum value in megahertz (MHz). The higher the SRF, the higher the inductor's effective frequency range. It is not recommended to use an inductor beyond its SRF limit."

Has this got anything to do with the SMPS chip, the UC3843? I looked at the datasheet and this states it operates at up to 500 KHz so does the Self Resonating Frequency need to be higher than this?

[iccaros]

for this design, I would not worry, you will not be running 100% duty cycle and that chip has a max at 500Khz, so they match, you can always use one with a higher resonant frequency but its the part they spec so I assume they have tested this design

[Cliff Schecht]

The UC3843 is an ancient design, back before designers made it easy to just copy parts out of a datasheet. This is a general purpose chip meant to handle most types of SMPS design (within a limit) and can be a buck, buck/boost, boost, flyback, forward converter, etc.. When I'm designing with a chip like this (an old Unitrode part), I tend to follow the Unitrode design seminar books which was also my first exposure to switching power supply design. There is a LOT of good info here and everything you need to design an SMPS based on this chip.

How are you handling the compensation for this design? Care to share a schematic of what you have so far?

[iccaros]

here is his schematic http://milas.spb.ru/~kmg/files/projects/200vdcdc/200dcdcSch.pdf

[Yazoo]

I searched the UK supplier, Farnell, and I think I've found suitable substitutes made by Panasonic. I couldn't find information of the SRF rating but the Pansasonic 1 mH equivalent has a DCR of 1 and an amperage rating of 0.5, the 100 uH has a DCR of 0.11 and an amperage rating of 1.7.