Selection of SMPS

During the last 10 years I have modified about 14 different types of SMPS and I can suggest you some criteria when you are going to choose a SMPS for modification.

First of all some good advices:
Do not open a SMPS box with a connected power cable
It is your responsibility when you modify a SMPS
Do not make a test with open box and mains connected, close the box

As several other radio amateurs I have opened up several PC SMPS's from different manufacturers and carefully examined them. The first sight in such a SMPS-box is very confusing but if you have a simplified schematic of the circuit you can identify several components etc. This is the beginning.

I have chosen SMPS from some basic criteria according which system components are used:

- SMPS rating (>200W)
- 5V diode amps (30A/40V)
- HF-filtering components
- Pulse Width Modulator IC (TL494, IR3M02, KA7500, KIA494, MB3759 or eq.)
- Comparator (LM339 or eq.)
- Two heat sinks (primary switching transistors and low voltage diodes)
- temperature controlled fan regulation is an advance
- no electrolytics looks like the examples beneath, if you want to use the PSU

It is not important to choose between an AT or ATX SMPS - normally the AT type is easier to modify.

My best choice is a Seventeam ST-230W - you can find this under other brands (IPEX and Compaq) here the type number commences with ST.
The SMPS is well-designed and easy to modify.
Don't modify the primary or high voltage side of the circuit and be quite sure the over voltage and over current circuit works well when finished the modifications.

Switch mode noise:

CE requirements to high frequency emission [CISPR 22 Class B]: upper threshold value is -61 dBm between 500 kHz and 30 MHz.

This noise level is not sufficient for a radio amateur!!!!

I prefer a noise level below -100 dBm measured with a Spectrum Analyzer and a "sniffer" on both output and mains cables.

A "must" is to connect at least two equal condensers from output terminals to chassis to reduce switch mode noise radiation from the output cables.

A very suitable condenser for the purpose

Example of details near the output terminals (Samlex SEC 1223)

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Updated September 15, 2011