switch mode power supplies and how to recycle -...

Switch Mode Power supplies and

How to Recycle themHow to Recycle them

Brian Flynn GM8BJF

Why PC Switchers?

• Ex PC switchers are readily available for almost

nothing

• Potentially useful as high current 12 V

supplies.supplies.

Basic Regulator circuits – Buck-boost

BasicRegulator circuits - Boost

Basic Regulator Circuits - Flyback

Regulator with Isolation - Forward

Converter

Forward Converter

Half-Bridge Converter

Other design Issues in SMPS

• Power Factor control (PFC)

• RFI/ EMI

• Regulator of low voltages (3.3V)

Sinusoidal Input current (PFC)

• Power electronics Make up a considerable

proportion of the electricity load.

• Current drawn by these devices is very peaky.

• Legislation drawn up in ~2002 to control this.• Legislation drawn up in ~2002 to control this.

Input Current Waveform

PFC Input Filter

RFI/EMI Suppression

Radiated Noise

• Radiated noise is stopped with screening

• Usually the case is quite sufficient.

• Conducted noise is more of an issue

• Input and output filters.• Input and output filters.

EMI Input filter

EMI Output Filter

Other EMI “Tricks”

More EMI “Tricks”

Onto Modifying PC ATX PSUs

Good Candidates for conversion

• PFC – Big iron choke on input!

• +12 V rail > 12 Amps

• Total output power > 250 W

• “Noise-Killer” = Quiet fan• “Noise-Killer” = Quiet fan

• Check for presence of input EMI filter

• Is it heavy ?

ATX Pinout

Pins of interest

• Yellow +12 V

• Black 0 V

• Red +5 V

• Green PS-ON• Green PS-ON

• Get rid of the rest!

• Grounding PS-ON (Green) should start PSU

• At this stage you will be left with a lot of useful hook-up wire.

Choosing a suitable Unit

Modifications

• Add load Resistor to 5 V rail

• Adjust output voltage

• Modify for 13.6 V on “12 V rail”

• Add screw terminals• Add screw terminals

• Add more output filtering

• Rewind transformer ???

• Depends how adventurous you are !

Improving Output Voltage Regulation

Improving Output Voltage Regulation

• Normally the regulation is done from the 5 V

• Sometimes combined with feedback from 12V

• Change the feedback to the 12 V rail only

Typical ATX Circuit Diagram

Another ATX Circuit

Part Circuit of Magamp 3.3 V Regulator

Demonstration !

I Bet You are thinking I cribbed the

diagrams out of a book!

Well I did!

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