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”

Other EMI “Tricks”

More 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

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

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!