Is 1/2 wave mains rectification compliant with standards for very low power?

Posted by: eco-ology on 09/10/2009

I am trying to redesign a universal AC to 5Vdc supply.[Regulated]
I only need circa 3mA @ 5Vdc and I do not need mains isolation because the 5V return needs to be connected to one pole of the incoming mains supply.
There are no user accessible connections.

My design goal is to keep below 0.1W @ 230V ac mains input, with a PF of at least 0.5

I have already met the above criteria with a full wave rectified quasi-resonant flyback mode design but I need to reduce the component cost and it seems crazy to build an isolated supply and then reference it back to the mains pole.

I can see that a 1/2 wave rectifier system could reduce cost by avoiding the need for optocouplers in the feedback loop. I need the design to be compliant with worldwide standards.

What do you recommend?

Comments

It sounds like our LinkSwitch-TN LNK302 device would work perfectly for this design. It can be configured as a self-powering Buck consuming only 50/80 mW at 230 VAC. You can easily use this device with off-the-shelf magnetics, or use our PIXls design software to generate a custom wound inductor. Even without half-wave rectification, our feedback circuit eliminates all opto-couplers.

For more information on LinkSwitch-TN see the webpage: http://www.powerint.com/en/products/linkswitch-family/linkswitch-tn

And for details on our design software, including basic tutorial videos, pleasee see: http://www.powerint.com/en/design-support/pi-expert-design-software

Cheers!

Hi PI-Jordi

Because I am connecting the 5Vdc return to one pole of the mains I will be shorting out part of the bridge rectifier if I use fullwave rectification.

However LNK302 and 1/2 wave would be OK, so many thanks for that suggestion. I was worried that 1/2 wave rectification did not comply with worldwide standards because it effectively injects DC back into the mains supply network.

I know that 1/2 wave rectification is frowned on nowadays but I don't know which standards[IEC or EN] apply........Do you have chapter and verse regarding the limits that are applied

Ned Leech

Hi Ned,

Unfortunately I can't quote you chapter and verse regarding the half-wave rectification issue. However, I have spoken with several engineers internally, and from all our experience we have no knowledge of any standard which directly prohibits the use of half-wave rectification. Almost all of our cell-phone chargers use half-wave rectification and have had no problems passing tests from regulators around the world. Also anecdotally: LED holiday lights are half-wave rectified and flicker at half the line frequency. If half-wave rectification were not allowed, these products would not be mass produced.

Half-wave rectification is not used in high power designs because of standards requirements on input power factor. There are currently no power factor regulations on low power designs.

Thanks Jordi.

Ned Leech