SMPS vs. transformless psu

smps vs psu

Hi all,
I need a power supply for a product, and considering two alternatives: transformless psu or SMPS. Can you please help me in this decision, based on your experience.

Input voltage: 85-250vrms
Output voltage: 3v
Output current: ~40ma

cost - for 1K unit, the price should be ~$0.5 (or less Smile )
The size of the psu must be small, since there is no much space in the product, hence - it also cannot produce much heat.

thanks,
Nir

transformless power supply

I’m in the process of a similar conundrum. Mine is 90-140VAC in 12VDC-15VDC out 5mA. I considered a shunt and a discrete HV regulator but thought I’d run through a transformer less PSU.

A couple of things one for 3V you definitely cant use a zener shunt. Zeners have a very soft knee at that low a voltage given your wide input voltage your zener current would vary widely and so would your zener voltage. (for transformerless)

A transformerless (capacitive dropper) front end Using a regulator (discrete or three terminal) the problem would be the input voltage to the regulator would vary widely with your line voltage likely exceeding the max input of any three terminal regulator I know of at high line in (with the exception of maybe TL783).So basically you would be using a discrete regulator and maybe with LM385-1.25V reference. You would likely need to use a TO220 pack for the pass device. You could I guess pre regulate with a shunt the input to your 3 Terminal reg but it would still be pretty dissipative for the shunt. By the time you also consider the capacitor tolerances for your x2 line caps it would probably be cheaper and less dissipative to use an offline controller for a non-isolated Buck or Buck/boost. As to whether are not you could hit your 50 cent target with either at 1k units good luck.

A switched doubler at the input would reduce the input variation you would have to design your capacitive PSU.Just another possibility.

Here or some of ONsemis offline controllers with integrated switch NCP10XX Series.

https://www.onsemi.com/PowerSolutio...P10&searchType=others&tabbed=Y&clearFilters=Y

ST has a Viper line and powerint also has controllers for non-isolated line conversion. Both have software packages to facilitate your design. Go to there web sites.