Hi,

I designed an electric vehicle charging station, that uses a 12V isolated AC/DC power supply. Unfortunatly, when high current is required (when i switch together 2 power relays), the voltage on the 12V line seems to dip for a fraction of a second. This is a problem, since it also causes the same effect on my 3.3V dc/dc converter that powers the MCU, that subsequently looses power and restarts. This does not happen often, but only when the relays are continiously turned on and off multiple times.

I am attaching the picture of the power supply schematic and the osciloscope measurments on the 12V line, at the moment the relays close.
The power supply that I am using is this: MP-LD05-23B12R2 https://si.farnell.com/multicomp-pro/mp-ld05-23b12r2/power-supply-ac-dc-12v-0-416a/dp/3583988

My question is what could cause this behaviour and how could i fix it?

My guess is that the power supply can not provide enough current and can not regulate the voltage. Is there a solution for this problem, that does not require a larger power supply? Perhaps a sufficiantly large capacitor can do the job?

 

C10 is showing 120μF.
Try increasing this to 1000μF first then try 4700μF.

 

In your diagram is not shown the time. But try to use low-ESR Capacitor in Power Supply for C10, C18.

 

If you really want to go medieval you could use a common mode choke and a differential mode choke with appropriate capacitor networks between them.

 

Use 230V AC coil relays (or contactors) and switch them with a triac/opto triac combination.