Safe & reliable AC power supply for public use application

Thread Starter

kramzar

Joined Mar 7, 2022
12
Hello,

I would like to get some secondary opinions on my power supply circuit that i have designed. I designed it with the idea that it will pass EMI certification and LV safety regualtions in mind. The supply will be used to power a control circuit for an electric car charger that is mounted in a public street lamp. power supply.PNG
This will be routed on a 2-layer pcb that has about 2W of power draw and some sensitive electronics on board (GSM, PLC modules).
I added Some Kemet X1 and Y1 safety capacitors rated for 400V and also 3 varistors to protect against transients (combined with a GDT).
I added a 1A slow blow non-replaceble fuse on both lines, since i cant guarantee the device will be wired with line and neutral spacified.
I chose an integrated power supply unit thats galvanicly seperated and has built-in EMI filtering in it.

If there is anything here that i am missing or is maybe unnecesary let me know. All feedback is apprechiated.
 
I have been helping a friend with this same thing. He's put up a 6KW solar array to power a free public charger. I built a PCB that implements the J1772 PWM protocol and varies the current limit based on insolation. But, I ran into the same problem you're talking about with safety. According to NEC, you have to include a "personnel protection device" by which they mean a GFCI. And I decided not to just implement GF in my PCBA because NEC says it has to be "listed." We looked all over for a 30 amp, 240 V GFCI and ended up deciding that the cheapest way to do it was to buy an off-the-shelf L2 EVSE. This has the advantage of giving you all of the protection regulators want. My PCBA tells the EVSE when to go by manipulating the CP line like an EV. Then it tells the car how much current it can have based on what's coming in from the sun.
 
Hello,

I would like to get some secondary opinions on my power supply circuit that i have designed. I designed it with the idea that it will pass EMI certification and LV safety regualtions in mind. The supply will be used to power a control circuit for an electric car charger that is mounted in a public street lamp. View attachment 279285
This will be routed on a 2-layer pcb that has about 2W of power draw and some sensitive electronics on board (GSM, PLC modules).
I added Some Kemet X1 and Y1 safety capacitors rated for 400V and also 3 varistors to protect against transients (combined with a GDT).
I added a 1A slow blow non-replaceble fuse on both lines, since i cant guarantee the device will be wired with line and neutral spacified.
I chose an integrated power supply unit thats galvanicly seperated and has built-in EMI filtering in it.

If there is anything here that i am missing or is maybe unnecesary let me know. All feedback is apprechiated.
I would probably include a common-mode choke if you want to knock out EMC.

The resistor at R2 is a good idea. I frequently do this when I worried about transients. But I would split the resistor into two, one in each leg and I would put the VDR between the resistors and the supply input. Also, 33R seems small if you're only pulling 2 watts through it. More like 330, I would think.

As a general rule, I think you should put your impedance (R2 and his brother and a common-mode choke) between the clamping components (VDR, gas tube) and the power supply. That way, the impendance can limit the current when the clamps clamp. Less current means less voltage getting through to the supply. The way you have it, the supply sees the full clamp voltage. Which, I know, is ignoring clamping effects that surely exist in the power supply. But you see what I mean, right?

Why is there only one gas tube hooked up with two VDRs? I don't think I've ever seen that. Why not three gas tubes, two common-mode and one differential?

Personally, I am not a fan of VDRs. They have wear-out mechanisms and they have a nasty habit of turning into low value resistors by which I mean electric heaters. Look into TVS diodes instead.
 

Thread Starter

kramzar

Joined Mar 7, 2022
12
I would probably include a common-mode choke if you want to knock out EMC.

The resistor at R2 is a good idea. I frequently do this when I worried about transients. But I would split the resistor into two, one in each leg and I would put the VDR between the resistors and the supply input. Also, 33R seems small if you're only pulling 2 watts through it. More like 330, I would think.

As a general rule, I think you should put your impedance (R2 and his brother and a common-mode choke) between the clamping components (VDR, gas tube) and the power supply. That way, the impendance can limit the current when the clamps clamp. Less current means less voltage getting through to the supply. The way you have it, the supply sees the full clamp voltage. Which, I know, is ignoring clamping effects that surely exist in the power supply. But you see what I mean, right?

Why is there only one gas tube hooked up with two VDRs? I don't think I've ever seen that. Why not three gas tubes, two common-mode and one differential?

Personally, I am not a fan of VDRs. They have wear-out mechanisms and they have a nasty habit of turning into low value resistors by which I mean electric heaters. Look into TVS diodes instead.
Thank you for your opinion. I will definitly add a common choke. Do you think a differential choke would also be necessary here?
As for the resistor, it is specified in the datasheet as the power supply is 5W max load. I will split it into 2 resistors however.

As for the VDR and the single GDT, i did it mainly to save space and cost. I have seen this single GDT configuration on a couple different designes, but i do not know how well it works or if it is even necesary.

The reason I chose the VDR instead of a TVS is that a VDR can usualy provide much higher energy disipations and larger transiants. I do not know what kind of transient events happen or how often they happen in public power installations. If the transiants are not very large then i would also much rather go with TVS diodes.
 
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