Grounding of multiple SMPS's

[schnell]

Hello,

We have a PCB , 26mm x 45mm (double-sided).
(This area does not include the offline flyback at the front end, because this question does not concern that part)

The secondary side of the PCB contains two switch-mode , 7W, Buck LED Drivers and one 5V, 250mW SMPS, and two microcontrollers.

The microcontrollers also read the average LED current with their ADC’s which read the LED Drivers' current sense resistor voltage via RC filters.

We are wondering about the best way to ground these converters?

Do you think that the gold line , in the picture below, is necessary to make the 5V ground the same as the other grounds?

Here is the diagram, which i wonder if it's right or not.?

http://i49.tinypic.com/2zhr60y.jpg

 

[itsmoked]

Yes you should make them all the same grounds. I would try to make the whole bottom of the board a giant ground plane. Your board is mostly all digital so I wouldn't knock myself out on isolating planes and allowing only single links to, say, the initial ground.

Do inspect your layout and make sure signal traces don't hack up your resulting ground plane in a way that could cause isolations that would be bad.

Yes, essentially you need the gold line - but a plane would be much better.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[schnell]

Thankyou,

-though i was worried that you have mostly agreed with the diagram, because in fact, due to severe PCB space limitations, it looks like we are going to have to go for this:-

http://i49.tinypic.com/2s9ypsw.jpg

….that is , unless any major problems can be seen with it?

There is no ground plane on this PCB as such , since the top & bottom of the double-sided PCB are too heavily populated with components and signal tracks.


-our first board came back after someone hurriedly , virtually auto-routed it, and the malfunction in the LED Drivers was totally unacceptable.

-We then manually re-wired this bad PCB and got it working, though the LED current was slightly jittery and uneven, -especially when both LED Drivers are running at the same time.

I wondered what can be recommended here for the best in terms of grounding a PCB with no room for large ground plane and multiple SMPS’s?

 

[MikeHalloran]

Next best system is 'radial distribution', where no two chips share impedance in the ground or supply circuits. On a larger scale, this is done by running power and ground leads on a rack to a single terminal block, with a very long screw to accommodate all the stacked up terminals.
But you're already pressed for space, so you may not be able to route individual power and ground traces to the edge of your board.

Unless you move them off the board, into the space above or below the board, with busbars or actual wires, which is sort of the next least awful way...



Mike Halloran
Pembroke Pines, FL, USA

 

[itsmoked]

eeeeuu. Always with the space problems. I have a guy who keeps designing large counter-top machines but never ever considers that you actually need a place to put the controls!!


Your second plan would work. Probably fine.

BTW: Why the 5V switcher? Why not use it as the 3.3 and gain a little board space by jettisoning the 3.3V?

Keith Cress
kcress -

http://www.flaminsystems.com