External Power Supply or Put Regulator on Main PCB?

[saad87]

I'm close to finishing my first 4-layer board. Initially, we felt that keeping the power supply as a separate unit would be better. This was because we wanted to avoid having heat on the main PCB - mostly because the area where the device is intended to be used is very hot and also quite dusty (heat from regulators would require ventilation, which would let a lot of dust in).

However, I was also told that by keeping the supply external we would need to have massive expensive capacitors on the main board to filter out noise.

So, as such, I'm debating weather I should keep my power regulators external or on the main PCB. I intend to go with linear regulators and require three rails - 1.8V, 3.3V and 5V. The majority of the current draw is on the 3.3 rail -- max 1A. The 5V is connected to a graphic LCD which draws a max. of 350mA. The 1.8V goes to the core of two CPLDs and I'm not yet sure how much this draws. Early power estimates suggest this is going to be less than 100mA.

I should add, the board is designed to be extendable. As in, additional daughter cards can be connected to it to extend it's functionality. However, these would not have a lot of current draw. But if I do go with regulators on the main PCB, I will obviously need to them on the daughter boards as well.

What would you folks think is a better solution? Is it possible that I can stick with linear regulators and use a package like D2Pak which allows good power dissipation and get away with not providing significant ventilation.

 

[yungman]

Do you have a lot of analog circuit on board? If not, use a switching regulator. Not nearly as hot. I quit using linear regulator long time ago. Good layout practice goes a long way in preventing noise.

 

[saad87]

Any regulator that you could recommend? I would prefer a easy to use one as this would be the first time I implement switchers!

 

[yungman]

Too late tonight, I'll see what I can find. They do have 78SRxx by Powertrend those days. Look at Linear technology for some ICs that easily implemented into a switching regulator.

 

[yungman]

OK, I order a lot from DigiKey. This is a link so you can choose DC DC converters:

http://search.digikey.com/us/en/cat/power-supplies-board-mount/dc-dc-converters/4325599

Just use this link and look for others you want. I order from them, they might not be the cheapest, but they are fast and reliable. I am particular impressed with them when I am doing design on my own and order small quantity low cost items. They are fast in delivery. We used them when I was working too.

 

[saad87]

Thanks. I've already found some that I like.

One question I had though - is it worth it to integrate the power circuitry on the PCB? What is the disadvantage of keeping the power supply external?

If I keep the PSU external, I don't need ventilation for the main circuitry and it can be effectively shut. As I mentioned, dust can be an absolute killer here and so I feel there is a real advantage of keeping the PSU separate.

But is there any major disadvantage of having it external? Would appreciate some advice on this.

 

[smk037]

How cool does your board need to stay?
How clean does your power have to be?
What is the function of your board?

I think these are questions that need to be answered before deciding whether or not the power circuit should be external.

I would stay away from linear regulators, and I don't think you will have a huge problem with heat.

This is a solution that I have used before on my boards.
Use this http://www.national.com/ds/LM/LM2576.pdf to regulate the input to 5V

and then use these http://www.linear.com/product/LTC3561A to regulate from 5V to your other voltages.

 

[smk037]

This is also one I would consider (instead of the 2576) depending on your power requirements.

http://www.national.com/ds/LM/LM5576.pdf

 

[yungman]

Thanks. I've already found some that I like.

One question I had though - is it worth it to integrate the power circuitry on the PCB? What is the disadvantage of keeping the power supply external?

If I keep the PSU external, I don't need ventilation for the main circuitry and it can be effectively shut. As I mentioned, dust can be an absolute killer here and so I feel there is a real advantage of keeping the PSU separate.

But is there any major disadvantage of having it external? Would appreciate some advice on this.

Disadvantage of multiple supplies externally is you need big power pins to bus the power in. Also for 3.3 and particular 1.8V, there is not much room for voltage drop along the trace to get below the limit. I made it a point to bus only one voltage eg. 24V and derive all the other voltage from that within the board. Using those switcher IC and put in the necessary components is cheap and small. You just need to be careful with the stability. People that design power supplies have a big mis-conception that if you put a big cap at the output, you drown out all oscillation. You have to know how to tame power supply just like taming an opamp.

Even using off the shelf DC to DC converter, you still need to be careful to draw the minimum current specified to make sure the converter remain stable. If you draw too little current, the converter may get unstable and burst into oscillation...Which people mistaken to be noisy. They just say the converter get noisy when drawing too little current!