Feasibility of a 12V 42W non-isolated supply
I've seen and implemented a small non-isolated supply before (5V 2W), where the incoming mains Neutral is one and the same as output DC ground - so there's a single flooded copper layer which acts as both. My question is, I now have a requirement for a 12V 42W output - is it still possible to have such a non-isolated supply?
Also, would I need PFC at 42W?
Comments
Thanks - that's reassuring.
The reason I'm looking to use a non-isolated supply here is that I have parts of the system that runs directly on mains, while other parts run on 12Vdc - there is no real need for isolation as everything is inside a product with an insulating housing, and it would be nice not to have to worry about maintaining the isolation throughout the system - one big Neutral/ground plane will do nicely for everything.
So I need to have incoming Neutral common with DC output ground - I guess that means I need to use half-wave rectification at the front of the power supply, since using a full bridge with 4 diodes would generally mean that output DC ground is one diode-drop away from incoming Neutral... again, I wonder if there are any reasons this might not be a good idea for a 42W supply?
As for the PFC question, the application is a domestic appliance - a health & beauty product, aimed at both European and US markets. I *think* we only need PFC above 75W? Unless the PF will be so poor that we'll need correction anyway to get decent efficiency?
I definitely wouldn't use half wave rectification...you can use a full bridge and still be considered non-isolated.
Even in non-isolated designs, while the neutral conductor is typical at "zero" volts, this can easily not be the case due to return currents in the neutral conductor. Even if the return currents (form other nearby appliacnes, equipment, factories, etc) raised the neutral conductor potential only by 1 volt, if you tied Nuetral to chassis ground you'd have a HUGE fault current due to the low impedance of the source.
Another benefit of going isolated as with a tranditional design, is that if you have any sensitive electronics in your product, the isolation from the mains provides additional protection for line surge/sag and ringwave events that could damage your product.
Another quick tip/note: avoid the pitfall of "one huge ground plane is going to fix all my problems." This is often not the case and I've seen many designs where employing a single large ground plane has caused more problems than it was intended to solve.
-The Traveler
It's definitely possible to do a non-isolated 42W design. For the power level though, you're still probably going to need to do a flyback design. Having it non-isolated though simplifies the feedback process as there wouldn't be an isolation barrier to cross with optocouplers.
The need for power factor correction depends on your industry, target application, etc. I'd need some additional information before I could advise you on that.
Just out of curiosity, why the need for a non-isolated 42W power supply?
-The Traveler