Non isolated buck power supply
Non isolated buck power supply
(OP)
After reading thread 248-226082 I am concerned about a design I am working on. It is a non-isolated off line buck converter for LED lighting. I understand the dangers of such a design, the system it powers has interlock switches to power it off when the unit is opened and is fully isolated from earth ground. It uses a full wave bridge rectifier to create the DC bus voltage. Member Skogsgurra replied "Using a single diode rectifier will make the Neutral available to the user. Using a bridge rectifier will not." Could someone explain why this would be so ?





RE: Non isolated buck power supply
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If we learn from our mistakes I'm getting a great education!
RE: Non isolated buck power supply
Look at it.
Try to picture where a ground could be.
You'll see the problem. It's pretty graphic.
If you proceed in this exercise to calling one of the AC lines the neutral and then bond it to earth you'll see neither of the rectified side's + or - can be successfully earthed.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Non isolated buck power supply
RE: Non isolated buck power supply
I nearly got my eye clobbered by an exploding LM741 op amp when I was getting ready to scope a circuit by connecting the ground lead to the circuit ground. BAM! Turned out the circuit had one leg of the AC tied to chassis, and it wasn't the neutral that day.
TTFN
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RE: Non isolated buck power supply
At this point I'm not sure what you're asking.
We explained you can't ground either side of the rectified output when not using isolation. In those cases the entire product needs to be insulated. Often TVs do this.
A non-transformer design has another issue. Normally the inrush to charge the caps is limited by the transformer saturating. Without the transformer you may need to add something to protect the diodes or use vastly larger diodes than otherwise.
As Scotty mentioned using a FWB is royal PIA for measuring and scope work.
Other than those points there isn't any particular problem.
Keith Cress
kcress - http://www.flaminsystems.com