Leakage Current
We use a PSU design that uses a TNY279 in a 12V isolated supply, this is similar to a lot of the designs you publish, our problem is leakage current. This supply is used to provide power to an isolated RS485 data link. When we have a number of units connected together, could be up to 128, the total leakage current seems to get to quite a high value. Can you recomend any way of reducing the leakage current.
Comments
I have attached the schematic of the power supply in question. I look forward to hearing your comments/
- Roger
Do you have any PI Expert Design files for this power supply?
If not, can you please provide me with the following additional information:
Transformer documentation (winding layouts, core/bobbin size, wire sizes, etc)
A PDF of your PCB layout
The general PSU specs (input voltage range, output voltage, output current, etc)
A couple of questions I have:
Why did you decide to go with a 15V output and then use a linear regulator to drop it down to 5V (The same for the 12V and 3.3V)? There is a huge efficiency loss with this setup. Was there some specific design requirements that called for this setup?
Also, with your setup of paralleling 128 units, why don't you have one larger supply or at least a few larger units instead of the individual smaller supplies?
I guess, in general, I'm kind of confused about the setup of this power supply. Any additional information or background you could provide would makes things a bit easier.
-The Traveler
To answer your questions in order.
No I do not have the PI Expert design files.
The transformers are Kashke 094.941.
Power supply spec, 15v @ 0.25A & 12v @ 0.3A
The equipment need two isolated supplies, and then needs two further supplies one on each of the two supplies to supply other electronics.
This unit can operate standalone or connected to an RS485 network, which is where it is possible to connect together 128 other units. This is where the problem with the seperate leakage current comes from.
I hope this helps.
-Roger
setsquare -
Based on the design of your power supply along with your description of how it's being used, I'm not sure there is much you're going to be able to do about the leakage current without some re-design of the power supply.
I fairly certain that the leakage current problems you're experiencing are probably due to the power supply topology you've chosen along with the off-the-shelf transformer you're using. It's near impossible to tell from Kashke's documentation (one of the problems with using off the shelf magnetics) what the winding arrangement is. Depending on how the transformers are built and how the windings are setup...this could be causing excessive primary to secondary capacitance which could be contributing to your leakage current (depending on how the outputs are referenced to chassis ground).
Are there any other components ahead of the power supply that could be contributing to the leakage current that arn't shown on your schematic?
Are you at all open to changing the design of your power supply?
-The Traveler
The schematic posted is the complete schematic of the power supply in the product.
I am willing to listen to suggestions that will correct / lower the leakage current if a change in the design will accomplish this then I will consider them. This is the first switched mode power supply we have used in any of our products, so any help is very useful.
_ Roger
What are the exact electrical specs/requirements of your power supply?
For the output, please let me know what you actually need for your application/design
Here are my guesses from our conversation so far...please update/correct them as you see fit.
AC Input: Low Line, High Line or Wide Range? (115, 230, or 115/230)
Output 1: 15V @ Xamps
Output 2: 5V @ Xamps
Output 3: 12V @ Xamps
Output 4: 5V @ Xamps
Output 5: 3.3V @ X amps
It looks like you need outputs 1 & 2 to share a common ground and outputs 3,4, and 5 to share a common ground.
Is that correct? Do you absolutely need to use the linear regulators? Do you have any specific ripple or regulation requirements?
Do you have any specific requirements for UVLO, OVP, OCP, Efficiency, no-load input power, etc?
-The Traveler
The specification of the power supply as far as I can determine.
AC Input :- Wide Range ( 85-265 )
Output 1 :- 15 Volts @ 250mA
Output 2 :- This needs to be stable as it drives sensitive linear circuits hence the regulator.
Output 3:- 12 Volts @ 330mA
Output 4 :- This also drives sensitive linear circuits hence regulator.
Output 5 :- Drives processor, regulator used to try to help to ensure noise does not get onto other circuits.
The two main outputs need to be isolated from each other as there is a possibility of the 0V line of the 15 volt supply getting connected to the incoming mains supply. This isolation prevents this accidental connection raiseing the other 0V line to mains potential.
You are correct in your assumption that OP 1 & 2 share a common ground as then do OP 3, 4 & 5.
Regulation 5%
Ripple 100mV
The efficiency should be as high as possible, as this power supply is used to supply a specialist lighting control the are run permanantly. Approx Eff of design 74/76%
There are no specific requirments for UVLO or OVP.
The Over Current Protection provided by the TNY270 is sufficient.
I hope this answers your questions.
- Roger
Leakage current is created by the line frequency component of your mains frequency creating a displacement current through the total Y-capacitance in your design. The main component of this Y capacitance is typically you input EMI filter and your Y cap across the safety isolation barrier. However, you can also have some stray Y capacitance created by PCB layout and other components.
To minimize your leakage current, you will have to reduce your total Y capacitance in each power supply. This can cause problems with EMI compliance though. If you can attach your design files, I might be able to make some suggestions.
-The Traveler