Reducing harmonics
I have designed a SMPS using LNK362.
140Vac - 290Vac input to 7Vdc 300mA output. Basically a NiCD battery charger.
How can I reduce the 3rd and 5th current harmonics.
The 3rd is at 75%. Within limit but uncomfortably close.
The 5th exceeds the limit at 62%.
The circuit uses a 100nF X2, Common mode choke, followed by full bridge and a 10uF storage cap.
The filter is shared by another circuit which can be on or off.
The problem is when the additional circuit is off and only the smps is running.
I also have a 2.2nF Y2 cap from primary to secondary.
Any advice you can give would be highly appreciated.
Thank you.
Comments
Hi Wesley,
please see attached data.
The circuit already uses an active pfc.
The application is for an maintained emergency fluorescent lamp.
The SMPS charges a 3.6V NiCD battery 24/7.
When operated normally from mains power, the fluorescent lamp operates from an ic controlled half bridge driver with integral active pfc control.
However, when the lamp is off, the smps continues to charge the battery without pfc.
I have a current design that passes all tests but uses 2 seperate dc rails.
What I am trying to achieve is a common bus for both the ballast and smps to reduce cost and components.
That is where the problem lies. A 10uF cap required for the ballast.
The main emphasis is on cost.
Hi,
Unfortunately I can only comment on technical issues relating to the operation of our devices. Good luck and I hope you can find a solution!
-Wesley
Hi Wesley,
Sorry I think you have misunderstood.
I am only interested in the operation of, and reducing harmonics, related to the LNK363.
The information given was just for background information.
The rest of the circuit is disabled when the harmonics are high.
All I am asking is how to reduce the harmonics with reference to LNK363.
What would influence the harmonics?
I have seen a similar design using similar size components and a TNY266 with 3rd and 5th harmonics < 45%.
The only way to reduce harmonics using this device is to apply passive filtering at the input or to add additional circuitry. The LNK-XT family, which LNK363 is a member of, has no built in PFC improving functionality.
Sorry about this.
There is only so much passive filtering can do to help improve the input current waveform (which I am assuming you're talking about). You may have to resort to some form of active PFC.
You may try some classic passive PFC solutions like a valley fill circuit but that may make your harmonics worse.
Can you post a 'scope waveform of what your are seeing?
-Wesley