AC input stage
Hi, I'm designing a low power ( 1/2 W ) power supply for a consumer electronics application using a LNK304 IC. I have a question regarding the AC input stage for the design.
On AN-37 you recommend a 2-capacitor, 2-resistor input filter for this power rating. Nevertheless, for output powers lower than 1/4 W the recommended circuit has only one capacitor and one resistor. The application requieres minimum PCB size and component volume, so I would be greatly benefited from eliminating an input capacitor and the 1W resistor.
I'd appreciate if you could explain me the purpose of these capacitors, and which consecuences can be expected if using the smaller input stage for a 1/2 W supply. Are they calculated to meet some requiered EMI performance?
Thanks in advance!
Comments
Thanks for your detailed reply, I will move on with the prototypes and check the results!
About your recommendations for EMI reduction: when you say "output rectifier ringing at turn-off", do you mean the ringing that can be seen in the drain-source voltage when inductor current reaches zero? (sorry, I didn't metion before that I'm using a LNK304 in a typical buck configuration). And what do you mean by "clamp circuitry"?
My mistake. The reccomendation for snubber/clamp circuitry is more applicable to flyback designs.
It is still a good idea to look at your drain voltage/current waveforms as well as the current/voltage waveforms across your main output diode in the buck configuration. Ringing can still occur during turn-off and you'll want to make sure this isn't contributing to your EMI.
-The Traveler
Ok, I'll keep this in mind. We really appreciate your help, thank you very much!
Regards,
The different input setups are suggestions for combatting EMI at different power levels. Ultimately, your design, transformer construction, parasitics, component choices and the configuration of your input stage will determine how well your power supply meets EMI requirements.
My suggestion would be to create an initial prototype and include provisions for different input stage components. If you can fit them into your required board space, great. If not, you can brainstorm the issue later. Once you get your initial prototype built, experiement with different input section configuration. See which one works best, which one uses the least components but still passes, etc.
At this point, if your design (with the provisions for extra components) fits in the required board space, you can just no-fill or short the unused/unecessary components. If your PCB is too large but you found that you didn't need all the extra components, create a second rev of the PCB without the extra components and do a quick re-layout of the pcb.
I dont' know the particulars of your design. But a couple of good things to keep an eye on that will effect conducted EMI: output rectifier ringing at turn-off (you might need some snubber components), your clamp circuitry (proper layout is key), minimizing loop areas of large di/dt traces and depending on your transformer construction consider taking the time to experiment with our e-Shield technology.
-The Traveler