EMI reality check needed

Go To Last Post
12 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Here is the system

uC + flyback converter

Those are the primary things in the system. The UC needs to measure analog signals with 8/10 bit accuracy. The flyback converter needs to output ~300v.

To save costs, ideally I want to put both on the same pcb and into the same case. This seems like a totally horrible idea to me, but I have no experience with high voltage flyback converters.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

If you sacrifice the efficency of the flyback
you can use "slow" switching and a good snubber
to reduce the dv/dt. And try to carefully layout
those loops with high di/dt.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

National and Linear Tech have excellent notes about switcher layout. Check the Linear app note "Switching Power Supplies for Poets" (or something close to that).

All that said, you are likely to have a real problem with the basic dV/dT. It takes very little stray coupling capacitance when the dv is so high. This is much more of an issue with boost than buck. Really don't have any good suggestions.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I was hoping the replies would be more like "you are fucking crazy, do not even try it" or "easy as fucking pie, just do it"

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

you are fucking crazy, just do it.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Number one rule to EMC - Electrons don't read f-ing schematics. You must think like an electron. As electrons flow out one part of your circuit, other electrons are back-filling to the source. The conductors you place are obvious. Be very conscious of the path of the back filling currents - the part of the board you didn't lay out, but the flow follows none the less.

official AVR Consultant
www.veruslogic.com

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

What is your absolute analog LSB value? Is it ~300V/2^10=293mV, or 5V/2^10=5mV, or something else? 300V/5mV is 96dB isolation, which sounds f'ing crazy to me; 300V/293mV is 60dB. Is this a good meter?

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I like this question.. maybe I'll try it.. will have to wait for weekend, though.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Tolan,

can you explain your system a little more?

Like input voltage/ power (PF?).
Output voltages/power.
Enclosure?
Product Class?
Isolation?

oddbudman

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

You might isolate the uP side from the high voltage. Also you might put a metal box/shield over the section with the high voltage section. It helps to visualize EMI as light shining all around in the enclosure.

http://focus.ti.com/docs/prod/folders/print/iso7520c.html

http://focus.ti.com/analog/docs/litabsmultiplefilelist.tsp?docCategoryId=1&familyId=897&literatureNumber=slla284

The ISO7520 and ISO7521 provide double galvanic isolation of up to 5 KVrms for 1 minute per UL. These digital isolators have two isolated channels with uni-directional (ISO7520) and bi-directional (ISO7521) channel configurations. Each isolation channel has a logic input and output buffer separated by a silicon oxide (SiO2) insulation barrier. Used in conjunction with isolated power supplies, these devices prevent noise currents on a data bus or other circuits from entering the local ground and interfering with or damaging sensitive circuitry.

Note: The devices have TTL input thresholds and require two supply voltages, 3.3V or 5V, or any combination. All inputs are 5-V tolerant when supplied from a 3.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Ok,

My new gameplan is to make the device as 2 separate parts with it's own housing. The box with the flyback converter will have a grounded box.

Once the project is complete and hits the market, I will look into a single enclosure solution.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

toalan,

Why bring 2 products to market? Won't this mean 2 lots of approval testing? 2 products to manage in production, 2 products to service once in the market ?

Your application might be different, but I've made some products with flyback converters and mcu onboard (secondary side) as lighting controllers. I've had these approved so they comply with EMC and LVD directive. This product was class 2 and used a plasic enclosure.

Toughest part for approval was the radiated emissions testing, but this can be managed with good board layout and decoupling.

What sort of power is your flyback converter?

oddbudman