SN6501: How do I reduce back emitted EMI due to SN6501 switching.

Hi Prasad,

Thanks for using TI E2E forums. We'll try to suggest some solutions for you. Let us say Vcc1 = local 5V supply = output of your DC-DC converter; Vcc2 = isolated 5V output generated by SN6501.

Option 1:

The 6501 uses a balanced push-pull topology, which is generally very helpful for a low baseline EMI signature to start-with.  It does have a fixed frequency and rise/fall times, so basically you only have two knobs to further reduce any small kickback voltage ( L *di/dt) generated  during switching. One is to use a transformer with low _leakage_ inductance (the L component) and the other knob is try and reduce the load current (the di component).  On top of this , you can consider using pi filters on the supply lines to target any problem frequencies.

Option 2:  

You can consider upgrading to SN6505, which offers additional features, but also provides additional ways to control/reduce EMI.  It comes in two versions, 6505A and 6505B, which differ in the clock frequency of the internal oscillator and you can pick one which suits your needs better, in terms of frequencies of concern in your system.  It also offers slew rate control  and has built-in Spread Spectrum Clocking,  both of which help in lowering EMI peaks.  You also can sync to an external clock for really tight control of system harmonics.

Debug steps to consider:

The DC-DC converter used for generating Vcc1 (local 5V supply per above) will also have its own signature. So it's important to understand how much EMI that contributes standalone. for this purpose, I recommend testing the module with a known good lab supply (5V), then testing it with a Vcc1 alone, testing it with Vcc2 alone, and then with everything combined, so you know precisely which component in your system is contributing. It is likely to be a combination of all of the above also.

Questions:

1) Could you please let us know if you are you trying to meet any emissions standards, or at this point you are focused on module functionality?

2) if possible, can you please provide a simplified block diagram of the system ? Always helps for analysis ...

Thanks,

Abhi

Hello Abhi,

Thanks for update !

Our main concern is to stop or reduce conducted EMI which travels backward to our backplane bus.

VDD1 = local 5V supply

VSS1 = GND of local 5V supply.

VDD2 = 5V isolated supply

VSS2 = Isolated GND

As per figure, if back noise is present on VDD1 and VSS1 it will travel to next module also via common backplane bus. So the other module may malfunction.

hence, we want to stop this back noise thrown by SN6501. Please guide.

Regards,

Prasad.