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SN6507: SN6507 vs SN6501

Theo Keinarth
Intellectual 325 points
Part Number: SN6507
Other Parts Discussed in Thread: SN6501, SN6505B

Tool/software:

Hi TI Team

I have the design kit SN6507 from Wuerth-Elektronik and I tested it with my own requirements. I need an isolated psu for an 4-20mA loop. 

Input voltage is 5V (5.5V). 
DC is default (48%).
R_SR is 18k for ~200V/µs.
R_CLK is 18k for ~600kHz
R_ILIM is 100k for ILIM ~250mA and 2.2µF as soft start capacitor for ~10ms.
Output voltage is 20-24V at max. 40mA.

As a transformer I used this one from WE ArtNr.:750317828 

Overall the circuit works fine but there are some things I don't understand.
If I short the output the SN6507 gets really hot (>150°C). I hoped the current limit would prevent this from happening. Is this normal?
I used a SN6501 before and since this one has no current limit and no over-temperature protection I thought I could switch to the SN6507 but it produces much more noise at the input and output with same circuit (different layout of course)? Is 5V to low for the SN6507? 

I removed alle noise filter components present on the WE-Eval-Board (RC-snubber etc.) because I did not need them on my SN6501 design.


Thank you in advance!

Regards Theo

  • 0
    TI__Mastermind 32975 points

    Cycle by cycle current limit shuts off the device when the current hits the defined threshold. The device tries to restart and assuming the short remains on the output, the shutdown/restart cycle repeats indefinitely. Similar to the same issue that exists with excessive heating of the current sense resistor used with peak current mode control. 

    Steve

  • 0 in reply to Steven Mappus1
    Intellectual 325 points

    Hi Steve

    Does the SN6507 produce more noise than the SN6501? I get almost no ringing with the SN6501. Maybe its the layout of the dev.-board from WE?
    Should I go for the SN6505B instead?

    Thank you in advance!

    Regards Theo

  • 0 in reply to Theo Keinarth
    TI__Mastermind 32975 points

    I would recommend not to remove the "noise components" from the Wurth EVM. SN6501 internal switches have higher RDS, weaker drive current and slower dv/dt edge rates compared to SN6507. Does this means SN6507 produces "more noise" - not necessarily? SN6507 internal switches have lower RDS, higher drive current and faster dV/dt which means the leakage inductance, parasitic transformer capacitance, parasitic PCB elements, etc are more easily stimulated by the faster switching of SN6507 compared to SN6501. This is why SN6507 has SSC and SR control functions aimed at reducing EM harmonics and such functions not needed or included within SN6501. My understanding is that the WE-Eval-Board is optimized to pass CISPR 32 CE/RE EMI so proceed with caution if you start to remove/change components Wurth might have in place for the purpose of mitigating EMI.

    Regards,

    Steve

  • 0 in reply to Steven Mappus1
    Intellectual 325 points

    Hi Steve

    Thanks for clarification. The reason I want to give Push-Pull a try is because of the lower noise compared to flyback. We currently use a RP-2424S/P from Recom which is really expensive and since we don need the high isolation also not necessary.

    The Problem is that I am really limited in space (the Recom is tiny even with the EMI components required). Adding all the additional passives Wuerth did on their EVAL board is not an option. So I changed everything to my design I already used with the SN6501. And with the slowest slew rate the outcome is OK and I can get rid of my current limit circuit I would need for the SN6501. Maybe ill try adding a tiny Ferrite (e.g. 74279228600) to the drive pins of the SN6507 and see if this makes a difference.

    Regards Theo