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SN6505A: Ex certified Isolated 28V Power Supply

Chethan K S
Prodigy 110 points
Part Number: SN6505A

Hello,

We are planning to use SN6505A for generating 28V,50mA isolated power supply. The key requirement here is to get compliance for IEC60079-11 for use in intrinsic safety application.

As per IEC60079, transformer construction has to meet one of the following

  • Winding shall be placed on one leg of the core side by side         or
  • Winding shall be placed on different legs of the core           or
  • Windings shall be wound one over another with >=0.5mm solid insulation between the windings.

If we go with >=0.5mm solid insulation between windings, leakage inductance will be high. Hence we are preferring push pull converter over flyback/flybuck.

1) Any issues with high turns ratio for SN6505A? 

2) Any minimum load required for SN6505A with 28V output? 

3) Can we give below specifications for transformer manufacturer.

  • Primary Inductance: 120uH ±10% @ 100KHz
  • Turns ratio: 1:6
  • Primary Voltage: ~5V 
  • Primary Saturation Current: >2.5A      (20% of L)
  • Primary Rated Current: 1A        (∆T = 20 C)
  • Secondary Voltage / Current: 28V / 50mA
  • Center Tap: Primary and Secondary

4) Since any isolation barrier requires 0.5mm solid insulation, we may not be able to use a HV capacitor across the transformer. Is there any concern regarding EMI/EMC for SN6505A.

5) I have one general query, is it possible to use a toroidal core with windings placed side by side with SN6505A. Any transformer design constraints here?

Thanks & Regards,

Chethan K S

  • 0
    TI__Expert 6955 points

    Chethan K S,

    1) There aren't issues with high turns ratio as long as you don't exceed 1A primary side current

    2) There isn't a minimum load dictated by the SN6505 but the transformer may not like a no-load condition. Sometimes transformers will need at least a 2 to 5mA load minimum to maintain a proper secondary side voltage.

    3)  Those specifications are fine. Be sure to specify your isolation requirements to the transformer manufacturer. Also specify your Vt requirements as pointed out in section 9.2.2.5.1 of the datasheet.  (let me know if you need further help with this).

    4) There isn't concern with EMI/EMC for SN6505A. Even if there is concern with emissions around the clock frequency and/or it's harmonics then one can always provide an external clock frequency.

    5) There aren't any mechanical constraints concerned with the transformer. The toroidal  transformer should be fine as long as it follows the electrical constraints dictated by the SN6505.

    Respectfully,

    Lucas