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LM5017: LM5017 reference design questions

Part Number: LM5017

Hello,

I'd like to use the LM5017 to generate a primary 5V supply and secondary isolated 5V supply from an input of 12 to 75V.

Looking at some reference designs and the LM5017 datasheet example for an isolated output (Fig. 17 of datasheet), I have some questions:

1. The datasheet example shows the transformer coils as opposite polarity (looking at the dots in the schematic).  Are both outputs positive?  I guess it just depends on how the secondary load is connected (regards polarity).  The LM5017ISO eval board also shows the transformer connected this way but some other reference designs do not (e.g. PMP7818, PMP7383).

2. Reference design PMP7383 shows diode D1 in the primary output path.  Why is this included?  This diode is not shown in other similar topology designs (e.g. PMP7942).

3. Reference design PMP7138 shows diode D2 connecting the 5V output back to the Vcc pin.  Why is the diode used in this circuit?  My understanding is that this diode can improve efficiency & power dissipation but only if the output is higher than the max. threshold voltage of Vcc.

Thanks,

Kieran

  • Hi Kieran,

    1. The rule for the transformer is that the dots go positive and negative in voltage together. Then it depends on how the secondary winding is connected. For a Fly-Buck converter, the secondary conducts when the low-side FET is on, meaning that VOUT is applied across the primary winding and Vout multiplied by the turns ratio appears on the secondary side.

    2. The series diode, designated D1 in the PMP7383 schematic is normallly not required. I think it's included here to prevent reverse current flow from the ouptut. Note that there is an output cap before the diode.

    3. It appears diode D1 in PMP7138 is included if the output voltage is adjusted higher such that external VCC bias is provided. As you pointed out, it will not conduct for a 5V output as the nominal VCC level is higher.

    Regards,
    Tim
  • Hi Tim,

    For 1, I think I see it now. The transformers are drawn differently (regards dot locations) between PMP7818 & PMP7383 vs. LM5017ISO circuit.

    Thanks,
    Kieran

    PS. You cant beat those Cork boys ;). I'm from Offaly but live in NZ. My brother worked at Artesyn for a while.