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SN6505B: Where does any stored energy in the transformer go, when power to the device is removed?

Kevin Broughton
Prodigy 40 points
Part Number: SN6505B

Tool/software:

Assuming a 5V supply that collapses rapidly when input power is removed, the device may be enabled or disabled.
When enabled, there will be energy stored in the transformer primary inductance as the 5V power rail source 
drops to zero. How is this residual energy in the transformer handled by the device, as it could potentially result
in a large overvoltage on the 5V rail and subsequent ringing? Are clamp diodes needed between D1/D2 pins and
5V to prevent this?

  • 0
    TI__Genius 10510 points

    Hello Kevin,

    The device will have internal under voltage lockout and will completely disabled once voltage falls below 1.7V. The energy build in transformer primary will transfer to secondary windings and charge the output capacitor. It is better to add snubbers in switch node. Adding snubbers in switch node can reduce voltage spikes and help reducing emissions, the same is explained in below application note.

    https://www.ti.com/lit/an/slla566a/slla566a.pdf

    Please make sure the transformer leakage inductance is <2% of primary inductance. Non-optimal or long traces between pin 1, pin 3 and transformer terminals can create parasitic inductance and such parasitic inductance has the same effect as leakage inductance. Please ensure the switch node spikes and operating voltage  is not going beyond recommended operating conditions. 

    Thanks. 

  • +1 in reply to Shinu.V.Mathew
    Prodigy 40 points

    Thanks, I checked the waveforms in our circuit as power is removed, and found it to be quite well behaved, so I don't think snubbers will be required.
    I can see an argument for not fitting clamp diodes on the primary side, as they will introduce a low impedance path which could cause energy to be diverted into the primary side, instead of decaying naturally in the secondary circuit.