Reverse CUrrent in sync buck charger system.

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Hi all I have a question about reverse current in sync buck charger.

Please refer to below articles and
https://www.ti.com/lit/ml/slyp089/slyp089.pdf

My question is in Fig. 22.

When a charger is started with battery connected, a huge negative current flows from the battery.
I don't understand how these huge current flows during startup,,,

Anyone can help me?
 

That current flows only if the buck converter is using a low-side FET as a synchronous rectifier to ground and not a diode. If the FET is on when the inductor current reverses direction then it will conduct large currents to ground from the battery. If a diode is used instead of a FET (at some loss in efficiency) then that path is blocked.
 

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