LM5143A-Q1: Diode emulation mode

Also, here is a waveform comparison of CCM and DCM mode.

Hello Sourish,

Please refer to the image,

When HS MOSFET is turned off and LS MOSFET is turned on , Low side MOSFET will act as freewheeling path for the inductor current and polarity of the inductor will reverse as shown in fig so the current direction would be inductor - Load -LS body diode - back to inductor.

But when you say negative current does it refer to the same current which I have shown in the image or is it different?

Hello Darshan,

The negative current refers to the inductor current flowing in anticlockwise direction (or opposite to what you have shown). In that case, the inductor first releases all the stored energy as long as the IL is positive.

Even in this waveform the part marked with red circle represents it.


Then due to inductor current being negative, the inductor stores energy in opposite direction of magnetic field. 

When the HS FET gets switched on, the Vin supply has to first overcome this opposite magnetic field formed in the inductor leading to extra losses. 

Negative current is facilitated if the LS FET remains on even when the inductor current changes its direction to anticlockwise (Negative).

So, to avoid such condition the LS FET is turned off at the zero crossing of inductor current and forcing it to emulate diode because of the body diode of the FET.

Hope this answers your question. 

Thanks and Regards,
Sourish Nandy.

Hello Sourish,

Thanks for sharing the valuable knowledge ,

adding to your explanation I have added my analysis please let me know whether my assumption is correct or not .

Once the Inductor current crosses zero i.e Negative ,Low side body diode will be reverse biased in that case , if the low side MOSFET is kept on then current will take a path in clockwise direction i.e - Inductor - Low side MOSFET D-S - and back to load so  current flowing opposite to the normal load direction will induce stored magnetic energy with the opposite polarity.

So in next cycle ,As soon as the HS FET turns ON, to establish forward current (positive direction into the load),it must 

• Cancel out the negative current first (discharge the opposite magnetic field).

• Build up new positive current.

This process will induce extar switching loss in HS MOSFET since time will be increased and at light load, this negative current doesn’t benefit the load — it’s just wasted cycling of energy.

Is my understanding based on your explanation is correct? - the reason to ask this questions is we are doing in depth analysis and calculation of MOSFET losses .