MPZ2012S221A datasheet(41/56 Pages) STMICROELECTRONICS

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Figure 46. Primary winding current peak reduction with higher leakage or higher secondary winding

resistance

Another effect of the leakage inductance is the ringing observed when the transition from the off-phase to

on-phase occurs. These oscillations are due to the LC circuit represented by the leakage inductance and the total

parasitic capacitance observed by the primary winding. This capacitance consists of several contributions:

•

Primary winding capacitance

•

Secondary winding capacitance, reflected to the primary side

•

Reverse capacitance of the Schottky diode, reflected to the primary side

The ringing at primary side could affect the regulation loop and falsely trigger the internal comparator, turning off

the high-side MOSFET although it should be kept on. This might result in undesired jitter at switch node.

Figure 47. Filtering effect of the snubber network

A proper masking time (TON MIN) filters these oscillations, which otherwise can affect the loop on the primary side.

Nevertheless, an RC snubber circuit (depicted in Figure 48. RC snubber network) to dump this ringing is always

recommended.

L6983I

Transformer selection

DS14040 - Rev 1

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MPZ2012S221A Datasheet(PDF) 41 Page - STMicroelectronics