L6986I datasheet(40/60 Pages) STMICROELECTRONICS

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Figure 46. Secondary winding current according to different leakage inductances (simulation)

LLEAKAGE = 2µH

LLEAKAGE = 200nH

LLEAKAGE = 50nH

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.

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

Nevertheless, a snubber circuit to dump this ringing is always recommended, as shown in figure below.

Figure 47. Snubber circuit

Capacitance

The parasitic capacitance of a real transformer mainly consists of the following contributions:

•

Capacitance across each winding (CP and CS, in the picture below) due to the capacitve coupling between

the coil and the core. As already mentioned, the winding capacitances are involved in the ringing observed

during transition from off to on phase of the primary side

•

Interwinding capacitance (CWW), that is the capacitance between windings. The interwinding capacitance

should be reduced in order to limit disturbance on the primary side due to possible steep voltage transitions

present in the load connected to the secondary output

L6986I

Design of the external components

DS13647 - Rev 1

page 40/60

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