A6986ITR datasheet(41/62 Pages) STMICROELECTRONICS

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As shown in Figure 45 (resulting from a simulation), the leakage inductance affects the shape of the secondary

winding current. In general, a very low leakage inductance implies that the current in the secondary winding can

quickly ramp up, allowing the charge of the secondary output capacitor and supporting the load current demand.

A high leakage inductance slows down the secondary winding current rise, limiting the charge delivery to the

output. Comparing solutions with different values of transformer leakage inductance shows that higher leakage

inductance transformers are characterized by poorer load regulation performances.

In addition, higher leakage inductance means a reduction of the effective current slope in the primary winding,

therefore increasing the duty cycle. As a consequence, the Schottky diode conduction time shortens, and higher

peak and RMS current are present at the secondary winding. Higher peaks at light load negatively impact the load

regulation.

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 in the primary side

•

Reverse capacitance of the Schottky diode, reflected in 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 the figure below.

Figure 47. Snubber circuit

Capacitance

A6986I

Design of the external components

DS13481 - Rev 2

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A6986ITR Datasheet(PDF) 41 Page - STMicroelectronics