A6986I datasheet(22/62 Pages) STMICROELECTRONICS

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Overcurrent protection at the secondary side

The increase of the secondary output current affects the current shape at primary side. In particular, the peak

currents in the high-side and low-side MOSFETs rise (in absolute value in the low-side, i.e. the current becomes

more negative), in accordance with equations (5) and (6)).

IPEAKHS= IOUT_pri+N∙IOUT_sec+∆IL2

(5)

IPEAKLS=−N∙IOUT_sec∙ 2D1−D −∆IL2+IOUT_prim

(6)

where IOUT_pri and IOUT_sec are respectively the primary and secondary output currents, N is the transformer turn

ratio, D the duty cycle and ΔIL the current ripple in the primary winding.

Figure 23 depicts how these peak currents vary depending on the secondary output current, under the specified

conditions.

Normally the negative current during the off-time is more critical and the secondary output is limited by relying on

the reverse current limit (typ. 1.9 A), as shown in the figure below.

A current drawn from the primary output helps to reduce the peak current in the low-side MOSFET [as shown by

equation (6)], hence allowing a higher current from the secondary output. Nevertheless, attention must be paid to

the IPEAKHS [equation (5)] that must be lower than the peak current limit.

Figure 24. Peak currents depending on the secondary output current (VIN = 12 V, VOUT_pri = 5.3, N = 6, fSW

= 500 kHz, no primary output current)

0.4

0.8

1.2

1.6

2.4

100

150

200

Secondary output current [mA]

IPEAK

reverse current

limit (typ.) exceeded

| IPEAK |

Reverse current limit

As mentioned in the previous section, the low-side MOSFET is protected against negative currents. This feature

limits the negative current in the MOSFETs especially during two events:

- Overvoltage at the primary output. Due to an overvoltage event the low-side MOSFET is kept on to discharge

the primary output.

- Overcurrent or short-circuit event at the secondary output, as described in the previous section.

5.10

Overvoltage protection

The overvoltage protection monitors the VFB pin and enables the low-side MOSFET to discharge the output

capacitor if the output voltage is 20% over the nominal value.

A6986I

Overvoltage protection

DS13481 - Rev 2

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