L6986ITR datasheet(21/60 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.

The Figure 24 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 Figure 24.

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

the equation (25)], hence allowing a higher current from the secondary output.

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.

4.10

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.

This is a second level protection and should never be triggered in normal operating conditions if the system

is properly dimensioned. In other words, the selection of the external power components and the dynamic

performance determined by the compensation network should guarantee an output voltage regulation within the

overvoltage threshold even during the worst-case scenario in term of load transitions.

Figure 25 shows the overvoltage operation during a negative steep load transient and designed with a not

optimized compensation network. This can be considered as an example for a system with dynamic performance

not in line with the load request.

L6986I

Overvoltage protection

DS13647 - Rev 2

page 21/60

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