The overvoltage protection is adapted for both flyback and forward mode auxiliary windings. In flyback mode, the

auxiliary winding is coupled with the same polarity as the output winding, so when the voltage is positive it reflects

the output voltage. In forward mode, the auxiliary winding has a reversed polarity, so when the voltage is positive

it reflects the input voltage. This option can be advantageous when the output voltage range is very wide. Both

protections for flyback and forward mode OV are active at the same time. The application circuit is designed to

allow only the proper protection to be triggered.

To reduce sensitivity to noise and prevent the protection from being erroneously activated, the OVP comparator

must be asserted for a time longer than TOVFILT for the protection to be triggered.

8.7.3.1

Flyback aux winding

For flyback windings the OVP condition is detected by comparing the voltage on OVP pin with the VOVTH

threshold.

8.7.3.2

Forward winding

The OVP pin is clamped at VOVCLAMP when sourcing current, avoiding it reaching negative voltages. The current

sourced by the clamp is measured and compared with IOVTH to detect the OV condition.

8.7.3.2.1

OV pin clamping

The OV pin is protected from overvoltage by an internal clamp at 3.3 V, able to sink up to 200 uA.

8.7.3.3

Soft-start OV Threshold selection

Since the primary side OV is active only during the primary side soft-start, it is suggested to set the threshold to a

value which lies between the secondary side turn-on threshold and the maximum secondary side voltage.

Values between 10 V and 15 V are typically good for many applications, including USB-PD with maximum output

voltage at 20 V: the higher voltages are reached only when the secondary side is controlling the conversion and

the primary side is disabled.

8.7.4

Ac brown-out protection

The ST-ONEMP device is equipped with brown-out protection to prevent the operation at too low ac input voltage.

Power conversion starts only when the voltage on HV becomes higher than VHVPk_BI.

If the voltage on HV remains below VHVPk_BO for more than TBODG the power conversion is turned off until the

voltage goes back above VHVPk_BI. The TBODG masking avoids unexpected turn-offs due to disturbances or line

dips.

8.7.5

Primary side overcurrent protection (OCP)

The primary side device includes protections to avoid uncontrolled current rise in the transformer.

8.7.5.1

First level overcurrent protection

The first protection level is provided by a clamp on the primary side current mode unit comparator which

guarantees that, after the blanking time is expired, the LON signal is turned off if the voltage on the ISEN pin

bypasses VIPKCLAMP. Even if this limit is reached, the device keeps operating unless other faults are triggered, for

example if the current limitation causes an unvervoltage on the output voltage.

8.7.5.2

Second level overcurrent protection (OCP)

The device is protected against short-circuit of the secondary rectifier, short-circuit on the secondary winding or

a hard-saturated transformer. A comparator monitors continuously the voltage on the CS pin and activates a

protection circuitry if this voltage exceeds VOCTH.

To distinguish an actual malfunction from a disturbance (e.g. induced during ESD tests), the first time the

comparator is tripped the protection circuit enters a “warning state”. If in the subsequent switching cycle the

comparator is not tripped, a temporary disturbance is assumed, and the protection logic is reset in its idle state; if

the comparator is tripped again a real malfunction is assumed and the device is stopped.

The fault is managed in auto-restart mode.

8.7.6

Primary side overtemperature protection

The primary side device can be protected from excess temperature by connecting a thermistor to the POTP pin.

ST-ONEMP

Fault management

DS13989 - Rev 2

page 24/39