STCMB1 datasheet(16/59 Pages) STMICROELECTRONICS

16 / 59 page

Application information

STCMB1

16/59

DocID028958 Rev 4

7.2

X-cap discharge

The compliance of consumer equipment with the safety regulation such as IEC 61010-1,

IEC 62368-1, and others, requires that, when the converter mains connector is removed

from the plug, the EMI filter capacitors connected across the line between the phase and the

neutral wires (the so called X-caps) have to be discharged bringing their voltage at a safe

level to avoid the user any risk of the electrical shock due to the energy stored in the

capacitors. This requirement is mandatory in case the total capacitance before the input

bridge exceeds 100 nF.

Figure 5. X-cap discharge operation

Typically this function is performed by means of a resistor in parallel but this method cannot

be applied in case the converter requires very low power consumption during the light-load

or no load operation, because the losses of the X-cap discharging resistor would be too

high.

To overcome this issue, enabling the user to design a high performance power supply the

STCMB1 device does this operation by means of the HV start-up system, allowing the

removal of the traditional X-cap discharge resistor. The STCMB1 internal circuits detect the

ac mains plug disconnection by sensing the voltage on the HV pin. After a detection time of

64 ms typical from the mains disconnection, the X-cap discharge operation is triggered and

the HV start-up system is turned on: a discharge current (4.2 mA minimum) is drawn from

the HV pin ensuring the X-cap discharge until the voltage on the HV pin falls below a safe

level (45 V maximum), within the regulation maximum discharging time. The current from

the HV pin will flow out from the VCC pin, to keep the IC correctly supplied until the end of

the discharge. An internal 15 V clamp is activated to limit the voltage on the VCC pin by

sinking the discharge current in excess of the IC consumption.

None additional component is needed by this function, totally embedded in the STCMB1.

STCMB1 Datasheet(PDF) 16 Page - STMicroelectronics