400EADBA0 datasheet(24/47 Pages) ONSEMI

24 / 47 page

NCP12400

www.onsemi.com

X2 Cap Discharge Feature

The X2 capacitor discharging feature is offered by usage

of the NCP12400. This feature save approx. 16 mW –

25 mW input power depending on the EMI filter X2

capacitors volume and it saves the external components

count as well. The discharge feature is ensured via the

start−up current source with a dedicated control circuitry for

this function. The X2 capacitors are being discharged by

current defined as Istart2 when this need is detected.

There is used a dedicated structure called ac line unplug

detector inside the X2 capacitor discharge control circuitry.

See the Figure 51 for the block diagram for this structure and

Figures 52, 53, 54 and 55 for the timing diagrams. The basic

idea of ac line unplug detector lies in comparison of the

direct sample of the high voltage obtained via the high

voltage sensing structure with the delayed sample of the high

voltage. The delayed signal is created by the sample & hold

structure.

The comparator used for the comparison of these signals

is without hysteresis inside. The resolution between the

slopes of the ac signal and dc signal is defined by the

sampling time TSAMPLE and additional internal offset NOS.

These parameters ensure the noise immunity as well. The

additional offset is added to the picture of the sampled HV

signal and its analog sum is stored in the C1 storage

capacitor. If the voltage level of the HV sensing structure

output crosses this level the comparator CMP output signal

resets the detection timer and no dc signal is detected. The

additional offset NOS can be measured as the VHV(hyst) on

the HV pin. If the comparator output produces pulses it

means that the slope of input signal is higher than set

resolution level and the slope is positive. If the comparator

output produces the low level it means that the slope of input

signal is lower than set resolution level or the slope is

negative. There is used the detection timer which is reset by

any edge of the comparator output. It means if no edge

comes before the timer elapses there is present only dc signal

or signal with the small ac ripple at the HV pin. This type of

the ac detector detects only the positive slope, which fulfils

the requirements for the ac line presence detection.

In case of the dc signal presence on the high voltage input,

the direct sample of the high voltage obtained via the high

voltage sensing structure and the delayed sample of the high

voltage are equivalent and the comparator produces the low

level signal during the presence of this signal. No edges are

present at the output of the comparator, that’s why the

detection timer is not reset and dc detect signal appears.

The minimum detectable slope by this ac detector is given

by the ration between the maximum hysteresis observed at

HV pin VHV(hyst),max and the sampling time:

Smin +

VHV(hyst),max

Tsample

(eq. 1)

Than it can be derived the relationship between the

minimum detectable slope and the amplitude and frequency

of the sinusoidal input voltage:

Vmax +

VHV(hyst),max

2 @ p @ f @ Tsample

2 @ p @ 35 @ 1 @ 10−3

(eq. 2)

+ 22.7 V

The minimum detectable AC RMS voltage is 16 V at

frequency 35 Hz, if the maximum hysteresis is 5 V and

sampling time is 1 ms.

The X2 capacitor discharge feature is available in any

controller operation mode to ensure this safety feature. The

detection timer is reused for the time limiting of the

discharge phase, to protect the device against overheating.

The discharging process is cyclic and continues until the ac

line is detected again or the voltage across the X2 capacitor

is lower than VHV(min). This feature ensures to discharge

quite big X2 capacitors used in the input line filter to the safe

level. It is important to note that it is not allowed to

connect HV pin to any dc voltage due this feature. e.g.

directly to bulk capacitor.

During the HV sensing or X2 cap discharging the VCC net

is kept above the VCC(off) voltage by the Self−Supply in any

mode of device operation to supply the control circuitry.

During the discharge sequence is not allowed to start−up the

device.

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