NCP1200A

http://onsemi.com

10

Power Dissipation

The NCP1200A is directly supplied from the DC rail

through the internal DSS circuitry. The average current

flowing through the DSS is therefore the direct image of the

NCP1200A

current

consumption.

The

total

power

If we operate the device on a 250 VAC rail, the maximum

rectified voltage can go up to 350 VDC. However, as the

characterization curves show, the current consumption

drops at high junction temperature, which quickly occurs

the 61 kHz version over a 1 nF capacitive load. As a result,

595

mW.

The

SOIC−8

package

offers

a

Adding some copper area around the PCB footprint will help

to 100

°C/W with 35 m copper thickness (1 oz.) or 6.5 mm x

6.5 mm with 70

m copper thickness (2 oz.). With this later

number, we can compute the maximum power dissipation

the

package

accepts

at

an

ambient

of

50

°C:

Pmax

+

TJmax

* TAmax

R

qJA

+ 750 mW

which is okay with

our previous budget. For the DIP8 package, adding a

min−pad area of 80 mm

from 100

°C/W to about 75°C/W.

In the above calculations, ICC2 is based on a 1 nF output

capacitor. As seen before, ICC2 will depend on your

MOSFET’s Qg: ICC2

shall thus accounts for the total gate−charge Qg your

MOSFET will exhibit. The same methodology can be

applied for the 100 kHz version but care must be taken to

and activated DSS in high−line conditions.

If the power estimation is beyond the limit, other solutions

are possible a) add a series diode with pin 8 (as suggested in

the above lines) and connect it to the half rectified wave. As

a result, it will drop the average input voltage and lower the

dissipation to:

350

@ 2

p

auxiliary winding to disable the DSS and decrease the power

thus that the rectified auxiliary voltage permanently stays

above 10 V (to not re−activate the DSS) and is safely kept

below the 16 V maximum rating.

Overload Operation

In applications where the output current is purposely not

controlled (e.g. wall adapters delivering raw DC level), it is

interesting to implement a true short−circuit protection. A

short−circuit actually forces the output voltage to be at a low

level, preventing a bias current to circulate in the

optocoupler LED. As a result, the FB pin level is pulled up

to 4.2 V, as internally imposed by the IC. The peak current

setpoint goes to the maximum and the supply delivers a

rather high power with all the associated effects. Please note

that this can also happen in case of feedback loss, e.g. a

broken optocoupler. To account for this situation,

NCP1200A hosts a dedicated overload detection circuitry.

Once activated, this circuitry imposes to deliver pulses in a

burst manner with a low duty cycle. The system

auto−recovers when the fault condition disappears.

During the startup phase, the peak current is pushed to the

maximum until the output voltage reaches its target and the

feedback loop takes over. This period of time depends on

normal output load conditions and the maximum peak

current allowed by the system. The time−out used by this IC

device internally watches for an overload current situation.

reached, the controller stops the driving pulses, prevents the

self−supply current source to restart and puts all the circuitry

in standby, consuming as little as 350

mA typical (ICC3

toward 0.