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NCP1028

High−Voltage Switcher for Medium Power Offline SMPS Featuring Low Standby Power

ON Semiconductor 

NCP1028

Fault Condition – Low Input Voltage

The NCP1028 includes a brown−out circuitry able to

protect the power supply in case of low input voltage

conditions. Figure 29 shows how internally the NCP1028

monitors the voltage image of the bulk capacitor. Below a

given level, the controller blocks the driving pulses, above

it, it authorizes them. The internal circuitry, depicted by

Figure 29a, offers a way to observe the high−voltage (HV)

rail. A resistive divider made of Rupper and Rlower, brings

a portion of the HV rail on pin 3. Below the turn−on level,

the 10 mA current source IBO is off. Therefore, the turn−on

level solely depends on the division ratio brought by the

resistive divider.

1 vin 2 vcmp

Vbulk

VDD

Rupper

IBO

ON/OFF

16.0 160

12.0 120

Vbulk = 100 V

8.00 80.0

Vbulk = 70 V

BO

+

4.00 40.0

−

BO

Rlower

+

0

0

21

VBO

Figure 29a. The internal brown−out

configuration with an offset current source.

20.0u

60.0u

100u

Time in Seconds

140u

180u

Figure 29b. Simulation results for 100/70 ON/OFF levels.

Figure 29.

To the contrary, when the internal BO signal is high, the IBO source is activated and creates an hysteresis. As a result,

it becomes possible to select the turn−on and turn−off levels via a few lines of algebra.

IBO is Off

IBO is On

V()) + Vbulk1

Rlower

Rlower ) Rupper

ǒ Ǔ V()) + Vbulk2

Rlower

Rlower ) Rupper

)

IBO

Rlower Rupper

Rlower ) Rupper

(eq. 1)

(eq. 2)

We can now extract Rlower from Equation 1 and plug it into Equation 2, then solve for Rupper:

Rupper + Rlower

Vbulk1−VBO

VBO

Rlower + VBO

Vbulk1−Vbulk2

IBO (Vbulk1−VBO)

If we decide to turn−on our converter for Vbulk1 equals 100 V and turn it off for Vbulk2 equals 70 V, then we obtain:

Rupper = 3.0 MW

Rlower = 18 kW

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