NCP1028 データシートの表示（PDF） - ON Semiconductor

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NCP1028

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

ON Semiconductor 

NCP1028

This difference might not be seen as a problem, but some

design specifications impose stringent conditions on the

maximum output current capability, regardless the line

input. Hence the need for an OPP input…

Since we want to limit the power to 12.8 W at high line,

let us calculate the needed peak current:

Ǹ From equation 6: Ipeak +

2Pout

FSWLph

=

693

mA

to

deliver 12.8 W at high line.

Compared to our 735 mA, we need to decrease the

setpoint by 6% roughly when Vin equals 350 Vdc.

The NCP1028 hosts a special circuitry looking at the

couple voltage/current present on pin 7. Figure 36 shows

how to arrange components around the controller to obtain

Over Power Protection.

Bulk

Current

Setpoint

Over Power OPP

Protection

GND

ROPPU

ROPPL

Figure 36. A resistive network reduces the

power capability in high−line conditions.

First, you need to know the required injected current and

the voltage across pin 7 to start activating OPP.

Experiments consist in wiring Figure 36 circuit and

running the power supply in conditions where it must shut

down (e.g. highest input voltage and maximum output

current per specification). For this, ROPPL can be put to

VDD

10 kW and ROPPU made of a series string of 4 1.0 MW

resistors plus a 10−turn 1.0 MW potentiometer set at its

maximum value. An amp−meter is inserted in series with

pin 7 and a volt−meter monitors its voltage with respect to

ground. Once the power supply is powered, slowly rotate

the potentiometer and observe both voltage and current

going up at pin 7. At a certain time, as voltage and current

increase, the controller will shut down the power supply.

The current at this time is the one we are looking for.

Suppose these experiments lead to 80 mA with a pin 7

activation voltage of 2.45 V. Final resistor equations are:

VbulkH = 375 Vdc ; the maximum voltage at which OPP

must shut down the controller

VbulkL = 200 Vdc ; the minimum voltage below which

OPP is not activated

IOPP = 80 mA

Vf = 2.45 V

; the current in pin 7

; the voltage of pin 7 at the above

condition

ROPPL

+

VbulkH−VbulkL

IOPP(VbulkL−Vf)

Vf

+

27

kW

(eq. 7)

ROPPH

+

ROPPL

VbulkL−Vf

Vf

+

2.2

MW

(eq. 8)

If the OPP feature is not needed for some designs, it is

possible to ground it via a copper wire to the adjacent

ground pin. This can help to develop a larger copper area

in an application where the thermal resistance is an

important parameter.

Ramp Compensation

When operating in Continuous Conduction Mode

(CCM), current−mode power supplies can exhibit

so−called sub−harmonic oscillations. To cure this problem,

the designer must inject ramp compensation. The ramp can

either be added to the current sense information or directly

subtracted from the feedback signal. Figure 37 details the

internal arrangement of the ramp compensation circuitry.

Gate Reset

IRR

Ramp

Vp

RR

Control

Figure 37. The Internal Feedback Chain and the Ramp Compensation Network

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