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NCP1072

High-Voltage Switcher for Low Power Offline SMPS

ON Semiconductor 

NCP1070, NCP1071, NCP1072, NCP1075, NCP1076, NCP1077

Fault Condition – Output Short−Circuit

As soon as VCC reaches VCC(on), drive pulses are

internally enabled. If everything is correct, the auxiliary

winding increases the voltage on the VCC pin as the output

voltage rises. During the start−sequence, the controller

smoothly ramps up the peak drain current to maximum

setting, i.e. IIPK, which is reached after a typical period of

1 ms. When the output voltage is not regulated, the current

coming through FB pin is below IFBfault level (35 mA

typically), which is not only during the startup period but

also anytime an overload occurs, an internal error flag is

asserted, Ipflag, indicating that the system has reached its

maximum current limit set point. The assertion of this flag

triggers a fault counter tSCP (53 ms typically). If at counter

completion, Ipflag remains asserted, all driving pulses are

stopped and the part stays off in trecovery duration (about

420 ms). A new attempt to re−start occurs and will last 53 ms

providing the fault is still present. If the fault still affects the

output, a safe burst mode is entered, affected by a low

duty−cycle operation (11%). When the fault disappears, the

power supply quickly resumes operation. Figure 26 depicts

this particular mode:

Figure 26. In Case of Short−Circuit or Overload, the NCP107X Protects Itself and the Power Supply Via a Low

Frequency Burst Mode. The VCC is Maintained by the Current Source and Self−supplies the Controller.

Auto−Recovery Over Voltage Protection

The particular NCP107X arrangement offers a simple

way to prevent output voltage runaway when the

optocoupler fails. As Figure 27 shows, an active zener diode

monitors and protects the VCC pin. Below its equivalent

breakdown voltage, that is to say 8.4 V typical, no current

flows in it. If the auxiliary VCC pushes too much current

inside the zener, then the controller considers an OVP

situation and stops the internal drivers. When an OVP

occurs, all switching pulses are permanently disabled. After

trecovery delay, it resumes the internal drivers. If the failure

symptom still exists, e.g. feedback opto−coupler fails, the

device keeps the auto−recovery OVP mode.

Figure 27 shows that the insertion of a resistor (Rlimit)

between the auxiliary dc level and the VCC pin is mandatory

a) not to damage the internal 8.4 V zener diode during an

overshoot for instance (absolute maximum current is

15 mA) b) to implement the fail−safe optocoupler protection

(OVP) as offered by the active clamp. Please note that there

cannot be bad interaction between the clamping voltage of

the internal zener and VCC(on) since this clamping voltage is

actually built on top of VCC(on) with a fixed amount of offset

(200 mV typical). Rlimit should be carefully selected to avoid

triggering the OVP as we discussed, but also to avoid

disturbing the VCC in low / light load conditions. The below

lines detail how to evaluate the Rlimit value...

Self−supplying controllers in extremely low standby

applications often puzzles the designer. Actually, if a SMPS

operated at nominal load can deliver an auxiliary voltage of

an arbitrary 16 V (Vnom), this voltage can drop below 10 V

(Vstby) when entering standby. This is because the

recurrence of the switching pulses expands so much that the

low frequency re−fueling rate of the VCC capacitor is not

enough to keep a proper auxiliary voltage. Figure 28

portrays a typical scope shot of a SMPS entering deep

standby (output un−loaded). Thus, care must be taken when

calculating Rlimit 1) to not trigger the VCC over current latch

(by injecting 6 mA into the active clamp – always use the

minimum value for worse case design) in normal operation

but 2) not to drop too much voltage over Rlimit when entering

standby. Otherwise, the converter will enter dynamic self

supply mode (DSS mode), which increases the power

dissipation. Based on these recommendations, we are able to

bound Rlimit between two equations:

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