NCP1000, NCP1001, NCP1002

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9

Current Limit Comparator and Power Switch Circuit

The NCP1000 series uses cycle−by−cycle current limiting

as a means of protecting the output switch transistor from

overstress. Current limiting is implemented by monitoring

the instantaneous output switch current during conduction,

and upon sensing an overcurrent condition, immediately

turning off the switch for the duration of the Oscillator

ramp−down period.

The Power Switch Circuit is constructed using a

SENSEFET

t allowing a virtually lossless method of

monitoring the drain current. A small number of the power

MOSFET cells are used for current sensing by connecting

their individual sources to a single ground referenced sense

at the noninverting input. If exceeded, the comparator

quickly resets the PWM Latch, thus protecting the Power

Switch Circuit. Figure 9 shows that this detection method

yields a relatively constant current limit threshold over

temperature. The high voltage Power Switch Circuit is

integrated with the control logic circuitry and is designed to

directly drive the converter transformer. The Power Switch

Circuit is capable of switching 700 V with an associated

drain current that ranges from 0.5 A to 1.5 A. Proper drain

voltage snubbing during converter start−up and overload is

mandatory for reliable device operation.

A Leading Edge Blanking circuit was placed in the current

sensing signal path to prevent a premature reset of the PWM

Latch. A potential premature reset signal is generated each

time the Power Switch Circuit is driven into conduction and

appears as a narrow voltage spike across current sense

capacitance, transformer interwinding capacitance, and

output rectifier recovery time. The Leading Edge Blanking

circuit has a dynamic behavior that masks the current signal

until the Power Switch Circuit turn−on transition is

completed.

The current limit propagation delay time is typically

220 ns. This time is measured from when an overcurrent

appears at the Power Switch Circuit drain, to the beginning

of turn−off. Care must be taken during transformer

saturation so that the maximum device current limit rating

is not exceeded. To determine the peak Power Switch Circuit

current at turn off, the effect of the propagation delay must

be taken into account. To do this, use the appropriate Current

Limit Threshold value from the electrical tables, and then

add the

DIpk based on the di/dt from Figure 16. The di/dt of

the circuit can be calculated by the following formula:

di dt (A

ms) + V L

where:

V is the rectified, filtered input voltage (volts)

L is the primary inductance of the flyback transformer

(Henries)

High Voltage Start−Up

The NCP1000−1002 contain an internal start−up circuit

that eliminates the need for external start−up components. In

addition, this circuit increases the efficiency of the supply as

it uses no power when in the normal mode of operation, but

instead uses the power supplied by the auxiliary winding.

Rectified, filtered ac line voltage is connected to pin 4. An

internal JFET allows current to flow from the start−up pin,

Figure 5 shows the startup current out of pin 1 which charges

the capacitor(s) connected to this pin.

The start circuit will be enhanced (conducting) when the

point the Power Switch Circuit will be disabled, and the unit

will generate voltage via the auxiliary winding to maintain

proper operation of the device. Figure 4 shows the charge

initially energized.

mode), the start circuit will again begin conducting, and will

The undervoltage lockout (UVLO) is designed to

guarantee that the integrated circuit has sufficient voltage to

be fully functional before the output stage is enabled. It

inhibits operation of the major functions of the device by

disabling the Internal Bias circuitry, and assures that the

Power Switch Circuit remains in its “off’’ state as the bias

voltage is initially brought up from zero volts. When the

NCP100x is in the “off’’ state, the High Voltage Start−up

circuit is operational. The UVLO is a hysteretic switch and

volts, the NCP100x will remain off until the upper threshold

of 8.6 volts is reached. At this time the power converter is

enabled and will commence operation. The UVLO will

voltage exceeds 7.5 volts. The temperature characteristics of

the UVLO circuit are shown in Figure 8.

If the converter output is overloaded or shorted, the device

will enter the auto restart mode. This happens when the

auxiliary winding of the power transformer does not have

below 7.5 volts the unit will shut down, and the High Voltage

cap up to 8.5 volts, which will clock the divide by eight

counter. The divide by eight counter holds the Power Switch

continue to discharge and recharge for eight consecutive

cycles. After the eighth cycle, the unit will turn on again. If

the fault remains, the unit will again cycle through the auto

restart mode; if the fault has cleared the unit will begin

normal operation. The auto restart mode greatly reduces the

power dissipation of the power devices in the circuit and