ncp1001 ON Semiconductor, ncp1001 Datasheet - Page 9

• Manufacturer Part Number: ncp1001
• Description: Integrated Off-line Switching Regulator
• Manufacturer: ON Semiconductor

Current Limit Comparator and Power Switch Circuit
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.
SENSEFETt 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
resistor, R
voltage across R
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.
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
resistor R
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.
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:
where:
The NCP1000 series uses cycle−by−cycle current limiting
The Power Switch Circuit is constructed using a
A Leading Edge Blanking circuit was placed in the current
The current limit propagation delay time is typically
V is the rectified, filtered input voltage (volts)
L is the primary inductance of the flyback transformer
(Henries)
pk
pk
. The spike is due to the MOSFET gate to source
. The current limit comparator detects if the
pk
exceeds the reference level that is present
di dt (A ms) + V L
NCP1000, NCP1001, NCP1002
http://onsemi.com
9
High Voltage Start−Up
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.
internal JFET allows current to flow from the start−up pin,
to the V
Figure 5 shows the startup current out of pin 1 which charges
the capacitor(s) connected to this pin.
voltage at Pin 1 (V
enhanced until the V
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
time for turn−on vs. V
initially energized.
mode), the start circuit will again begin conducting, and will
charge up the V
V
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
will hold the device in its “off’’ state any time that the V
voltage is less than 7.5 volts. As the V
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
allow the unit to continue to operate as long as the V
voltage exceeds 7.5 volts. The temperature characteristics of
the UVLO circuit are shown in Figure 8.
will enter the auto restart mode. This happens when the
auxiliary winding of the power transformer does not have
sufficient voltage to support the V
chip. Once the chip is operational, if the V
below 7.5 volts the unit will shut down, and the High Voltage
Start−up circuit will be enabled. This will charge the V
cap up to 8.5 volts, which will clock the divide by eight
counter. The divide by eight counter holds the Power Switch
Circuit off. This causes the V
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
CC
The NCP1000−1002 contain an internal start−up circuit
Rectified, filtered ac line voltage is connected to pin 4. An
The start circuit will be enhanced (conducting) when the
If the V
The undervoltage lockout (UVLO) is designed to
If the converter output is overloaded or shorted, the device
Limiter and Undervoltage Lockout
CC
CC
voltage drops below 7.5 volts (e.g. current limit
pin at a current of approximately 3.0 mA.
CC
CC
cap until the 8.5 volt limit is reached.
CC
) is less than 7.5 volts. It will remain
CC
voltage reaches 8.5 volts. At this
capacitance when the unit is
CC
cap to discharge. It will
CC
CC
requirements of the
increases past 7.5
CC
voltage falls
CC
CC
CC