NCP1280
http://onsemi.com
11
TYPICAL CHARACTERISTICS
Figure 27. Outputs Rise Time versus Load
Capacitance
C
L
, LOAD CAPACITANCE (pF)
200150100500
0
10
20
30
40
50
60
80
Figure 28. Outputs Fall Time versus Load
Capacitance
C
L
, LOAD CAPACITANCE (pF)
200150100500
0
5
10
15
20
25
35
70
t
on
, OUTPUTS RISE TIME (ns)
30
t
off
, OUTPUTS FALL TIME (ns)
T
J
= −40°C
T
J
= 25°C
T
J
= 125°C
T
J
= −40°C
T
J
= 25°C
T
J
= 125°C
1751257525
1751257525
Measured from 10% to 90% of V
OH
V
AUX
= 12 V
Measured from 90% to 10% of V
OH
V
AUX
= 12 V
DETAILED OPERATING DESCRIPTION
Introduction
An NCP1280 based system offers significant efficiency
improvements and system cost savings over a converter
using a traditional forward topology. The NCP1280
provides two control outputs. OUT1 controls the primary
switch of a forward converter. OUT2 has an adjustable
overlap delay, which can be used to control an active
clamp/reset switch or any other complementary drive
topology, such as an asymmetric half−bridge. In addition,
OUT2 can be used to control a synchronous rectifier
topology, eliminating the need of external control circuitry.
Other distinctive features include: two mode overcurrent
protection, line under/overvoltage detectors, fast line
feedforward, soft−start and a maximum duty cycle limit.
The Functional Block Diagram is shown in Figure 2.
The features included in the NCP1280 provide some of
the advantages of Current−Mode Control, such as fast line
feedforward, and cycle by cycle current limit. It eliminates
the disadvantages of low power jitter, slope compensation
and noise susceptibility.
Active Clamp Topology
The transformer reset voltage in a traditional forward
converter is set by the turns ratio and input voltage. Where
as the reset voltage of an active clamp topology is constant
over the converter off time and only depends on the input
voltage and duty cycle. This translates into a lower voltage
stress on the main switch, allowing the use of lower voltage
MOSFETs. In general, lower voltage MOSFETs have lower
cost and ON resistance. Therefore, lower system cost and
higher efficiency can be achieved. In addition, the lower
voltage stress allows the converter to operate at a higher duty
cycle for a given primary switch voltage stress. This allows
a reduction in primary peak current and secondary side
voltage stress as well as smaller secondary inductor size.
High Voltage Startup Regulator
The NCP1280 contains an internal 700 V startup regulator
that eliminates the need for external startup components. In
addition, this regulator increases the efficiency of the supply
as it uses no power when in the normal mode of operation,
but instead uses power supplied by an auxiliary winding.
The startup regulator consists of a constant current source
that supplies current from the input line voltage (V
in
) to the
capacitor on the V
AUX
pin (C
AUX
). The startup current is
typically 13.8 mA. Once V
AUX
reaches 11 V, the startup
regulator turns OFF and the outputs are enabled. When V
AUX
reaches 7 V, the outputs are disabled and the startup regulator
turns ON. This “7−11” mode of operation is known as
Dynamic Self Supply (DSS). The V
AUX
pin can be biased
externally above 7 V once the outputs are enabled to prevent
the startup regulator from turning ON. It is recommended to
bias the V
AUX
pin using an auxiliary supply generated by an
auxiliary winding from the power transformer. An
independent voltage supply can also be used. If using an
independent voltage supply and V
AUX
is biased before the
outputs are enabled or while a fault is present, the One Shot
Pulse Generator (Figure 2) will not be enabled and the
outputs will remain OFF.
As the DSS sources current to the V
AUX
pin, a diode should
be placed between C
AUX
and the auxiliary supply as shown
in Figure 29. This will allow the NCP1280 to charge C
AUX
while preventing the startup regulator from sourcing current
into the auxiliary supply.
