NCP1200
APPLICATIONS INFORMATION
INTRODUCTION
The NCP1200 implements a standard current mode
architecture where the switch−off time is dictated by the
peak current setpoint. This component represents the ideal
candidate where low part−count is the key parameter,
particularly in low−cost AC−DC adapters, auxiliary
supplies etc. Due to its high−performance High−Voltage
technology, the NCP1200 incorporates all the necessary
components normally needed in UC384X based supplies:
timing components, feedback devices, low−pass filter and
self−supply. This later point emphasizes the fact that ON
Semiconductor’s NCP1200 does NOT need an auxiliary
winding to operate: the product is naturally supplied from
the high−voltage rail and delivers a V
CC
to the IC. This
system is called the Dynamic Self−Supply (DSS).
Dynamic Self−Supply
The DSS principle is based on the charge/discharge of the
V
CC
bulk capacitor from a low level up to a higher level. We
can easily describe the current source operation with a bunch
of simple logical equations:
POWER−ON: IF V
CC
< V
CCOFF
THEN Current Source
is ON, no output pulses
IF V
CC
decreasing > V
CCON
THEN Current Source is
OFF, output is pulsing
IF V
CC
increasing < V
CCOFF
THEN Current Source is
ON, output is pulsing
Typical values are: V
CCOFF
= 11.4 V, V
CCON
= 9.8 V
To better understand the operational principle, Figure 15’s
sketch offers the necessary light:
V
CCOFF
= 11.4 V
10.6 V Avg.
V
CCON
= 9.8 V
V
CC
ON
OFF
Current
Source
Output Pulses
10.00M
30.00M
50.00M
70.00M
90.00M
Figure 15. The Charge/Discharge Cycle
Over a 10
mF
V
CC
Capacitor
The DSS behavior actually depends on the internal IC
consumption and the MOSFET’s gate charge, Qg. If we
select a MOSFET like the MTD1N60E, Qg equals 11 nC
(max). With a maximum switching frequency of 48 kHz (for
the P40 version), the average power necessary to drive the
MOSFET (excluding the driver efficiency and neglecting
various voltage drops) is:
Fsw
@
Qg
@
V
cc
.
0.16 = 256 mW. If for design reasons this contribution is
still too high, several solutions exist to diminish it:
1. Use a MOSFET with lower gate charge Qg
2. Connect pin through a diode (1N4007 typically) to
one of the mains input. The average value on pin 8
mains PEAK
. Our power contribution
becomes
p
example drops to: 160 mW.
Dstart
1N4007
2*V
with
Fsw = maximum switching frequency
Qg = MOSFET’s gate charge
V
CC
= V
GS
level applied to the gate
To obtain the final driver contribution to the IC
consumption, simply divide this result by V
CC
: Idriver =
Fsw
@
Qg
= 530
mA.
The total standby power consumption
at no−load will therefore heavily rely on the internal IC
consumption plus the above driving current (altered by the
driver’s efficiency). Suppose that the IC is supplied from a
400 V DC line. To fully supply the integrated circuit, let’s
imagine the 4 mA source is ON during 8 ms and OFF during
50 ms. The IC power contribution is therefore: 400 V
.
4 mA
C3
4.7
mF
400 V
+
1
NCP1200
Adj
2 FB
3 CS
HV 8
NC 7
V
CC
6
EMI
Filter
4 GND Drv 5
Figure 16. A simple diode naturally reduces the
average voltage on pin 8
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