NCP1200
APPLICATIONS INFORMATION
INTRODUCTION
Dynamic Self−Supply
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 DSS principle is based on the charge/discharge of the
V
bulk capacitor from a low level up to a higher level. We
CC
can easily describe the current source operation with a bunch
of simple logical equations:
POWER−ON: IF V < V
THEN Current Source
CC
CCOFF
is ON, no output pulses
IF V decreasing > V
OFF, output is pulsing
THEN Current Source is
THEN Current Source is
CC
CCON
IF V increasing < V
CC
CCOFF
ON, output is pulsing
Typical values are: V
= 11.4 V, V
= 9.8 V
CCOFF
CCON
To better understand the operational principle, Figure 15’s
sketch offers the necessary light:
the high−voltage rail and delivers a V to the IC. This
CC
system is called the Dynamic Self−Supply (DSS).
V
= 11.4 V
CCOFF
V
CC
10.6 V Avg.
V
CCON
= 9.8 V
ON
OFF
Current
Source
Output Pulses
50.00M 70.00M
10.00M
30.00M
90.00M
Figure 15. The Charge/Discharge Cycle
Over a 10 mF VCC 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:
. 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
2 * V
mains PEAK
becomes
. Our power contribution
p
example drops to: 160 mW.
Fsw @ Qg @ V
with
cc
Fsw = maximum switching frequency
Qg = MOSFET’s gate charge
Dstart
1N4007
V
CC
= V level applied to the gate
GS
To obtain the final driver contribution to the IC
C3
4.7 mF
400 V
+
NCP1200
consumption, simply divide this result by V : Idriver =
CC
HV
NC
1
2
3
4
8
7
6
5
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
Adj
FB
CS
V
CC
EMI
Filter
GND Drv
Figure 16. A simple diode naturally reduces the
average voltage on pin 8
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