NCP1201
http://onsemi.com
10
DETAILED OPERATING DESCRIPTION
Introduction
The NCP1201 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 criteria,
particularly in low−cost AC−DC adapters, auxiliary supplies
etc. Due to its high−performance High−Voltage technology,
the NCP1201 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 NCP1201 does NOT need an auxiliary
winding to operate: the device is self supplied from the
high−voltage rail and delivers a V
CC
to the IC. This system
is named 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 following
simple logic equations:
POWER−ON: IF
V
CC
<
V
CCOFF
THEN
Current Source is ON, no output pulses
IF VCC decreasing >
V
CCON
THEN
Current Source is OFF, output is pulsing
IF VCC increasing <
V
CCOFF
THEN
Current Source is ON, output is pulsing
Typical values are:
V
CCOFF
= 12.5 V,
V
CCON
= 10.5 V
To better understand the operation principle, Figure 27
sketch offers the necessary explanation,
Figure 27. The Charge/Discharge Cycle Over a 10
m
F V
CC
Capacitor
10 mS
30 mS
50 mS
70 mS
90 mS
Current
Source
OFF
V
CC
Output Pulses
Vripple = 2 V
VCC
OFF
= 12.5 V
VCC
ON
= 10.5 V
ON
The DSS behavior actually depends on the internal IC
consumption and the MOSFET’s gate charge Qg. If we
select a MOSFET like the MTP2N60E, Qg max equals
22 nC. With a maximum switching frequency of 70 kHz for
the oscillator 60 kHz, the average power necessary to drive
the MOSFET (excluding the driver efficiency and
neglecting various voltage drops) is:
Pdriver
+
Fsw(max)
Qg
VCC
(eq. 1)
Where,
P
driver
= Average Power to drive the MOSFET
F
sw(max)
= Maximum switching frequency
Qg = MOSFET’s gate charge
V
CC
= VGS level applied to the gate of the MOSFET
To obtain an estimation of the driving current, simply
divide Pdriver by V
CC
,
Idriver
+
Fsw(max)
Qg
+
1.54 mA
(eq. 2)
The total standby power consumption at no−load will
therefore heavily rely on the internal IC current
consumption plus the driving current (altered by the driver’s
efficiency). Suppose that the IC is supplied from a 350 VDC
line. The current flowing through pin 8 is a direct image of
the NCP1201 current consumption (neglecting the
switching losses of the HV current source). If I
CC2
equals
2.1 mA @ T
A
= 25
°
C, then the power dissipated (lost) by the
IC is simply: 350 V x 2.1 mA = 735 mW. For design and
reliability reasons, it would be interesting to reduce this
source of wasted power. In order to achieve that, different
methods can be used.
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
becomes:
VmainsPEAK
2
p
(eq. 3)