NCP1215
http://onsemi.com
9
Primary Current Sensing
The primary current sensing circuit is shown in Figure 19.
CS
R
shift
V
shift
R
CS
V
CS
GND
+
−
To Latch
I
primary
Figure 19. Primary Current Sensing
12.5
m
A
B
50
m
A
Feedback Loop
Control
FB
When the primary switch is ON, the transformer current
flows through the sense resistor R
cs
. The current creates a
voltage, V
cs
which is negative with respect to GND. Since
the comparator connected to CS pin requires a positive
voltage, the voltage V
shift
is developed across the resistor
R
shift
by a current source which level−shifts the negative
voltage V
cs
. The level−shift current is in range from 12.5 to
50
m
A depending on the Feedback Loop Control block
signal (see more details in the Feedback Loop Control
section).
The peak primary current is thus equal to:
Ipk
+
Rshift
RCS
· ICS
(eq. 1)
A typical CS pin voltage waveform is shown in Figure 20.
Figure 20. CS Pin Voltage
0
t
Switch
Turn−on
I
shift
= 12.5
m
A
I
shift
= 50
m
A
V
Figure 20 also shows the effect of the inductor current of
differing output power demand.
The primary current sensing method we described, brings
the following benefits compared to the traditional approach:
•
Maximum peak voltage across the current sense resistor
is determined and can be optimized by the value of the
shift resistor.
•
CS pin is not exposed to negative voltage, which could
induce a parasitic substrate current within the IC and
distort the surrounding internal circuitry.
•
The gate drive capability is improved because the
current sense resistor is located out of the gate driver
loop and does not deteriorate the turn−on and also
turn−off gate drive amplitude.
Gate Driver
The Gate Driver consists of a CMOS buffer designed to
directly drive a power MOSFET.
It features an unbalanced source and sink capabilities to
optimize turn ON and OFF performance without additional
external components. Since the power MOSFET turns−off
at high drain current, to minimize its turn−off losses the sink
capability of the gate driver is increased for a faster turn−off.
To the opposite, the source capability is lower to slow−down
power MOSFET at turn−on in order to reduce the EMI noise.
Whenever the IC supply voltage is lower than the
undervoltage threshold, the Gate Driver is low, pulling down
the gate to ground. It eliminates the need for an external
resistor.
Startup Circuit
An external startup resistor is connected between high
voltage potential of the input bulk capacitor and Vcc supply
capacitor. The value of the resistor can be calculated as
follows:
Rstartup
+
Vbulk
*
Vstartup
Istartup
(eq. 2)
Where:
V
startup
V
cc
voltage at which IC starts operation
(see spec.)
I
startup
Startup current
V
bulk
Input bulk capacitor’s voltage
Since the V
bulk
voltage has obviously much higher value
than V
startup
the equation can be simplified in the following
way:
Rstartup
+
Vbulk
Istartup
(eq. 3)
The startup current can be calculated as follows:
Istartup
+
CVcc
Vstartup
tstartup
)
ICC−start
(eq. 4)
Where:
C
Vcc
Vcc capacitor value
t
startup
Startup time
I
CC−start
IC current consumption (see spec.)
