NCP4304A, NCP4304B
www.onsemi.com
13
APPLICATION INFORMATION
General Description
The NCP4304A/B is designed to operate either as
a standalone IC or as a companion IC to a primary side
controller to help achieve efficient synchronous
rectification in switch mode power supplies. This controller
features a high current gate driver along with high-speed
logic circuitry to provide appropriately timed drive signals
to a synchronous rectification MOSFET. With its novel
architecture, the NCP4304A/B has enough versatility to
keep the synchronous rectification efficient under any
operating mode.
The NCP4304A/B works from an available bias supply
with voltage range from 10.4 V to 28 V (typical). The wide
V
CC
range allows direct connection to the SMPS output
voltage of most adapters such as notebook and LCD TV
adapters. As a result, the NCP4304A/B simplifies circuit
operation compared to other devices that require specific
bias power supplies (e.g. 5 V). The high voltage capability
of the V
CC
is also a unique feature designed to allow
operation for a broader range of applications.
Precise turn off threshold of the current sense comparator
together with accurate offset current source allows the user
to adjust for any required turn off current threshold of the SR
MOSFET switch using a single resistor. Compared to other
SR controllers that provide turn off thresholds in the range
of −10 mV to −5 mV, the NCP4304A/B offers a turn off
threshold of 0 mV that in combination with a low R
DS(on)
SR
MOSFET significantly reduces the turn off current
threshold and improves efficiency.
To overcome issues after turn on and off events, the
NCP4304A/B provides adjustable minimum on time and off
time blanking periods. Blanking times can be adjusted
independently of IC V
CC
using resistors connected to GND.
If needed, blanking periods can be modulated using
additional components.
An ultrafast trigger input helps to implement synchronous
rectification systems in CCM applications (like CCM
flyback or forward). The time delay from trigger input to
driver turn off event is 10 ns (typicaly). Additionally, the
trigger input can be used to disable the IC and activate a low
consumption standby mode. This feature can be used to
decrease standby consumption of an SMPS.
Finally, the NCP4304A/B features a special input that can
be used to automatically compensate for SR MOSFET
parasitic inductance effect. This technique achieves the
maximum available on-time and thus optimizes efficiency
when a MOSFET in standard package (like TO−220 or
TO247) is used. If a SR MOSFET in SMT package with
negligible inductance is used, the compensation input is
connected to GND pin.
Zero Current Detection and Parasitic Inductance
Compensation
Figure 40 shows the internal connection of the ZCD
circuitry on the current sense input. The synchronous
rectification MOSFET is depicted with it’s parasitic
inductances to demonstrate operation of the compensation
system.
Figure 40. ZCD Sensing Circuitry Functionality
−
+
−
+
SR MOSFET
LDRAIN LSOURCE
M1
RSHIFT_CS
I
shift_CS
+
+Vout
GND
DRV
CS
COMP GND
Vdd
ZCD SET
ZCD RESET
To Internal Logic
V
th_cs_off
+
−
+
−
V
REF
= V
th_cs_on
I
shift_CS
100 mA
−1
LCOMP
