FAN604
www.onsemi.com
11
APPLICATIONS INFORMATION
FAN604 is an offline PWM controller which operates in
a quasi-resonant (QR) mode and significantly enhances
system efficiency and power density. Its control method
is based on the load condition (valley switching with
fixed blanking time at heavy load and valley switching
with variable blanking time at medium load) to maximize
the efficiency. It offers constant output voltage (CV)
regulation through opto-coupler feedback circuitry.
Line voltage compensation gain can be programmed by
using an external resistor to minimize the effect of line
voltage variation on output current regulation due to turn-
off delay of the gate drive circuit.
FAN604 incorporates HV startup and accurate brown-in
through HV pin. The brown-in voltage is programmed by
using an external HV pin resistor. The constant current
regulation (CCR), which sets the maximum output
current level, is programmable via an external resistor
connected to the CCR pin.
Protections such as VDD Over-Voltage Protection (VDD
OVP), VS Over-Voltage Protection (VS OVP), VS Under-
Voltage Protection (VS UVP), internal Over-Temperature
Protection (OTP), Brownout protection and externally
triggered shut-down (SD) function improve reliability.
Basic Operation Principle
Quasi-resonant switching is a method to reduce primary
MOSFET switching losses low line is more effective. In
order to perform QR turn-on of the primary MOSFET,
the valley of the resonance occurring between
transformer magnetizing inductance (Lm) and MOSFET
effective output capacitance (Coss-eff) must be detected.
parasitictransMOSFETOSSeff CCCC -OSS
effOSSmresonance CLt
2
For heavy load condition (50%~100% of full load), the
blanking time for the valley detection is fixed such that
the switching time is between 1/fBNK-MAX and 1/fBNK-MAX
+ tresonance and primary side peak current will be
modulated by voltage level of feedback. For the medium
load condition (25%~50% of full load), the blanking time
is modulated as a function of load current such that the
upper limit of the blanking frequency varies from fBNK-
MAX as load decreases where the blanking frequency
reduction stop point is fBNK-MIN. For the light load
condition (5%~25%)), the blanking time for the valley
detection is fixed such that the switching time is between
fBNK-MIN and fBNK-MIN + tresonance and primary side peak
current will be modulated by the function of VCS-IMN
modulation, as shown in Figure 22
Burst Mode Operation
Figure 21 shows when VFB drops below VFB-Burst-L, the
PWM output shuts off and the output voltage drops at a
rate which is depended on the load current level. This
causes the feedback voltage to rise. Once VFB exceeds
VFB-Burst-H, FAN604 resumes switching. When the FB
voltage drops below the corresponding VCS-IMIN-L, the
peak currents in switching cycles are limited by VCS-IMIN-
L regardless of FB voltage. Thus, more power is delivered
to the load than required and once FB voltage is pulled
low below VFB-Burst-L, switching stops again. In this
manner, the burst mode operation alternately enables and
disables switching of the MOSFET to reduce the
switching losses.
Output Voltage
VFB
VFB-Burst-H
VFB-Burst-L
VCS-IMIN-L
Figure 21 Burst-Mode Operation
Deep Burst Mode
FAN604 enters deep burst mode if FB voltage stays
lower than VFB-Burst-L for more than tDeep-Burst-Entry (640 µs).
Once FAN604 enters deep burst mode, the operating
current is reduced to IDD-Burst (300 μA) to minimize power
consumption. Once feedback voltage is more than VFB-
Burst-H, power-on-reset occurs within a time period of tDeep-
Burst-Exit (25 μs) and IC resumes switching with normal
operating current, IDD-OP.
IPK
VDS
fBNK-MAX =1/ tBNK-MIN tEXT tEXT tEXT
Fixed Blanking Time Modulated Blanking Time
tBNK tBNK tBNK
Fixed Blanking Time
VFB
fBNK-MIN = 1/tBNK-MAX
Figure 22 Frequency Fold-back Function