ISL6730A, ISL6730B, ISL6730C, ISL6730D
12
FN8258.1
August 8, 2013
harmonic distortion minimum, it is desirable to set the control
bandwidth much lower than twice of the line frequency. The
recommended voltage loop bandwidth is 10Hz.
During start-up, the compensation capacitors and the charging
current from the error amplifier sets the input power increase
rate. Thus, soft-start is achieved.
The COMP is discharged during shutdown and fault conditions.
Light Load Efficiency Enhancement
For PC, adaptor and TV applications, it is desirable to achieve
high efficiency at light load conditions and low standby current.
The ISL6730A, ISL6730B can enter light load efficiency mode
automatically.
The voltage error amplifier output, COMP, is an indicator of the
average input power level. The controller compares the V(COMP)
and V(SKIP). If V(COMP)-1V is less than V(SKIP)*0.25, the PFC
controller stops gate switching and the COMP pin voltage is
clamped to V(SKIP)+0.6V. ISL6730A/B use a fixed V(SKIP), which
is 1.4V; for ISL6730C/D, the SKIP function are disabled.
The controller exits skip mode when V
FB
drops to 88% (typical) of
the reference voltage or when the sensed returned current
exceeds 29µA.
Protection Circuits
Input Brownout, BO Protection
Brownout occurs when there is a drop in the line voltage. The BO
pin is a dual function pin. The BO pin detects the brownout
condition and shuts down the gate driver and controller. During
normal operation, the BO pin is used to compensate the effect of
the input line voltage change on the voltage loop. To keep the
harmonic distortion low, the corner frequency formed by the R
BO
and C
BO
should be lower than 6Hz.
The BO pin is the output of the average voltage of the rectified
voltage. The PFC controller is turned off when the BO pin drops
below 0.4V. This protects the PFC power stage to enable
operation at or below brownout condition for long periods of
time. The controller resumes operation when the BO pin returns
to 0.5V.
The BO pin is usually connected to GND through a capacitor, C
BO
.
To avoid distortion on the VIN pin, select C
BO
so that:
Overcurrent Protection
The peak current limiting function prevents the inductor from
saturation. The gate driver turns off when the current goes above
the current limit.
Overpower Protection
The overpower protection is implemented by limiting the COMP
pin voltage higher than 3.85V (typical).
Overvoltage Protection
If the voltage on the FB pin exceeds the reference voltage by about
4%, the gate driver is turned off. The controller resumes normal
operation after the FB pin drops below reference voltage.
Over-Temperature Protection
The ISL6730A, ISL6730B, ISL6730C, ISL6730D is protected
against over-temperature conditions. When the junction
temperature exceeds +160°C, the PWM shuts down. Normal
operation is resumed when the junction temperature decreases
below +135°C.
Application Guidelines
Layout Considerations
As in any high frequency switching converter, layout is very
important. Switching current from one power device to another
can generate voltage transients across the impedances of the
interconnecting bond wires and circuit traces. These
interconnecting impedances should be minimized by using wide,
short printed circuit traces. The critical components should be
located as close together as possible using ground plane
construction or single point grounding.
Figure 14 shows the critical power components; Q
1
, D and C
OUT
.
To minimize the voltage overshoot, the interconnecting wires
indicated by heavy lines should be part of the ground or the
power plane in a printed circuit board. The components shown in
Figure 14 should be located as close together as possible. Please
note that the capacitors C
VCC
and C
O
each represent numerous
physical capacitors. Locate the ISL6730A, ISL6730B, ISL6730C,
ISL6730D within 2 inches of the MOSFET, Q
1
. The circuit traces
for the MOSFETs’ gate and source connections from the
ISL6730A, ISL6730B, ISL6730C, ISL6730D must be sized to
handle up to 1.5A peak current.
Component Selection Guidelines
A 300W, universal input, PFC converter design is provided for
demonstration. The design method is for a continuous current
mode power factor correction boost converter with the
ISL6730B/D. The switching frequency is 62kHz.
FIGURE 14. CRITICAL CURRENT POWER COMPONENTS
Q
1
C
OUT
L
D
GATE
VCC
C
VCC
