ISL6742 Intersil Corporation, ISL6742 Datasheet - Page 16

• Manufacturer Part Number: ISL6742
• Description: Advanced Double-Ended PWM Controller
• Manufacturer: Intersil Corporation

Figure 15, below, illustrates a master-slave current sharing
method.
In parallel and redundant applications the IShare signals
from each power supply are connected together. Each
power supply produces a voltage proportional to its average
output current on IOUT, and through limiting resistor R3, on
IShare. The unit with the highest IShare signal (and highest
output current) sources current onto the IShare Bus, and is
identified as the master unit. The units with lower IShare
signals do not source current onto IShare, and are identified
as slave units. Each slave unit compares the master’s
IShare signal with its own, and if there is sufficient difference,
turns Q1 on which pulls down on the feedback voltage.
Reducing the feedback voltage causes the output voltage to
appear low, the feedback loop compensates by increasing
the output voltage, and the output current increases. Each
slave unit will increase its output voltage until its output
current is nearly equal to that of the master.
The difference between the master’s output current and that
of a slave unit is set by R1 and R2. Some difference is
required to prevent undesirable switching of master and
slave roles. This difference also prevents operation of the
current sharing circuitry when a power supply is operating
stand alone.
The maximum output voltage that a slave can induce in its
output is controlled by R6 and the output voltage feedback
divider. Typically the maximum allowed output voltage
increase is limited to a few percent, but must be greater than
FIGURE 15. MASTER-SLAVE CURRENT SHARING USING
U2A
+
-
1
2
3
4
5
6
7
8
CS
IOUT
AVERAGE CURRENT
ISL6742
(>>R3)
U1
S&H
4x
R4
VDD
VOLTAGE ERROR AMPLIFIER
16
15
14
13
12
11
10
9
R3
R1
INVERTING (-) INPUT
DISCONNECT IF P/S FAILS
OR IS TURNED OFF
16
(>>R1)
R2
BIAS
(>>R1)
-
U2B
+
C1
R5
R6
Q1
FEEDBACK
VOLTAGE
OUTPUT
DIVIDER
ISHARE
R7
VOUT
ISL6742
the tolerance of the feedback and reference components
and any distribution drops between units. If remote sensing
is used, the adjustment range must also include the
difference in distribution drops between the power supply
outputs and the remote sensing location. The current limit
circuit must limit the voltage change to less than the output
overvoltage threshold or an overvoltage condition can be
induced.
Amplifier U2A sets the scaling factor from IOUT to IShare
and increases the current sourcing capability of IShare. U2B
is a low bandwidth amplifier that sets the frequency
response and gain of the current share circuitry. The current
share bandwidth must be much lower than the voltage
feedback loop bandwidth to ensure overall stability. The gain
is set by R1 and R5, and the bandwidth by R5 and C1.
The disconnect in series with IShare may be omitted for
power systems that do not require fault isolation. The
disconnect switch is normally implemented with MOSFET or
JFET devices.
Average Current Mode Control
The average current signal produced on IOUT may also be
used for average current mode control rather than peak
current mode control. There are many advantages to
average current mode control, most notably improved noise
immunity and greater design flexibility of the current
feedback loop compensation. Figure 16 portrays the
concept.
Instead of being compared to a peak current sense signal as
it would be in a peak current mode control configuration, the
voltage amplifier output is integrated against the average
output current. The voltage loop compensation and the
current loop compensation may be adjusted independently.
The voltage error amplifier programs the average output
current of the supply, and its maximum output level
determines the maximum output current. Either IOUT or the
voltage EA output must be scaled appropriately to achieve
the desired current limit setpoint. The offset voltage shown in
Figure 16 must be provided to compensate for input offset
voltage of the current amplifier to ensure that zero duty cycle
operation is achievable.
VERR
FIGURE 16. AVERAGE CURRENT MODE CONTROL
CURRENT ERROR
AMPLIFIER
C2
U2
+
-
R3
R4
OFFSET
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IOUT
VOLTAGE ERROR
R2
AMPLIFIER
U1
+
-
C1
VOUT
REF
July 25, 2005
FN9183.1
R1
Rb