AN-1077 데이터 시트보기 (PDF) - International Rectifier
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AN-1077 Datasheet PDF : 20 Pages
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Application Note AN-1077
value for Rf. As such, selection of switching fre-
quency is at the discretion of the user with considera-
tion to overall converter design, with particular
consideration of EMI and efficiency requirements.
There are essentially two levels of current limita-
tion provided by the IR1150. There is a “soft” current
limit, which is essentially a duty cycle limiting fold back
type: the converter duty cycle is limited to the point
where output power is limited and the output voltage
begins to decrease.
There is also a “peak” current limit feature which
immediately terminates the present drive pulse once
the peak limit threshold, ≈ -1.0V, is exceeded.
The current sense resistor is selected based at
minimum input voltage and maximum output power in
order to guarantee normal operation under this condi-
tion.
The current amplifier has a DC gain GDC=2.5, is
internally compensated and bandwidth limited above
280kHz. The operation of the OCC control IC is based
on peak current mode, therefore the switch current
can be used in alternative to the inductor current as
an input to the ISNS pin.
The range for the current sense voltage VSNS is
between 0V and -1V, and care must be taken when
using a current transformer, to meet this range.
Figure 7 - Oscillator Frequency vs.
Programming Resistor Rf
A chart plotting Rf value vs. frequency is provided
to determine the appropriate resistor value for the
desired switching frequency.
Typical design tradeoffs relative to switching fre-
quency must be carefully considered when selecting
an optimum switching frequency for a particular con-
verter design. Some key considerations would be;
Optimized inductor size, power dissipation, and
cost EMI requirements, (EN55011, lower limit of
150kHz)
Switching loss in the power switch increase with
switching frequency
For the design example of this application note,
we will select the switching frequency to be 100kHz, a
good tradeoff between EMI performance, optimized
inductor, and power switch losses.
Current Loop and Overcurrent
Protection
The current sense pin ISNS is the input to the cur-
rent sense amplifier and the overcurrent protection
comparator.
Vm
GDCRSIS
Vm
Vm
Vm - GDCRSIS
time
VGATE
Ton
Ts
time
Figure 8 - Ramp relation with duty cycle
The current sense resistor determines the point of
soft over-current, that is the point at which input cur-
rent will be limited and the output voltage will drop.
The worst case is at low line when the current is
the highest and also the boost factor of the converter
is higher. The current sense resistor RS must be de-
signed so that at the lowest input line and largest load,
the converter will be able to maintain the output volt-
age.
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