mic9131 Micrel Semiconductor, mic9131 Datasheet - Page 16

• Manufacturer Part Number: mic9131
• Description: Mic9131 High-voltage, High-speed Telecom Dc-to-dc Controller
• Manufacturer: Micrel Semiconductor

MIC9131
If the current sense transformer were not used, the sense
resistor would dissipate 1.7 watts.
Slope Compensation
Power supplies using peak current mode control techniques
require slope compensation when they are operating in
continuous mode and have a duty cycle greater than 50%.
Without slope compensation, the duty cycle of the power sup-
ply will alternate wide and narrow pulses commonly referred
to as subharmonic oscillations. Even though the MIC9131
operates below a 50% duty cycle, slope compensation adds
the benefits of improved transient response and greater
noise immunity in the current sense loop (especially when
the current ramp is shallow). Slope compensation can be
implemented by adding an optimum 1/2 of the inductor cur-
rent downslope, reflected back to the current sense input. In
real world applications, 2/3 of the inductor current downslope
is used to allow for component tolerances.
Slope compensation at the ISNS input may be implemented
by using a resistor and capacitor as shown in Figure 12. The
rectangular waveshape of the gate drive output is integrated
by the resistor/capacitor filter, which results in a ramp used
for the slope compensation signal. When the gate drive and
the current signal at the sense resistor goes low, the capaci-
tor is discharged to 0V.
The procedure outlined below demonstrates how to calculate
the component values.
Compute the inductor current downslope as seen at the cur-
rent sense input.
where :
M9999-080206
P
For a flyback, buck or forward mode topology the
V
V
L is the inductance of the output inductor (or the
R
M
DISS
O
D
SENSE
2 =
is the forward voltage drop of the rectifier diode
is the output voltage
=
dt
di
MIC9131
I
=
inductor downslope is equal to:
RMS
=
V
I
SENSE
Gate Drive
V
SENSE
2
O
(pin 16)
(pin 14)
×
L
+
R
I
SENSE
Figure 12
V
SNS
D
=
0 82
.
5
=
3 25
=
.
0 164
R2
C1
.
2
R1
×
0 164 1 7
Ω
.
R
SENSE
=
.
W
16
For a boost topology, the inductor downslope is:
In a transformer isolated topology, the downslope must be
reflected back to the primary by the turns ratio of the trans-
former. The reflected downslope is:
where : Ns/Np is the turns ratio of the secondary winding
The reflected downslope is multiplied by the current sense
resistor to obtain the downslope at the current sense input
pin (ISNS).
The required downslope of the compensation ramp at the
ISNS input is:
R1 is know if a value for the resistor between the current
sense resistor and the Isns pin, has already been selected.
If not chose a value of 1k, which will minimize any offset
and signal degradation at the ISNS pin. Select a value of
C1 to minimize signal degradation from the cutoff frequency
of R1/C1. The bandwidth should be at least six times the
switching frequency.
The slope of the generated compensation ramp is:
Solving for R2 and assuming R2 is much greater than R1.
where: V
M
M
M
secondary winding inductance for the flyback
topology)
M2 is the inductor current downslope
to the primary winding.
M2
reflected to the secondary side of the current
sense transformer.
I
where Rs is the value of the current sense resistor.
M
C
where: f
supply (not the oscillator frequency)
R
drive waveform
SNS SLOPE
GATE_DRIVE
1
2
2 =
2
3 =
3
REFLECTED
=
REFLECTED
=
=
_
2
I
V
dt
di
SNS SLOPE
× ×
GATE_DRIVE
V
S
GATE DRIVE
=
π π
_
is the switching frequency of the power
V
=
1
M
OUT
f
S
M
_
=
3
is the amplitude of the gate
×
2
is the inductor curent downslope
×
M
REFLECTED
R
−
×
C
2
L
1
×
V
1
0 67
×
IN
R2 R1
.
×
Np
Ns
+
R
R1
+
1
V
D
×
R
×
S
R2 C1
1
×
August 2006
Micrel, Inc.