A switched-mode power supply
(SMPS) employing peak current-mode
control (PCMC) requires a ramp signal to
slope-compensate the inner (current) control
loop. Slope compensation stabilizes
the current loop, which would otherwise
exhibit subharmonic oscillation when
operating in continuous conduction mode
with a duty factor near or above 50%.
Also, the current that should be controlled
is the average value of inductor
current, which is directly related to load
current since the output capacitor must
have zero average current.
Using PCMC, the difference between
peak and average values of inductor current
is an error that should be corrected.
Slope compensation corrects this peakto-
average inductor current error.
Slope compensation is implemented by
summing a periodic ramp with the sensed
current signal. One way to create the
ramp is to use the R-C oscillator waveform
generated by the control IC.1,2 The oscillator
is generally buffered by an emitter follower
and fed into a resistor, summing the
oscillator ramp with the sensed current
signal, which feeds another resistor. The
sum is sent to the control IC's sensed current
pin, ISNS.
This technique loads the oscillator
and introduces noise, though, which can
falsely trigger the pulse-width modulation
(PWM) control. Recent application
notes from PCMC IC makers suggest
against loading the oscillator, even with
an emitter follower buffer.3,4
The recommended way to obtain the
ramp is to integrate the MOSFET's gatedrive
output with an R-C network. The
gate-drive output is a rectangular pulse of
0 to about 10 V in amplitude. Proper
selection of R and C values integrates this
pulse and produces a ramp waveform.
When the pulse goes low, a diode discharges
the capacitor, and the ramp
returns to near zero.
This method is quite effective, but a
problem occurs if the SMPS input voltage
varies considerably. If the input ranges
from 9 to 16 V, and the control IC is driven
directly from this varying input, the slope
of the compensating ramp will vary directly
with the input voltage. This results in
overcompensation and actually increases
peak-to-average error.
The circuit in Figure 1, however, generates
a ramp whose slope is independent
of the input voltage. It does so by charging
a capacitor with a constant-current
source using the control IC's (U1's) PWM
gate drive (pin 6, OUT) as an input.
Suppose that the input voltage to the
SMPS (pin 7 on U1, whose connection has
been omitted for clarity), is at the 9-V minimum.
When the pulse is high, at around
7.5 to 8.0 V, the network R1-Q1-Q2-R2
forms a constant-current source that feeds
capacitor C1. For R1 = 220 Ω (and VBE =
0.65 V), the current is about 2.95 mA.
When fed into C1 for a time t, this constant
current produces a ramp voltage across C1
in accordance with the equation:
Q = C x V = I x t, or V = (I x t)/C
and the slope of the ramp is:
V/t = I/C
So for the example circuit, the slope is
2.95 mA/1.5 nF =1.97 V/µs.
For a switching period of 3.0 µs (333
kHz) and a duty factor of 2/3, the on-time
of the PWM pulse is 2.0 µs. Therefore, the
slope-compensating ramp's peak is 3.94
V. When the pulse goes low, diode D1 conducts
and discharges C1 to near 0 V. R1
and C1 must be chosen so the ramp's
peak voltage doesn't equal or exceed the
minimum input voltage, which is 9.0 V in
this case. Also, R3 and C1 must be chosen
so R3 doesn't load down C1.
When the input is at its maximum,
16.0 V, the ramp slope is still 1.97 V/µs,
which would not be the case when using
an R-C network. The ramp is inputted to
resistor R3, and the sensed current
waveform, ISNS_UNFLTRD, is inputted to
R4. The sum of the two appears at U1
pin 3, which is the control IC's currentsense
input. The measured waveform
showed no change when varying the
input from 9 to 16 V (Fig. 2).
References:
- The UCC38C42 Family Of High-Speed, BiCMOS Current Mode PWM Controllers - Application Note; Bill Andreycak, Texas Instruments, SLUA-257, Feb. 2002
- Practical Considerations In Current-Mode Power Supplies;
Bill Andreycak, Texas Instruments, 2001
- Ramp Compensation for the NCP1200, AND8029/D;
Christophe Basso, On Semiconductor, March 2001, Rev. 1
- Single CS3842A Provides Control for 500-W/200-kHz Current-
Mode Power Supply; CS3842AN/D, On Semiconductor,
April 2001, Rev. 1