SLUA110 Datasheet, PDF (2/19 Pages) Texas Instruments – PRACTICAL CONSIDERATIONS IN CURRENT MODE POWER SUPPLIES

APPLICATION NOTE
Parameters Required for
Slope Compensation Calculations
Slope compensation can be calculated after specific
parameters of the circuit are defined and calculated.
SECTION
Control
PARAMETER
T on (Max) Oscillator
AV Oscillator (PK-PK Ramp Amplitude)
I Sense Threshold (Max)
Output
V Secondary (Min)
L output
I AC Secondary
(Secondary Ripple Current)
General
R Sense (Current Sensing Resistor)
M (Amount of Slope Compensation)
N Turns Ratio (NP / Ns)
Once obtained, the calculations for slope compensation
are straightforward, using the following equations and
diagrams.
U-111
Figure 4. Simplified Circuit
Step 1. Calculate the Inductor Downslope
S(L) = di/dt = VSEd-SEC
(Amps/Second)
Step 2. Calculate the Reflected Downslope
to the Primary
S(L)’ = S(L)/N
(Amps/Second)
Step 3. Calculate Equivalent Downslope Ramp
V S(L)’ = S(L)’ l R sense
(Volts/Second)
Step 4. Calculate the Oscillator Charge Slope
V S(OSC) = d (vosc) / T on
(Volts/Second)
Step 5. Generate the Ramp Equations
Using superposition, the circuit can be illustrated as:
Figure 3. General Circuit
Resistors R1 and R2 form a voltage divider from the oscilla-
tor output to the current limit input, superimposing the
slope compensation on the primary current waveform.
Capacitor C1 is an AC coupling capacitor, and allows the
AC voltage swing of the oscillator to be used without add-
ing offset circuitry. Capacitor C2 forms an R-C filter with R1
to suppress the leading edge glitch of the primary current
wave. The ratio of resistor R2 to R1 will determine the exact
amount of slope compensation added.
For purposes of determining the resistor values, capacitors
CT (timing), Cl (coupling), and C2 (filtering) can be
removed from the circuit schematic. The oscillator voltage
(Vosc) is the peak-to-peak amplitude of the sawtooth
waveform. The simplified model is represented sche-
matically in the following circuit.
These calculations can be applied to all current mode con-
verters using a similar slope compensating scheme.
3-l07