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SLUA110 PDF预览

SLUA110

更新时间: 2024-11-25 22:17:59
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德州仪器 - TI /
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19页 781K
描述
PRACTICAL CONSIDERATIONS IN CURRENT MODE POWER SUPPLIES

SLUA110 数据手册

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U - 1 1 1  
APPLICATION NOTE  
PRACTICAL CONSIDERATIONS IN  
CURRENT MODE POWER SUPPLIES  
Constant Output Current  
Introduction  
To maintain a constant AVERAGE current, independent of  
duty cycle, a compensating ramp is required. Lowering the  
This detailed section contains an in-depth explanation of  
the numerous PWM functions, and how to maximize their  
usefulness. It covers a multitude of practical circuit design  
considerations, such as slope compensation, gate drive  
circuitry, external control functions, synchronization, and  
paralleling current mode controlled modules. Circuit dia-  
grams and simplified equations for the above items of inter-  
est are included. Familiarity with these topics will simplify  
the design and debugging process, and will save a great  
deal of time for the power supply design engineer.  
error voltage precisely as a function of  
the pulse width sooner. This narrows the duty cycle cre-  
ating a CONSTANT output current independent of or  
will terminate  
This ramp simply compensates for the peak to aver-  
age current differences as a function of duty cycle. Output  
currents and are now identical for duty cycles  
and  
I. SLOPE COMPENSATION  
Current mode control regulates the PEAK inductor current  
via the ‘inner’ or current control loop. In a continuous mode  
(buck) converter, however, the output current is the AVER-  
AGE inductor current, composed of both an AC and DC  
component.  
While in regulation, the power supply output voltage and  
inductance are constant. Therefore,  
/
and  
dl/dT, the secondary ripple current, is also constant. In a  
constant volt-second system, dT varies as a function of  
the basis of pulse width modulation. The AC ripple  
current component, dl, varies also as a function of dT in  
accordance with the constant  
Average Current  
At high values of  
and the secondary is at its maximum. This is represented  
graphically by duty cycle D1, the corresponding average  
current II, and the ripple current d(l1). As  
its minimum at duty cycle, the ripple current also is at its  
minimum amplitude. This occurs at duty cycle D2 of aver-  
age current I2 and ripple current d(I2). Regulating the  
peak primary current (current mode control) will produce  
different AVERAGE output currents I1, and I2 for duty  
cycles D1 and D2. The average current INCREASES with  
duty cycle when the peak current is compared to a fixed  
error voltage.  
Figure 2. Constant Average Current  
Determining the Ramp Slope  
the AC current in both the primary  
Mathematically, the slope of this compensating ramp must  
be equal to one-half (50%) the downslope of the output  
inductor as seen from the control side of the circuit. This is  
proven in detail in “Modelling, Analysis and Compensating  
of the Current Mode Controller,” (Unitrode publication U-97  
and its references). Empirically, slightly higher values of  
slope compensation (75%) can be used where the AC  
component is small in comparison to the DC pedestal, typi-  
cal of a continuous converter  
decreases to  
Circuit Implementation  
In a current mode control PWM IC, the error voltage is gen-  
erated at the output of the error amplifier and compared to  
the primary current at the PWM comparator At this node,  
subtracting the compensating ramp from the error voltage,  
or adding it to the primary current sense input will have the  
same effect: to decrease the pulse width as a function of  
duty cycle (time). It is more convenient to add the slope  
compensating ramp to the current input. A portion of the  
oscillator waveform available at the timing capacitor (CT)  
will be resistively summed with the primary current. This is  
entered to the PWM comparator at the current sense input.  
Figure 1. Average Current Error  
3-106  

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