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Disadvantages and problems of current mode

source:Industry News     Popular:adapter     release time:2021-04-22 09:01:20     Article author:sznbone    

  It can be seen from the figure that the power adapter current mode keeps the switch tube peak current constant, that is, the power adapter controls the output inductor current peak value to be constant to ensure the required DC average load current output, so as to maintain the corresponding DC output voltage and voltage error amplifier determination The output voltage values are the same.

Disadvantages and problems of current mode(图1)

  The DC load current is the average value of the output inductor current, and the constant peak value of the switch tube current only constant the peak value of the inductor current, and cannot guarantee the constant average value of the inductor current, that is, the output load current cannot be kept constant. In this way, in the above-mentioned unimproved current mode circuit, the change of the DC input voltage will cause an instantaneous change of the DC output voltage. After a short delay, this output voltage change will be adjusted by the feedback loop of the error amplifier. Because it is this voltage feedback loop that ultimately determines the output voltage.

  Supplement: This involves the problem of "the ratio of peak to average value". This problem comes from keeping the inductor peak current constant and cannot guarantee its average current constant. The reason is that the change of the duty cycle will change the average current, while the peak current remains the same. . When the duty cycle changes in a wide range, the problem of sub-harmonic instability will be caused, but it can be solved by slope compensation.

  Although the inner loop of the camera power adapter controlling the peak current can keep the inductor peak current constant, it may not necessarily provide the correct average inductor current corresponding to the output voltage, resulting in another change in the output voltage. Repeated adjustments will cause the output voltage to oscillate when the input voltage changes, and it will last for a period of time. As for the specific situation, it can be explained by the rising and falling slopes of the output inductor current in the figure.

  Problems with the current mode of the power adapter.

  (A) Output inductor current waveform under different input voltages. In current mode, the inductor peak current is constant. When the DC input voltage is the lowest, it is the largest, and the corresponding inductor average current is l; as the DC input voltage increases, the on-time will decrease to maintain a constant output. But the corresponding inductor average current L is smaller than L. Since the output voltage is related to the average value (not the peak value) of the inductor current, oscillations will be caused when the input voltage changes. m2 is the falling slope of the inductor current, m1 is the rising slope of the inductor current at low voltage input, and m1 is the rising slope of the inductor current at high voltage input.

  (B) When the duty cycle is less than 0.5, the initial inductor current disturbance l1 to the next cycle will cause a smaller l2, until it finally disappears.

  (C) When the duty cycle is greater than 0.5, the initial disturbance of the inductor current l3 to the next cycle will cause a greater disturbance L4, which will eventually decay but form an oscillation.

  The figure shows the rise and fall slopes of the current mode inductor current at two different input voltages. m2 is the falling slope: m2=dI/dt=V. /L. It can be seen that it does not change with the input voltage. When the input voltage is higher, the on-time L is shorter; when the input voltage is lower, the on-time t is longer.

  Since the peak current of the transistor is limited by the PwM comparator, the peak inductor current is constant, as shown in the figure. The voltage outer loop keeps the output voltage V unchanged, so that the DC input V of the comparator is constant. The constant V keeps the peak value of V unchanged, so that the peak current of the captive tube and output inductor is also constant.


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