These propagation delays can be several hundred nano seconds long. Using Equation 3 we can calculate the slope of the primary current, where VIN is the rectified dc line voltage, LP is the transformer primary inductance, and dt is the total propagation delay.
With a fixed propagation delay (dt in Equation 3), as VIN
increases the slope of the primary current also increases. Because the
of the propagation delay, the peak current at the maximum VIN will be higher than the peak current at the minimum VIN because of the overshoot (Figure 2).
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Figure 2. Propagation delay vs. VIN.
The result is that the input and output power increases as the input
line voltage increases. An example illustrates the problem. The peak
primary current (Equation 4
) is based on these systems requirements:
For peak current-mode control, after we calculate the peak current, we can size the current sense resistor (Equation 5
is the PWM current limit comparator voltage reference (0.5V). The peak current overshoot at the minimum input voltage is:
At the maximum input line the peak current is (Equation 6