Design How-To

# Improving peak current-mode control

Calculate slope

These propagation delays can be several hundred nano seconds long. Using **Equation 3** we can calculate the slope of the primary current, where V_{IN} is the rectified dc line voltage, L_{P} is the transformer primary inductance, and *dt* is the total propagation delay.

(3)

With a fixed propagation delay (dt in **Equation 3**), as V_{IN}
increases the slope of the primary current also increases. Because the
of the propagation delay, the peak current at the maximum V_{IN} will be higher than the peak current at the minimum V_{IN} because of the overshoot (**Figure 2**).

**Click on image to enlarge.**

*Figure 2. Propagation delay vs. V*

_{IN}.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:

(4)

For peak current-mode control, after we calculate the peak current, we can size the current sense resistor (

**Equation 5**).

(5)

V

_{CS}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**):

(6)