5.4 Transformer Current Measurement

Peak Current Mode control may be implemented by sensing transformer primary side current. This design has a Current Transformer (CT) that provides an isolated and fast current signal with low latency for controlling the current loop on cycle-by-cycle basis.

A series DC blocking capacitor only allows AC currents to pass through the main transformer. The CT has a 1:100 turns ratio. A total load resistor value of 22.5Ω is used to terminate the current transformer output. Designers should ensure proper power rating of termination resistors. The output signal, P_CT (TP400), is given a dynamic offset by adding a passive network connected to 3.3V.

This results in a 500 mV offset when P_CT = 0V and a 0V offset when P_CT = 3.3V. It serves as a low-cost method to provide an offset in PCMC.

Figure 5-5. Current Transformer with Load Resistor

Figure 5-6. Dynamic Offset for PCMC- Peak Current Mode Control