Boundary mode operation reduces converter size and improves regulation
A LT8300 flyback converter turns on its internal switch immediately after the secondary side current reduces to zero and turns off when the switch current reaches the pre-defined current limit. Thus, it always operates at the transition of continuous conduction mode and discontinuous conduction mode, commonly referred to as boundary mode or critical conduction mode.
Boundary mode control is a variable frequency current mode switching scheme. When the internal power switch turns on; the transformer current increases until its preset current limit set point is reached. The voltage on the SW pin rises to the output voltage divided by the secondary-to-primary transformer turns ratio plus the input voltage. When the secondary current through the diode falls to zero, the SW pin voltage falls below VIN
. The internal DCM comparator detects this event and turns the switch back on, thus repeating the cycle.
Boundary mode returns the secondary current to zero at the end of every cycle, resulting in the parasitic resistive voltage drop not causing load regulation errors. Furthermore, the primary flyback switch is always turned on at zero current and the output diode has no reverse recovery loss. This reduction in power loss allows the flyback converter to operate at a relatively high switching frequency, which in turn reduces the transformer size when compared to lower frequency alternative designs. Figure 3
shows the SW voltage and current along with the current in the output diode.
Figure 3: Flyback converter waveforms in boundary mode
The load regulation is excellent with boundary mode operation because the reflected output voltage always samples at the diode current zero-crossing. The LT8300 typically provides better than ±2 percent load regulation as shown in Figure 4
Figure 4: Load & line regulation curves of figure 1 schematic