The UCG28826 is a peak current mode control QR flyback converter. The converter starts by turning on the primary-side integrated GaN HEMT. The current in the transformer primary side winding I_PRI increases with a slope dependent on V_IN and primary magnetizing inductance L_M and equals V_IN/L_M. Once I_PRI reaches the peak value I_PK,PRI, the GaN HEMT turns off. By flyback action, the secondary winding voltage increases and turns on the synchronous rectifier (SR) FET body diode to clamp to output voltage V_OUT. During this time, the secondary winding current reduces from secondary peak current I_PK,SECwith a slope V_OUT/L_S, where L_Sis the secondary winding inductance. The switch node voltage is equal to the sum of V_IN and primary to secondary turns ratio N times V_OUT, called the plateau voltage. Once the secondary current reduces to zero, L_M and total switch node capacitance C_SW begin to resonate to cause magnetizing ring. The UCG28826 turns on the primary GaN HEMT at a valley in this magnetizing ring to reduce the turn-on switching losses.

Figure 6-3 Flyback converter waveforms

Hence, in UCG28826, in every cycle, the primary GaN HEMT turns off after reaching the peak current threshold and turns on next at a target valley in magnetizing ring during discontinuous mode (DCM) operation. The instantaneous primary peak current and target valley is determined by the feedback (FB) pin voltage as per the control law of Figure 6-4. Connect the FB pin to the optocoupler collector. The negative feedback loop sets FB pin voltage to the required value to support a certain V_OUT and P_OUT on the output. The device can operate in one of the four modes of operation: burst, frequency foldback, valley switching or continuous conduction mode, as described later.

Figure 6-4 UCG28826 Control Law

In the control law shown in Figure 6-4, the UCG28826 offers flexibility to select a max peak current I_PK,MAX to optimize for the switching frequency at rated load and transformer size. For each I_PK,MAX setting, a scaling ratio of 1/3rd or 1/4th is available for the minimum peak current I_PK,MIN. This I_PK,MIN value determines the switching frequency and losses in light load conditions when the flyback converter is operating in frequency foldback or burst modes. For all values of I_PK,MAX and I_PK,MIN, the slope of control law peak current vs feedback voltage remains the same, as per Equation 1. Table 6-5 shows the threshold voltages for transition between different modes and between valleys for different peak current settings of Table 6-2.