The high voltage battery in an electric vehicle is trending to 800V. To leverage the high frequency performance benefits of 650V GaN, the PSFB topology is rearchitected to the SHB. Conventional PSFB control results in ZVS, however this creates a voltage imbalance on the split input capacitors. This voltage imbalance results from the circulation current during the 1-D phase always being drawn in the same direction (into or out) of the capacitor midpoint.

Many ways can achieve midpoint voltage balance of input capacitors. Remember the basic principle for the sequence is:

• I_MID average is 0A
• I_D,RMS is the same for each FET
• ZVS is achieved for each FET

Table 2-10 shows the difference with and without midpoint voltage balance control. When input 400V, the differential voltage between top and bottom capacitor is as high as 28V, not to mention 800V application. Applying voltage balance control is necessary.

Table 2-10 Effect of Voltage Balance Control

Control Scheme	VCAP_TOP	VCAP_BOT	VCAP_DIFF
Without voltage balance control	218V	190V	28V
With voltage balance control	204V	202V	2V

Figure 2-19 to Figure 2-21 show the midpoint voltage at various input voltages. One can conclude that midpoint average voltage is very close to half of input voltage and the maximum deviation is no more than 5V.

Figure 2-19 Midpoint voltage at 400V_in

Figure 2-20 Midpoint voltage at 600V_in

Figure 2-21 Midpoint voltage at 800V_in