**3. Static Switch Design**

The static nature of an SSCB requires the use of a high-side switch capable of a continuous on-time. The most common high-side drive configuration is the bootstrap circuit as it has a low propagation delay. Furthermore, it is simple and inexpensive. However, without modifications, the bootstrap topology is incapable of the continuous on-time needed for SSCB applications [19].

The introduction of an isolated voltage bias to the bootstrap topology provides the continuous operation needed, improves isolation, and simplifies the overall circuit [20]. Several considerations for the voltage bias must be met for successful implementation, and a careful selection process should be adhered to. The bias must be isolated as the secondary voltage will swing with the voltage at the emitter of the IGBT. The converter must have high dv/dt tolerances to keep voltage spikes across the bootstrap capacitor below those that would damage the high-side driver. As the IGBT is a voltage-controlled device, the converter must also have a low voltage drift under no-load conditions, preferably under 10%, in order to maintain the desired output of the converter.

In essence, the converter's main purpose is to maintain the charge lost due to the gate operation and leakage currents on the bootstrap capacitor, and the following equations found in [19] have been modified to reflect the voltage bias and the static nature of the SSCB. The capacitor is selected based on the gate charge of the transistor and is governed by the following equations:

$$\mathbf{C}\_{\mathbb{K}} = \mathbf{Q}\_{\mathbb{K}} / \mathbf{V}\_{\text{bias}}.\tag{2}$$

$$\mathcal{L}\_{\text{boot}} \ge 10 \ast \mathcal{C}\_{\mathfrak{F}'} \tag{3}$$

where, Cg is the total gate capacitance, Qg is the gate charge of the transistor, Vbias is the voltage supplied by the converter, and Cboot is the capacitance value of the bootstrap capacitor. Likewise, the high-side gate driver source and sink requirements are dependent on Qg and are expressed as

$$\mathbf{I}\_{\text{source}} = \mathbf{1}.5 \ast \mathbf{Q}\_{\text{ $\mathcal{H}$ }} \mathbf{t}\_{\text{SW\\_con}\_{\text{ $\mathcal{H}$ }}} \tag{4}$$

$$\mathbf{I}\_{\rm sink} = \mathbf{1}.5 \,\ast \,\mathbf{Q}\_{\rm g} \mathbf{t}\_{\rm sw\\_off\\_t} \tag{5}$$

where Isource and Isink are the source and sink requirements of the high-side gate drive, respectively; 1.5 is an empirical constant; and tsw\_on/tsw\_off are the transition times experienced when switching the transistor on or <sup>o</sup>ff, respectively. Finally, a minimum gate resistance must be implemented to ensure that the source and sink currents do not exceed that of the maximum rating of the high-side gate drive and is governed by the following:

$$\text{Rg\\_min} \ge \text{Vbias/lsource/sirk}\_\prime \tag{6}$$

where Rg\_min is the minimum gate resistance.

The simplified circuit diagram utilizing an isolated 15 V 1:1 REPCOM DC/DC converter, single output FAN7371MX high-side driver, and IXXH110N65C4 IGBT is shown in Figure 4.

**Figure 4.** Simplified high-side switch design circuit capable of continuous on-time.

The designed static switch circuit is capable of operating with voltages up to a maximum of 600 V, and the gate driver can supply up to 4 A for source and sink operations. The voltage bias runs at a 1:1 conversion and operates at 15 V and 1 W. The gate charge, Qg, of the IXXH110N65C4 IGBT is 167 nC. A 0.1 uF ceramic capacitor was selected for Cboot, and a gate resistance of 4.7 ohms was implemented.
