5.1 Bootstrap Resistor Selection

Considering Figure 5-1, when the low side MOSFET (Q2, Q4, or Q6) turns on, VS pulls to GND and the bootstrap capacitor (C_B1, C_B2, and C_B3) is charged. When the high side MOSFET (Q₁, Q₃, and Q₅) is turned on, VS swings above VCC and the charge on the bootstrap capacitor (C_Bx) provides current to drive the IC high side gate driver. The first charge of C_Bx from VCC through the bootstrap resistor (R_B1, R_B2, and R_B3) and bootstrap diode (D_B1, D_B2, and D_B3) occurs when power is first applied and low side turns on the first time. At this time the charge current is the largest as, typically, C_Bx is not discharged fully at each cycle during normal operation.

A bootstrap resistor (R_Bx) is included in the bootstrap circuit to limit the inrush current that charges C_Bx when VS pulls below VCC. This inrush current is largest with the first charge. Limiting inrush current is desirable to limit noise spike on VS and COM, potentially causing shoot-through. The amplitude and length of time of the inrush current is determined mostly by the component value of R_Bx and C_Bx, as well as the VCC level. The aim in resistor selection for the application is to slow down the inrush current but have limited effect on the RC time constant of charging C_Bx.

Typically, values for R_Bx are 3Ω to 10Ω, enough to dampen the inrush current but have little effect on the V_BS turn ON. Below are some scopeshots illustrating the effect of different R_Bx values.

Table 5-1.