5.2 Bootstrap Capacitor Selection

The initial step in determining the value of the bootstrap capacitor is to determine the minimum voltage drop (ΔV_BS) that can be guaranteed when the high side device is turned on. In other words, the minimum gate-source voltage (V_GSmin) must be greater than the UVLO of the high side circuit, specifically V_BSUV– level. Therefore, if V_GSmin is the minimum gate-source voltage such that:

V_GSmin > V_BSUV–

Then:

$∆V_{BS}=V_{CC}-V_F-V_{GSmin}-V_X$

Where:

• V_CC is the supply voltage to the MCP14H2304
• V_F is the voltage drop across the bootstrap diode (DBS)
• V_X is the voltage drop across the MOSFET

V_X is calculated as the current seen across low side MOSFET multiplied by its R_DSON and is simply V_CE-ON at the specify output current if an I_GBT were used instead.

In addition to the voltage drops across these components, other factors that cause V_BS to drop are leakages, charge required to turn on the power devices, and duration of the high-side on time. The total charge (QT) required by the gate driver then equals:

$QT=QG+QLS+\left[I_{LKN}\right]\times t_{HON}$

Where:

• QG is the gate charge of power device
• QLS is the level shift charge required per cycle
• t_HON is the high-side ON time
• I_LKN is the sum of all leakages that include:
  • I_GSS/I_GES: Gate-source leakage of the power device
  • I_{LK_DB}: Bootstrap diode leakage
  • I_{LK_IC}: Offset supply leakage of HVIC
  • I_QBS: Quiescent current for high side supply

QLS is not listed in the datasheet. Depending on the process technology, it could range anywhere from 3-20 nC for 500V to 1200V process respectively. Assuming a value of 10 nC for Microchip’s 600V process should be sufficient with added margin.

From the basic equation, then the minimum bootstrap capacitor is calculated as:

$C_{Bmin}\ge \frac{QT}{∆V_{BS}}$