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[FAQ] Can I connect an external bootstrap diode in parallel with the internal diode?

Other Parts Discussed in Thread: LMG1205, LM5113-Q1, LM5108

FAQ: Can I connect an external bootstrap diode in parallel with the internal diode?

Several of TI’s half-bridge gate drivers incorporate the bootstrap diode necessary to generate the high-side bias for the high side channel. Examples of these drivers include UCC2728x, UCC2720x, UCC2721x and LM5108, as well as some of our half bridge drivers for GaN FETs such as LMG1205 and LM5113-Q1. 

The integrated bootstrap diodes in TI drivers are capable of handling the peak currents observed in most typical applications. However, in applications where switching frequencies are very high, for example in excess of 1MHz, and a bootstrap capacitor with large capacitance is used, the diode’s peak forward and peak reverse currents could be very high. In such applications, it may be necessary to use an external fast recovery or schottky diode in parallel with the internal diode, or to at least make a provision for such diode on the board, to reduce power losses in the gate driver associated with the diode.

The figure below shows an example on how to connect the external diode (DBOOT).

Select a fast recovery or schottky diode that has a low forward voltage drop to reduce the power loss during charging of the bootstrap capacitor. Another key consideration is the diode’s reverse blocking voltage rating, which must be higher than the maximum switch node voltage, including transients that occur during switching.

It is also recommended to add a current-limiting resistor (RBOOT) in series with DBOOT to reduce the inrush current and limit the slew rate of VHB-HS during each switching cycle. The resistor chosen must be able to withstand the high power dissipation during the first charging sequence of the bootstrap capacitor. Follow the recommendation in the driver’s datasheet to select the appropriate resistor value.

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