SLUAAT9 January   2024 UCC27201A

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Start-Up Operation
  5. 2Design Considerations
  6. 3Proper Sizing of Bootstrap and VDD Capacitors
  7. 4High Power Continuous Operation
  8. 5Design Considerations
  9. 6Summary
  10. 7References

3 Proper Sizing of Bootstrap and VDD Capacitors

Correct sizing of the bootstrap capacitor and VDD capacitors is important in any application but even more so in the bidirectional DC-DC converters. The following is a quick review of the process to select the bootstrap and VDD capacitor values. The application note “Bootstrap circuitry selection for half-bridge configurations” is also a good reference on this topic.

Initially, calculate the allowable voltage drop on the bootstrap capacitor from charging the MOSFET QG.

Equation 1. ∆VHB=VDD-VF-VHBmin

Where...

  • VDD is the supply voltage of the gate driver device
  • VF is the bootstrap diode forward drop
  • VHBmin is the minimum HB bias voltage to drive the MOSFETs

Next determine the total charge needed per switching cycle from the bootstrap capacitor, normally this is dominated by the MOSFET QG.

Equation 2. QTOTAL=QG+IHBS x DMAXfSW+ IHBfSW

Where...

  • QG is the total MOSFET gate charge on the high-side driver output
  • IHBS is HB to VSS leakage current from the data sheet
  • DMAX is the converter maximum duty cycle on the high-side MOSFET
  • IHB is the HB quiescent current from the data sheet

Lastly, the target minimum bootstrap capacitor value can be determined from the following equation.

Equation 3. CBOOTmin=QTOTALΔVHB

The minimum bootstrap capacitance value is obtained from the above analysis to provide adequate bootstrap bias charge to properly drive the high-side MOSFET QG with adequate VGS. The capacitor must be a high-quality dielectric such as X7R or better and the designer needs to consider tolerance with temperature and voltage applied and add some margin.

If the design cannot accommodate the suggested timing of the starting pulses of the LI input when a phase is enabled or a low frequency LI pulse cannot be accommodated to keep the HB-HS bias capacitor charged while idle, we advise to make provisions for an external boot diode in parallel with the internal boot diode in the driver. Also, selection of a 100V half-bridge driver such as the LM5101A, UCC2720x(A), UCC27211A, UCC27301A or UCC27311A is important.

The external boot diode must be a Schottky diode with a voltage rating adequate to meet the maximum voltage on HS in the worst-case operating condition including any HS voltage overshoot. Also the external diode VF must be such that the external diode conducts the majority of the bootstrap charging current to reduce the stress on the driver internal diode. Placing the external diode close to the driver VDD and HB pins is important so a small package device is best for this.

Figure 3-1 shows the diode forward voltage curves of a good candidate for a 100V Schottky external diode which is in a small SOD123 package. Also shown in red are the driver internal boot diode VF parameters at 80mA and 100uA.

You can see the red markers of the driver internal boot diode VF is noticeably higher than the external diode which is the target. Also keep in mind the small package which is needed for close placement to the driver IC and choose a diode with high peak current capability since the boot diode can have high peak current during the initial charging pulse charging the bootstrap capacitor from 0V.

GUID-20240111-SS0I-RXZ9-WCBK-XKPVLQFZ2SKW-low.svgFigure 3-1 VF Vs IF of Example External Boot Diode