SNVAA94 November   2023 LM5113-Q1 , LMG1205 , LMG1210

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1 Introduction
  5. 2 Bootstrap Overcharge
  6. 3 Modeling Bootstrap Overcharge
  7. 4 Changing Bootstrap Components
  8. 5 Zener Diode Method
  9. 6 Schottky Diode Method
  10. 7 Overvoltage Clamp Method
  11. 8 Active Switch Method
  12. 9 Synchronous GaN Bootstrap Method
  13. 10Other Methods of Preventing Bootstrap Overcharge
    1. 10.1 Reducing Dead Time
    2. 10.2 Opting for a Bias Supply
    3. 10.3 Adjusting for Gate Voltage
  14. 11Summary
  15. 12References

8 Active Switch Method

The LMG1210 uses a different method to prevent bootstrap overcharge. Like LMG1205, LMG1210 uses a switch in series with the bootstrap diode path. However, LMG1210 switches on when the low-side output (LO) is high, unlike LMG1205, which switches only with overvoltage on Cboot.

GUID-20231012-SS0I-ZFSD-XKDD-HR61P1WPMDZ3-low.svg Figure 8-1 LMG1210 Functional Block Diagram Showing Series Switch Used to Prevent Bootstrap Overcharging

Bootstrap overcharging happens during the dead time because of the GaN FET third-quadrant behavior that creates large negative voltages on HS. Blocking the bootstrap diode during the dead time on every cycle prevents any possibility of overcharging. When LO is high, the dead time must be over. So, attaching the bootstrap switch state to LO consistently keeps the switch in the correct state. Figure 8-2 shows how conduction is blocked during the HS undershoot event and overcharging is prevented.

GUID-20231012-SS0I-FLWS-8D0J-XHPGQHVKZSGD-low.svg Figure 8-2 Waveform Highlighting the Bootstrap Charging Window (in Green)
GUID-20231012-SS0I-ZWKG-R9BP-6NJCTPPKD8HB-low.svg Figure 8-3 Capture Showing Bootstrap Current (blue) and Bootstrap Voltage (red) With (A) and Without (B) a Series Switch

Figure 8-3 compares the same system with and without the series switch. The system behaves correctly with the switch—where the bootstrap capacitor charges when LO is high and reaches a steady state of approximately 4.4 V. This same system overcharges without the switch past 6 V and only charges during the dead time.

One benefit of avoiding dead-time conduction is the reduction of reverse recovery of the bootstrap diode. When the bootstrap diode is permitted to conduct before HS rises, the diode builds up a significant current. The bootstrap diode has a reverse recovery event when HO is turned on and HS rises. Reference Figure 8-3, where there is nearly 1 A of reverse recovery current, as opposed to the circuit with the switch, which shows a nominal reverse current. Preventing reverse recovery events is another benefit of this active switching technique.

A downside to this active switch method is that the method does not allow productive overcharging. Therefore, the high-side GaN FET gate voltage is always VDD minus a diode drop. A lower gate voltage means the high-side GaN FET has a higher resistance and experiences more conduction losses.

The active switch method does not rely on a fixed threshold voltage and has no response time issue like the overvoltage method. Additionally, the active switch is better for supporting both 5 V and 6 V gate GaN FETs because the switch lacks a fixed threshold.