Active bootstrapping drivers
A circuit to enhance the driving capability of conventional inverting bootstrapping GaN drivers. When the inverting driver input is logic high and the driver output is off, the voltage stored on the first bootstrap capacitor for turning on the high side (pull-up) FET of the inverting driver is charged to the full supply voltage using an active charging FET, instead of using a diode or diode-connected FET in a conventional bootstrapping driver. The gate voltage of the active charging FET is bootstrapped to a voltage higher than supply voltage by a second bootstrap capacitor that connects to the inverting driver input, which is at a logic high. The second bootstrap capacitor is charged by an additional diode or diode-connected FET connected to the supply voltage when the inverting driver input is a logic low.
1. A bootstrapping gate driver for a high side FET having a drain terminal connected to a supply voltage, a source terminal connected to an output, and a gate terminal, the bootstrapping gate driver comprising:
an input for receiving a control signal; and
a bootstrap capacitor electrically connected between the supply voltage and ground when the control signal corresponds to a charging phase, thereby charging the capacitor, the capacitor being electrically connected to the gate terminal of the high side FET when the control signal corresponds to a driving stage;
an active charging circuit comprising: (i) a first charging FET, a diode or a diode-connected FET, and a second bootstrap capacitor for charging the first bootstrap capacitor up to the supply voltage without a diode voltage drop when the control signal corresponds to the driving stage; and (ii) a second charging FET to mitigate slow charging of the first bootstrap capacitor.
2. The bootstrapping gate driver of claim 1 , further comprising a resistor electrically connected between the supply voltage and the gate of the high side FET to decrease static current consumption.
3. The bootstrapping gate driver of claim 1 , further comprising a low side FET for turning off the high side FET, the low side FET having a drain terminal connected to the source of the high side FET and the output, a gate terminal connected to the input, and a source terminal connected to ground.
4. The bootstrapping gate driver of claim 1 , further comprising a second input and a second active charging circuit including a third active charging FET, a second diode or diode-connected FET, and a third bootstrap capacitor, the third bootstrap capacitor being connected to and controlled by a third active charging FET and a second diode or diode-connected FET, the third active charging FET being controlled in turn by the voltage on the second bootstrap capacitor.
5. The bootstrapping gate driver of claim 4 , wherein the additional stage of the active charging circuitry comprises a FET that is smaller than the high side FET, and the secondary stage comprises a FET that is smaller than the FET of active charging circuit, and wherein the bootstrap capacitor is smaller than the bootstrap capacitor of the additional stage of the active charging circuitry.
6. The bootstrapping gate driver of claim 1 , wherein the active charging circuit is cascaded such that the bootstrapping gate driver comprises at least one additional stage of the active charging circuitry.
7. The bootstrapping gate driver of claim 1 , wherein the active charging circuit is cascaded, such that the bootstrapping gate driver comprises at least one additional stage of the active charging circuitry, and the charging FET for charging the first bootstrap capacitor in each driver stage is controlled by the first bootstrap capacitor in the preceding driver stage.
8. The bootstrapping gate driver of claim 1 , wherein the FETs of the bootstrapping gate driver are gallium nitride (GaN) FETs.