Method and circuit for recharging a bootstrap capacitor using a transfer capacitor
A circuit including and a method utilizing an improved bootstrap topology provide power to a high side (HS) driver for high efficiency applications. The improved bootstrap topology includes a transfer capacitor to store charge and to recharge a bootstrap capacitor, which provides power to the HS driver. The improved bootstrap topology also includes a resistor connected to the transfer capacitor to charge the transfer capacitor from a voltage source and to isolate the transfer capacitor from high voltage pulses.
1. A circuit comprising:
a high side driver for driving a high side transistor to couple a load output to a supply voltage;
a bootstrap capacitor coupled to a bootstrap input of the high side driver;
a transfer capacitor coupled to the bootstrap capacitor to charge the bootstrap capacitor, the transfer capacitor having a first transfer capacitor terminal and a second transfer capacitor terminal, the second transfer capacitor terminal directly connected to a low side power input of a low side driver;
a first resistor coupled to the first transfer capacitor terminal, the transfer capacitor configured to be charged from a voltage source through the first resistor, wherein the first resistor is configured to effectively decoupled the transfer capacitor from the voltage source during a voltage transient event; and
a second resistor through which the transfer capacitor is configured to charge the boot capacitor.
2. The circuit of claim 1 further comprising:
a low side transistor coupled to the load output, wherein the low side driver is coupled to the low side transistor for driving the low side transistor, wherein the transfer capacitor is connected to a low side terminal of the low side transistor.
3. The circuit of claim 1 further comprising:
a nonlinear device coupled to the bootstrap capacitor and to the transfer capacitor, wherein the transfer capacitor charges the bootstrap capacitor through the nonlinear device.
4. The circuit of claim 3 wherein the nonlinear device is a diode.
5. The circuit of claim 3 wherein the nonlinear device is a transistor.
6. The circuit of claim 3 wherein the transfer capacitor and the first resistor supplying current to the transfer capacitor protect against the application of a high voltage across the nonlinear device.
7. A method comprising:
charging a transfer capacitor, the transfer capacitor having a first transfer capacitor terminal and a second transfer capacitor terminal, the second transfer capacitor terminal directly connected to a low side power input of a low side driver;
discharging the transfer capacitor to charge a bootstrap capacitor, wherein the discharging the transfer capacitor to charge the bootstrap capacitor is performed via a nonlinear device coupled to the bootstrap capacitor and to the transfer capacitor, wherein the nonlinear device is a transistor;
supplying power from the bootstrap capacitor to a high side driver circuit;
providing a control signal from the high side driver circuit to a high side transistor; and
applying load power to a load via the high side transistor.
8. The method of claim 7 wherein the transfer capacitor is connected to a low side terminal of a low side transistor.
9. The method of claim 7 wherein a first resistor is coupled to the transfer capacitor terminal, the transfer capacitor configured to be charged from a voltage source through the first resistor, wherein the first resistor is configured to effectively decoupled the transfer capacitor from the voltage source during a voltage transient event.
10. The method of claim 7 wherein the transfer capacitor and a resistor supplying current to the transfer capacitor protect against the application of a high voltage across the nonlinear device.