High-efficiency drive circuit and bidirectional FET
A drive system suitable for motors and the like employs bidirectional FETs with active gate current sourcing and sinking to eliminate series diode losses. In one embodiment, the bidirectional FETs have floating field plates that can be dynamically biased according to device polarity.
1 . An electrical power converter circuit comprising:
a set of bidirectional FET semiconductor switches each providing a semiconductor substrate having a first channel electrode and second channel electrode separated along a channel through the substrate and a gate electrode positioned along the channel between the first and second channel electrodes, where current flowing in a first polarity from the first channel electrode to the second channel electrode and current flowing in a second polarity from the second channel electrode to the first channel electrode are both controlled by a gate voltage applied to the gate electrode to switch between current flow and no current flow states; and
a gate drive circuit providing a gate drive signal to the gates of each of the bidirectional FET semiconductor switches dynamically referenced to a varying selected one of the first channel electrode and second channel electrode depending on a polarity of current flow between the first and second channel electrodes, the gate drive circuit including semiconductor switches sourcing and sinking current to and from the gates.
2 . The electrical power converter circuit of claim 1 wherein the gate drive circuit provides a switching speed in excess of 1 kHz.
3 . The electrical power converter circuit of claim 1 wherein the electrical power converter circuit provides an average output current in excess of 1 A and an average output voltage in excess of 100 V.
4 . The electrical power converter circuit of claim 1 further including a semiconductor switch drive providing a control signal to the semiconductor switches having an on-state for sourcing current to the gate by the semiconductor switches and off-state for sinking current from the gate by the semiconductor switches during each polarity of current flow.
5 . The electrical power converter circuit of claim 4 further including an electrical isolator providing galvanic isolation between the control signal and the semiconductor switches.
6 . The electrical power converter circuit of claim 4 further including a galvanically isolated DC power supply providing a voltage to the semiconductor switches.
7 . The electrical power converter circuit of claim 6 wherein the semiconductor switches provide series-connected solid-state switches communicating with the gate at their junction, with a first switch controlling current flow from the DC power supply to the gate and a second switch controlling current flow from the gate to a ground referenced to a given one of the first and second channel electrodes having a lower voltage.
8 . The electrical power converter circuit of claim 7 wherein the first switch is an NPN transistor and the second switch is a PNP transistor.
9 . The electrical power converter circuit of claim 7 further including a diode half-bridge providing series-connected diodes having at their junction the ground reference with a first diode connected between the ground reference and a first channel electrode communicating with a cathode side of the first diode, and a second diode connected between the ground reference and a second channel electrode communicating with a cathode side of the second diode.
10 . The electrical power converter circuit of claim 1 wherein the bidirectional FET semiconductor switches further include at least one field plate positioned over the channel to at least one side of the gate and free from a fixed galvanic connection to any of the gate electrode and the first and second channel electrodes.
11 . The electrical power converter circuit of claim 10 wherein the electrical power converter circuit provides a biasing switch connecting the at least one field plate to a given one of the first and second channel electrodes having a lower voltage.