Driving semiconductor switches in power converters

Apparatus for converting power from an input for delivery to an output, the apparatus comprising:

• a power train including one or more input switches constructed and arranged to selectively connect a resonant circuit including a transformer to the input, and one or more output switches constructed and arranged to selectively connect the transformer to the output;
• a switch timing controller constructed and arranged to generate timing signals to operate the power train in a series of converter operating cycles;
• one or more switch drivers having an input connected to receive selected ones of the timing signals from the switch timing controller;
• wherein at least some of the input switches or the output switches, or both include a switch control terminal, a common terminal, and a switched terminal that is controlled by a respective switch driver;
• each switch driver including a first input for receiving a switch control signal from the switch controller, an output for connection to the control terminal of its respective switch, a reference terminal for connection to the common terminal of its respective switch, a second input for sensing circuit conditions at the switched terminal of its respective switch, and a power input for receiving power to operate, the switch driver including:
• a first current sink connected to sink a first current from the output in response to an OFF signal at the first input;
• a threshold detector connected to the second input and constructed and arranged to provide a ZVS signal indicating that a voltage, Vsw, across the switched terminal and the common terminal of its respective switch is at or below a predetermined threshold;
• a first current source connected to deliver a first current to the output in response to an ON signal at the first input if the ZVS signal is received from the threshold detector;
• a slope detector connected to the second input and constructed and arranged to provide a slope signal indicating the rate of change of the voltage, Vsw, across the switched terminal and the common terminal of its respective switch;
• a second current source connected to deliver a second current to the output in response to an ON signal at the first input, the second current being adjusted by the slope signal.

Apparatus for converting power from an input for delivery to output terminals, the apparatus comprising:

• a switching power conversion stage having a power train including a plurality of distributed output switches constructed and arranged to selectively conduct power from the power train to the output terminals;
• each output switch including a switch control terminal, a common terminal, and a switched terminal;
• each output switch being driven by a respective switch driver physically located proximal to said switch;
• a switch timing controller constructed and arranged to generate timing signals to operate the power train including the plurality of distributed output switches, in a series of converter operating cycles;
• a gate drive transformer having a primary winding connected to receive ON and OFF signals from the switch timing controller and a plurality of secondary windings each connected to deliver ON and OFF signals to one or more respective switch drivers;
• wherein each switch driver includes a first input connected to a respective secondary winding of the gate drive transformer to receive the ON and OFF signals from the timing controller, an output for connection to the control terminal of its respective switch, a reference terminal for connection to the common terminal of its respective switch, a second input for sensing circuit conditions at the switched terminal of its respective switch, and a power input connected to the respective secondary winding of the gate drive transformer for receiving power to operate,
• the switch driver including circuitry operative to:
• quickly turn OFF the respective switch in response to the OFF signal at the first input, turn ON the respective switch in response to an ON signal at the first input if the voltage (Vsw) across the switched and common terminals of the respective switch is below a predetermined threshold;
• and control a rate of change of voltage at the output for slowly turning ON the respective power switch in response to an ON signal at the first input if the voltage (Vsw) across the switched and common terminals of the respective switch is above the predetermined threshold.

A method of operating a power converter comprising:

• providing a plurality of power switches to operate a power train, each power switch having a respective gate, source, and drain terminals, and a drain-source voltage, Vsw;
• providing a switch timing controller constructed and arranged to provide ON and OFF switch control signals to operate the power train;
• providing a switch driver for operating a respective power switch;
• providing each switch driver a first input for receiving the switch control signals, an output for connection to the gate terminal, a common terminal for connection to the source terminal, and a second input for sensing the drain terminal, of the respective power switch;
• using the switch driver to quickly turn off the respective power switch in response to the OFF signal at the first input, turn on the respective power switch in response to an ON signal at the first input if the drain-source voltage, Vsw, of the respective power switch is below a predetermined threshold, and control a rate of change of voltage at the output for slowly turning ON the respective power switch in response to an ON signal at the first input if the drain-source voltage, Vsw, of the respective power switch is above the predetermined threshold.

A method of operating a power converter comprising:

• providing a plurality of power switches to operate a power train, including an inductive component, each power switch having a respective gate, source, and drain terminal, a drain-to-source output capacitance, a drain-source voltage, Vsw, and a transition voltage, Vtr, for the output capacitance, Coss, wherein Coss=c1 when Vsw=Vtr, Coss=c2 when Vsw=40V, and c1=2×c2;
• providing a switch timing controller constructed and arranged to provide ON and OFF switch control signals to operate the power train;
• providing a switch driver for operating a respective power switch;
• the switch driver having a first input for receiving the switch control signals, an output for connection to the gate terminal, a common terminal for connection to the source terminal, and a second input for sensing the drain terminal, of the respective power switch;
• using the switch driver to quickly turn off the respective switch in response to the OFF signal at the first input, turn on the respective switch in response to an ON signal at the first input if the drain-source voltage, Vsw, of the respective switch is below a predetermined threshold, and control a rate of change of voltage at the output for slowly turning ON the respective power switch in response to an ON signal at the first input if the drain-source voltage, Vsw, of the respective switch is above the predetermined threshold;
• and
• setting the predetermined threshold to a voltage greater than the transition voltage for the output capacitance of the respective switch, such that the respective switch is turned ON to discharge its output capacitance, Coss, at a drain-source voltage, Vsw, that approximates the transition voltage.