The development of high-efficiency power supplies in combination with a requirement of higher power density is a continuing goal in the field of power electronics. A power converter is a power processing circuit (or power supply) that converts an input voltage waveform into a specified output voltage waveform. The power converter generally provides a stable, well-regulated output voltage waveform. In many applications requiring a DC output, a switched-mode DC/DC converter is frequently employed. The switched-mode DC/DC converter generally includes an inverter, an input/output isolation transformer and a rectifier and filter (including a capacitor) on a secondary side of the isolation transformer. The inverter generally includes a switching device, such as a power Metal-Oxide Semiconductor Field-Effect Transistor ("MOSFET"), Insulated-Gate Bipolar Transistor ("IGBT") or a MOS-Controlled Thyristor ("MCT"), that converts the DC input voltage to an AC voltage. The input/output isolation transformer, then, transforms the AC voltage to another value and the rectifier generates the desired DC voltage at the output of the power converter. Conventionally, the rectifier includes a plurality of rectifying diodes that conduct the load current only when forward-biased in response to the input waveform to the rectifier.
One of the principal contributors to the inefficiencies associated with the switched-mode power converter is the losses associated with the turn-on and turn-off of the switching device. This is particularly true when the power converter is operated under light- or no-load conditions.
In the past, there have been attempts to reduce the losses associated with the switching devices (e.g., the power MOSFETs) by decreasing the turn-on and turn-off losses associated therewith. For instance, a number of power MOSFET gate drive schemes use an active switch between the gate and source of the power MOSFET to provide fast turn-off thereof. One type of power MOSFET gate drive scheme clamps the gate at about zero volts with respect to the source. However, this type of gate drive scheme provides a very small margin for error. Depending on the actual threshold voltage of the power MOSFET, a fluctuation in the gate-source voltage (e.g., only 1.5 volts) thereacross (induced by noise or parasitic elements in the circuit) may cause spurious turn-ons of the power MOSFET.
The noise susceptibility in the power MOSFET gate drive circuits become increasingly troublesome as the switching speed of the power MOSFET and power level of the switched-mode power converter increase. The noise susceptibility is magnified when the power MOSFET is employed with an isolating gate-drive transformer. High current switching paths and parasitic couplings superimpose spurious spikes into the gate drive circuits. Under certain conditions, noise voltage pulses in the gate drive circuit (depending on the polarity and amplitude) may induce the power MOSFET to turn on at inappropriate times. Turning on the power MOSFET at an inappropriate time may lead to a catastrophic failure in the switched-mode power converter.
Accordingly, what is needed in the art is a controller for a switching device having an isolated control terminal that substantially reduces inadvertent turn-on of the switching device.