An electrical power converter is a circuit that converts electrical power having one voltage and current characteristic into electrical power having a specified output voltage and current characteristic. In applications requiring conversion of electrical power from Direct Current to Direct Current (DC/DC power converters), switch mode DC/DC converter are frequently employed. A DC/DC converter is typically used to convert an unregulated source of voltage into a regulated source of constant voltage. A switch mode DC/DC converter can include a transformer having primary and secondary windings and a solid state power switch coupled to the primary windings that controls the energy transfer from the primary to the secondary windings. Certain switch mode DC/DC converters employ a duty cycle modulator (DCM) device that controls the switching of the power switch. The DCM device varies the duty cycle of the pulse to define the ratio of switch on time over the switching period and control the output voltage of the DC/DC converter. However, in many applications the need to increase switching frequencies results in an increase in switching losses. Therefore, DC/DC converter power designers employ a variety of schemes to eliminate or minimize losses associated with the DC/DC converters.
A forward converter is one type of DC/DC converter. The forward converter is a switch mode DC/DC converter that employs a power switch and a transformer to convert the input voltage into an output voltage. The transformer enables isolation of the input circuitry from the output circuitry. The forward converter is a common technique of converting electrical power from one DC voltage to another. The active clamp circuit is one technique for reducing power loss and voltage stress on the power transistors of this type of power converter. The active clamp circuit limits the peak voltage of the power transistor during switching cycles and facilitates the balancing of magnetic fields in the power transformer to allow for slightly smaller transformers. This allows a designer to employ lower voltage rating power transistors in the DC/DC converter. The lower voltage rating power transistors are capable of handling more current and power.
However, the active clamp circuit is not very popular in forward converters since it is difficult to drive the clamping circuit. For example, if a p-type Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) device operating in an enhancement mode is employed to limit the peak voltage of an n-type MOSFET power transistor, a second power supply is needed to provide a voltage below ground to drive the gate of the p-type MOSFET device. If an n-type MOSFET device operating in enhancement mode is employed to limit the peak voltage of a power transistor, another transformer is necessary to drive the gate of the n-type MOSFET device, so that the gate-to-source voltage of the n-type MOSFET device remains stable.
Another type of power converter is the double forward converter. The double forward converter provides two power pulses to the output within one switching cycle. Hence it is inherently more efficient than the classic forward converter and its modern derivatives. This type of power converter requires the active clamp circuit. The double forward converter includes a transformer having a single primary winding and two secondary windings for each output. A main switch is connected in series with the primary winding. The main switch is controlled by a duty cycle modulator control circuit. A clamping switch is coupled across the main switch through a capacitor. The capacitor and clamp switch are employed to automatically transfer energy stored in the transformer primary winding, while the main switch is off, back to the voltage source connected to the transformer primary winding and also to limit the peak voltage of the main switch. The clamping switch of the double forward converter is also difficult to drive without a negative power supply or additional transformer.