One embodiment of a prior power converter 100 that may operate in transition mode is schematically depicted in FIG. 1. In this regard, transition mode may be defined as a boundary mode between a discontinuous operating mode and continuous operating mode. The power converter 100 of FIG. 1 includes a transformer (T1) 110, two MOSFETS (M1, M2) 120 and 122, four diodes (D2, D4, D5, D6) 130-136, three capacitors (C3, C4, C5) 140-144, and a resistor (R5) 152. The power converter 100 is shown connected to a direct current voltage source 160. The transformer 110 includes primary and secondary windings (T1-1, T1-2) 110A and 110B. The MOSFETS (M1, M2) 120 and 122 may be referred to herein as the primary MOSFET (M1) 120 and the auxiliary MOSFET (M2) 122. In FIG. 1, parasitic capacitances and inherent body diode characteristics of the primary and auxiliary MOSFETS (M1, M2) 120 and 122 are not illustrated. Diodes (D4, D5, D6) 132-136 and capacitor (C5) 144 together comprise driving circuitry for auxiliary MOSFET (M2) 122 to control switching of auxiliary MOSFET (M2) 122 between an on and an off state, preferably under substantially Zero Voltage Switching (ZVS) conditions. Rectifier diode (D2) 130 and smoothing capacitor (C3) 140 together comprise secondary side rectification means.
As depicted in FIG. 1, the prior transition mode power converter 100 technology has external capacitor (C5) 144 and switch diode (D4) 132 arranged in series with the gate input capacitance of auxiliary MOSFET (M2) 122. This serial combination (C5, D4, M2 gate input capacitance) is in parallel with snubber capacitor (C4) 142 to divide the high voltage on snubber capacitor (C4) 142 into a certain level of voltage to safely drive the gate of auxiliary MOSFET (M2) 122 to an on state. MOSFET (M2) 122 can be turned off when the voltage across snubber capacitor (C4) 142 drops to zero using the other two switch diodes (D5 and D6) 134 and 136.
The divider circuit used in the prior transition mode power converter of FIG. 1 is subject to the tolerance of auxiliary MOSFET (M2)'s 122 gate input capacitance, which usually is not clearly defined and can vary by drain-source voltage as well as lot by lot and vendor by vendor. Additionally, the voltage on snubber capacitor (C4) 142 can vary significantly with the output load and the input line voltage as well. Thus, the auxiliary MOSFET (M2)'s 122 gate voltage level can have significant variation, especially if the output load range is large. Considerations such as these can make the prior power converter 100 of FIG. 1 less desirable for higher power applications.