Technique for Preventing Core Saturation in Push-Pull Converters
Original Publication Date: 1984-Jun-01
Included in the Prior Art Database: 2005-Feb-04
The unbalance of transformer flux in push-pull switching regulator is a major cause of switching transistor failures and malfunction. In standard push-pull operation, the PWM (pulse-width modulated) function provides the "ON" time that should give the right voltage to both switching transistors T1 and T2. However, symmetry is not actually achieved in terms of flux in spite of equally wide pulses, because of differences in storage times and (Vce) sat between transistors T1 and T2. Because of these two factors, pulses applied to the bases of transistors, will, in turn, result in unequal flux swings. This situation leads to a DC operating point for the flux that limits the flux swing. In bad cases, this means very different IE currents for both T1 and T2 with the transformer core driven into saturation.