SN6501DBVT datasheet(14/29 Pages) TI1

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A’

VIN

IN = VP + VDS

CR1

CR2

VIN

VOUT

CR1

CR2

VIN

VOUT

SN6501

SLLSEA0E – FEBRUARY 2012 – REVISED JANUARY 2013

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Figure 47. Switching Cycles of a Push-Pull Converter

When Q1 conducts, VIN drives a current through the lower half of the primary to ground, thus creating a negative

voltage potential at the lower primary end with regards to the VIN potential at the center-tap.

At the same time the voltage across the upper half of the primary is such that the upper primary end is positive

with regards to the center-tap in order to maintain the previously established current flow through Q2, which now

has turned high-impedance. The two voltage sources, each of which equaling VIN, appear in series and cause a

voltage potential at the open end of the primary of 2×VIN with regards to ground.

Per dot convention the same voltage polarities that occur at the primary also occur at the secondary. The

positive potential of the upper secondary end therefore forward biases diode CR1. The secondary current starting

from the upper secondary end flows through CR1, charges capacitor C, and returns through the load impedance

RL back to the center-tap.

When Q2 conducts, Q1 goes high-impedance and the voltage polarities at the primary and secondary reverse.

Now the lower end of the primary presents the open end with a 2×VIN potential against ground. In this case CR2

is forward biased while CR1 is reverse biased and current flows from the lower secondary end through CR2,

charging the capacitor and returning through the load to the center-tap.

CORE MAGNETIZATION

Figure 48 shows the ideal magnetizing curve for a push-pull converter with B as the magnetic flux density and H

as the magnetic field strength. When Q1 conducts the magnetic flux is pushed from A to A’, and when Q2

conducts the flux is pulled back from A’ to A. The difference in flux and thus in flux density is proportional to the

product of the primary voltage, VP, and the time, tON, it is applied to the primary: B ≈ VP × tON.

Figure 48. Core Magnetization and Self-Regulation Through Positive Temperature Coefficient of RDS(on)

This volt-seconds (V-t) product is important as it determines the core magnetization during each switching cycle.

If the V-t products of both phases are not identical, an imbalance in flux density swing results with an offset from

the origin of the B-H curve. If balance is not restored, the offset increases with each following cycle and the

transformer slowly creeps toward the saturation region.

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