Datasheet

R
DS
B
H
A
A’
V
IN
V
P
V
DS
V
IN
= V
P
+ V
DS
C
CR
1
CR
2
Q
1
Q
2
V
IN
V
OUT
R
L
C
CR
1
CR
2
Q
1
Q
2
V
IN
V
OUT
R
L
SN6501
SLLSEA0F –FEBRUARY 2012–REVISED AUGUST 2013 www.ti.com
Figure 63. 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 V
IN
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 Q
2
, which now
has turned high-impedance. The two voltage sources, each of which equaling V
IN
, appear in series and cause a
voltage potential at the open end of the primary of 2×V
IN
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 CR
1
. The secondary current starting
from the upper secondary end flows through CR
1
, charges capacitor C, and returns through the load impedance
R
L
back to the center-tap.
When Q
2
conducts, Q
1
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×V
IN
potential against ground. In this case CR
2
is forward biased while CR
1
is reverse biased and current flows from the lower secondary end through CR
2
,
charging the capacitor and returning through the load to the center-tap.
CORE MAGNETIZATION
Figure 64 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 Q
1
conducts the magnetic flux is pushed from A to A’, and when Q
2
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, V
P
, and the time, t
ON
, it is applied to the primary: B ≈ V
P
× t
ON
.
Figure 64. Core Magnetization and Self-Regulation Through Positive Temperature Coefficient of R
DS(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.
16 Submit Documentation Feedback Copyright © 2012–2013, Texas Instruments Incorporated
Product Folder Links :SN6501