2003 Sep 09
9
Philips Semiconductors
Product specification
GreenChip
II SMPS control IC
TEA1506P; TEA1506AP;
TEA1506T; TEA1506AT
Valley switching
A new cycle starts when the power MOSFET is switched
on (see Fig.9). After the ‘on-time’ (which is determined by
the ‘sense’ voltage and the internal control voltage), the
switch is opened and the secondary stroke starts. After the
secondary stroke, the drain voltage shows an oscillation
with a frequency of approximately
where L
p
is the primary self inductance of the transformer
and C
d
is the capacitance on the drain node.
As soon as the oscillator voltage is high again and the
secondary stroke has ended, the circuit waits for the
lowest drain voltage before starting a new primary stroke.
This method is called valley detection.
Figure 9 shows the drain voltage together with the valley
signal, the signal indicating the secondary stroke and the
oscillator signal.
In an optimum design, the reflected secondary voltage on
the primary side will force the drain voltage to zero. Thus,
zero voltage switching is very possible, preventing large
capacitive switching losses
and allowing high frequency operation, which results in
small and cost effective inductors.
1
2
π
×
L
p
C
d
×
(
)
×
-----------------------------------------------
P
1
2
---
C
V
2
×
f
×
×
=
handbook, full pagewidth
drain
secondary
stroke
MGU235
secondary
ringing
primary
stroke
valley
B
A
secondary
stroke
oscillator
A: Start of new cycle at lowest drain voltage.
B: Start of new cycle in a classical PWM system at high drain voltage.
Fig.9 Signals for valley switching.
Summary of Contents for SY33
Page 18: ...7ÿSchematic Diagram ...
Page 20: ......
