Valley Switching; Signals For Valley Switching - TCL SY33 Service Manual

Philips Semiconductors
GreenChip™II SMPS control IC

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 is the primary self inductance of the transformer
p
and C is the capacitance on the drain node.
d
As soon as the oscillator voltage is high again and the
secondary stroke has ended, the circuit waits for the
handbook, full pagewidth
A: Start of new cycle at lowest drain voltage.
B: Start of new cycle in a classical PWM system at high drain voltage.
2003 Sep 09
1
---------------------------------------------- -
2 π × × ( ×
L
p
primary secondary
stroke
drain
valley
secondary
stroke
oscillator
Fig.9 Signals for valley switching.
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
)
C
d
capacitive switching losses
and allowing high frequency operation, which results in
small and cost effective inductors.
secondary
stroke ringing
9
Product specification
TEA1506P; TEA1506AP;
TEA1506T; TEA1506AT

1
× ×
P = -- - C V

2
B A
MGU235

2
×
f
