AN10881
All information provided in this document is subject to legal disclaimers.
© NXP B.V. 2011. All rights reserved.
Application note
Rev. 2 — 26 September 2011
49 of 102
NXP Semiconductors
AN10881
TEA1713 resonant power supply control IC with PFC
8.4 HBC oscillator
The slope controlled oscillator determines the switching frequency of the half-bridge. The
oscillator generates a triangular waveform at the external capacitor Cf
min
.
8.4.1 Presettings
Two external components determine the frequency range:
•
Capacitor at CFMIN
Sets the minimum frequency in combination with an internally trimmed current source.
•
Resistor at RFMAX
Sets the frequency range and, in combination with CFMIN, the maximum frequency.
The oscillator frequency depends on the charge and discharge current of the capacitor on
CFMIN. This (dis)charge current consists of a fixed part which determines the minimum
frequency, and a variable part which depends on the value of the resistor on RFMAX and
the voltage at pin RFMAX.
•
The voltage on RFMAX is 0 V when the oscillator frequency is minimum.
•
The voltage on RFMAX is 2.5 V when the oscillator frequency is maximum.
•
The value of the resistor on RFMAX determines the relationship between VRFMAX
and the frequency. It also determines the maximum frequency when RFMAX = 2.5 V.
The maximum frequency of the oscillator is independent of the settings on CFMIN and
RFMAX and is limited internally to a minimum of 500 kHz.
shows the
relationship between VRFMAX and f
HB
for three different values of C
fmin
and R
fmax
.
8.4.2 Operational control
During operation, the state of the half-bridge node HB controls the oscillator is. An internal
slope detection circuit monitors the voltage on HB to achieve this.
The charge current of the oscillator is initially set to a low value of 30
A. After the start of
the half-bridge slope has been detected, the charge current is increased to the normal
value that corresponds to the working frequency at that moment. Feedback on SNSFB
controls the working frequency. Normally, the half-bridge slope starts directly after the
switch-off of the MOSFET, the time with the low oscillator current (30
A) being negligible.
Fig 27. Frequency relationships
f
HB,limit
f
max,B
V
fmax
V
RFMAX
A
curve
C
fmin
R
fmax
A
high
high
B
low
low
C
low
too low
B
C
f
max,A
f
min,
B and C
f
min,A
0
f
HB
001aal037
