Over Current Protection; Soft Start; Resonance Correction - Toshiba TP43H60 Service Training

Over Current Protection

Figure 10 is the schematic diagram for the over
current protection and soft start circuits. The over
current detect (OC) senses excess current in the
LC series resonant circuit. As current in the LC
series resonant circuit increases, a voltage devel-
ops at the over current protect (OC) terminal pin
10. Resistor R870 and C874 detect the current
flow through the LC circuit. Resistor R866 samples
the voltage and applies it to pin 10. Capacitor C867
is a filter to reduce ripple. Depending on the input
voltage on pin 10, the over current protect responds
in one of two ways:
(1) OC Low Threshold Voltage: +2V
When the input voltage at the OC terminal is
higher than +2V, the voltage at the soft start (Css)
terminal pin 5 lowers and the soft start engages.
By reengaging the soft start, the main oscillator fre-
quency increases, reducing the current flow through
the LC circuit. The soft start continues to operate
until the voltage on pin 10 drops below 2V.
(2) OC High Threshold Voltage: +2.5V
If the input voltage at the OC terminal exceeds
+2.5V, the oscillator frequency increases to its maxi-
mum frequency and C866 discharges rapidly.
When the Css terminal voltage decreases to 0.7V,
the circuit resets and C866 charges again. The main
oscillator frequency decreases gradually. If this
condition continues, the latch engages and oscilla-
tion stops.

Soft Start

Still referring to Figure 10. At start-up, the soft
start is engaged by capacitor C866 on pin 5, soft
start (Css) terminal. While capacitor C866 charges,
the switching frequency increases to reduce surge
current through the MOSFETs. Once C866 is fully
charged, the switching frequency goes to its nor-
mal operating frequency (approximately 70-80
kHz).
165V from D801
R872
Q801
OC
10
OC
5
C866
Over Current Protect and Soft Start

Resonance Correction

By monitoring the current flow through pin 11, The
resonance correction (RC) block prevents the
oscillator's switching frequency from dropping
below the LC resonate frequency. As the switch-
ing frequency decreases, current increases through
pin 11 of Q801. When the switching frequency
comes close to resonance, the RC block engages
the over current protect to increase the switching
frequency and move it away from resonance.
10
OSC
Control
RC
Css
11 RC
To pin 14,
Q801
C872
R866
R870
C867
Figure 10
OSC
T862
C874
C870