Mitsubishi Electric HITACHI VS-50111 Troubleshooting Manual page 22

V20 Chassis Technical Training
and Troubleshooting Guide
• Under normal load conditions, some
drive cycles to the MOSFET are dis-
abled. Output from the MOSFET is
cycled On and Off at approximately a 60
KHZ rate.
• If the load is light, even more drive cycles
to the MOSFET are disabled. A typical
On/Off rate from the MOSFET is 33
KHZ.
• Under heavy load, very few drive cycles
to the MOSFET are disabled. The
output from the MOSFET approaches
the 132 KHZ produced by the oscillator.
A Simplified Functional Block diagram of the
Standby Regulator IC is shown in Figure 2. The
internal oscillator outputs two signals, a Clock sig-
nal and a DC Max signal.
Both signals are generated as long as power is applied
to the IC. The Clock signal triggers the start of each
cycle. During the clock signal, the EN input voltage at
pin 4 is checked. The voltage at the EN input is con-
trolled by feedback from the DC output supply.
If pin 4 is 1 volt or higher, the S input of an internal FF
is driven High through the Enable AND gate. The Q
output of the FF goes High, turning On the MOSFET.
The MOSFET conducts until the drain current reaches
a predetermined limit. At his point a Current Compara-
tor outputs a High that is directed to the R input of the
FF. The Q output goes Low and the MOSFET turns
Off.
If for some reason the current limit is not detected,
the DC max oscillator signal will reset the FF and
shut off the MOSFET before the next clock pulse.
The current limit point is derived from the present
operating conditions.
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