19
MNL107-900-000D
4.7
CLOSING THE LOOP
In order to provide a clean output with very good regulation, two feedback loops are used. One
feedback loop is a fast loop that takes the IGBT output and closes the loop to the error amplifier.
This loop corrects for switching effects on a pulse-by-pulse basis. The second feedback loop
corrects for regulation. It is a slow loop that takes the output at the output transformer and
converts it from RMS to DC to compare to and modify the reference. Current feedback from the
current sense transformer is used in another loop to effectively limit the output current. The output
of this transformer is converted from RMS to DC and is used to reduce the reference once a
preset limit is reached. The later action causes that output voltage to reduce if the current
attempts to increase further.
4.8 CURRENT
LIMIT
There are two distinct current limits:
Overload-
The overload limit uses the RMS/DC converter output of the output current sense
transformer. When the DC voltage exceeds 2.5V (approximately 120% of nominal), the reference
gets overridden and the output goes into constant current mode. The voltage decreases to force
the unit to hold the current at 120% of nominal. The RMS/DC converter has a filter with an
equivalent 200 millisecond delay. This means that as long as the RMS output current does not
exceed 120% of nominal for more than 200 milliseconds, the output will not be affected.
Short Circuit Protection-
Short circuit protection is a circuit that protects the unit and load from
short circuits of greater than 300% of nominal current. The IGBT drivers have internal circuitry to
sense de-saturation of the IGBT. De-saturation is caused by excessive output current. The driver
over-current outputs are used to set a latch. The latch is used to inhibit the PWM signal to the
IGBT drivers, which immediately disables the output inverter. Power must be cycled to reset this
type of overload.
Note: A “constant current” overload may cause the “SYSTEM” status light to flash or grow dim.
A short circuit overload will cause the “SYSTEM” to go off and stay off until reset.
4.9 OVER
TEMPERATURE
The unit contains a temperature sensor that is located on the IGBT heat sink. If an excessive
temperature is sensed, the sensor output which is “OR-ed” with the over current latch signal,
turns off the output until the temperature has decreased to an acceptable limit.
4.10 HOUSEKEEPING
SUPPLIES
Three small, fly-back based DC/DC switching power supplies generate all the DC voltages
needed to run the internal circuits. The DC to DC converter assembly has a small
A
housekeeper
@
that runs from the input DC and provides control power to the converter. Another low power
DC/DC convertor provides standby power for the front panel control board (107-920 -00X). This
converter takes power either from the AC or DC inputs to supply monitoring circuits at times.
An additional 30W fly-back supply located on the output inverter board (107-752-000) handles the
rest of the operating power including the cooling fan. This supply runs from the bulk 150Vdc
power.
