DCM4623xD2K26E0y7z
Use 0 V for ∆VOUT-LL when load is above 10ꢀ of rated load. See section
on light load boosting operation for light load effects on output voltage.
Design Guidelines
Building Blocks and System Design
The DCM™ converter input accepts the full 160 to 420 V range, and it
generates an isolated trimmable 24.0 Vdc output.
Output Current Limit
The DCM features a fully operational current limit which effectively
keeps the module operating inside the Safe Operating Area (SOA) for
all valid trim and load profiles. The current limit approximates a
“brick wall” limit, where the output current is prevented from
exceeding the current limit threshold by reducing the output voltage
via the internal error amplifier reference. The current limit threshold
at nominal trim and below is typically 120ꢀ of rated output current,
but it can vary between 100ꢀ to 141ꢀ. In order to preserve the SOA,
when the converter is trimmed above the nominal output voltage,
the current limit threshold is automatically reduced to limit the
available output power.
The DCM converter provides a tightly regulated output voltage with
regulation accuracy of 1ꢀ for all line conditions and for any load
above 10ꢀ the rated load.
The DCM4623xD2K26E0y7z is designed to be used in applications
where the output power requirements are up to 500 W.
Soft Start
When the DCM starts, it will go through a soft start. The soft start
routine ramps the output voltage by modulating the internal error
amplifier reference. This causes the output voltage to approximate a
piecewise linear ramp. The output ramp finishes when the voltage
reaches either the nominal output voltage, or the trimmed output
voltage in cases where trim mode is active.
When the output current exceeds the current limit threshold, current
limit action is held off by 1ms, which permits the DCM to
momentarily deliver higher peak output currents to the load. Peak
output power during this time is still constrained by the internal
Power Limit of the module. The fast Power Limit and relatively slow
Current Limit work together to keep the module inside the SOA.
Delaying entry into current limit also permits the DCM to minimize
droop voltage for load steps.
During soft-start, the maximum load current capability is reduced.
Until Vout achieves at least VOUT-FL-THRESH, the output current must be
less than IOUT-START in order to guarantee startup. Note that this is
current available to the load, above that which is required to charge
the output capacitor.
Sustained operation in current limit is permitted, and no derating of
output power is required.
Trim Mode and Output Trim Control
When the input voltage is initially applied to a DCM, and after tINIT
elapses, the trim pin voltage VTR is sampled. The TR pin has an
internal pull up resistor to VCC, so unless external circuitry pulls the
pin voltage lower, it will pull up to VCC. If the initially sampled trim
pin voltage is higher than VTRIM-DIS, then the DCM will disable
trimming as long as the VIN remains applied. In this case, for all
subsequent operation the output voltage will be programmed to the
nominal. This minimizes the support components required for
applications that only require the nominal rated Vout, and also
provides the best output setpoint accuracy, as there are no additional
errors from external trim components
Some applications may benefit from well matched current
distribution, in which case fine tuning sharing via the trim pins
permits control over sharing. The DCM does not require this for
proper operation, due to the power limit and current limit behaviors
described here.
Current limit can reduce the output voltage to as little as the UVP
threshold (VOUT-UVP). Below this minimum output voltage
compliance level, further loading will cause the module to shut
down due to the output undervoltage fault protection.
Line Impedance, Input Slew rate and Input Stability Requirements
Connect a high-quality, low-noise power supply to the +IN and –IN
terminals. Additional capacitance may have to be added between +IN
and –IN to make up for impedances in the interconnect cables as
well as deficiencies in the source.
If at initial application of VIN, the TR pin voltage is prevented from
exceeding VTRIM-EN, then the DCM will activate trim mode, and it will
remain active for as long as VIN is applied.
VOUT set point can be calculated using the equation below:
Excessive source impedance can bring about system stability issues
for a regulated DC-DC converter, and must either be avoided or
compensated by filtering components. A 1 µF input capacitor is the
minimum recommended in case the source impedance is
insufficient to satisfy stability requirements.
VOUT-FL = 9.98 + (18.779 • VTR/VCC
)
(1)
Note that the trim mode is not changed when a DCM recovers from
any fault condition or being disabled.
Module performance is guaranteed through output voltage trim
range VOUT-TRIMMING. If VOUT is trimmed above this range, then certain
combinations of line and load transient conditions may trigger the
output OVP.
Additional information can be found in the filter design application
note:
www.vicorpower.com/documents/application_notes/vichip_appnote23.pdf
Please refer to this input filter design tool to ensure input stability:
http://app2.vicorpower.com/filterDesign/intiFilter.do.
Overall Output Voltage Transfer Function
Taking trim (equation 1) into account, the general equation relating
the DC VOUT to programmed trim (when active), load is given by:
Ensure that the input voltage slew rate is less than 1V/us, otherwise a
pre-charge circuit is required for the DCM input to control the input
voltage slew rate and prevent overstress to input stage components.
VOUT = 9.98 + (18.779 • VTR/VCC) + ∆VOUT-LL
(2)
Finally, note that when the load current is below 10ꢀ of the rated
capacity, there is an additional ∆V which may add to the output
voltage, depending on the line voltage which is related to light load
boosting. Please see the section on light load boosting below for
details.
DCM™ DC-DC Converter
Rev 1.0
Page 17 of 23
12/2017
VICOR [ VICOR CORPORATION ]
