DCM4623XD2K53EYZZ Datasheet, PDF (20/25 Pages) Vicor Corporation – Fully operational current limit

temperature fault is registered, the powertrain immediately stops
switching, the output voltage of the converter falls, and the converter
remains disabled for at least time t_FAULT. Then, the converter waits for
the internal temperature to return to below T_INT-OTP before
recovering. Provided the converter is still enabled, the DCM will
restart after t_INIT and t_ON.
Output Overvoltage Fault Protection (OVP)
The converter monitors the output voltage during each switching
cycle by a corresponding voltage reflected to the primary side control
circuitry. If the primary sensed output voltage exceeds V_OUT-OVP, the
OVP fault protection is triggered. The control logic disables the
powertrain, and the output voltage of the converter falls.
This type of fault is latched, and the converter will not start again
until the latch is cleared. Clearing the fault latch is achieved by either
disabling the converter via the EN pin, or else by removing the input
power such that the input voltage falls below V_IN-INIT.
External Output Capacitance
The DCM converter internal compensation requires a minimum
external output capacitor. An external capacitor in the range of 220
to 2200 µF with ESR of 10 mΩ is required, per DCM for control loop
compensation purposes.
However some DCM models require an increase to the minimum
external output capacitor value in certain loading and trim
condition. In applications where the load can go below 25% of rated
load but the output trim is held constant, the range of output
capacitor required is given by C_{OUT-EXT-TRANS} in the Electrical
Specifications table. If the load can go below 25% of rated load and
the DCM output trim is also dynamically varied, the range of output
capacitor required is given by C_{OUT-EXT-TRANS-TRIM} in the Electrical
Specifications table.
Light Load Boosting
Under light load conditions, the DCM converter may operate in light
load boosting depending on the line voltage. Light load boosting
occurs whenever the internal power consumption of the converter
combined with the external output load is less than the minimum
power transfer per switching cycle. In order to maintain regulation,
the error amplifier will switch the powertrain off and on repeatedly,
to effectively lower the average switching frequency, and permit
operation with no external load. During the time when the power
train is off, the module internal consumption is significantly
reduced, and so there is a notable reduction in no-load input power
in light load boosting. When the load is less than 25% of rated Iout,
the output voltage may rise by a maximum of 5.05 V, above the
output voltage calculated from trim, temperature, and load line
conditions.
Thermal Design
Based on the safe thermal operating area shown in page 5, the full
rated power of the DCM4623xD2K53Eyzz can be processed provided
that the top, bottom, and leads are all held below 79°C. These curves
highlight the benefits of dual sided thermal management, but also
demonstrate the flexibility of the Vicor ChiP platform for customers
who are limited to cooling only the top or the
bottom surface.
The OTP sensor is located on the top side of the internal PCB
structure. Therefore in order to ensure effective over-temperature
fault protection, the case bottom temperature must be constrained
by the thermal solution such that it does not exceed the temperature
of the case top.
DCM™ DC-DC Converter
Page 20 of 25
Rev 1.1
01/2017
DCM4623xD2K53Eyzz
The ChiP package provides a high degree of flexibility in that it
presents three pathways to remove heat from internal power
dissipating components. Heat may be removed from the top surface,
the bottom surface and the leads. The extent to which these three
surfaces are cooled is a key component for determining the
maximum power that is available from a ChiP, as can be seen from
Figure 20.
Since the ChiP has a maximum internal temperature rating, it is
necessary to estimate this internal temperature based on a real
thermal solution. Given that there are three pathways to remove heat
from the ChiP, it is helpful to simplify the thermal solution into a
roughly equivalent circuit where power dissipation is modeled as a
current source, isothermal surface temperatures are represented as
voltage sources and the thermal resistances are represented as
resistors. Figure 20 shows the "thermal circuit" for a 4623 ChiP DCM,
in an application where both case top and case bottom, and leads are
cooled. In this case, the DCM power dissipation is PD_TOTAL and the
three surface temperatures are represented as T_{CASE_TOP}, T_{CASE_BOTTOM},
and T_LEADS. This thermal system can now be very easily analyzed
with simple resistors, voltage sources, and a current source.
This analysis provides an estimate of heat flow through the various
pathways as well as internal temperature.
Power Dissipation (W)
Thermal Resistance Top
ΦINT-TOP°C / W
MAX INTERNAL TEMP
Thermal Resistance Bottom
ΦINT-BOTTOM°C / W
T_{CASE_BOTTOM}(°C)
+
–
Thermal Resistance Leads
ΦINT-LEADS°C / W
T_LEADS(°C)
+
–
T_{CASE_TOP}(°C)
+
–
Figure 20 — Double side cooling and leads thermal model
Alternatively, equations can be written around this circuit and
analyzed algebraically:
T_INT – PD₁ • Φ_INT-TOP = T_{CASE_TOP}
T_INT – PD₂ • Φ_INT-BOTTOM = T_{CASE_BOTTOM}
T_INT – PD₃ • Φ_INT-LEADS = T_LEADS
PD_TOTAL = PD₁+ PD₂+ PD₃
Where T_INT represents the internal temperature and PD₁, PD₂, and
PD₃ represent the heat flow through the top side, bottom side, and
leads respectively.
Power Dissipation (W)
Thermal Resistance Top
ΦINT-TOP°C / W
Thermal Resistance Bottom
ΦINT-BOTTOM°C / W
T_{CASE_BOTTOM}(°C)
MAX INTERNAL TEMP
Thermal Resistance Leads
ΦINT-LEADS°C / W
T_LEADS(°C)
+
–
T_{CASE_TOP}(°C)
+
–
Figure 21 — One side cooling and leads thermal model
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