DCM300P280x500A40
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 tFAULT. Then, the converter waits for
the internal temperature to return to below TINT-OTP before
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.
recovering. Provided the converter is still enabled, the DCM will
restart after tINIT and tON
.
Output Overvoltage Fault Protection (OVP)
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 PDTOTAL and the
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 VOUT-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 VIN-INIT
.
three surface temperatures are represented as TCASE_TOP, TCASE_BOTTOM
and TLEADS. This thermal system can now be very easily analyzed
with simple resistors, voltage sources, and a current source.
,
External Output Capacitance
The DCM converter internal compensation requires a minimum
external output capacitor. An external capacitor in the range of 200
to 2000 µF with ESR of 10 mΩ is required, per DCM for control loop
compensation purposes.
This analysis provides an estimate of heat flow through the various
pathways as well as internal temperature.
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 10% of rated
load but the output trim is held constant, the range of output
capacitor required is given by COUT-EXT-TRANS in the Electrical
Specifications table. If the load can go below 10% of rated load and
the DCM output trim is also dynamically varied, the range of output
capacitor required is given by COUT-EXT-TRANS-TRIM in the Electrical
Specifications table.
Thermal Resistance Top
MAX INTERNAL TEMP
ΦINT-TOP°C / W
Thermal Resistance Bottom
Thermal Resistance Leads
ΦINT-BOTTOM°C / W
ΦINT-LEADS°C / W
+
–
+
–
+
–
T
CASE_BOTTOM(°C)
TLEADS(°C)
TCASE_TOP(°C)
Power Dissipation (W)
Burst Mode
Under light load conditions, the DCM converter may operate in burst
mode depending on the line voltage. Burst mode 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 burst mode. When
the load is less than 10% of rated Iout, the output voltage may rise by
a maximum of 2.95 V, above the output voltage calculated from trim,
temperature, and load line conditions.
Figure 20 — Double side cooling and leads thermal model
Alternatively, equations can be written around this circuit and
analyzed algebraically:
TINT – PD1 • ΦINT-TOP = TCASE_TOP
TINT – PD2 • ΦINT-BOTTOM = TCASE_BOTTOM
TINT – PD3 • ΦINT-LEADS = TLEADS
PDTOTAL = PD1+ PD2+ PD3
Where TINT represents the internal temperature and PD1, PD2, and
PD3 represent the heat flow through the top side, bottom side, and
leads respectively.
Thermal Design
Based on the safe thermal operating area shown in page 5, the full
rated power of the DCM300P280x500A40 can be processed provided
that the top, bottom, and leads are all held below 90°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
Thermal Resistance Top
MAX INTERNAL TEMP
ΦINT-TOP°C / W
Thermal Resistance Bottom
Thermal Resistance Leads
ΦINT-BOTTOM°C / W
ΦINT-LEADS°C / W
+
–
+
–
T
CASE_BOTTOM(°C)
TLEADS(°C)
TCASE_TOP(°C)
Power Dissipation (W)
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.
Figure 21 — One side cooling and leads thermal model
DCM™ DC-DC Converter
Rev 1.1
vicorpower.com
800 927.9474
Page 20 of 25
10/2015
VICOR [ VICOR CORPORATION ]
