NBM3814X60E12A7YZZ Datasheet, PDF (18/23 Pages) Vicor Corporation – Non-Isolated, Fixed-Ratio DC-DC Converter

NBM3814x60E12A7yzz
Filter Design
A major advantage of NBM systems versus conventional PWM
converters is that the auto-transformer based NBM does not
require external filtering to function properly. The resonant LC
tank, operated at extreme high frequency, is amplitude modulated
as a function of HI side voltage and LO side current and efficiently
transfers charge through the auto-transformer. A small amount
of capacitance embedded in the HI side and LO side stages of the
module is sufficient for full functionality and is key to achieving
power density.
This paradigm shift requires system design to carefully evaluate
external filters in order to:
n Guarantee low source impedance:
To take full advantage of the NBM module’s dynamic
response, the impedance presented to its HI side terminals
must be low from DC to approximately 5MHz. The
connection of the bus converter module to its power
source should be implemented with minimal distribution
inductance. If the interconnect inductance exceeds
100nH, the HI side should be bypassed with a RC damper
to retain low source impedance and stable operation. With
an interconnect inductance of 200nH, the RC damper
may be as high as 1µF in series with 0.3Ω. A single
electrolytic or equivalent low-Q capacitor may be used in
place of the series RC bypass.
n Further reduce HI side and/or LO side voltage ripple without
sacrificing dynamic response:
Given the wide bandwidth of the module, the HI side source
response is generally the limiting factor in the overall
system response. Anomalies in the response of the HI side
source will appear at the LO side of the module multiplied by
its K factor.
n Protect the module from overvoltage transients imposed
by the system that would exceed maximum ratings and
induce stresses:
The module high/low side voltage ranges shall not be
exceeded. An internal overvoltage lockout function
prevents operation outside of the normal operating HI side
range. Even when disabled, the powertrain is exposed
to the applied voltage and power MOSFETs must
withstand it.
Total load capacitance of the NBM module shall not exceed the
specified maximum. Owing to the wide bandwidth and small LO
side impedance of the module, low-frequency bypass capacitance
and significant energy storage may be more densely and efficiently
provided by adding capacitance at the HI side of the module. At
frequencies <500kHz the module appears as an
impedance of R_LO between the source and load.
Within this frequency range, capacitance at the HI side appears as
effective capacitance on the LO side per the relationship
defined in Eq. (13).
C_{LO_EXT} =
C_{HI_EXT}
(13)
K²
This enables a reduction in the size and number of capacitors used
in a typical system.
Thermal Considerations
The VIA™ package provides effective conduction cooling from
either of the two module surfaces. Heat may be removed from the
top surface, the bottom surface or both. The extent to which these
two surfaces are cooled is a key component for determining the
maximum power that can be processed by a VIA, as can be seen
from specified thermal operating area in Figure 1. Since the VIA has
a maximum internal temperature rating, it is necessary to estimate
this internal temperature based on a system-level thermal solution.
To this purpose, 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 19 shows the “thermal circuit” for the VIA module.
R_{JC_TOP}
R_HOU
P_DISS
R_{JC_BOT}
s
+
T_{C_TOP}
–
s
–
T_{C_BOT}
+
Figure 19 — Double sided cooling VIA thermal model
In this case, the internal power dissipation is P_DISS, R_{JC_TOP} and
R_{JC_BOT} are thermal resistance characteristics of the VIA module and
the top and bottom surface temperatures are represented as T_{C_TOP},
and T_{C_BOT}. It is interesting to notice that the package itself provides
a high degree of thermal coupling between the top and bottom
case surfaces (represented in the model by the resistor R_HOU). This
feature enables two main options regarding thermal designs:
n Single side cooling: the model of Figure 19 can be simplified by
calculating the parallel resistor network and using one simple
thermal resistance number and the internal power dissipation
curves; an example for bottom side cooling only is shown in
Figure 20.
In this case, R_JC can be derived as following:
RJC =
(R_{JC_TOP} + R_HOU) • R_{JC_BOT}
R_{JC_TOP} + R_HOU + R_{JC_BOT}
(14)
NBM^™ in a VIA Package
Page 18 of 23
Rev 1.1
05/2016
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