BCM3814B60E10A5C02 Datasheet, PDF (18/38 Pages) Vicor Corporation – Isolated, Fixed-Ratio DC-DC Converter

BCM3814x60E10A5yzz
A similar exercise should be performed with the additon of a
capacitor or shunt impedance at the high voltage side of the BCM.
A switch in series with V_HI is added to the circuit. This is depicted in
Figure 18.
S
+
VHinI _–
CC
S^BA^CMC
K^K=⁼1¹/3^/62
V_VLOout
Figure 18 — BCM with HI side capacitor
A change in V_HI with the switch closed would result in a change in
capacitor current according to the following equation:
I^{c (t)} = ^{C dVdtHI}
(7)
Assume that with the capacitor charged to V_HI, the switch is
opened and the capacitor is discharged through the idealized BCM.
In this case,
Low impedance is a key requirement for powering a high-
current, low-voltage load efficiently. A switching regulation stage
should have minimal impedance while simultaneously providing
appropriate filtering for any switched current. The use of a BCM
between the regulation stage and the point of load provides a
dual benefit of scaling down series impedance leading back to
the source and scaling up shunt capacitance or energy storage
as a function of its K factor squared. However, the benefits are
not useful if the series impedance of the BCM is too high. The
impedance of the BCM must be low, i.e. well beyond the
crossover frequency of the system.
A solution for keeping the impedance of the BCM low involves
switching at a high frequency. This enables small magnetic
components because magnetizing currents remain low. Small
magnetics mean small path lengths for turns. Use of low loss
core material at high frequencies also reduces core losses.
The two main terms of power loss in the BCM module are:
n No load power dissipation (P_{HI_NL}): defined as the power
used to power up the module with an enabled powertrain
at no load.
n Resistive loss (R_LO): refers to the power loss across
the BCM module modeled as pure resistive impedance.
P_dissipated = P_{HI_NL} + P_RLO
(10)
Therefore,
I_c= I_LO ^• K
(8)
substituting Eq. (1) and (8) into Eq. (7) reveals:
I_LO =

KC^{2 •}
dV_LO
dt
(9)
The equation in terms of the LO side has yielded a K² scaling factor
for C, specified in the denominator of the equation.
A K factor less than unity results in an effectively larger capacitance
on the low voltage side when expressed in terms of the high
voltage side. With a K = 1/6 as shown in Figure 18,
C = 1µF would appear as C = 36µF when viewed from the low
voltage side.
P_{LO_OUT} = P_{HI_IN} – P_dissipated = P_{HI_IN} – P_{HI_NL} – P_RLO
(11)
The above relations can be combined to calculate the overall
module efficiency:
h
= P^LO_OUT = P^{HI_IN – PHI_NL – PRLO}
P_{HI_IN} P_{HI_IN}
(12)

_{= V}^{HI • IHI – PHI_NL – (ILO)2 • RLO}
V^{HI • IHI}
( ) ⁼ _{1 – P} ^{HI_ NL}_V+HI
(I_LO)²
• ^IHI
•
R_LO
BCM^® in a VIA Package
Page 18 of 38
Rev 1.1
05/2016
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