BCM6123XD1E1368YZZ Datasheet, PDF (25/30 Pages) Vicor Corporation – Isolated Fixed-Ratio DC-DC Converter

BCM6123xD1E1368yzz
A similar exercise can be performed with the additon of a capacitor
or shunt impedance at the primary of the BCM. A switch in series
with V_PRI is added to the circuit. This is depicted in Figure 21.
A change in V_PRI with the switch closed would result in a change in
S
+
VV_Pi_Rn_{I –}
C
B_SACM_C
K^K =⁼ 1¹/^/3³²2
V_VSoECut
Figure 21 — BCM with primary capacitor
capacitor current according to the following equation:
I_C (t)
=
C
dV_PRI
dt
(7)
Assume that with the capacitor charged to V_PRI, the switch is
opened and the capacitor is discharged through the idealized
BCM. In this case,
I_C = I_SEC • K
(8)
substituting Eq. (1) and (8) into Eq. (7) reveals:
C
I_SEC(t) = _{K 2}
•
dV_SEC
dt
(9)
The equation in terms of the secondary 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 secondary when expressed in terms of the primary. With
K = 1/32 as shown in Figure 21, C = 1µF would appear as
C = 1024µF when viewed from the secondary.
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, these benefits are
not achieved 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 SAC low involves
switching at a high frequency. This enables the use of 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 are:
nnNo load power dissipation (P_{PRI_NL}): defined as the power used to
power up the module with an enabled powertrain at no load.
nnResistive loss (P_RSEC): refers to the power loss across the BCM
modeled as pure resistive impedance.
P = P + P _DISSIPATED
PRI_NL
RSEC
(10)
Therefore,
P = P – P = P – P – P (11) _{SEC_OUT}
PRI_IN
DISSIPATED
PRI_IN
PRI_NL
RSEC
The above relations can be combined to calculate the overall
module efficiency:
η
=
P_{SEC_OUT}
P_{PRI_IN}
=
P_{PRI_IN} – P_{PRI_NL} – P_RSEC
P_{PRI_IN}
(12)
( ) ₌ V_PRI • I_PRI – P_{PRI_NL} – I_SEC ² • R_SEC
V_PRI • I_PRI
( ) ^{= 1 –}
( ) P_{PRI_NL} + I_SEC ² • R_SEC
V_PRI • I_PRI
BCM^® Bus Converter
Page 25 of 30
Rev 1.1
04/2017
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