BCM3814x60E15A3yzz
A similar exercise can be performed with the addition of a
capacitor or shunt impedance at the high voltage side of the
BCM. A switch in series with VHI is added to the circuit. This is
depicted in Figure 21.
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.
S
S
BCM
SAC
V
Vout
+
–
LO
K = 1/4
C
C
K = 1/32
VVin
HI
A solution for keeping the impedance of the BCM 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.
Figure 21 — BCM with HI side capacitor
The two main terms of power loss in the BCM module are:
A change in VHI with the switch closed would result in a change in
capacitor current according to the following equation:
ꢀnNo load power dissipation (PHI_NL): defined as the power used to
power up the module with an enabled powertrain at no load.
ꢀnResistive loss (PRLO): refers to the power loss across the BCM
dVHI
module modeled as pure resistive impedance.
IC (t) = C
(7)
dt
PDISSIPATED = PHI_NL + PR
(10)
LO
Assume that with the capacitor charged to VHI, the switch is
opened and the capacitor is discharged through the idealized
BCM. In this case,
Therefore,
PLO_OUT = PHI_IN – PDISSIPATED = PHI_IN – PHI_NL – PR
(11)
LO
IC = ILO • K
(8)
The above relations can be combined to calculate the overall
module efficiency:
substituting Equation 1 and 8 into Equation 7 reveals:
C
dVLO
dt
PLO_OUT
PHI_IN
PHI_IN – PHI_NL – PR
PHI_IN
ILO(t) =
•
(9)
K2
LO
η =
=
(12)
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 = 1/4 as shown in Figure 21, C = 1µF would
2
VHI • IHI – PHI_NL – I
• RLO
( LO)
=
VHI • IHI
appear as C = 16µF when viewed from the low voltage side.
2
PHI_NL + I
• RLO
( LO)
= 1 –
( )
VHI • IHI
BCM® in a VIA Package
Page 20 of 41
Rev 2.0
02/2018
VICOR [ VICOR CORPORATION ]
