BCM400x500y1K8A3z
Sine Amplitude Converter™ Point of Load Conversion
R
SEC
1.76 nH
22.6 mΩ
= 1.3 nH
IOISUECT
ROUT
LSEC_OUT_LEADS
= 6.7 nH
LPRI_IN_LEADS
+
+
CPRI_INT_ESR
21.5 mΩ
C
R SEC_INT_ESR
COUT
R
138 mΩ
1/8 • VPRI
CIN
510 µΩ
V•I
K
1/8 • ISEC
CPRI_INT
0.37 µF
CSEC_INT
25.6 µF
+
–
+
–
CIN
COUT
VSEC
VOUT
VPRI
VIN
IPRI_Q
IQ
25 mA
LPRI_INT = 0.56 µH
–
–
Figure 16 — BCM module AC model
The Sine Amplitude Converter (SAC™) uses a high frequency resonant
tank to move energy from Primary to secondary and vice versa. (The
resonant tank is formed by Cr and leakage inductance Lr in the power
transformer windings as shown in the BCM module Block Diagram).
The resonant LC tank, operated at high frequency, is amplitude
modulated as a function of input voltage and output current. A small
amount of capacitance embedded in the primary and secondary stages
of the module is sufficient for full functionality and is key to achieving
high power density.
The use of DC voltage transformation provides additional interesting
attributes. Assuming that RSEC = 0 Ω and IPRI_Q = 0 A, Eq. (3) now
becomes Eq. (1) and is essentially load independent, resistor R is now
placed in series with VIN
.
RIN
R
SAC™
The BCM400x500y1K8A3z SAC can be simplified into the preceeding
model.
SAC
VSEC
Vout
+
–
K = 1/8
K = 1/32
Vin
VPRI
At no load:
VSEC = VPRI • K
(1)
(2)
Figure 17 — K = 1/8 Sine Amplitude Converter
with series input resistor
K represents the “turns ratio” of the SAC.
Rearranging Eq (1):
The relationship between VPRI and VSEC becomes:
VSEC
K =
•
•
K
VSEC = (VPRI – IPRI RIN)
(5)
VPRI
Substituting the simplified version of Eq. (4)
(IPRI_Q is assumed = 0 A) into Eq. (5) yields:
In the presence of load, VOUT is represented by:
VSEC = VPRI • K – ISEC • RSEC
2
(3)
(4)
•
•
•
VSEC = VPRI K – ISEC RIN K
(6)
and IOUT is represented by:
I
PRI – IPRI_Q
ISEC
=
K
ROUT represents the impedance of the SAC, and is a function of the
RDSON of the input and output MOSFETs and the winding resistance of
the power transformer. IQ represents the quiescent current of the SAC
control, gate drive circuitry, and core losses.
BCM® Bus Converter
Page 19 of 25
Rev 1.4
07/2015
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