V048K015T80 Datasheet, PDF (14/20 Pages) Vicor Corporation – VI Chip - VTM Voltage Transformation Module

Application Note (continued)
PRELIMINARY
Using the VTM with a controlled DC source
The VTM may be applied to provide a low output voltage at high
current from a controlled 48V source. The overall efficiency of a
power system based on Factorized Power Architecture, using
VTMs, exceeds the efficiency of power systems based on
Distributed Power Architecture, using DC-DC converters, or the
Intermediate Bus Architecture, using non-isolated POL converters.
The VTM has a very low output resistance, ROUT. This will cause
the output voltage to change slightly with load current unless
ROUT is compensated by a control loop.Without compensation, the
output voltage of the VTM can be expressed as follows:
VOUT = K • Vf – ROUT • ILOAD
The fast VTM transient response eliminates the need for large
amounts of capacitance at the point of load (POL). Energy
storage capacitors may be added at the input of the VTM, where
they are more effective. Capacitors used at the VTM input get
reflected at the output multiplied by (1/K)². For example, a 1 µF
capacitor placed at the input of a VTM with a K=¹/32 has the
effective energy storage of 1,000 µF at its output. Since the VTM
has a 1 MHz bandwidth, only high frequency decoupling with
ceramic capacitors is necessary at the POL, even with fast
switching, dynamic loads.
The following describes typical applications for a VTM powered
from a 48 V source, such as DC-DC converters or PRMs.
Open Loop Application
where Vf is the factorized bus input voltage to the VTM.
For the V048K015T80, this equation becomes:
VOUT = ¹/32 • Vf – 0.0013 • ILOAD
The voltage of the source may be set to a value that produces the
desired VTM output voltage at the nominal load current:
Vf = (VOUT + ROUT • INOM)/K ;
= 32 • (VOUT + 0.0013 • INOM)
The voltage of the source may be set by using trim up (Ru) or
trim down (Rd) resistors or it may be actively controlled by a
suitable voltage applied at a source voltage control node (SC).
In an open loop implementation, the VTM is connected at the
output of a DC-DC converter or controlled voltage source (Fig.
29). The no load output of the VTM is the voltage of the source
multiplied by the K factor of the VTM. The K factor of the
V048K015T80 is ¹/32; hence, at 48 Vdc from the source, the
VTM’s output is ¹/32 • 48V = 1.5 Vdc. The output of the VTM
can be set over the range of 1.0 to 1.8 Vdc, at no load, and 0.9 to
1.7 V, at full load, by controlling the Vf of the source from 32 V
to 57.6 V.
+Out
+S
DC-DC
Converter
or
SC
Voltage Source
–S
–Out
Ru
Vf
Rd
+IN
V=K•Vf – ROUT • ILOAD
+OUT
VTM
K=¹/32
LOAD
– OUT
–IN
Figure 29—Open loop operation with DC-DC converter or other controlled voltage source.
45 ^{Vicor Corporation Tel: [phone] vicorpower.com}
V•I Chip Voltage Transformation Module Rev. 1.6
Page 14 of 20