PRM48AF480T400A00_15 Datasheet, PDF (31/44 Pages) Vicor Corporation – High Efficiency Converter

PRM48Ay480x400A00
Adaptive Loop Compensation (Adaptive Loop Operation)
A factorized power system naturally has a DC load line associated
with it since the regulator stage (PRM) is positioned before the
isolation and voltage transformation stage (VTM) Consider for a
moment a factorized power system that has the following
parameters:
n V_F = 40 V
n K_VTM=1/4
n R_{OUT_VTM} =10 mohm @ 25°C
At no load the output voltage at the load will be equal to 10 V
(V_F • K_VTM). With increasing load current, the output voltage at the
load will drop at a rate proportional to the VTMs R_OUT. It should be
noted that the R_OUT has a positive temperature coefficient and so the
DC load line changes with temperature.
If the presence of this load line is acceptable for your application,
then the PRM can be configured by way of the TRIM pin alone.
Please refer to the Trimming the Output Voltage section for details.
In this case both the AL and VT pins should be left open.
If the presence of this load line is undesirable, the load line can be
eliminated by way of the PRMs Adaptive Loop (AL) engine. The AL
engine measures the output current of the PRM and accordingly
increases the output voltage of the PRM in order to regulate the
PRMs output resistance to a fixed negative resistance, R_{LL_AL}, settable
by way of the AL pin. R_{LL_AL} should be sized to exactly cancel the
R_OUT of the VTM at 25°C. The AL engine is also able to account for
the positive temperature coefficient of R_OUT by way of its VT pin
which will be explained shortly.
Setting the Adaptive Loop Load Line (Adaptive Loop Operation)
To determine an appropriate value for the compensation slope
(R_{LL_AL}) it helps to reflect the VTM’s output resistance to the input
side of the VTM. A resistance on the output side of the VTM is scaled
by the VTMs transformer ratio (K_VTM) squared as defined
by equation (2):
( ) ^{RLLAL = ROUT_REFL =ROUT_VTM_25C •}
1²
K_VTM
(2)
Where
R_{OUT_VTM} is the VTM output resistance at 25°C
K_VTM is the VTM transformer ratio V_IN/V_OUT
PRM and VTM Output Voltage
Adaptive Loop Comensation Example
3
2
1
Increases ^{with LoPadRMtoOcoumtppuet nsate for VTM ROUT}
Compensated VTM Output
0
-1
DecUrneacsoemsewnsitahteLdoaVdTMduOe _utotpRut
Adaptive Loop
compensation brings
output into regulation
OUT
-2
-3
0
20
40
60
Load Current (%)
VTM V_OUT (Uncompensated)
PRM V_OUT
80
100
VTM V_OUT (Regulated)
Figure 27 — Adaptive Loop Compensation Illustration
For our hypothetical VTM from above (with K_VTM = 1/4 and
R_{OUT_VTM} = 10 mΩ) the output resistance reflected over to the input
would be equal to 160 mΩ. For this example, R_{LL_AL} should be set
to -160 mΩ to approximately cancel at 25°C the inherent load line
from the VTM.
R_{LL_AL} is set by the voltage difference between the AL pin and SGND
pin, V_AL, per the following formula:
R_{LL_AL} = V_AL • (-0.5) Ω/V
(3)
V_AL ≤ 3.10 V
Where V_AL is the voltage on the AL pin
V_AL is sampled by a 10-bit ADC, whose input is connected to V_{CC_INT}
through a 10 kΩ pull up resistor. This pull up disables the AL engine
when the AL pin is left open. V_AL can be actively set with a DAC that
is ground referenced to SGND. V_AL can be passively set by connecting
a resistor, R_AL, from AL to SGND such that the voltage divider made
with V_{CC_INT} and the 10 kΩ pull up yields the desired V_AL. The
formula for calculating this resistor is provided in Equation (4).
R_AL = ^{10 kΩ ∙VAL}
(4)
V_{CC_INT} – V_AL
ON/OFF
CONTROL
SGND
R_TRIM
R_AL
SGND
Vin
C_IN
GND
PRM
ENABLE
TRIM
AL
SHARE/
CONTROL NODE
VAUX
REF/
REF_EN
VT
VC
IFB
Adaptive Loop Temperature Feedback
VTM Start Up Pulse
+IN
+OUT
VF: 20 V to 55 V
L_F
C_F
–IN
SGND
–OUT
SGND
VTM
V_OUT
TM
+OUT
VC
PC
C_OUT
+IN
–IN
–OUT
PRIMARY
SECONDARY
ISOLATION BOUNDRY
SEC_GND
Figure 28 — PRM-VTM Adaptive Loop Example
PRM^TM Regulator
Page 31 of 44
Rev 1.4
11/2015
vicorpower.com
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