Datasheet

SLUU166A - December 2003 − Revised February 2004
7
UCC39002 Advanced Load-Share Controller User’s Guide, HPA027A
6 Design Procedure
The following is a step-by-step design procedure on how to determine the appropriate components to parallel
power modules for load sharing. The user’s guide is stated for the first module and the circuit is repeated for
each of the remaining two modules.
In order to accurately current share between power modules, specific parameters must be known:
D V
OUT
= nominal output voltage of the modules to be paralleled
D I
OUT(max)
= maximum output current of each module to be paralleled
D ∆V
ADJ(max)
= maximum voltage adjustment range of the power module to be paralleled
D N = number of modules to be paralleled
D VDD = bias voltage for the UCC39002 controllers
D The transfer function of the power modules between their positive voltage sense and power output
terminals.
6.1 Measuring the Module’s Unity Gain Crossover Frequency
Power modules usually have a very low bandwidth to ensure proper operation with a variety of loads. The
transfer function is determined using a network analyzer and injecting a small signal across a 20-Ω to 50-Ω
resistor placed between the positive sense terminal and the positive voltage output terminal as shown in
Figure 2. The resultant bode plot will show the dc gain and the unity gain crossover frequency of the module.
Expect the module’s crossover frequency to be within the range from 10 Hz to 30 kHz. The desired crossover
frequency for the load-share loop is set well before the crossover frequency of the modules. This is
accomplished by adding a zero to the compensation of the transconductance error amplifier as described in the
error amplifier section. The unity gain crossover frequency is unique to the specific module and must be
measured for each module type.
LOAD
DC−DC
Module
V
IN
50Ω
NETWORK
ANALYZER
XFRMR
+Vout
−Vout
+Sense
−Sense
Channel
A
Channel
B
Source
Out
Figure 2. Measuring the Unity Gain Crossover Frequency