SLUA110 Datasheet, PDF (17/19 Pages) Texas Instruments – PRACTICAL CONSIDERATIONS IN CURRENT MODE POWER SUPPLIES

APPLICATION NOTE
In reality, small offets of ± 10 millivolts exist in each PWM
amplifier and comparator. As the common error voltage,
(Ve) traverses through the IC’s circuitry, its accuracy de-
creases by the number and quality of gates in its path. The
maximum error occurs at the lowest common mode ampli-
fier voltage, approximately 1 volt. The ± 20 millivolt offset
represents a ± 2% error at the PWM comparator. At higher
common mode voltages, typical of full load conditions, the
error voltage (Ve) is closer to its maximum of 4 volts. Here
the same ± 20 millivolts introduces only ± 0.5% error to the
signal.
The other input to the PWM comparator, Vr, is the voltage
developed by the primary current flowing through the cur-
rent sense resistor(s). In many applications, a 5% tolerance
resistor is utilized resulting in a ± 5% error at the PWM
comparator’s “current sense” or ramp input.
Pulse width is determined by comparing the error voltage
(Ve) with the current sense voltage, (Vr). When equal, the
primary current is therefore the error voltage divided by the
current sense resistance; Ip = Ve/Rs. Output current is
related to the primary current by the turns ratio (N) of the
transformer. Sharing of the load, or total output current is
directly proportional to the sharing of the total primary cur-
rent. The previous equations and values can be used to
determine the percentage of sharing between modules.
U-111
Primary current, Ip = Ve/Rs. Introducing the tolerances,
Ip’ = Ve (± 2%) / Rs (± 5%); therefore Ip’ = Ip (± 7%)
The primary currents (hence output currents) will share
within ± seven percent (7%) of nominal using a five per-
cent sense resistor. Clearly, the major contribution is from
the current sense circuitry, and the PWM IC offsets are
minimal. Balancing can be improved by switching to a
tighter tolerance resistor in the current sense circuitry.
The control-to-output gain (K) decreases with increasing
load. At high loads, when primary currents are high, so is
the error amplifier output voltage, (Ve). With a typical value
of four volts, the effects of the offset voltages are minimized.
This helps to promote equal sharing of the load at full
power, which is the intent behind paralleling several
modules.
For demonstration purposes, four current mode push-pull
power supplies were run in parallel at full power. The pri-
mary current of each was measured (lower traces) and
compared to a precision 1 volt reference (upper trace). The
voltage differential between traces is displayed in the
upper right hand corner of the photos. Using closely
matched sense resistors, the peak primary currents varied
from a low of 2.230A to 2.299 amps. Calculating a mean
value of 2.270 amps, the individual primary currents
shared within two percent, indicative of the sense resistor
tolerances.
Unit 1
Unit 2
Unit 3
Figure 46. Primary Currents - Parallel Operation
Unit 4
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