Power Supplies
The design of the isolated, unregulated power source to the AMC1305 analog supply input closely follows
the
TIPD121 Design Reference Guide,
0-5 A, Single-Supply, 2 kV Isolated Current Sensing Solution
.
The
transformer driver is used because it is designed for low-power, push-pull converters with
input voltages in the range of 3 V to 5.5 V; such voltage ranges fit well within the AMC1305 controller-side
supply range. Three important components in the dc-dc converter are the isolation transformer, the
rectifier diode, and the linear regulator.
4.1
Transformer Selection
To prevent the isolation transformer from saturating, its volt-seconds (V-t) product must be greater than
the maximum volt-seconds product applied by the SN6501. The maximum voltage delivered by the
SN6501 is the nominal converter input plus a 10% margin. The maximum time this voltage is applied to
the primary is half the period of the lowest frequency at the specified input voltage. The minimum
switching frequency of the SN6501 at 5-V operation is 300 kHz. Therefore, the transformer minimum V-t
product under these conditions, as determined by equations (1) and (2) in the
, is 9.1
V
μ
s. The specified V-t product of the isolation transformer selected (DA2304) is well above this 9.1-V
μ
s
requirement.
When searching for a suitable transformer, the minimum turns ratio required must be determined; such a
ratio allows the push-pull converter to operate over the specified current and temperature range. The
minimum turns ratio required can be expressed through the ratio of secondary to primary voltage
multiplied by a correction factor that takes into account the transformer typical efficiency. Equations (3)
through (8) in the
show the specific requirements for determining the minimum turns
ratio for a given application. The DA2304 has a 1:2.2 turns ratio; such a ratio produces an unregulated,
open-circuit voltage output well within the input range of the low-dropout regulator used in this design.
4.2
Rectifier Diode Selection
The chosen rectifier diode must possess low forward voltage to provide as much voltage to the converter
output as possible. When used in high-frequency switching applications, the rectifier must also possess a
short recovery time. Schottky diodes meet both of these requirements. The MBR0520L with a typical
forward voltage of approximately 200 mV at 8-mA forward current is used in this design.
illustrates the forward voltage versus forward current characteristics of the MBR0520L diode.
5
SBAU237 – September 2014
AMC1305xxEVM User's Guide
Copyright © 2014, Texas Instruments Incorporated
