Rainbow Electronics MAX13256 - Manual (Page 14)

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MAX13256
36V H-Bridge Transformer
Driver for Isolated Supplies
Isolated RS-485/RS-232 Data Interfaces
The MAX13256 provides power for multiple transceivers
in isolated RS-485/RS-232 data interface applications.
The 300mA output current capability of the MAX13256
allows multiple RS-485/RS-232 transceivers to operate
simultaneously.
PCB Layout Guidelines
As with all power-supply circuits, careful PCB lay-
out is important to achieve low switching losses and
stable operation. For thermal performance, connect the
exposed pad to a solid copper ground plane.
The traces from ST1 and ST2 to the transformer must be
low resistance and inductance paths. Place the trans-
former as close as possible to the MAX13256 using short,
wide traces.
When the device is operating with the internal oscillator,
it is possible for high-frequency switching components
on ST1 and ST2 to couple into the CLK circuitry through
PCB parasitic capacitance. This capacitive coupling can
induce duty-cycle errors in the oscillator, resulting in a DC
current through the transformer. To ensure proper opera-
tion, ensure that CLK has a solid ground connection.
Exposed Pad
Ensure that the exposed pad has a solid connection to
the ground plane for best thermal performance. Failure
to provide a low thermal impedance path to the ground
plane results in excessive junction temperatures when
delivering maximum output power.
Component Selection
Transformer Selection
Transformer selection for the MAX13256 can be simpli-
fied by the use of a design metric, the ET product. The
ET product relates the maximum allowable magnetic flux
density in a transformer core to the voltage across a wind-
ing and switching period. Inductor magnetizing current in
the primary winding changes linearly with time during the
switching period of the device. Transformer manufactur-
ers specify a minimum ET product for each transformer.
The transformer’s ET product must be larger than:
ET = V
DD
/(2 x f
SW
)
where f
SW
is the minimum switching frequency of the
ST1/ST2 outputs (255kHz (min)) when the internal oscil-
lator is used or one-half of the clock frequency when an
external clock source is used.
Choose a transformer with sufficient ET product in the
primary winding to ensure that the transformer does not
saturate during operation. Saturation of the magnetic
core results in significantly reduced inductance of the
primary, and therefore a large increase in current flow.
This can cause the current limit to be reached even when
the load is not high.
For example, when the internal oscillator is used to drive
the H-bridge, the required transformer ET product for an
application with V
DD
(max) = 36V is 70.6VFs. An applica-
tion with V
DD
(max) = 8.8V has a transformer ET product
requirement of 17.3VFs.
In addition to the constraint on ET product, choose a
transformer with a low DC-winding resistance. Power
dissipation of the transformer due to the copper loss is
approximated as:
P
D_TX
= I
LOAD2
x (R
PRI
/N 2 + R
SEC
)
where R
PRI
is the DC winding resistance of the primary,
and R
SEC
is the DC winding resistance of the second-
ary. In most cases, an optimum is reached when R
SEC
=
R
PRI
/N 2 . For this condition, the power dissipation is equal
for the primary and secondary windings.
As with all power-supply designs, it is important to opti-
mize efficiency. In designs incorporating small trans-
formers, the possibility of thermal runaway makes low
transformer efficiencies problematic. Transformer losses
produce a temperature rise that reduces the efficiency of
the transformer. The lower efficiency, in turn, produces
an even larger temperature rise.
To ensure that the transformer meets these require-
ments under all operating conditions, the design should
focus on the worst-case conditions. The most stringent
demands on ET product arise for minimum input volt-
age, switching frequency, and maximum temperature
and load current. Additionally, the worst-case values for
transformer and rectifier losses should be considered.
The primary should be a single winding; however, the
secondary can be center-tapped, depending on the
desired rectifier topology. In most applications, the phas-
ing between primary and secondary windings is not sig-
nificant. Half-wave rectification architectures are possible
with the MAX13256; however, these are discouraged.
If a net DC current results due to an imbalanced load,
the average magnetic flux in the core is increased. This
reduces the effective ET product and can lead to satura-
tion of the transformer core.