AHP2805DXCH Datasheet, PDF (9/11 Pages) International Rectifier – HIGH RELIABILITY HYBRID DC/DC CONVERTERS

AHP28XXD Series
General Application Information
The AHP28XXD series of converters are capable of
providing large transient currents to user loads on demand.
Because the nominal input voltage range in this series is
relatively low, the resulting input current demands will be
correspondingly large. It is important therefore, that the line
impedance be kept very low to prevent steady state and
transient input currents from degrading the supply voltage
between the voltage source and the converter input.
In applications requiring high static currents and large
transients, it is recommended that the input leads be made
of adequate size to minimize resistive losses, and that a
good quality capacitor of approximately100µfd be connected
directly across the input terminals to assure an adequately
low impedance at the input terminals. Table 2 relates
nominal resistance values and selected wire sizes.
Table 2. Nominal Resistance of Cu Wire
Wire Size, AWG
24 Ga
22 Ga
20 Ga
18 Ga
16 Ga
14 Ga
12 Ga
Resistance per ft
25.7 mΩ
16.2 mΩ
10.1 mΩ
6.4 mΩ
4.0 mΩ
2.5 mΩ
1.6 mΩ
As an example of the effects of parasitic resistance,
consider an AHP2815D operating at full power of 100 W.
From the specification sheet, this device has a minimum
efficiency of 83% which represents an input power of more
than 120 W. If we consider the case where line voltage is
at its’ minimum of 16 volts, the steady state input current
necessary for this example will be slightly greater than 7.5
Amps.
If this device were connected to a voltage source with 10
feet of 20 gauge wire, the round trip (input and return)
would result in 0.2 Ω of resistance and 1.5 volts of drop
from the source to the converter. To assure 16 volts at
the input, a source closer to 18 volts would be required. In
applications using the paralleling option, this drop will be
multiplied by the number of paralleled devices. By choosing
14 or 16 gauge wire in this example, the parasitic resistance
and resulting voltage drop will be reduced to 25% or 31%
of that with 20 gauge wire.
Another potential problem resulting from parasitically
induced voltage drop on the input lines is with regard to
the operation of the enable 1 port. The minimum and
maximum operating levels required to operate this port
are specified with respect to the input common return line
at the converter. If a logic signal is generated with respect
to a ‘common’ that is distant from the converter, the effects
of the voltage drop over the return line must be considered
when establishing the worst case TTL switching levels.
These drops will effectively impart a shift to the logic levels.
In Figure VI, it can be seen that referred to system ground,
the voltage on the input return pin is given by
e_Rtn = I_Rtn • R_P
Therefore, the logic signal level generated in the system
must be capable of a TTL logic high plus sufficient additional
amplitude to overcome e_Rtn. When the converter is inhibited,
I_Rtn diminishes to near zero and e_Rtn will then be at system
ground.
Incorporation of a 100 µfd capacitor at the input terminals
is recommended as compensation for the dynamic effects
of the parasitic resistance of the input cable reacting with
the complex impedance of the converter input, and to
provide an energy reservoir for transient input current
requirements.
Figure VI. Problems of Parasitic Resistance in input Leads
R_p
e_source
R_p
System Ground
I_in
Vin
100
I_Rtn
µfd
e_Rtn
Rtn
Case
Enable 1
Sync Out
Sync In
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