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9

AHP28XXD Series

General Application Information

Table 2. Nominal Resistance of Cu Wire

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.

Wire Size, AWG

Resistance per ft

24 Ga

25.7 m

Ω

22 Ga

16.2 m

Ω

20 Ga

10.1 m

Ω

18 Ga

6.4 m

Ω

16 Ga

4.0 m

Ω

14 Ga

2.5 m

Ω

12 Ga

1.6 m

Ω

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

Vin

Rtn

Case

Enable 1

Sync Out

Sync In

R

p

R

p

I

Rtn

I

in

e

source

System Ground

e

Rtn

100

µfd

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

Therefore, the logic signal level generated in the system

must be capable of a TTL logic high plus sufficient additional

amplitude to overcome e

I

ground.

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 approximately100

µfd 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.

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.