Ibex Manufacturing, Inc.
Application Note 9
Battery Backup for PBXX-30/24AC and PBXX-50/24AC Power Supplies
This note discusses possible ways to provide battery backup for the PBXX-30/24AC and PBXX-50/24AC series of power supplies.

These supplies are normally powered by 24VAC or low-voltage DC. There are separate inputs for the two types of power. Both AC and DC may be applied to the supply at the same time - there are on-board blocking rectifiers to "wire-OR" them together. Whichever voltage is higher will supply most of the input power. The battery is normally connected to the DC input to provide current to the power supply in the event of an AC failure. The equivalent input circuit is:



With this type of circuit, when AC power is applied, it must result in a higher voltage at the filter capacitor than the battery voltage. Otherwise, the power supply continues to draw current from the battery even though AC power is active.

With a nominal AC supply of 24VRMS, the voltage at the filter capacitor can be as low as 26.4VDC under low-line and full-load conditions. Therefore, a backup battery cannot have a voltage higher than this without causing the power supply to draw current from it. The filter capacitor voltage can be measured on the circuit board at test point T3 (near the input power connector).

If a 24V battery is connected to the DC input while the battery is being charged, then the charger supplies current to the power supply as well as charging -current to the battery (normal charging voltage for a 24V SLA battery is 27.6V - 30V).

Although it may be tempting to use a higher-voltage AC source to get around this problem, with an AC voltage higher than 24VRMS (nominal), under high-line and low-load conditions, the voltage at the filter capacitor can exceed the 36VDC limit specification of the power supply.

A relay can be used to disconnect the battery while it is being charged, but this introduces a switching delay if the AC power fails.

If the switching delay of a relay is unacceptable, then, for power supply voltages higher than 15V, it may be necessary to use a charger/battery combination to power the supply and not use the AC input. The power supply is approximately 90% efficient, so the DC input power needs to be approximately 10% higher than the total output power.

An 18V SLA battery system is appropriate for backing up a 15V (or lower) power supply voltages. The battery can be charged while connected to the power supply without exceeding the 26.4V level discussed above. A 12V SLA battery system is appropriate for backing up a 9V (or lower) power supply voltages.

If it is desirable to power the supply with DC system power and still have battery backup, it is permissible to parallel the two AC input terminals and apply up-to 36VDC. Both AC input terminals must be used in parallel to allow two of the bridge rectifiers to share the current. Any DC input source is returned to the common output-voltage returns.

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