Agilent Technologies E3630A Load Considerations, Pulse Loading, Reverse Current Loading

Figure 3. Current Limit Characteristic of the 6V Supply

this region. If the line voltage is maintained in the upper end of the input voltage range, however, the supply probably will operate within its specifications

Connecting Load

Each load should be connected to the power supply output terminals using separate pairs of connecting wires. This will minimize mutual coupling effects between loads and takes full advantage of the low output impedance of the supply. Load wires must be of adequately heavy gauge to maintain satis- factory regulation at the load.

Each pair of connecting wires should be as short as possible and twisted or shielded to reduce noise pick-up. If a shield is used, connect one end to the supply ground terminal and leave the other end unconnected.

If load considerations require locating output power distribu- tion terminals at a distance from the power supply, then the power supply output terminals should be connected to the remote distribution terminals by a pair of twisted or shielded wires and each load should be connected to the remote distri- bution terminals separately.

Parallel Operation

Two or more supplies can be connected in parallel to obtain a total output current greater than that available from one sup- ply. The total output current is the sum of the output currents of the individual supplies. The output voltage controls of one power supply should be set to the desired output voltage, and the other supply set for a slightly larger output voltage. The supply set to the lower output voltage will act as a constant voltage source, while the supply set to the higher output will act as a current-limited source, dropping its output voltage until it equals that of the other supply. The constant voltage source will deliver only that fraction of its rated output current

necessary to fulfill the total current demand.

Series Operation

Series operation of two or more power supplies can be accomplished up to the output isolation rating of any one sup- ply to obtain a higher voltage than that available from a single supply. Series connected supplies can be operated with one load across both supplies or with a separate load for each supply. The power supply has a reverse polarity diode con- nected across the output terminals so that if operated in series with other supplies, damage will not occur if the load is short-circuited or if one supply is turned on separately from its series partners. When this connection is used, the output volt- age is the sum of the voltages of the individual supplies. Each of the individual supplies must be adjusted in order to obtain the total output voltage.

LOAD CONSIDERATIONS

This section provides information on operating your supply with various types of loads connected to its output.

PULSE LOADING

The power supply will automatically cross over from constant- voltage to current-limit operation in response to an increase in the output current over the preset limit. Although the preset limit may be set higher than the average output current, high peak currents (as occur in pulse loading) may exceed the pre- set current limit and cause crossover to occur and degrade performance.

REVERSE CURRENT LOADING

An active load connected to the supply may actually deliver a reverse current to the supply during a portion of its operating cycle. An external source can not be allowed to pump current into the supply without risking loss of regulation and possible damage to the output capacitor of the supply. To avoid these effects, it is necessary to preload the supply with a dummy load resistor so that the supply delivers current through the entire operating cycle of the load devices.

OUTPUT CAPACITANCE

An internal capacitor across the output terminals of the supply helps to supply high-current pulses of short duration during constant-voltage operation. Any capacitance added externally will improve the pulse current capability, but will decrease the load protection provided by the current limiting circuit. A high- current pulse may damage load components before the aver- age output current is large enough to cause the current limit- ing circuit to operate.

REVERSE VOLTAGE PROTECTION

A diode is connected across the output terminals with reverse polarity. This diode protects the output electrolytic capacitors and the series regulator transistors from the effects of a reverse voltage applied across the output terminals. Since series regulator transistors can not withstand reverse voltage either, diodes are also connected across them. When operat- ing supplies in parallel, these diodes protect an unenergized supply that is in parallel with an energized supply.

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