AUTO-SERIES Operation

Auto-series operation permits equal or proportional voltage sharing, and allows control of output voltage from one master unit. The voltage of the slaves is determined by the setting of the front panel VOLTAGE control on the master and voltage divider resistor. The master unit must be the most positive sup- ply of the series. The output CURRENT controls of all series units are operative and the current limit is equal to the lowest setting. If any output CURRENT controls are set too low, auto- matic cross over to constant current operation will occur and the output voltage will drop. Figure 12 and Figure 13 show the rear panel switch settings and terminal connections for Auto-series operation of two supplies and three supplies. This mode can also give ±voltage tracking operation of two supplies with two separate loads.

Mixed model numbers may be employed in auto-series combi- nation without restriction, provided that each slave is specified as being capable of auto-series operation. If the master supply is set up for constant current operation, then the master-slave combina- tion will act as a composite constant current source.

Total output voltage to ground must not exceed 240 Vdc.

Determining Resistors. External resistors control the fraction (or multiple) of the master unit's voltage setting that is supplied from the slave unit. Notice that the percentage of the total output volt- age contributed by each supply is independent of the magnitude of the total voltage. For two units in auto-series the ratio of R1 to R2 is

(R1+R2)/R1	= (Vo/Vm)
R2/R1	= (Vs/Vm)
Where Vo	= auto-series voltage = Vs + Vm
Vm	= master unit's output voltage
Vs	= slave unit's output voltage

For example, using the E3617A as a slave unit and putting R2=50 kΩ (1/4 watt), then from the above equations,

R1 = R2(Vm/Vs) = 50(Vm/Vs) kΩ

In order to maintain the temperature coefficient and stability perfor- mance of the supply, choose stable, low noise resistors.

It is recommended to connect a 0.1 μF capacitor in paral- lel with R2 in two supplies operation or R2 and R4 in three supplies operation to ensure the stable operation.

Setting Voltage and Current. Use the master unit's controls to set the desired output voltage and current. The VOLTAGE control of the slave unit is disabled. Turning the voltage control of the master unit will result in a continuous variation of the output of the series combination, with the contribution of the master's output voltage to that of the slave's voltage always remaining in the ratio of the external resistors. Set the CURRENT control of slave unit

above the master unit's current setting to avoid having the slave switch to CC operation.

When in CC operation the combined output current is the same as the master unit's current setting, and when in CV operation the combined output voltage is the sum of the master unit's and the slave unit's output voltages.

Overvoltage Protection. Set the OVP shutdown voltage in each unit so that it shuts down at a voltage higher than its output voltage during auto-series operation. When a master unit shuts down, it pro- grams any slave units to zero output. When a slave unit shuts down, it shuts down only itself (and any slaves below it in the stack). The master (and all slaves above the shut-down slave) continues to sup- ply output voltage.

MASTER POWER SUPPLY

MASTER

LOCAL

M/S 1

M/S 2

SENSE

OUT

-S

VREF A1 A2 A3

A4 A5

SLAVE

REMOTE

LOAD

SLAVE POWER SUPPLY

MASTER

LOCAL

M/S 1

M/S 2

SENSE

OUT

-S

VREF A1 A2 A3

A4 A5

SLAVE

REMOTE

Figure 12. Auto-Series Operation of Two Supplies

MASTER POWER SUPPLY

+ _

MASTER

LOCAL

M/S 1 M/S 2

SENSE

OUT

-S

CC VREF A1 A2

SLAVE

REMOTE

LOAD

SLAVE POWER SUPPLY(S1)

+ _

MASTER

LOCAL

M/S 1 M/S 2

SENSE

OUT

-S

CC VREF A1 A2

SLAVE

REMOTE

SLAVE POWER SUPPLY(S2)

+ _

MASTER

LOCAL

M/S 1 M/S 2

SENSE

OUT

-S

CC VREF A1 A2

SLAVE

REMOTE

Vo=Vm(1+ R2+ R2R4)

Where

Vo = Auto-Series voltage = Vm + Vs1 + Vs2

R1R3

Vm = master unit's output voltage

Vs1 = slave(S1) unit's output voltage

Vs2 = slave(S2) unit's output voltage

Figure 13. Auto-Series Operation of Three Supplies

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