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Design IdeasJanuary 2, 1997 |
In the design of a unipolar power supply, you
usually consider only the need to deliver energy from the supply to the load.
Instances do arise, however, in which the supply may need to receive energy from
the load. Commonly, the response of the unipolar supply is to interrupt the
energy transfer, because the output voltage becomes larger than the adjusted
output value. This situation is very undesirable, because the power supply
itself is no longer controlling the output voltage. The active-preload circuit
in Figure 1 allows a unipolar supply to sink current when necessary.
The circuit essentially connects a shunt regulator, which controls the output
current of the power supply, to the output of the supply.
When the power supply sources current to the load, IC1 amplifies the positive voltage developed across RS, the sensing resistor (polarity shown in parentheses). IC1 compares the sensed signal with the negative reference voltage VR, using error amplifier IC2. The output of IC2 swings to the positive limit, turning off transistor Q1, thus disabling the circuit. The circuit is then in its inactive state and does not interfere with normal operation when the supply sources current.
When the supply tries to sink current, the voltage across the sensing resistor changes polarity. The error amplifier then swings negative, to a level that permits Q1 to sink the current coming from the load minus a very small value determined by ISR=VR/(RSK1). The closed-loop feedback of the active-preload circuit constantly maintains this small current that is absorbed either by the power supply or an added resistor. Capacitor C1 tailors the bandwidth of the circuit to a value greater than that of the power supply itself. You can modify the circuit to accommodate an npn bipolar transistor or a power MOSFET. You can also add current-limiting protection. To work with 0V output, you can return the collector of Q1 to a negative supply. (DI #1970)
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