In the past a dc power supply only needed to deliver power, now loads can return power.

The only way to cope with this new challenge is integrating an electronic load in the power supply, called a power sink. Without the sink-capability the output voltage will start rising and get out of control.

Dc Motors are more and more controlled by a pulse width modulation (PWM) circuit; the advantage is a flexible loss-less speed control.

Car makers make use of this technique to make new solutions possible for pumps, electric steering, brakes, windscreen wipers, hybrid cars and more. Also energy is conserved, this means less heat dissipation.

The special behaviour of a PWM controlled motor is the return of power during a braking action.

In Fig.1 you can see the typical load current, in phase I the motor accelerates; in phase II it has constant speed with a certain load and in phase III the motor brakes and the current becomes negative.

Normally the output circuit of a power supply is not designed to absorb current returned by the load. The only path left for the negative load current is into the output capacitor Co, so it will charge and the voltage will rise without any control from the power supply, following the formula dv/dt = i/C.

To solve this problem a power sink is added, and integrated in the voltage control of the power supply. So the output voltage is kept to the desired voltage, whether the operation mode is sink or source. Dynamically the system reacts fast. In this example the load current is switched between positive and negative (a harsh condition). On the output voltage only a slight variation is visible (upper trace). On a normal unit the voltage would rise uncontrolled.

Test Systems require a test-time as short as possible. For each new item to be tested the voltage often has to be programmed down to zero. A normal power supply has a problem because it cannot quickly discharge the output capacitor Co. A Power Sink will make it possible to do fast down-programming at light or no-load conditions.

Also for generating fast simulation voltages, like the battery voltage of a starting car (ISO7637), a power sink is indispensable. 

Delta Elektronika BV is based in Zierikzee, Netherlands, www.DeltaPowerSupplies.com