Here’s a common situation: When testing your device, you need the voltage on the output of the power supply to quickly change from 0 V to 48 V. You pull out a trusted power supply and set it up to do the test. You know your power supply can quickly slew up from 0 V to 48 V. Even though your device has a capacitor on its input, the power supply can source a lot of current and drive up the voltage on the capacitor. Happily, you can make the required voltage change in 20 ms.
Now, you want to go in the other direction and change the voltage on your device from 48 V back down to 0 V. In this case, though, your trusted power supply can’t quickly change its voltage downward. Why? The capacitor on your device is charged up to 48 V, and the output filter capacitor in the power supply is charged up to 48 V. Even though the power supply tries to change its output voltage, the energy stored in those capacitors has nowhere to go. The result is that it takes 200 ms for the output to creep back down.
Rise And Fall Time Specs
You can observe this behavior in the specifications of many power supplies (see the table). These fall time specifications are clearly saying that the stored energy in the system needs to be dissipated but has nowhere to go.
When your device is drawing full current (i.e., full load) out of the power supply, then the energy is drawn away through your device and the fall time is reduced. When your device is drawing light current (i.e., 10% load or no load), the electrons stored in the capacitors can’t get out fast enough, resulting in an excruciatingly slow change in voltage (see the figure).
To improve fall times, power supply designers have employed bleed resistors in their power supply output stages. These bleed resistors provide a path for the current to flow back into the power supply to dissipate the energy stored in its output capacitor and maybe even the energy stored in your device’s input capacitor. However, simple current bleed resistors could not provide a fast down-programming result.
More advanced power supply designs utilize a circuit called an active down-programmer. Rather than a simple bleed resistor, the down-programmer is a transistor circuit that acts like an electronic load built into the power supply. The active down-programmer is turned on when the power supply needs to lower its output voltage, It rapidly draws off the energy stored in the output capacitor and can even dissipate energy stored in your device’s input capacitor. Effectively, it sinks the current from the capacitors back inside the power supply.
Of course, the down-programmer circuit has its limits. As an electronic load inside the power supply, it must dissipate the energy as heat. This means the down-programmer circuit gets hot, needs a heatsink, and adds to the size and weight of the power supply. The added heat within the power supply means it generally has larger fans to manage the heat. Active down-programmers may be limited in the amount of current they can draw (typically 10% of rated power supply output current), or they may be limited to the total energy they can draw (stated in terms of duty cycle of down-programming time to up-programming time).
Benefits Of Active Down-Programmers
Advanced power supplies equipped with down-programmers can provide equally fast rise and fall times and thus be tens to hundreds of times faster than a power supply without an active down-programmer. Note that a bipolar amplifier/power supply also can sink current back into the power supply. That’s why bipolar amplifier/power supplies are viewed as really fast power supplies and used in applications where the voltage on the device under test needs to change rapidly.
Once you have a power supply with a down-programmer, it can quickly up-program from V1 to V2 volts and also quickly down-program from V2 to V1 volts. If your device needs fast slew voltage to operate properly, this is an important capability. You can find advanced power supplies that can slew up and down in 20 ms or faster, with some able to achieve sub-millisecond rise and fall times.
Beyond a single up- or down-programming step, a power supply with fast rise and fall times can be used to generate waveforms. The waveforms would be limited to only positive voltage and would not be able to swing between +V1 and –V1 to create an ac waveform, such as a sine wave. However, a waveform limited to unipolar voltages (i.e., always positive) is still quite useful as a power waveform generated to apply input voltage transients on your device.
For example, if you were testing automotive electronics, the disturbances on the 12-V main voltage of the car during ignition could be simulated to test if your device would reset when the car starts. Rise and fall times in the millisecond or sub-millisecond range translate into waveform bandwidths of several kilohertz. While not arbitrary waveform generator speeds, these kilohertz waveforms have power behind them (equal to the wattage of the power supply) unlike the few milliwatts you might get out of a function generator.