Test engineers turn to programmable power supplies when they need to apply power to their device under test (DUT). Sometimes, the DUT has a small amount of stored energy that needs to be removed during the test. Taken to an extreme, the DUT may itself be an energy generator, whereby testing requires the continuous absorption of power (i.e., loading down the DUT).
Let’s consider the case of the DUT having a large capacitor on the main dc power input. When powering the DUT, the power supply must first charge the DUT’s large input capacitor, let’s say to 48 V. With a properly sized power supply, you can source a lot of current and drive up the voltage on the capacitor. A typical power supply can make the required voltage change of 0 to 48 V in 20 ms.
Later in the test, you want to go in the other direction and change the voltage on your DUT from 48 V back down to 10 V. The stored energy in the capacitor must go somewhere to discharge the capacitor and bring its voltage down. If no special provision is made in the power supply, the capacitor’s only discharge path is to discharge back into the DUT. If the DUT is a power-hungry device, the capacitor will discharge quickly and the voltage on the capacitor will drop until the power supply’s regulated output-voltage set point of 10 V is reached. At that moment, the energy will resume flowing from the power supply into the DUT’s capacitor to maintain the 10 V on the capacitor.
Capacitor Discharge
In most power supplies, some provision is made to discharge the capacitor. The simplest and oldest method is to use a bleed resistor. This is a fixed-value resistor that’s across the terminals of the power supply internal to the supply. The purpose of the bleed resistor is to draw down the power supply’s own output filter capacitor. However, the bleed resistor also forms a path for the energy to flow out of the DUT back into the power supply.
Bleed resistors are very simple, but they don’t work very well. The amount of current flowing in the resistor is proportional to the voltage across the resistor (which is the programmed output voltage of the power supply). So, at low voltage, the current is low and the bleed down time is lengthy. Also, they typically can’t handle a lot of current. Lastly, as a fixed value resistor, the bleed resistor doesn’t allow for any flexibility or control over the bleed-down operation.
A better approach is 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. The down-programmer rapidly draws off the energy stored in the output capacitor and can even dissipate energy stored in your DUT’s input capacitor. Effectively, it sinks the current from the capacitors back inside the power supply.