Showing posts with label voltage response time. Show all posts
Showing posts with label voltage response time. Show all posts

Monday, March 23, 2015

Use slew rate control to cleanly power up and reduce peak inrush current of your DUTs

Previously on Watt’s Up? a colleague wrote about how the current limit setting affects a power supply’s voltage response time (click here to review). In this posting he clearly shows how a low current limit setting can greatly slow down the output voltage turn on response time when powering up your DUT.

While this is generally true and good advice, especially for basic performance power supplies, there are additional things to consider when working with high performance power supplies models, as you will see.

Many basic performance power supplies tend to have larger output filter capacitors in order to achieve lower output noise performance. A disadvantage of having a large output capacitor is that it slows down the output voltage response speed of the power supply. Basic performance power supplies can have turn on response times on the order of a 100 milliseconds.

High performance power supplies operate by a somewhat different set of rules. In comparison to basic performance power supplies they typically have much smaller output capacitors and they are designed to have output turn on and turn off response times on the order of a millisecond or less.

However, absolute fastest is not always the best and that is why fast, high performance power supplies also usually incorporate an output voltage slew rate control as well. This allows you to optimize the output turn on and turn off speed for your particular application. This lets you take advantage of the faster output speed you have available, without it being overkill and cause other problems.

The two most common problems that arise when powering up and powering down many DUTs are related to charging and discharging the input filter capacitor incorporated into them. They are:
  • High peak inrush (and discharge) currents due to the high dV/dt slew rate being applied
  • Power supply CC-CV mode cross over issues resulting from the high peak inrush current

To illustrate, the turn on characteristic of our N6762A power supply was captured when powering up a load consisting of a 1,200 microfarad capacitor in parallel with a 10 ohm resistor. The N6762A was set to 10 volts and its voltage slew rate set to maximum.  This was captured using the N6762A’s digitizing voltage and current readback together with the 14585A software, shown in Figure 1.

Figure 1: N6762A power supply turn on response set to maximum slew rate into parallel RC load

The vertical markers have been placed at zero and maximum voltage points of the turn on ramp. The peak inrush current reaches 3.7 amps and the peak voltage overshoots to 11.06 volts, 10% over the 10 volt setting. The overshoot is a result of the power supply crossing over into current limit during the ramp up and allowing the voltage to rise to 11.06 volts before the voltage control loop regains control to bring the output back down to 10 volts. It also takes a little while for the voltage to settle after the peak overshoot. Both the overshoot voltage and peak inrush current can be problems when powering up a DUT. These occur as a result of having too fast of a voltage slew rate when powering the DUT.

To address the problem we then set the N6762A’s slew rate to a more acceptable value of 2,000 volts/second. The turn on voltage and current were again captured and are shown in Figure 2. As can be seen the voltage overshoot is eliminated and the inrush current has been reduced to a more moderate 3.3 amps.

Figure 2: N6762A power supply turn on response set to 2,000 V/s slew rate into parallel RC load

So in closing high performance power supplies have a significant advantage in their output response speed, in comparison to basic power supplies. And while faster is usually better, absolute fastest may not be best, and this applies to the output response time of power supplies as well! But by having the ability to set the output slew rate on high performance power supplies gives you the ability to optimize its speed for your given application, providing for the best possible outcome possible!


Tuesday, June 25, 2013

Current limit setting affects voltage response time

The current limit setting in a power supply is primarily used to protect the device under test (DUT) from excessive current. You should set your current limit setting higher than the maximum amount of current you expect your DUT to draw, but low enough so that if your DUT fails as a short or low impedance, it does not draw an amount of current that can damage wires, connectors, or the DUT itself due to excessive current. The power supply will limit the current at the current limit setting and reduce the voltage accordingly. If you want, you can turn on over-current protection (OCP) and then the power supply output will turn off if the output transitions into constant current (CC) mode. For previous posts on this topic, click here and here.

Current limit plays an important role in protecting your DUT. But you should also know that the current limit setting can affect the voltage response time, specifically the up-programming speed. Voltage up-programming speed is the time it takes the output voltage to go from a lower voltage to a higher voltage. For example, the up-programming output response time for an Agilent N5768A power supply (rated for 80 V, 19 A, 1520 W) is specified to be no more than 150 ms with a full load (settling band is 1% of the rated output voltage). This spec assumes the current limit is set high enough to not limit the current. The output capacitor of this power supply will draw current as the voltage on the cap rises (Ic = C * dVc/dt). The output current and the cap current flow through the current monitoring resistor which is where the current is measured and compared to the current limit setting. See Figure 1. Therefore, the output cap current adds to the output current and can cause the power supply to momentarily go into CC mode as the output cap charges. If this happens, the output voltage will rise more slowly than if the power supply stayed in constant voltage (CV) mode the entire time the output voltage was rising and charging the output cap.

So, the current limit setting can slow down the voltage response time if set too low causing the power supply to momentarily go into CC mode as the output voltage is rising and the output cap is charging. This effect is shown in Figure 2 for various current limit settings on the N5768A power supply. As you can see, the lower the current limit setting (Iset), the longer it takes for the voltage to reach its final value.

If fast up-programming response time is important to you in your power supply application, make sure you set your current limit high enough to provide current to your DUT and to charge the power supply’s output capacitor without going into CC mode. Once the output voltage reaches its final value, you can always lower the current limit again to properly protect your DUT.