Why Does My Power Supply Overshoot at Current Limit? Insights on Mode Crossover

One often encountered issue with power supply use is expecting that the current limit will clamp the current to no greater than the set value, only to discover the current initially overshoots when the DUT demands current in excess of the set limit. In some cases the short surge of excess current may be enough to damage a sensitive DUT. Those experienced with power supplies will recognize this as a dynamic characteristic of mode crossover.

What is mode crossover? Mode crossover is the transition point between Constant Voltage (CV) and Constant Current (CC) modes. The dynamic response characteristic of mode crossover is an aspect that separates real-world from ideal-world power supplies. To start it will be helpful to review a previous posting on “How Does a Power Supply regulate its Output Voltage and Current?” Here it is shown there are two control loops in most power supplies, one for regulating the voltage and one for regulating the current. Only one is in control at any given time while the other is “open loop”. The error amplifier that is open loop is up against it stops. When load conditions change such that the power supply transitions through mode crossover the open loop error amplifier needs to recover and gain control of the output. In the more common case of the power supply operating as a voltage source there can be a current overshoot during the brief moment when the load increases beyond the power supply’s current limit setting. Conversely, for a current source, there can be a voltage overshoot during the brief moment when the load decreases, causing the output voltage to rise to the voltage limit setting.

The magnitude of the overshoot depends on many factors relating to both the power supply and the DUT. Supplementary circuitry usually surrounds the error amplifiers to clamp them from being driven into saturation or cutoff so that they can more quickly recover when needed. Amplifiers are carefully selected for their recovery characteristics. Careful design is required to assure a stable transition between modes during crossover while at the same time minimizing the delay and overshoot.  The magnitude of the overshoot also depends on how quickly and to what extent the DUT transitions between loading conditions.

Figure 1 shows the mode crossover current overshoot of a 50 volt, 3 amp general purpose power supply, set for 10 volts and 1 amp output.  The loading DUT is an electronic load set to transition from no load to 10 amps with a slew of 0.8 amps per microsecond. This loading represented a worst case for all practical purposes. When the load transitions to full (i.e. overload) it takes about 6 milliseconds for the current limit control loop to fully take over and bring the current down. During this mode crossover period the current overshoot plateaus at 5 amps, which is the gross current limit capacity of the power supply. Basically this is the point where the power supply runs out of drive.

Figure 1: Constant voltage to constant current mode crossover for 10 V, 1A power supply settings

In Figure 2 the power supply current limit was reduced to 0.1 amps and the mode crossover was again captured. This had an interesting impact on the current overshoot. While the peak current still hit a plateau of 5 amps, the duration of the overshoot was considerably reduced to about 0.5 milliseconds.  The reason for this is there was a much larger difference driving the error amplifier’s input, causing it to transition more quickly. The peak level remained unchanged as it is determined by the power supply’s gross current limit capacity, which is fixed.

Figure 2: Constant voltage to constant current mode crossover for 10 V, 0.1A power supply settings

The extent of an overshoot during mode crossover depends on the power supply as well as the DUT. A power supply optimized for voltage sourcing usually has very little voltage overshoot at mode crossover, but then can have significant current overshoot, as we see here. Conversely, a power supply optimized for current sourcing usually has very little current overshoot at mode crossover, but then can have significant voltage overshoot. Higher performance power supplies may provide faster and better mode crossover performance, but this usually comes at greater expense. Some useful things to do include:

·         Be aware that overshoot during mode crossover is a reality that exists in most all power supplies

·         Try not to oversize the power supply. Be aware that the peak level of voltage or current during mode crossover may be governed more by the maximum voltage and current ratings of the power supply and less by the settings. Using an oversized power supply with its limit set to 5% of its capacity will likely yield a much larger overshoot than a smaller one with it limit set to 50% of its capacity.

·         Understand the nature of your DUT, behavior or fault modes that may cause it to draw an overload, and how sensitive it is to an overload

·         If your DUT is sensitive to an overload, include evaluating the response characteristics of mode crossover as part of your evaluation, using realistic conditions that reflect the characteristics of your DUT.

Recognizing that there is dynamic response characteristics associated with mode crossover of “real-world” power supplies, and they need to be considered, may save a lot of surprise and frustration later on!

Ideal Versus Real: Understanding Some Fundamentals When Selecting Power Supplies

Introduction
In college, we learned about electronics using ideal components: pure resistors without series inductance, pure capacitors without ESR, op amps with infinite gain and zero offset voltage. For power supplies, the situation was no different: a constant voltage source with zero output impedance, unlimited current compliance, and infinite bandwidth. With components like these, how difficult could it be to design electronic circuits and systems?

Then, we got jobs as engineers in the real world and discovered things like temperature coefficients in resistors, dielectric absorption in capacitors, and phase shifts in the gain of amplifiers. Power supplies did not escape the omnipotent forces determined to destroy our idealistic view of electronics. Non-zero output impedance, output current limitations, and finite bandwidth have all conspired to make our lives a little more complicated when applying power supplies. The effects of these non-ideal power supply attributes and others are discussed in this post.

The Ideal Voltage Source
An ideal voltage source would maintain its output voltage constant irrespective of the loading conditions. For example, if the source were a 5 V DC source, the output would measure exactly 5.0 V with no current flowing, or with 1 A flowing, or 10 A, or 500 A, and so on. Additionally, when the load current changed from one value to another, such as from 5 A to 10 A, the output voltage would be maintained at exactly 5.0 V, unperturbed throughout the change. See Figure 1a.

The Real Voltage Source
Unfortunately, power supplies like the ideal one described above do not exist in real life. Real power supplies try to maintain a constant voltage on their outputs by employing a feedback loop that monitors the output voltage, compares that voltage to a reference, and continuously makes adjustments based on the difference. They also have to be designed to fit in a limited space, with limited input power, and limited ability to dissipate the inevitable heat generated internally. Consequently, real power supplies have limited current compliance, finite output impedance, and finite bandwidth. The effects of these attributes become apparent when drawing current from the power supply, whether that is a static current or dynamic current. For example, a 5.0 V output at no load with 10 milliohms of output impedance will drop to 4.9 V with a 10 A static load. The output voltage will continue to decrease as the current increases. See Figure 1b.

With dynamic loads, the non-ideal nature of the real power supply output becomes more evident. Consider the output voltage behavior shown in Figure 1b immediately following the load current changes. The voltage overshoots and undershoots of the real source are a result of its non-zero output impedance which is a function of frequency (Zo(f)) and is determined by the internal feedback loop used to maintain the output voltage.

Power Supply Output – Deviant Behavior?
When selecting a power supply to meet your needs, first make sure you know what output voltage deviations you can tolerate. Evaluate your needs with respect to both static and dynamic conditions. For example, some devices, such as cell phones, have a low voltage detection circuit built-in. Make sure you are aware of the voltage level at which this circuit takes effect and how long the voltage must be below that level for the circuit to trip. The power supply used for testing should be selected to maintain its output voltage to meet your needs under changing load current conditions, especially to avoid nuisance tripping of a low voltage detection circuit. The load regulation (or load effect) specification tells you how well the power supply will maintain its output voltage when subjected to static load changes. The transient response specification will tell you how long it will take for the output voltage to recover to within a voltage band around the output voltage following a current change. Power supplies with different performance levels have correspondingly different specifications as shown in the table.

Other Non-ideal Attributes to Consider
In addition to the output voltage response to static and dynamic load changes, real power supplies also exhibit many other non-ideal behaviors. Line regulation, output noise, and cross regulation for multiple output power supplies are some examples.

• Line regulation is a measure of how the output voltage responds statically to input line voltage changes. It is primarily caused by finite loop gain, with some secondary effects from internal bias supply line regulation effects.
• Output noise is usually specified in either peak-to-peak volts, or rms volts, or both, and within a specified bandwidth such as 20 Hz to 20 MHz. Output noise has many sources, including residual effects from rectification circuits, internal digital circuits, and even the op amps themselves that are used for output voltage regulation.
• On a multiple output power supply, cross regulation is a measure how one output voltage responds to load current changes on the other output(s).

Clearly, for all of these attributes, the lower the specified behavior, the more “ideal” the power supply. While it may be tempting to look for a power supply with the lowest specifications in all of these areas, it is always prudent to evaluate your true needs, and make your selection based on those needs. Since tradeoffs must often be made, knowing your requirements will always make the selection process easier by broadening the choices when compared to just looking for the best specifications in all areas.

Other, more subtle behavior of non-ideal power supply outputs can also be important, depending on your application:

• Overshoots at AC (or DC) input turn-on and turn-off should be considered.
• Output voltage behavior when the power supply enters or leaves a current limit condition (mode crossover overshoots) can sometimes cause problems.

These behaviors are often unspecified by the power supply manufacturer. However, relying on reputable power supply vendors helps avoid problems since these vendors frequently take steps during the design process to minimize these effects.

Wrap-up
Obviously, real power supplies don’t behave like ideal power supplies. Sometimes this non-ideal behavior makes a difference in your application, and sometimes it does not. When selecting a power supply, it is important for you to know your true requirements in order to make the selection process go as smoothly as possible and to avoid overspending. A power supply’s specifications outline its non-ideal behavior, so look for specifications that meet your needs. Also realize that there are unspecified performance issues that could be important in your application as well. If you don’t see the specification for which you have interest, ask your power supply vendor about parameters you feel are important in your application.