November 6, 2018 by Ron Stull
Welcome to Part 2 of our "Pushing the Limits" series, where we delve into a question we hear often at CUI, "What if I operate my power supply outside of a certain specification range?" In Part 1 we covered the input voltage specification. Now in Part 2 we will look at output current and the problems that can occur when exceeding the output current specification.
The rated output current is one of the most important specs when selecting a power supply. It plays a large part in determining the size and cost of the unit, which leads designers to choose power supplies that have just enough current to meet their requirements. In these cases, it is tempting for a designer to select a power supply that addresses the "normal" operating current in order to save on cost and size, while assuming that it can handle the peak currents for a short time. This same line of thinking goes for the minimum current limit as well. However, exceeding either maximum or minimum current specs can lead to several issues including degradation of performance, protected shutdown, or even component failure.
Efficiency, regulation, and electromagnetic emissions (EMI) are some of the most important specifications affected when operating a power supply outside of its rated output current.
As the output current goes up, so does the output power. If the efficiency were fixed across load, the additional current would lead to a linear increase in power dissipation within the supply. This added power loss causes an increase in component temperature rise which can lead to thermal failure. In practice it is unlikely that the efficiency will remain constant and, as shown in the graph below, it is common for a power supply to reach peak efficiency before maximum load, resulting in efficiency that decreases past its rated current. This leads to an exponential increase in power dissipation with respect to load increases, causing the maximum temperature to drop much faster than if the efficiency were constant. In addition to the thermal concerns, the dropping efficiency could cause the power supply and/or system to fail efficiency regulations. As the graph further shows, when operating the power supply 20% beyond the rated load of 200 W, the efficiency drops a full percentage point below its 91% specification. This results in a 30% increase in power dissipation.
Load regulation is another problematic spec when operating outside the rated output current. The load regulation tells the user the maximum amount that the output voltage can be expected to change as the load changes between no load and full load. The graph below shows an example of the load regulation of a 200 W ac-dc power supply. This particular power supply has an output voltage that drops with increased current. However, this is not always the case as some power supplies will see the output voltage increase with load. Either way, operating outside of the specified current range may cause the output voltage to move outside of its load regulation specification, leading to problems in applications that cannot accept voltages outside of this range.
Applications with tight input ranges often take advantage of external voltage sense connections, which will regulate the output voltage at the load rather than at the output of the power supply. With external sensing, a drop in output voltage that would normally occur between the power supply and load is compensated by increasing the output voltage of the supply. As a result, there is often a specification for the maximum voltage that can be compensated for in order to prevent damage to the power supply due to an increased voltage.
In power supplies with a minimum current rating, operating below this limit may also cause the unit to operate outside of its regulation specification. These power supplies are often small, less expensive units with simple control schemes that are not designed to handle the issues that occur at light loads. A minimum current can also be specified in multiple output power supplies, which is needed to regulate the secondary outputs within their specified limits.
A final, less obvious issue related to output current is increased EMI. Switching power supplies are electrically noisy devices and a lot of board space is devoted to filter components to help them meet regulatory requirements; usually just enough to pass the required testing. Even when operating within the specified load range, issues still arise when used with certain loads. In general, the magnitude of EMI is expected to increase with load and operating beyond the maximum load can push EMI above the failure threshold. This is further compounded if the filter becomes less effective at higher loads. Increased currents and/or temperatures in these components can also alter their values and change the filter's response.
The specification related issues described above all assume that the power supply will allow the user to operate above the maximum output current. However, most power supplies are equipped with some form of over current protection that prevents the load from exceeding a certain current threshold.
Some power supplies have a well-defined threshold closer to the rated output beyond which the over current protection kicks in. Users that attempt to size their power supply for nominal current and let peak current needs exceed the current rating may also see the output shut down due to this protection.
Other protection schemes have wider tolerances that allow the load to significantly exceed the maximum rated output. Threshold differences between individual supplies can cause issues if the protection is enabled in some supplies but not in others. If the output does not shut down, the supply will operate above its maximum current resulting in spec related issues or failure.
In addition, more complex power supplies offer protection against current falling below the minimum rating while others will disable operation completely under these conditions. Power supplies that cannot reliably regulate themselves at light loads will lead to excessive voltage on the output which could also trigger a protection.
While the previous issues will not always cause a failure or damage to power supply components, many components will experience extra voltage and/or current stress as a result of the increased load current, putting them at greater risk.
With increased output current comes similar increases in component currents throughout the power train. Components such as MOSFETs, diodes, resistors, and even copper traces will see increased power dissipation and heat because of the increased current. Diodes and other components with a fixed voltage, will observe a linear increase in power dissipation, while MOSFETs and components with resistive elements will show an exponential rise in power dissipation with respect to load increases. In both cases this will lead to increased temperature rise, reduced reliability, and increased risk of failure.
Magnetic components such as chokes and transformers, while experiencing increased conduction losses like the previous components, may also encounter increased core loss and be pushed into saturation, producing further losses and heat generation. Saturated magnetics could also cause the power supply to cease functioning or generate increased currents in other components, such as the MOSFETs and diodes. For example, in a buck converter, the ripple current is directly related to the inductance. When the inductance starts dropping the peak currents in the MOSFET and diode will increase as a result.
In addition to discrete magnetic components, there are also parasitic inductances, such as the leakage inductance of the transformer. These parasitic components cause voltage spikes when the switch changes state and the magnitude of this spike increases with load. In the case of the transformer leakage, the voltage spike is applied across the MOSFET and may cause it to fail if too large. Other components, like those that sense voltages and currents, will sense these voltage spikes which will result in the controller receiving incorrect voltage and current information – leading to poor performance or failure.
Power, size and cost are all important factors when choosing a power supply. Unfortunately improving one often inversely affects the others, with more power typically meaning a larger and/or more expensive power supply. Even so, users will often try to force all three factors opening themselves up to potential problems. The output current is one such area that affects just about every component in the power supply. Some effects are obvious while others are easily overlooked and cause immediate or long-term issues. Before operating outside of the output current rating of a power supply the user should consult with the power supply manufacturer to understand the risks of doing so or to seek out an alternative solution.
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