April 5, 2022 by Bruce Rose - 7 Minute Read
As engineers we are often looking at ‘what if’ scenarios to ensure our designs work under as many application conditions as possible. For conditions under which our designs do not work properly, we at least do not want either our product to become damaged or our designs to damage something else. With power supplies one of the concerns often is “what happens if the output load current from my power supply exceeds the power supply’s current rating”. In this article we will discuss common methods employed in power supply designs when the rated load is exceeded.
Although we most often call them “power supplies” we normally are actually referring to “voltage supplies”. The difference between the two designs is an ideal ‘power supply’ will deliver constant power to a load regardless of the load voltage or load current (Figure 1). An ideal voltage supply will provide a constant voltage to a load regardless of the load current (Figure 2). We will not be able to design, build or buy an ideal voltage supply because an ideal voltage supply will have the ability to deliver unlimited current (and power) to a load. Even though we mentioned we are discussing voltage supplies rather than power supplies, we will continue to generically use (misuse) the term ‘power supply’ in our discussions.
As was mentioned above, all power supplies will have some form of current limiting and most of the time it will be controlled and non-destructive. This discussion will focus on limiting the output current from a power supply to protect the power supply in the event the load demands too much current and would otherwise damage the power supply. Input current limiting is often implemented in power supplies in the form of a fuse in series with one or both input conductors. This current limiting is to protect the upstream power source and conductors, it is assumed the power supply with the input fuse(s) is already damaged if the input current drawn is high enough to blow the input fuse(s). Some applications require a tightly controlled current limit in order to operate properly, i.e. charging batteries. It will be necessary to have a discussion with the power supply vendor if a tightly controlled current limit is required, most power supplies employ loose current limiting that is present only to protect the power supply from being damaged. Some common methods of limiting the output current from a power supply include fuse current limiting, constant current limiting, fold-back current limiting, and hiccup current limiting.
Perhaps the simplest form of power supply output current limiting is to place a fuse in series with the output terminal of the power supply (Figure 3). This method would be effective but is not often employed in power supplies because it is relatively easy to draw excessive load current (i.e., shorting the output terminals or plug of the power supply) and accidentally blow the fuse. In addition, the output current limiting function in the power supply protects the internal semiconductor components from being damaged due to excessive load current. It may be difficult to select a fuse which will blow fast enough to protect the internal semiconductors but not blow when a motor starts up or load capacitors are being charged. Fuses work well to protect conductors and not as well to protect semiconductors.
A common method that has been implemented to limit the output current from power supplies is to monitor the output current and reduce the output voltage when the current limit is reached while maintaining the maximum output current (Figure 4). In this implementation, the output voltage during current limiting is dependent upon the impedance presented by the load during the current limit operation. This current limit method is relatively simple to implement but stresses components in the output current path as the power supply operates at its maximum current during current limit operation. This version of current limiting may be the best choice when the load draws a short burst of excessive current, such as during motor start-up or bypass capacitor charging. Users who do not realize the power supply is in current limit mode may believe the supply is not working properly because the output voltage is less than the data sheet specified level when the supply is operating in current limit mode.
To address the component stress issues present in constant current limiting and mentioned above, some power supplies are designed with fold-back current limiting. This implementation can be confusing to the user. In a fold-back current limited power supply the output voltage and output current are both reduced after maximum output current is detected (Figure 5). A standard behavior for a fold-back current limited power supply does not exist and thus the user will need to read the data sheet to understand how the supply they have selected will behave. This method of power supply output current limiting may cause problems when the load is a motor starting up or a large amount of input bypass decoupling capacitors being charged. The fold-back behavior may cause confusion to the user if the supply is in current limit mode and the user is trying to understand why the power supply is not producing the proper output voltage or current.
Perhaps the most common implementation for current limiting now employed in power supplies is known as hiccup mode. This mode of over-current protection can be thought of as an active version of fuse protection mentioned earlier in this discussion. With hiccup mode current limiting protection, the output voltage of the power supply is shut down when an over-current situation is detected. After a specified waiting time the output voltage of the power supply is re-established. If the over-current situation still exists, then the supply repeats the shut-down and wait process. If the over-current situation no longer exists, then the power supply continues operating in the normal mode (Figure 6).
The hiccup mode of over-current protection is easy to implement in the voltage regulator controller chip and minimizes over-current stress on the components in the power supply output power path. Hiccup mode over-current protection can be an issue for motor start-up loads and some situations with large banks of input filter capacitors. For applications with motor loads, if the motor does not start-up adequately during the ON time of the power supply the motor will slow down during the OFF time of the output voltage of the power supply and again not start during the next hiccup cycle. Under this condition the motor never starts because of the OFF time in the power supply output voltage (Figure 7).
A similar issue may arise when the load is a large value of input filter capacitance and a load current is also present. When the power supply output voltage is first applied to the discharged capacitors the current drawn by the capacitors can be large enough to cause the power supply to go into over-current operation. During the OFF time of the power supply, if a load current is present in addition to the capacitors the load current can discharge the capacitors sufficiently during the OFF time of the power supply such that the capacitors never are able to charge up to the power supply output voltage (Figure 8).
If the load current is low enough (or not present) the capacitors may charge in a stair-step fashion due to the pulses of power supply output voltage and the power supply and load will operate properly after the initial start-up delay (Figure 9).
Power supply output current limiting is present on all power supplies. It is generally beneficial for the user to understand what type of current limiting is used in their supply and thus how the supply will behave during an output over-current situation. Observing the output voltage of a power supply with just a DMM (Digital Multi-Meter) may cause confusion to the user regarding what is happening when the output voltage is not the value specified in the power supply data sheet.
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