Current Limit Characterization

Current limit measurements demonstrate the degree to which a constant voltage power supply limits its maximum output current to a preset value. This preset value can be fixed or variable throughout a specified range. There are basically three types of current limiting design implementations:

1.Conventional current limiting power supplies

2.CV/CC mode power supplies

3.Foldback current limiting power supplies

Conventional current limiting power supplies and CV/CC mode power supplies are very similar in function. These implementations generally vary only in the degree of regulation in the constant current operating region (see Figure 5) and in the ability of the user to adjust the CC operating point (CV/CC power supplies). A rounded crossover knee and sloping current limit characteristic denotes less precise current regulation. In comparison, a sharp knee and vertical current limit characteristic denote a higher degree of current regulation. The foldback current limiting power supply employs a technique that enables both the output voltage and current to decrease simultaneously for load resist- ances below the crossover value. The purpose of current limiting is to provide protection for the power supply and the device being powered (assum- ing the current limit value is below the maximum current rating of the device).

Test Overview/ Procedures

A measurement of the output voltage and current of the power supply under test is required while decrementing the electronic load resistance (or current in CC mode) by steps from an initial value that produces the power supply’s full rated voltage output (see Figure 6). The voltage will remain con- stant until the compliance current (output current of the power supply) increases to the preset current limit value. The crossover region or current limit has been reached when the rated output voltage of the power supply changes by a degree greater than the load regulation specification. At the current limit knee, the compliance current and output volt- age behavior is determined by the type of current limiting circuit implemented in the power supply design (see Table 3).

Table 3. Typical Test Results of Standard Current Limiting Implementations

	I Compliance (or Iout)
Current Limiting Method	at Minimum Load Resistance

CV/CC	Remain constant (CC mode)
Conventional Current Limiting	Typically ≤ (105%) Imax
Current Foldback	Typically foldback is ≤ (50%) Imax

Figure 5. Typical Operating Characteristics of Three Types of Current
Limiting Power Supplies	7

AN 372-1 specifications

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