Basic selection criteria for switching power supply
Basic basis for selecting switching power supply
The voltage and current range are the two easiest indicators to determine, as long as they are calculated based on the power consumption of the circuit. Consideration should also be given to testing high and low supply voltage extremes.
Most fixed power sources allow output voltage to vary within a range of ± 10%. If this does not meet the circuit requirements, a power source with adjustable output or a larger range of variation can be selected.
If the combined device is powered by this power source, 75% to 90% of the maximum current required for the device is provided by one power source, and the insufficient part can be connected to two or more power sources in parallel.
Expansion and Safety of Switching Power Supplies
1. Parallel or series operation
When a power supply cannot meet the required voltage or current range, two or more power supplies (or different outputs of the same power supply) can be connected in parallel or series for use. In this working mode, the connection between the voltage stabilization and control circuits between each power module still exists, except that one power supply serves as the main control party and the other power supply serves as the controlled party.
2. Overload protection
Because a power supply needs to supply different circuits for use, the current flow rate of these circuits may be unknown. To avoid damage to the power supply, it is necessary to set the range of protection circuits.
Almost all power supplies have the following characteristics: when exceeding the output range, either the output remains at the maximum output value or the power is automatically turned off. Some programmable power supplies can automatically set the type of stable output of the power supply in addition to setting the output range through a program. That is to say, when the voltage or current required by the external circuit exceeds the set limit, the power supply can automatically change from a constant voltage source to a constant current source or from a value current source to a constant voltage source.
Adding a protective diode to the power supply can prevent damage caused by accidentally connecting the polarity of the external power supply. Thermal sensors can also be used to prevent the power supply from burning out due to continuous operation under overload or ineffective cooling.
Potential root causes of damage within switching power supplies
1. Pulsation and noise
An ideal DC power supply should provide pure DC, but there are always some interferences, such as pulsating currents and high-frequency oscillations superimposed on the output port of the switching power supply. These two types of interference, combined with the peak noise generated by the power supply itself, cause intermittent and random drift of the power supply.
2. Stability
When the line voltage or load current changes, the output voltage of the DC power supply will also fluctuate. The degree of voltage stabilization is determined by the parameters of the voltage stabilization circuit, which refer to the capacity of the filtering capacitor and the rate of energy release.
If a relatively constant power supply is used to power the power supply, only basic load stabilization is required. The size of stability is generally defined as the percentage of output voltage or the change in voltage at no load or full load.
3. Internal impedance
A relatively large internal resistance of the power supply has two disadvantages for the load. Firstly, it is not conducive to the operation of the load regulator circuit. Moreover, any change in the load current will cause fluctuations in the output of the DC power supply. This fluctuation has the same impact on the test results as the impact of pulses and noise on the test results.
4. Switching power supply transient response or recovery
The magnitude of the transient response and recovery time of the power supply indicates the ability of the power supply regulator circuit to restore normal voltage when the output load suddenly changes. There are two parameters to calibrate the transient response and recovery of the power supply: one is the deviation value output when the load suddenly changes; The second is the time it takes for the output to recover to its original value. For the sake of uniformity, when the load changes by 10%, the output deviation is usually calibrated using the millivolts of the output deviation from the peak voltage, and the recovery time is calibrated using the millivolts used to restore the output to normal value. Some other manufacturers use larger load current changes to measure recovery time. For example, the time taken to restore normal values when the output current changes by 50% to 100%.
