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How to choose the appropriate switching power supply

People in the electrical circuit module design or to the new product finalize the design, rarely seriously consider the choice of matching power supply, until found that the problem in the power part, to re-evaluate the problem.In order to help people, this paper describes the definition of dc power supply parameters and the potential impact on the test unit (UUT) or other circuit systems.

The basic basis of power supply selection

The voltage and current range, which are the two easiest indicators to determine, can be calculated based on the power consumption of the circuit.However, if only a single dc voltage is used to power the UUT, consider testing the UUT's high and low supply voltage extremes.

Most fixed power supplies allow a range of output voltage of 10%. If this is not sufficient for the circuit requirements, power supplies with adjustable output or with a greater range may be selected.

If the power supply is used to supply the combined device, 75% to 90% of the maximum current required by the device is supplied by one power source, and the insufficient part can be connected with two or more power sources.The power supply for the semi-fixed test bench must have a capacity of at least 125% of the current required for the test bench.

1. Working mode

Almost all dc power supplies operate in constant voltage source mode, which means that the output voltage remains constant over the entire range of current variations.There is also a batch of power supply can also work within a certain range in value stream source mode.

The output range of the power supply is not limited to the voltage or current output capacity of the power supply itself, but also related to the working state of the power supply. In the adaptive mode, the power supply can automatically adjust the output range of the voltage or current under the premise of constant capacity.

A small number of power supplies also act as a load on electronic equipment. In this mode, this power supply can be used to test other constant-voltage or constant-current sources.

2. Response time

When the power supply can be remotely controlled or is part of a test system, the delay time from issuing the control command until the power supply output stabilizes to the desired value is a very important factor (for example, turning on or changing the voltage).Response times range from milliseconds to tens of milliseconds.A small amount of power may have a rapid conversion rate exceeding IV/us.These power supplies can be used to produce high-power waveform generators of any kind.

3. Resolution

The minimum adjustable increment of output voltage or current specified by a technical parameter.Some sources have the same resolution across the entire output range, while others have different resolutions across different output ranges.

Accuracy, expansion and safety of power supply

1. Remote sensing characteristics

In order to accurately control the voltage or current on the load, any changes in the circuit can be fed back to the regulator circuit through a separate sensing channel, thus correcting any voltage drop changes between the UUT and the power supply.

2. Programmable

Some sources make the output programmable by changing the dc voltage or resistance.In some applications, the IEEE488 or rs-232 interface is used to select different test schemes or different output voltage values from a given program.Part of the power supply also has the ability to set the storage, the change of the output can be done by program control or step operation.

3. 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 in series for use.In this mode of operation, the connection between the voltage stabilizer and the control circuit between the power supply modules still exists, except that one power supply is used as the main controller and the other power supply is used as the controlled one.

4. Overload protection

Because a power supply needs to be supplied to different circuits, the current flow rate of these circuits may be unknown, and in order to avoid damage to the power supply, the range of protective circuits needs to be set.

Almost all power supplies have the following characteristics: when out of the output range, either the output remains at the maximum output value, or turn off the power supply itself.In addition to the output range of some program-controlled power supply, the stable output type of power supply can also be set automatically.In other words, 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.

Protection diodes are added to the power supply to prevent damage caused by misconnecting the polarity of the external power supply.The thermal sensor can also be used to prevent the power supply from burning out due to the continuous operation of the power supply in an overload state or ineffective cooling.

Potential sources of damage within the power supply

1. Pulsation and noise

The ideal dc supply should provide pure dc, however there are always some disturbances such as pulsating currents and high-frequency oscillations superimposed on the SMPS output port.These two disturbances, coupled with the high peak noise generated by the power supply itself, contribute to the intermittent and haphazard drift of the power supply (PARD).

The amount of PARD allowed in the power supply output is greatly affected by the echo sensitivity of the UUT, and if the test circuit contains a high-gain amplifier, any part of the PARD within the specified band of the amplifier will be amplified.If PARD's magnitude is large and a special filter is not supplied to the primary amplifier, the entire test result will be inaccurate.

For linear power supplies, pulsation is the most general form, and the magnitude can be represented by the RMS value or the RMS percentage of the output voltage Vrms/Vout.The VPP representation is more accurate for switching power supplies.However, the manufacturer of linear switching power supply will generally give two values of RMS and peak to peak difference or either of them.

2. Stability

When the line voltage or load current elbow changes, the output voltage of the dc power supply will also fluctuate.The degree of voltage regulation is determined by the parameters of the voltage regulation circuit, which refers to the capacity of the filter capacitor and the rate of energy release.

If the power supply is supplied from a relatively constant source, only basic load stabilization is required.The magnitude of the stability is generally defined as the percentage of the output voltage, or the variation of the voltage, at no load or at full load.

Internal impedance

The relatively large internal resistance of the power supply has two disadvantages for the load. First, it is not conducive to the work of the load voltage stabilizing circuit. What's more, any change in the load current will lead to the fluctuation of the output of the dc power supply.

4. Transient response or recovery of power supply

The magnitude of the power supply transient response and recovery time indicates the ability of the power supply voltage stabilizing circuit to restore normal voltage when the output load changes suddenly.There are two parameters to calibrate the power supply transient response and recovery: one is the output deviation value when the load changes suddenly;The second is the time it takes to restore the output to its original value.For the sake of unification, in general, when the load changes by 10%, the output deviation from the peak voltage is calibrated with the millivolt value, and the recovery time is calibrated with the millivolt value used to restore the output to the normal value.Other manufacturers use larger load current variations to measure recovery time.Take, for example, the time it takes to return to normal with a 50% to 100% change in the output current.

Sanyi technology of safety series switching power supply will be in strict accordance with the above requirements.

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