The function and performance index and test method of dc power supply.

1. Basic principle of dc power supply.

Dc voltage stabilizer is composed of power frequency transformer, rectifier filter circuit and voltage regulator circuit.

(1) the function of dc stabilized voltage power supply converter is to transform the ac voltage of the power grid 220V into the ac voltage Ui required by the rectifier filter circuit. The power ratio of the transformer subside to the original side is P2/ P1= eta, and in the formula is the efficiency of the transformer.

(2) rectifier filter circuit: the rectifier circuit converts the ac voltage Ui to the pulsed dc voltage. After filtering the larger ripple components, the output ripple is less than the dc voltage U1. The commonly used rectifier filter circuit is full wave rectifier filter, bridge rectifier filter and so on.

Each filter capacitor C satisfies rl-c = (3~5) T/2, or medium T is input ac signal cycle, and RL is the equivalent load resistance of rectifier filter circuit.

(3) three-terminal integrated voltage regulator: the commonly used integrated voltage regulator has a fixed three-terminal voltage regulator and a adjustable three-terminal voltage regulator. The common adjustable positive pressure integrated voltage stabilizer has CW317 (LM317) series, whose output voltage ranges from 1.25V to 37 volts, and the simplest circuit external components require only a fixed resistor and a potentiometer. Its chip has transition, overheating and safe working area protection, the maximum output current is 1.5a. As shown in the typical circuit, the expression of the output voltage Uo is: Uo = 1.25 (1+R2/R1).

In the formula, R1 generally takes 120-240 ohms, and the voltage difference between the output end and the adjusting end is the reference voltage of the voltage regulator (typical value is 1.25V).

2. Performance index and test method of dc current.

The technical indexes of dc power supply are divided into two kinds: one is the characteristic index, including the allowable input voltage, output voltage, output current and output voltage regulation range, etc.; Is another kind of quality index, which is used to measure the stability of the output dc voltage, including the voltage coefficient (or voltage regulation rate), the output resistance (or current regulation), ripple voltage (around and random drift) and temperature coefficient. The test circuit is shown in figure 3.

Dc power performance index test circuit.

(1) ripple voltage: the ac voltage component superimposed on the output voltage. An oscilloscope is used to observe that its peak value is generally millivolt. It can also be used to measure its effective value by ac millivoltmeter, but because the ripple is not a sine wave, there is a certain error, and the ripple voltage vp-p of general dc power supply is less than 10mV.

(2) voltage stabilizing coefficient: when the load current and ambient temperature are constant, the relative change of input voltage causes the relative change of output voltage.

(3) the voltage adjustment ratio: the input voltage relative changes is the plus or minus 10% of output voltage relative variation, coefficient of voltage and voltage regulation rate shows that the input voltage change on the influence of the output voltage, so just test one of them.

(4) output resistance and current adjustment rate.

Output resistance and the amplifier output resistance is the same, its value is at constant when the input voltage, output voltage variation and the absolute value of the ratio of the output current variation. Current regulation: output current from 0 to the maximum output voltage generated by the relative changes in value. Both the output resistance and the current adjustment rate indicate the effect of load current on the output voltage, so only one of them can be tested.

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