What is the difference between a linear DC power supply and a switch mode power supply?
Linear power supply
The main circuit of a linear power supply is as follows:
That is to say, only a portion of the 220V mains power is added to the primary of the main transformer after being controlled by a thyristor. When the output voltage Uo is high, the conduction angle of the thyristor is large, and most of the mains voltage is "let go" by the thyristor (as shown in the above figure), so the voltage applied to the primary of the transformer, i.e. Ui, is high. Of course, after rectification and filtering, the output voltage is also relatively high.
When the output voltage Uo is very low, the conduction angle of the thyristor is very small, and the vast majority of the mains voltage is "blocked" by the thyristor (as shown in the figure below), only allowing a very low voltage to be applied to the primary of the transformer, that is, Ui is very low. Of course, after rectification and filtering, the output voltage is also very low.
The various PWM integrated chips currently used in the production of switch mode power supplies are mainly designed from the perspective of small output voltage variation range and relatively stable output current.
But the so-called PWM chip is a pulse width modulator. When the output voltage is high and the output current is high, the switching tube inside the power supply has a longer turn-on time and a shorter turn off time:
If the output voltage and current continue to decrease, the control pulse needs to continue to narrow, but the PWM circuit can no longer meet this requirement. At this point, the circuit becomes intermittent, as shown in:
Pulses can sometimes be intermittent and intermittent, causing noise and increased ripple in the power supply, resulting in poor electrical performance. The so-called 'low-end instability' has actually become an unqualified product. In order to solve this problem, our company has taken new technological measures to better address it (not further elaborated).
Comparison between Linear Power Supply and Switching Power Supply
1. Linear power supplies have good accuracy (1-3 orders of magnitude better than switch mode power supplies), low ripple, good adjustment rate, low external interference, and are suitable for various occasions.
2. Linear power devices operate in a linear state, so the losses are relatively higher compared to switching power supplies, and switching power supplies have better efficiency.
3. The size of switch mode power supplies is smaller than linear power supplies, but switch mode power supplies have problems with polluting the power grid and radiation interference.
4. Switching power supplies are not suitable for continuous adjustable starting from 0 voltage when outputting high voltage and high current, but they are suitable for fixed output or relatively fixed output with less requirement for radiation interference.
5. Linear power supplies are relatively easy to repair. However, switch mode power supplies pose certain difficulties in maintenance due to their dense components. In addition, due to the stark difference between circuits and linear power supplies, maintenance personnel require a high level of technical proficiency, requiring the use of an oscilloscope to observe the working status of each point in the circuit
