What are the differences between linear DC power supply and switching power supply?

Regarding the circuit structure, whether it is a linear power supply or a switching power supply depends on the specific occasion and should be used reasonably. These two circuits are widely used both internationally and domestically, and each has its own characteristics. Linear power supplies are widely used for their high precision and superior performance. Switching power supplies are also widely used in many occasions where the output voltage and output current are relatively stable because they eliminate the bulky power frequency transformer and reduce the volume and weight to varying degrees.

1. Linear Power Supply

The main circuit of the linear power supply is as follows:

Generally speaking, a thyristor is a device that controls voltage. Since the conduction angle of the thyristor can be controlled by a circuit, as the output voltage Uo changes, the conduction angle of the thyristor also changes. The voltage Ui applied to the primary of the main transformer also changes accordingly.

That is, only a part of the ~220V mains voltage is added to the primary of the main transformer after being controlled by the thyristor. When the output voltage Uo is higher, the thyristor conduction angle is larger, and most of the mains voltage is "released" by the thyristor (as shown in the figure above), so the voltage added to the primary of the transformer, that is, Ui, is higher, which of course means that the output voltage is higher after rectification and filtering.

When the output voltage Uo is very low, the conduction angle of the thyristor is very small, and most of the AC voltage is "cut off" by the thyristor (as shown in the figure below), allowing only a very low voltage to be applied to the primary of the transformer, that is, Ui is very low. Of course, the output voltage after rectification and filtering is also very low.

In fact, connecting a high-power transistor in series at the output end of the thyristor power supply (actually multiple transistors in parallel) will form a linear power supply. As long as the control circuit outputs a small current to the base of the transistor, it can control the transistor to output a large current, so that the power supply system can be stabilized again on the basis of the thyristor power supply. Therefore, the voltage regulation performance of this linear regulated power supply is 1-3 orders of magnitude better than that of the switching power supply.

2. Switching Power Supply

The main circuit of the switching power supply is as follows:

It can be seen from the circuit that the city power becomes 311V high voltage after rectification and filtering . After the K1~K4 power switch tubes work in order, it becomes a pulse signal added to the primary of the high-frequency transformer . The pulse height is always 311V. When K1 and K4 are turned on, the 311V high-voltage current flows into the primary of the main transformer through K1 in a positive direction and flows out through K4, forming a positive pulse in the primary of the transformer. Similarly, when K2 and K3 are turned on, the 311V high-voltage current flows into the primary of the main transformer through K3 in a reverse direction and flows out through K2, forming a reverse pulse in the primary of the transformer. In this way, a series of positive and negative pulses are formed in the secondary of the transformer, and a DC voltage is formed after rectification and filtering. When the output voltage Uo is higher, the pulse width is wide, and when the output voltage Uo is lower, the pulse width is narrow. Therefore, the switch tube is actually a device that controls the width of the pulse.

The various PWM integrated chips currently used in the production of switching power supplies are mainly designed from the perspective of a small output voltage variation range and a 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 large, the switch tube inside the power supply is turned on for a longer time and turned off for a shorter time:

When the output power is small, the pulse width is narrower:

But this pulse width cannot be narrowed infinitely. The range of pulse width change, that is, the adjustment range, is only 10% -90% . This feature determines that this PWM chip is not suitable for a so-called continuously adjustable power supply that starts from 0 voltage. For example, a 500V5A switching power supply, when its output reaches 500V5A, the control pulse is the widest, as shown in the following figure: