The characteristics of linear regulated DC power supplies are: output voltage is lower than input voltage; fast response speed and low output ripple; low noise during operation; relatively low efficiency (LDOs, which are now commonly seen, were developed to address efficiency issues); and high heat generation (especially in high-power power supplies), indirectly increasing thermal noise in the system. Switching power supplies utilize modern power electronics technology to control the on/off time ratio of the regulating transistor to maintain a stable output voltage. Switching power supplies are generally composed of a pulse-width modulation (PWM) control IC and MOSFETs. Their characteristics are: output voltage can be higher than input voltage; high noise during operation; high efficiency; small size; and low temperature rise. Compared to linear power supplies, the cost of both increases with output power, but at different rates. At a certain output power point, the cost of linear power supplies is actually higher than that of switching power supplies; this point is called the cost inversion point. With the development and innovation of power electronics technology, switching power supply technology is also constantly innovating, and this cost inversion point is increasingly shifting towards lower output power, providing a broad development space for switching power supplies.
A linear power supply mainly includes a power frequency transformer, an output rectifier filter, a control circuit, and a protection circuit.
A switching power supply mainly includes an input grid filter, an input rectifier filter, an inverter, an output rectifier filter, a control circuit, and a protection circuit.
Linear power supplies first transform AC power through a transformer, then rectify and filter it through a rectifier circuit to obtain an unstable DC voltage. To achieve a high-precision DC voltage, voltage feedback adjustment of the output voltage is necessary. This power supply technology is mature, achieving high stability and minimal ripple, and it lacks the interference and noise of switching power supplies. However, its disadvantages include the need for a large and bulky transformer, and the required filter capacitors are also quite large and heavy. Furthermore, the voltage feedback circuit operates in a linear state, resulting in a voltage drop across the regulating transistor. When outputting a large operating current, this leads to excessive power consumption of the regulating transistor, low conversion efficiency, and the need for a large heatsink. This type of power supply is unsuitable for the needs of computers and other similar devices and will gradually be replaced by switching power supplies.
The difference between linear power supplies and switching power supplies lies in their operation. Linear power supplies first transform AC power through a transformer, then rectify and filter it through a rectifier circuit to obtain an unstable DC voltage. To achieve a high-precision DC voltage, voltage feedback adjustment is necessary to regulate the output voltage. From a performance perspective, this technology is mature, achieving high stability and minimal ripple, and it lacks the interference and noise of switching power supplies. Voltage feedback circuits, on the other hand, operate in a linear state, resulting in a voltage drop across the regulating transistor. When outputting a large operating current, the power consumption of the regulating transistor becomes too high, leading to low conversion efficiency.
In simple terms, voltage regulation in a linear power supply can be viewed as resistance regulation, which is equivalent to changing the voltage by adjusting a sliding rheostat. In contrast, a switching power supply changes the voltage by adjusting the switching frequency. Furthermore, compared to linear power supplies, the cost of both increases with output power, although the rates of increase differ.
(1) The cost of linear power supplies is higher than that of switching power supplies at a certain output power point.
Therefore, with the development and innovation of power electronics technology, switching power supply technology has continued to break through and innovate. This cost issue has instead led to the shift of switching power supply technology towards lower output power, providing a wide range of development opportunities for switching power supplies.
(2) The relationship between power electronic equipment and people’s work and life is becoming increasingly close. Electronic equipment cannot do without reliable power supply. After entering the 1980s, computers fully realized the use of switching power supply. In the 1990s, switching power supply entered various electronic and electrical fields.
(3) In fact, from the schematic diagram of the switching power supply, we can see that it does not use a bulky power frequency transformer, and because the power dissipation on the regulating transistor is greatly reduced, a large heat sink is unnecessary. This makes the switching power supply smaller and lighter. However, the biggest advantage of the switching power supply is its low power consumption and high efficiency. In the switching power supply circuit, the transistor, under the excitation signal, repeatedly repeats the switching state of 'on' and 'off', with an extremely fast switching speed and a frequency of only 50Hz, which greatly improves the power supply efficiency.
(4) Wide voltage regulation range of the switching power supply. The output voltage of the switching power supply is adjusted by the duty cycle of the excitation signal, and changes in the input signal voltage can be compensated by frequency modulation or pulse width modulation. In this way, it can still ensure a relatively stable output voltage when the mains voltage fluctuates greatly.
(5) The operating frequency of switching power supplies is currently basically 50kHz, which is 1000 times that of linear regulated power supplies. This improves the filtering efficiency after rectification by almost 1000 times. Even with half-wave rectification followed by capacitor filtering, the efficiency is improved by 500 times. Under the same ripple output voltage, the capacitance of the filter capacitor when using a switching power supply is only 1/500 to 1/1000 of that in a linear regulated power supply.
