The linear power supply can only output lower voltage than the input voltage, so all the linear power supply requires the input voltage is higher than the output minimum differential voltage, this differential voltage is called the drop voltage, this parameter determines the efficiency of the linear power supply and the maximum power loss
Taking a linear DC power supply with a voltage of 9V and a maximum current of 2A as an example, the drop voltage of the linear regulator is generally 2V. If lithium batteries are selected, when the voltage at both ends of each battery drops to 4.2V and the discharge begins, the normal operating voltage of the circuit needs at least 11V voltage (9V load plus 2V drop voltage), so at least three lithium batteries are needed. At this time, the minimum input voltage of the circuit is 11V, the maximum is 12.6V, the load power (when the battery is discharged) is 18W, and the linear regulator consumes 4-7.2W when the battery is discharged, so the linear regulator efficiency is only 70%+.
If the energy provided by each cell is reduced, the efficiency of the circuit will be improved. In theory, if the battery is operated at 11V, the efficiency of the circuit can be improved and the cost can be reduced (at the expense of the number of charge and discharge cycles of the battery). In fact, the minimum charging voltage of the battery reaches and works at 9V, which seriously reduces the battery charging efficiency and life. Moreover, this part of the battery is lost because the energy generated by the efficiency problem is all heat, so a linear regulator that provides a constant voltage from the battery is quite expensive.
The above can also be achieved with a simple switching power supply, switching devices using FET, its conduction is only 0.008 euro, the maximum power consumption on the switching tube compared with the linear power supply is much smaller, circuit efficiency up to 99% or more. More importantly, compared with linear power supply, which requires at least three cells, switching power supply has almost the same circuit efficiency whether using single or multiple cells.
Transformer is an essential component of linear DC power supply, the output power of 1kW linear power supply, the transformer weight is about 453g, if the requirements of multiple operating frequency will be more bulky, the required semiconductor components need a large number of heat sinks and fans, different input voltage levels of linear power supply need electronic switches to switch. In contrast, the switching power supply circuit that can work at different voltages and frequencies does not require electronic switching switches, weighs no more than 220g, the volume is only 1/4 of the linear power supply, and the switching power supply cost is much less than the linear power supply cost.
Switching power supply also has its own disadvantages, such as the output contains high frequency noise, linear power supply is quieter than switching power supply, in susceptible to noise interference in analog circuits, linear power supply is the best choice. Another disadvantage of the switching power supply is that if the load current or input voltage is abrupt, its recovery time is longer than that of the linear power supply. In order to achieve maximum efficiency, most of today's power supply systems first use switching power supply for voltage modulation.
In the above example, we assume that the power loss of the switch tube is fixed. In a more detailed analysis, the loss of the switch tube during the turn-on and turn-off process and the power loss required to drive the switch tube should be included. In addition, there are still dedicated linear regulators with very low drop voltages for small power applications, so the linear supply is still suitable for small power applications.
Summary: Linear power supply circuit is simple, less noise, volume and weight heat, poor efficiency; Switching power supply circuit is complex, high voltage components, switching noise, small volume light weight, less heat, high efficiency.