Basic working principle of linear power supply
As mentioned earlier, power supplies are divided into two categories, linear power supplies and switching power supplies. Although switching power supplies are more complicated than linear power supplies, their basic working principles are the same. In fact, they are all closed-loop negative feedback. The actual function of negative feedback is to stabilize the output voltage.
Linear power supplies are the step-down power supplies we mentioned earlier, that is to say, the output voltage is lower than the input voltage of the power supply.
There are also two types of linear power supplies: parallel and series. The voltage adjustment unit of the parallel power supply is connected in parallel with the load, and the voltage regulator tube connected in parallel with the load shunts the load current to keep the load voltage stable when the input voltage or load current changes.
The series power supply is more efficient than the parallel power supply, and its voltage adjustment unit uses active devices to be connected in series between the input power supply and the load.
The series adjustment unit works in a linear mode, which means that the adjustment unit is not designed to be in a full-on or full-off working mode, but in a "partially on" working mode. The negative feedback loop determines the degree of conduction of the regulation unit to maintain the output voltage stability.
The core device of the negative feedback loop is a high-gain operational amplifier. We usually call it a voltage error amplifier, which is the voltage adjustment device in the above figure. Its function is to continuously compare the output voltage with a stable reference voltage. , when the output has only a few millivolt errors, the output can be adjusted by changing the conduction voltage of the series adjustment unit. The stable voltage reference is connected to the positive input terminal of the operational amplifier. The divided output voltage is close to the voltage reference and connected to the negative input terminal of the operational amplifier. When the output voltage is stable, the divided output voltage is equal to the reference voltage.
The gain of the voltage error amplifier is to amplify the difference between the voltage reference and the output voltage many times, and the amplified error voltage directly controls the on-resistance of the series adjustment unit to maintain the rated output voltage. If the load increases, the output voltage will drop, then the output of the amplifier will increase, allowing more current to flow to the load; similarly, if the load decreases, the output voltage will increase, and the output of the amplifier will allow the pass cell to flow to the load current decreases.
The response speed of the voltage error amplifier to output changes and the control accuracy of the output voltage depend on the feedback compensation design of the error amplifier. The size of the negative feedback compensation is determined by the voltage dividing resistor and the resistance between the negative input terminal and the input terminal of the voltage error amplifier. It determines the size of the DC gain and the accuracy of the output voltage, and also determines the gain and bandwidth of the amplifier at high frequencies, that is, the response time or transient response time to load changes.
The working process of a linear power supply is very simple, that is, to use voltage negative feedback to stabilize the output voltage of the power supply.
