The power supply is an indispensable component of various electronic devices, and its performance is directly related to the technical specifications of the electronic device and whether it can work safely and reliably. Because the key components inside the switching power supply work in the high-frequency switching state, the power consumption is small, the conversion rate is high, and the volume and weight are only 20% to 30% of the linear power supply, so it has become the mainstream product of stabilized power supply.
1. The composition of the switching power supply
The switching power supply is composed of four parts: main circuit, control circuit, detection circuit and auxiliary power supply
1. Main circuit
Inrush current limit: limit the inrush current on the input side at the instant of power-on.
Input filter: Its role is to filter the clutter existing in the power grid and hinder the feedback of the clutter generated by the machine to the power grid.
Rectification and filtering: Directly rectify the AC power of the power grid to smoother DC.
Inverter: Turn the rectified DC power into high-frequency AC power, which is the core part of high-frequency switching power supply.
Output rectification and filtering: According to the needs of the load, provide a stable and reliable DC power supply.
2. Control circuit
On the one hand, it samples from the output terminal, compares it with the set value, and then controls the inverter to change its pulse width or pulse frequency to stabilize the output. On the other hand, according to the data provided by the test circuit, the protection circuit identifies and provides The control circuit performs various protection measures on the power supply.
3. Detection circuit
Provide various parameters and various instrument data in operation in the protection circuit.
4. Auxiliary power supply
Realize the software (remote) start of the power supply to supply power for the protection circuit and control circuit (PWM and other chips).
two. The working principle of switching power supply
A switching power supply uses a power semiconductor device as a switching element, and periodically switches the switch to control the duty cycle of the switching element to adjust the output voltage. The switching element is repeatedly turned on and off at a certain time interval. When the switch is turned off, the input power Vi provides energy to the load RL through the switch S and the filter circuit. When the switch S is turned off, the energy storage device in the circuit (The circuit composed of L1, C2, and diode D) The energy stored when the switch is turned on is released to the load RL, so that the load gets continuous and stable energy.
VO = TON / T * Vi
VO is the average voltage across the load
TON is the time each time the switch is turned on
T is the duty cycle of the switch
It can be seen from the formula that changing the ratio of the switch-on time and the duty cycle, the average voltage between VO also changes accordingly. Therefore, as the load and input power supply voltage changes, the ratio of TON and T is automatically adjusted to maintain the output voltage VO change. Changing the on-time TON and the duty cycle ratio changes the duty cycle of the pulse. This method is called Time Ration Control (TRC).
According to the TRC control principle, there are three ways:
1. Pulse width modulation (Pulse With Modulation, abbreviated as PWM): The switching period is constant, and the duty cycle is changed by changing the pulse width.
2. Pulse Frequency Modulation (Pulse Frequency Modulation, abbreviated as PFM): The conduction pulse width is constant, and the duty cycle is changed by changing the switching working frequency.
3. Mixed modulation: the conduction pulse width and the switching working frequency are not fixed, and can change each other. It is a mixture of the above two methods.
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