1 Introduction to the principle of switching power supply
There are many kinds of switching power supply circuits, but the most common one is the flyback converter. The principle is shown in Figure 1. The power input is first rectified, then filtered, then passed through the transformer and primary switch, and the primary controller; this control The duty cycle of the switch is changed according to the feedback signal, and the feedback signal is derived from the secondary feedback.
Figure 1 Principle circuit of switching power supply
Although an inductor can be used, the design shown uses an unisolated transformer. Isolation design is more common in offline devices. In off-line devices, the transformer has an isolation function that facilitates duty cycle adjustment. The flyback switching power supply can operate in both discontinuous conduction and continuous conduction modes. The discontinuous conduction mode is shown in Figure 2. Ilm and Vlm are the current through the transformer's magnetizing inductance and the applied voltage.
Figure 2 Two conduction modes of the switching power supply
When the switch is closed, a voltage is applied across the primary of the transformer because the secondary diode is turned off and the transformer acts like an inductor. The current through the primary coil rises while the energy is stored in the magnetic flux. When the switch is turned off, the secondary diode turns on and the current drops as it passes through the secondary because the energy is converted to the secondary bulk capacitor. If the current drops to zero after passing through the magnetizing inductance region, this is a discontinuous conduction mode. If the magnetizing current does not drop to zero, as shown in Figure 3, the system operates in continuous conduction mode.
Figure 3 Discontinuous conduction mode of the switching power supply
Both modes have their own advantages and disadvantages and can be selected according to design requirements. You can choose a continuous mode design with a large load, or a discontinuous mode design with a small load. There are two control modes, voltage and current. In the voltage mode, the secondary voltage is fed back to directly control the duty cycle. In the current mode, the secondary voltage is fed back to control the maximum switching current, that is, the PWM part of the control IC. The switch is closed and the switch is opened when the current reaches the limit set by the feedback.
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