Design of single-ended flyback switching power supply based on TOPSwitch and PI Expert

Switching power supplies are widely used in various electrical equipment and systems due to their small size, light weight and high efficiency. The quality of their performance is directly related to the realization of the entire system function. There are many types of switching regulated power supplies. Among them, single-ended flyback switching power supplies are widely used in the field of small power supplies due to their simple circuits, fewer components required, and the ability to provide multiple isolated outputs.

Traditional single-ended flyback power supplies are generally composed of PWM control chips (such as UC3842) and power switch tubes (MOSFET is generally used when the frequency is high). The design of the PWM chip control loop is complex, which can easily cause system instability. The power switch tube sometimes requires an external drive circuit. In addition, the design of the flyback transformer is also a difficult point, which often leads to an extension of the power supply design cycle. With the advent of a new generation of monolithic switching power supplies represented by TOPSwitch produced by PI, many of the above problems have been well solved. Using TOPSwitch to design switching power supplies not only has fewer components, simpler structure, less heat generation, and more reliable operation, but also the matching software design platform Pl Expert makes the design of transformers extremely easy. The combination of the two has become an efficient switching power supply design solution. This article uses a specific design example to explain in detail how to use TOPSwitch and PI Expert to design a switching power supply, and verifies it through experiments.

1 Basic working principle of single-ended flyback power supply

The basic working principle of a single-ended flyback switching power supply is relatively simple. The single-ended flyback circuit based on TOPSwitch using a basic feedback circuit is shown in Figure 1, where transformer T1 has three functions: energy storage, primary-secondary isolation, and voltage conversion.

When TOPSwitch is turned on, the secondary rectifier diode D2 is cut off, and the energy of the output load is provided by C1; after TOPSwitch is turned off, the magnetic flux in the transformer core decreases, the polarity of the secondary winding voltage is reversed, the rectifier diode begins to conduct, and the energy stored in the transformer is delivered to the load, while replenishing the energy previously reduced by C1.

According to whether the primary current of the transformer is reduced to zero, the single-ended flyback switching power supply can be divided into two operating modes: intermittent and continuous. Different operating modes have a certain impact on the efficiency of the entire power supply and the selection of related parameters.

2 Introduction to TOPSwitch and PI Expert

The TOPSwitch series of monolithic switching power supplies is a new type of switching power supply chip developed by Power Integrations (PI) in the United States. It integrates various functional modules required for offline switching power supplies into one chip, including high-voltage power field effect transistor MOSFET, PWM controller, high-frequency oscillator, high-voltage startup bias circuit, reference voltage, error amplifier, parallel bias regulator for loop compensation, and various protection circuits.

TOPSwitch has three main pins, namely DRAIN (D), SOURCE (S) and CONTROL (C). Additional pins include over-voltage and under-voltage detection (L), current limit (X) and frequency selection (F).

When in use, a 47μF bypass capacitor should be connected between C and S, and the charging process of the capacitor is used to realize the soft start of the circuit. The CONTROL pin is the error amplifier and feedback input pin to realize duty cycle adjustment control.

PI Expert is an interactive power supply design software platform developed by PI specifically for TOPSwitch. It is easy to use, flexible and convenient, and is an efficient switching power supply design tool.

3 Design practice

This paper designs a single-ended flyback multi-output switching power supply, which is used as a control and drive power supply in the electrical transmission system. Its input voltage is AC 85~265V, and the entire circuit structure is simple, reliable, and has over-voltage and under-voltage protection functions. The outputs are electrically isolated from each other. The output power of the control power supply is large and the voltage regulation accuracy is ±5%, and the output power of the drive is small and the voltage regulation accuracy is ±10%.

3.1 Circuit structure selection

The circuit structure of the single-ended flyback switching power supply can be divided into the following types according to the different feedback methods used:

(1) Primary/Basic type, using windings for feedback;

(2) Primary/Bnhanced type, using windings and a Zener diode for feedback;

(3) Opto/Zener type, using optocouplers and Zener diodes for feedback;

(4) Opto/TL43l type, using optocouplers and precision reference source TL43I for feedback.

The performance of the above structures is enhanced in turn, but the cost and complexity also increase in turn.

The Primary/Basic type structure is the simplest, requiring only one resistor, using the magnetic coupling relationship between the windings, and directly using the windings for feedback (as shown in Figure 1). This design adopts this structure, and adds a voltage regulator chip (such as 7805) to the +5V control power supply that requires higher voltage regulation accuracy. This not only ensures that the required output has sufficient voltage regulation accuracy, but also eliminates the feedback optocoupler and precision reference source TL43l. The circuit structure is simple, the operation is reliable, and it is also desirable from a cost perspective.

The system circuit diagram is shown in Figure 2.

In Figure 2, P6KE200 and BYV26C form a shutdown overvoltage absorption circuit, R1 is the over-voltage and under-voltage detection resistor, R2 is the overcurrent detection resistor, NF is the feedback winding, and RF is the feedback resistor.