The design and production of switch mode power adapters should start from the main circuit, where the power conversion circuit is the core of the switch mode power adapter. The structure of a power conversion circuit, also known as a switching power supply topology, has multiple types. The topology also determines the corresponding PWM controller and output rectifier/filter circuit. The following is a general process for designing and manufacturing switch mode power adapters.

1. Determine the structure of the circuit

Whether it is an AC/DC switching power supply or a DC/DC switching power supply, its core is a DC/DC converter. Therefore, the circuit structure of the switching power adapter refers to the structure of the DC/DC converter. The commonly used topology of DC/DC converters in switching power supplies is as follows:

a) Buck Converter, also known as buck voltage regulator
b) Boost Converter, also known as Boost Voltage Stabilizer
c) Flyback Converter
d) Forward Converter
e) Half Bridge Converter
f) Full Bridge Converter
g) Push pull converter

Step-down and step-up converters are mainly used in DC/DC switching power supplies where input and output do not require isolation; Flyback converters are mainly used in low-power AC/DC or DC/DC switching power supplies that require input and output isolation; Forward converters are mainly used in high-power AC/DC or DC/DC switching power supplies that require isolation for input and output; Half bridge and full bridge converters are mainly used in high-power AC/DC or DC/DC switching power supplies that require input and output isolation, where full bridge converters can provide greater output power than half bridge converters; Push pull converters are mainly used in DC/DC or DC/AC switching power supplies with lower input voltages that require isolation for input/output.

The output voltage of a step-down converter is lower than the input voltage, while the output voltage of a step-up converter is greater than the input voltage. In DC/DC converters with isolation transformers such as flyback, forward, half bridge, full bridge, and push-pull, it is convenient to achieve voltage reduction, boost, and polarity transformation of the power supply by adjusting the ratio of the first and second turns of the high-frequency transformer. This type of converter can be either a boost type, a buck type, or a polarity conversion type. When designing a switch mode power adapter, the first step is to choose a suitable circuit structure based on the input voltage, output voltage, output current, and whether electrical isolation is required.

2. Select control circuit

The switch mode power adapter achieves voltage conversion by controlling the on and off time of power MOSFETs or power transistors. Its control methods mainly include pulse width modulation, pulse frequency modulation, and hybrid modulation. Pulse Width Modulation, abbreviated as PWM; Pulse Frequency Modulation, abbreviated as PFM; Hybrid modulation is a method in which the pulse width and switching frequency are not fixed and can be changed from one another.

The PWM method has a fixed switching frequency and adjusts the duty cycle by changing the pulse width. Therefore, the switching period is also fixed, which provides convenience for designing filtering circuits, making it the most commonly used method. Currently, most switch mode power adapters use this method. To facilitate the design of switch power supply manufacturers, many IC manufacturers have designed PWM controllers as integrated circuits to facilitate the selection of power supply factories. In early power adapter products, we commonly used PWM controllers as follows:

a) Self excited oscillation type RCC control circuit
b) TL494 Voltage type PWM control circuit
c) SG3525 voltage type PWM control circuit
d) UC3842 Current Mode PWM Control Circuit
e) TOPSwitch II series PWM control circuit
f) PWM control circuit of TinySwitch series

The self-excited oscillation type RCC control circuit utilizes the positive feedback winding of the high-frequency transformer to achieve saturated conduction of the power switching transistor by starting the resistor, and the de saturation characteristic of the power switching transistor to achieve turn-off. Pulse width modulation is achieved by controlling the base current of the power switching transistor. It has the characteristics of simple structure and low cost, and is suitable for application in low-power flyback switching power adapters, such as standby power supplies for various electrical devices, mobile phone chargers, etc.

TL494 is a voltage type PWM control circuit chip with a fixed oscillation frequency, which includes all the control functions required by switching power supplies. It is widely used in push-pull, half bridge, and full bridge topology switching power supplies. The built-in power transistor can provide a driving capacity of 500mA, with two output modes of push or pull, suitable for driving bipolar power switching transistors. Suitable for constructing high-power switching power supplies.

SG3525 is also a voltage type PWM control circuit chip, which is an improved product of SG3524. The functions of SG3524 are basically the same as those of TL494. The SG3525 has a built-in soft start circuit with input undervoltage locking function, which can achieve pulse by pulse shutdown. Its driving output stage adopts a push-pull circuit structure, with a current filling/pulling capacity of over 200mA and a faster turning off speed. Not only can it drive bipolar power switching transistors, but it is also more suitable for driving field-effect power transistors (MOSFETs) to achieve higher switching frequencies and power conversion efficiency.

UC3842 is a current type PWM control chip circuit, which has advantages such as fewer pins, simple peripheral circuits, excellent performance, and low unit price. It is suitable for constructing low-power single ended flyback switching power supplies and is currently a preferred model for single ended PWM control circuits. This circuit has an undervoltage locking function and a high current totem pole output structure, suitable for driving bipolar power transistors and field-effect power transistors. Its current mode control mode makes it easy to limit the peak current of each cycle, effectively preventing magnetic saturation of high-frequency transformers and improving the reliability of switch mode power adapters.

The TOPSwitch II series single-chip switching power supply integrates all the functions of a PWM control system into a three terminal chip. It includes a pulse width modulator, field-effect power transistor, automatic bias circuit, protection circuit, high-voltage starting circuit, and loop compensation circuit. The output end can be completely isolated from the power grid through a high-frequency transformer. Externally, only rectifier filters, high-frequency transformers, drain clamp protection circuits, feedback circuits, and output rectifier filtering circuits are needed to form a flyback switching power supply adapter.

The TinySwitch series single-chip switching power supply is a high-efficiency, low-power, and low-cost four terminal single-chip switching power supply specific IC launched by PI Company in the United States. The TinySwitch series adds an enabling end to the TOPSwitch II series three terminal single-chip switching power supply, making it more convenient and flexible to use. The control system actually adopts a skip cycle method to achieve the voltage stabilization process, equivalent to a pulse frequency modulator. This series of products is particularly suitable for making micro switch power adapters or standby power supplies below 10W.

When designing a switch mode power adapter, it is necessary to choose a suitable PWM control circuit based on the topology of the main circuit, the size of the output power, and the application field of the power supply.