Power supplies for personal computers are built on transformerless network connection schemes (in fact, this is a pulsed power supply device). They are characterized by a high value of efficiency, small overall dimensions and low weight. However, a transformerless power supply has a significant drawback - impulse noise, which negatively affects other PC circuits and other household electronic devices. Therefore, each such product undergoes a serious test of electromagnetic compatibility with other equipment. In addition, the transformerless power supply does not have galvanic isolation in terms of the electrical circuit from the supply voltage. This, in turn, requires the use of special safety measures during the repair of such a product.
Transformerless power supply: the main functional parts
The device consists of the following components:
- network rectifier;
- anti-interference filter;
- start-up circuit;
- capacitive filter;
- voltage converter based on a pulsed power inverter;
- output voltage generating circuit;
- management scheme;
- a chain for generating and transmitting feedback signals to a control circuit.
Additional items
Depending on the model and functional purpose, a transformerless power supply unit may contain additional circuits, such as a linear stabilizer in discrete or integrated design, overload protection of input and output voltage and current, as well as noise suppression circuits. In addition, such PSUs may include circuits for generating special control signals that ensure coordination of the operation of the device with the fed circuit.
Principle of operation
Switching universal power supplies to obtain a DC voltage carry out a triple conversion of the input voltage. First, it straightens and smoothes. The result is a DC voltage, after which the formation of rectangular pulses with a frequency of tens of kilohertz occurs. After that, the received pulses are transformed to the secondary side with the corresponding coefficient, and rectification and smoothing occur again. The main node of transformerless power supplies are key voltage converters, and first of all their power unit (output stage of high power). These cascades are usually divided into single-cycle and push-pull.
Adjustable Power Supplies
To adjust the values of the output voltage in transformerless devices, the method of pulse-width modulation is most often used , which consists in changing the duration of the pulses, as well as the pauses between them. In this case, the conversion frequency remains unchanged. The relationship between values such as pause duration and pulse depends on the value of the output voltage. It automatically changes to maintain the output voltage within the rated value.