A two-switch forward converter based on SG3525 control

The two-switch forward converter is widely used in high-input medium- and high-power power products due to its low switching voltage stress, strong internal bridge-to-bridge capability, and high reliability. In the switching power supply system, the design of the pulse width modulator is a key issue. The pulse width modulator used in the system described in this paper is the second generation product of the American Silicon General Company, SG3525, which is an excellent performance, full-featured and versatile. Strong monolithic integrated PWM controller. Because it is simple, reliable, and easy to use and flexible, it greatly simplifies the design and debugging of the pulse width modulator.

The topology of the two-switch forward converter is shown in Figure 1. The basic working principle is: S1 and S2 are simultaneously turned on and turned off at the same time. When S1 and S2 are turned on, the power supply outputs power to the load via the transformer and charges C. When S1 and S2 are turned off, the output current continues to flow through D4, and the excitation current of the transformer winding returns to the power source through D1-VIN-D2. Since the conduction of D1 and D2 causes the switching tubes S1 and S2 to withstand only the supply voltage. Although this double-tube single-ended forward circuit uses one more switch tube, its voltage is half lower than that of the single tube, and the transformer has one flux reset winding, so it is suitable for occasions with high input voltage.

Figure 1 Dual-tube forward circuit topology

With the development of power electronics technology, various high-power fully-controlled devices have emerged one after another, among which MOS-type power transistors have developed very rapidly due to its high withstand voltage, low drive power, good frequency response characteristics and short switching time. It is often used as a switching tube in switching regulated power supplies and DC chopper circuits. The switching tube is controlled by pulse width modulation (PWM). Silicon General Electric has designed a second-generation integrated circuit pulse width controller for high-frequency power MOSFET driving, of which SG3525 can be used to drive N-channel power MOSFETs.

As can be seen from the block diagram of Figure 2, the SG3525 mainly consists of a reference regulator source, an oscillator, an error amplifier, a PWM comparator and a latch, a phase splitter, a NOT gate circuit, and a totem output circuit.

Figure 2 SG3525 internal structure

Drop Cable

Drop Cable,Fiber Drop Cable,Bus Drop Cable,Fiber Optic Drop Cable

Huizhou Fibercan Industrial Co.Ltd , https://www.fibercannetworks.com