Double-switch forward converter and its application design - Power Circuit - Circuit Diagram

A single-switch (or single-transistor) forward converter is one of the most basic types of transformer-based isolated buck converters, and is widely used in applications that require large step-down ratios. The advantages of this converter include the need for a single ground reference transistor and the non-pulse output current to reduce the rms ripple current content of the output capacitor. However, the power capability of this converter is less than the half-bridge or full-bridge topology, and the transformer requires a core reset, limiting the maximum duty cycle of this converter to approximately 50%. In addition, metal oxide semiconductor field effect transistor (MOSFET) switches have drain voltage variations that are twice or more than the input voltage, making this topology difficult to use in higher input voltage applications.

In the forward converter, the magnetic core of the transformer is magnetized in one direction, and corresponding measures are required to reset the core to the initial value in each switching cycle. Otherwise, the excitation current will increase in each switching cycle, and several cycles are experienced. This will saturate the core and damage the switching device. In contrast, if there is a core reset, the current will not increase in each switching cycle, and the voltage will be inverted based on the magnetizing inductance (Lmag) and the core will be reset. Figure 1 takes a single-switch forward converter as an example. It briefly compares the circuit diagram with no core reset and core reset and the magnetizing inductor current waveform.

There are three common standard core reset techniques, three windings, resistor, capacitor, diode (RCD) clamp and two-switch forward. See Figure 1b) for the circuit diagram of the tertiary winding core reset technique. This technique can provide a duty cycle greater than 50%, but the peak voltage of switch Q1 may be greater than twice the input voltage, and the transformer has three windings to make the transformer structure. More complex. The RCD clamp core reset technique also enables duty cycles greater than 50%, but requires equations and simulations to verify the correctness of the reset and complicate the design process. The cost of the RCD clamp technology is lower than that of the tertiary winding technique, but the power consumption is affected by the clamp resistor in the reset circuit.

Since the NCP1252 is a two-switch forward converter, the maximum voltage limit of the power MOSFET as a switch is the input voltage. Usually the drain-to-source breakdown voltage (BVDSS) imposes a derating factor equal to 15%. If we choose a 500 V power MOSFET, the maximum voltage after derating should be: 500 V x 0.85 = 425 V. The power MOSFET we chose is the FDP16N50 in the TO220 package. The BVDSS is 500 V, the on-resistance (RDS(on)) is 0.434 Ω (@Tj=110°C), and the total gate charge (QG) is 45 nC. The drain-to-drain charge (QGD) is 14 nC.

For the calculation of the conduction loss and switching conduction loss of the MOSFET, see equations (13) to (14):