When one also considers solving the saturation problem that occurs in a flyback converter, a forward converter is suitable for regulation up to hundreds of Watts. The circuit diagrams below show a typical flyback converter and forward converter. Note the polarity of the transformers on the input and output sides; the primary and secondary windings transformer have the same winding direction in order to ensure the currents have the same polarity, and a ratio of N:1 primary:secondary is normally used to analyze this circuit.
The PWM signal is applied to the gate on Q1. The topology is similar to that in a flyback converter, although there is an additional diode and inductor on the output side, as well as the use of a three-winding transformer versus a two-winding transformer. Compared to a bust-boost switching regulator, a forward converter takes advantage of a transformer for galvanic isolation to suppress conducted EMI from propagating to the output side of the converter.
The inductor on the output side, the duty cycle of the PWM signal, and the switching frequency are the primary factors that determine the level of regulation enforced on the output current.
Very simply, using a higher frequency PWM signal and larger output inductor will reduce the peak-to-peak fluctuation on the output current, which appears as a triangle wave. The ripple on the output current and its defining equation are shown in the graph below.
Note that N is the turns ratio shown in the above forward converter circuit diagram. Output current fluctuations from a forward converter. The primary simulations involved in analyzing a forward converter are time-domain simulations with a PWM signal.
Using a DC sweep allows you to examine power conversion efficiency and determine when the output starts to behave like a highly nonlinear circuit. Transient analysis is also critical for examining the effectiveness of your PWM frequency you use. The transient response time will be a limiting factor that determines the switching frequency you can use.
Normally, you would reduce the transient response time equal to the RC time constant from the capacitor and load resistance by using a smaller capacitor across the output, but this causes greater ripple to be seen on the output. One option is to use a lower PWM frequency and add filtration on the output to suppress residual switching noise and PWM ripple.
You can reach even higher power efficiencies with similar performance in other areas by using an active-clamp forward converter. This design only requires a two-winding transformers, but it forces transformer reset with multiple FETs on the input and output sides of the converter. A circuit diagram for this converter is shown below. Circuit diagram for an active-clamp forward converter.
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To avoid signal integrity problems in their layouts, design engineers need to be familiar with PCB design guidelines for high speed. The rectified output from the transformers secondary winding is connected to the load. As per the above block diagram, when the switch is turned ON, the input is applied to the primary winding of the transformer and a voltage is appeared at the secondary winding of transformer.
Therefore, the dot polarity of the windings of transformer is positive, due to this the diode D1 gets forward biased. Then the output voltage of the transformer is fed to the low pass filter circuit which is connected to the load. When switch is turned OFF, the current in the windings of transformer comes down to zero assuming the transformer to be ideal.
The forward converter said to be in powering mode when the transistor is in ON state. In this condition, the supply voltage is connected to the primary side winding of the transformer and also diode D1 gets forward biased in this condition. Diode D2 will not conduct in this condition, as it will remain reversed biased.
Both the windings starts conducting simultaneously when transistor is in ON state. And, this output voltage is applied to the secondary circuit, which consists of L-C filter. The maximum received output voltage, in case of ideal transformer, at the load will be:. As the transistor turns off, the current of windings of transformer falls to zero ideally. D1 will be reversed biased in this condition, therefore separates the output section of circuit from the transformer and the input.
However, the inductor at the secondary side maintains a continuous flow of current through the freewheeling diode D2.
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