Flyback Converter

The physical design of the transformer is critical in flyback converters. If they are not properly designed physically, excessive voltage spikes could be generated that would adversely affect the reliable operation of the semiconductor components (refer to Section 3.5.8). 

Figure 3-27 The effects of interleaving on the electrical waveforms of an off-line flyback converter. (Note spike amplitude and overall degree of "ringing.")...

Watt, Universal AC Input, Multiple-output Flyback Converter... 

Rule of thumb for MOSFET-based flyback converters ... 

The topology is going to be an isolated, multiple output flyback converter that must meet the safety requirements of UT, CSA, and VDE. These considerations affect the design of the final packaging, transformer, and voltage feedback designs. 

Figure 4-3 Major parasitic elements within converters (a) buck converter (b) flyback converter.

Figure 4-5 Lossless snubber for a one-transistor forward or flyback converter.

Figure 4-7 An active clamp used in a one-transistor forward or a flyback converter.

A15 Watt, ZVS Quasi-resonant, Current-mode Controlled Flyback Converter... 

Determining the core size. TDK rates its cores by the amount of power that can be handled by the core in a one-transistor forward converter. Its volume requirements are very similar to a flyback converter. The EPC core that rated at 15 W or greater is the EPC 17 core size. The part numbers for this assembly are core, PC40EPC17-Z bobbin,BER17-llllCPH and clamp, FEPC17-A. 

B.2.2 Voltage-mode Controlled Flyback Converter and Current-mode Controlled Forward-mode Converter Control-to-Output Characteristics... 

The operation of a discontinuous-mode, flyback converter is quite different from that of a forward-mode converter, and likewise their control-to-output characteristics are very different. The topologies that fall into this category of control-to-output characteristics are the boost, buck/boost, and the flyback. The forward and flyback-mode converters operating under current-mode control also fall into this category. Only their dc value is determined differently. Their representative circuit diagram is given in Figure B-12. 

The dc gain exhibited by the power section of the switching power supply of a current- or voltage controlled flyback converter is approximately... 

The output filter pole in both voltage-mode controlled flyback converter and the current-mode controlled forward and flyback is highly dependent on the equivalent resistance of the load. This means that when the load current increases or decreases, the location of the output filter pole moves. The filter pole can be found from... 

Figure B-12 The control-to-output model for a voltage-mode controlled flyback converter.

Figure B-19 An example of single-pole with in-band gain limiting compensation used with a voltage/current-mode controlled flyback converter.

The next task is to determine the plaeement of the eompensating zero and pole within the error amplifier. The zero is plaeed at the lowest frequency manifestation of the filter pole. Since for the voltage-mode controlled flyback converter, and the current-mode controlled flyback and forward converters, this pole s frequency changes in response to the equivalent load resistance. The lightest expected load results in the lowest output filter pole frequency. The error amplifier s high frequency compensating pole is placed at the lowest anticipated zero frequency in the control-to-output curve cause by the ESR of the capacitor. In short ... 

Figure E-1 The radiated spectrum of a typical off-line PWM flyback converter.

Figure E-2 The radiated spectrum of a ZVS QR off-line flyback converter.

The last filter that will be looked at in this chapter is the EMI filter. This filter is commonly used on the input of a power circuit to reduce conducted and reflected emissions. For instance, a flyback converter can draw current from the bus that looks like a sawtooth waveform with... 

The following circuit (Fig. 4.72) shows the implementation of a quasi-resonant flyback converter featuring the STR6600. 

Figure 4.72 Circuit schematic for quasi-resonant flyback converter.

Figure 4.82 Actual schematic of the two-phase discontinuous flyback converter.

The discontinuous mode flyback converter is especially well suited to the task, since it has a wide dynamic operating range also providing input-output isolation while maintaining the simplest topology. In addition the input can be made to look resistive if both the on time and the frequency are fixed. The downside of the flyback converter is that it is best suited to lower power requirements. 

Figure 4.83 SPICE model of a single phase discontinuous flyback converter.

Figure 4.84 PSpice model of a two phase voltage-mode flyback converter.

The peak input current of the discontinuous flyback converter, with a fixed duty cycle and fixed frequency, is defined by... 

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