A switching transformer, also known as a pulse transformer, is a type of transformer used in switching power supplies, meaning it operates in a switching state. It typically operates in a pulse state, with frequencies ranging from tens of Hz to several kiloHz.
Regarding the classification of switching transformers, based on input voltage, there are single-ended and double-ended types; based on output voltage, there are forward and flyback types. Single-ended and double-ended switching power supplies, or forward and flyback switching power supplies, use switching transformers that differ significantly in their operating principles.
When the input voltage of a switching transformer is a DC pulse voltage, it is called a unipolar pulse input, and a switching power supply with this unipolar pulse input is called a single-ended transformer switching power supply. When the input voltage of a switching transformer is an alternating positive and negative pulse voltage, it is called a bipolar pulse input, and a switching power supply with this bipolar pulse input is called a double-ended transformer switching power supply. When the primary coil of the transformer is being excited by a DC pulse voltage, the secondary coil of the transformer is just having power output, and this switching power supply is called a forward transformer switching power supply. When the primary coil of the transformer is being excited by a DC pulse voltage, the secondary coil of the transformer does not provide power output to the load, but only provides power output to the load after the excitation voltage of the primary coil of the transformer is turned off, this type of transformer switching power supply is called a flyback switching power supply.
Let the cross-section of the core of a switching transformer be S. When a rectangular pulse voltage of amplitude U and width τ is applied to the primary coil of the switching transformer, an excitation current flows through the primary coil. Simultaneously, a magnetic field is generated in the core of the switching transformer, and the core is magnetized. Under the action of a magnetic field with strength H, a magnetic flux with magnetic induction intensity B is generated, referred to simply as magnetic flux, denoted by "φ". The process by which the magnetic induction intensity B or magnetic flux φ changes under the action of the magnetic field strength H is called the magnetization process. The so-called excitation current is the current that magnetizes and demagnetizes the transformer core.
Working principle of switching transformer
220V AC power is rectified and filtered to become 310V DC. This DC voltage is applied to the collector of a switching transistor through the primary winding of a switching transformer. Simultaneously, a bias voltage is applied to the base of the switching transistor, causing it to conduct. Due to the instantaneous conduction of the switching transistor, a current is generated in the primary winding of the switching transformer connected to the transistor's collector. This current generates a magnetic field in the transformer's core, which in turn generates a voltage in the feedback coil. This voltage is applied to the base of the switching transistor. This positive feedback voltage further increases the transistor's conductivity, strengthens the magnetic field, and increases the feedback voltage. The transistor continues to conduct until saturation. At saturation, the current through the transistor's collector is at its maximum, but the amplitude of current change decreases until it stops changing. Once the primary current of the switching transformer stops changing, the induced magnetic field disappears, and the feedback voltage also disappears. As the feedback voltage disappears at the base of the switching transistor, it rapidly transitions from a deep saturation state back to the conducting state. The collector current decreases rapidly, and this decreasing current generates a magnetic field on the core that is opposite to the one described above. This opposite magnetic field also generates a feedback voltage opposite to the one described above on the feedback coil. This voltage is applied to the base of the switching transistor, accelerating the transition from conducting to cutting off. After the switching transistor cuts off, the current disappears, the feedback voltage disappears, and the switching transistor, under the influence of DC bias, begins another cycle of conducting, saturating, conducting, and cutting off. This process repeats continuously, which is the working principle of a switching power supply.
Flyback: A flyback switching power supply refers to a switching power supply that uses a flyback high-frequency transformer to isolate the input and output circuits. "Flyback" means that when the switching transistor is on, the inductor in series in the output circuit is in a discharging state when the input is high; conversely, when the switching transistor is off, the inductor in series in the output circuit is in a charging state when the input is high.
Working principle: The primary and secondary windings of the transformer have opposite polarities, which is probably where the name "Flyback" comes from: a. When the switching transistor is turned on, the current in the primary inductor of the transformer begins to rise. At this time, due to the same polarity of the secondary windings, the output diode is turned off, the transformer stores energy, and the load is powered by the output capacitor. b. When the switching transistor is turned off, the voltage induced in the primary inductor of the transformer reverses direction. At this time, the output diode turns on, and the energy in the transformer supplies power to the load through the output diode, while simultaneously charging the capacitor to replenish the energy lost earlier. Evolution of the flyback circuit: It can be seen as an isolated Buck/Boost circuit:
In a flyback circuit, the output transformer T not only provides electrical isolation and voltage matching, but also stores energy. The former is a property of the transformer, and the latter is a property of the inductor. Therefore, some people call it an inductive transformer, and sometimes I also call it an asynchronous inductor.
Forward transformer switching power supplies have relatively good transient control characteristics and output voltage load characteristics, resulting in more stable operation and less output voltage fluctuation. They are frequently used in applications with high requirements for output voltage parameters.
A forward converter transformer switching power supply refers to a power supply where the secondary coil of the transformer is outputting power while the primary coil is being excited by a DC voltage.
A flyback transformer can be viewed as an inductor with a voltage-transforming function, forming a buck-boost circuit. A forward transformer only has a voltage-transforming function and can be considered as a buck circuit with a transformer. A flyback converter connects the negative terminal of the first rectifier diode to an electrolytic capacitor on the secondary side, while a forward converter connects it to an inductor.
In general, forward and flyback converters operate on different principles. A forward converter operates on both the primary and secondary windings; if the secondary winding is not operating, a freewheeling inductor provides current. It typically operates in CCM (Continuous Current Management) mode. The power factor is generally not high, and the input/output ratio is proportional to the turns ratio and duty cycle. A flyback converter operates on the primary winding while the secondary winding is not operating, with both windings operating independently. It typically operates in DCM mode. Theoretically, it has a unity power factor, but the transformer inductance is smaller, and an air gap is required. Therefore, it is generally suitable for small to medium power applications. Most power supply manuals will provide detailed explanations and design formulas.
A forward transformer is ideal, as it does not store energy. However, because the magnetizing inductance (Lp) is finite, the magnetizing current causes the core voltage (B) to be large. To avoid flux saturation, the transformer needs an auxiliary winding for flux reset. A flyback transformer, on the other hand, can be viewed as a coupled inductor; the inductor first stores energy and then releases it. Since the input and output voltages of a flyback transformer have opposite polarities, the secondary winding can provide a reset voltage to the core when the switching transistor is turned off. Therefore, a flyback transformer does not require an additional flux reset winding.
Main differences
The main difference between forward and flyback converters lies in their operating modes of the high-frequency transformers, although they operate in the same quadrant. In a forward converter, energy is transferred to the load simultaneously when the primary-side switching transistor is turned on; when the transistor is turned off, the transformer's energy is demagnetized through a magnetic reset circuit. In a flyback converter, energy is stored in the transformer when the primary-side switching transistor is on, but it is not applied to the load. When the transistor is turned off, the transformer's energy is released to the load side. In a forward converter switching power supply, the extra diode is a freewheeling diode, and an additional energy storage inductor is typically added to the output section. The most important difference between forward and flyback converters is that the primary and secondary windings of the transformer are out of phase.
Biggest difference
The biggest difference between forward and flyback converters is that when the switching transistor is off, the forward converter's output is mainly maintained by the energy storage inductor and freewheeling diode, while the flyback converter's output is mainly maintained by the energy released from the transformer secondary winding. Forward circuits are not suitable for multiple outputs. To use pulse width modulation (PWM) for voltage regulation in a forward circuit, an inductor must be connected in series after the secondary rectification; otherwise, the output voltage is mainly determined by the input and has little impact on the pulse width, which only affects the output ripple. Problems with using a forward circuit for multiple outputs include: if each output does not use an inductor, there is no voltage regulation effect on input changes, and the safety of a switching power supply is lacking. If an inductor is added to each output, the output voltage is theoretically related to the load size, and the loops not participating in the feedback will be malfunctioning.
Flyback circuits are inherently suitable for multi-output voltage regulation. A flyback circuit first stores energy, then supplies that energy to each output according to its voltage ratio. Initially, we can assume the output ratio of each output is constant (actually there may be errors, see below), and the energy is distributed according to the principle of giving more current to the output that needs it most.
Regarding feedback
The feedback path is always very accurate because it's based on the specified feedback signal, but the feedback path must have some load. Otherwise, it will increase the imbalance between the output paths. Multi-path weighted feedback can be used to make the vector sum of the errors zero. Simply put, it balances the errors across the paths; the path with the higher weight has the higher accuracy. The transformer adheres to the principle that the ratio of each transient voltage equals the coil ratio; this is one of the most frequently used conditions in understanding transformers.
About switching power supplies
In a forward converter, the secondary circuit of the transformer operates when the switching transistor is on, while in a flyback converter, the secondary circuit operates when the switching transistor is off (the switch is off, energy is transferred from the primary stage to the secondary stage; when the secondary stage is active, the switch closes, and the primary stage inductor stores energy). Forward converters require a freewheeling diode at the output, while flyback converters do not (their transformer windings are also different; the primary and secondary windings of a flyback transformer operate at opposite times, while the primary and secondary windings operate at the same times). The biggest problem with forward converters compared to flyback converters is the use of more components. Although the number may seem small, these are all essential and significantly more expensive. Forward converters require a large energy storage filter inductor and a freewheeling diode. The output voltage of a forward converter is modulated by the duty cycle much less than that of a flyback converter. Therefore, forward converters require a higher amplitude error signal for duty cycle control, and a larger gain and dynamic range for the error signal amplifier.
In order to reduce the excitation current of the transformer and improve its operating efficiency, the volt-second capacity of the transformer in a forward converter switching power supply is generally larger. In addition, in order to prevent the back electromotive force generated by the primary coil of the transformer from breaking down the switching transistor, the transformer in a forward converter switching power supply has an additional back electromotive force absorption winding compared to the transformer in a flyback converter switching power supply. Therefore, the transformer in a forward converter switching power supply is larger than the transformer in a flyback converter switching power supply.
A more significant drawback of forward converter switching power supplies is that the back electromotive force (EMF) voltage generated by the primary coil of the transformer is higher when the control switch is turned off compared to that generated by flyback converter switching power supplies. This is because the duty cycle of the control switch in a typical forward converter switching power supply is around 0.5, while the duty cycle in a flyback converter switching power supply is typically much smaller.
Application differences
Forward transformers do not store energy; they only handle coupling transmission. Flyback transformers, on the other hand, store the energy generated during the turn-on process and release it during the turn-off process. Forward windings are in phase, while flyback windings are out of phase. Forward transformers do not require inductance adjustment, while flyback transformers do. Forward transformers exhibit residual magnetism, necessitating a demagnetizing circuit to prevent saturation. Since they do not store energy, they require an energy storage coil and a freewheeling diode, whereas flyback transformers do not.
Flyback converters are mainly used in applications below 150-200 watts, while forward converters are used in applications from 150W to several hundred watts. The reason flyback converters are more widely used is because power supplies below 100W are more common in our daily lives, making them more prevalent. The principle is the same: one stores energy and then transforms the voltage through a turns ratio, while the other transforms the voltage directly through the turns ratio. In a forward converter, both terminals of the primary winding are positive, hence the name "forward"; in a flyback converter, one terminal is positive and the other is negative, hence the name "flyback."
Flyback converters are suitable for low power applications, are inexpensive, and relatively easy to debug, making them commonly used in low-power power supplies. The differences between them are: Regarding the main transformer, forward converters require an additional demagnetizing winding, although some use two diodes in the main winding for demagnetization. Regardless, forward power supplies must include a demagnetizing circuit. Flyback converters do not require an additional output energy storage inductor because the energy is stored in the secondary coil. Forward converters require an additional output energy storage inductor, and the rectifier section needs a freewheeling diode.
