The demand for electrical and electronic power sources in innovative automotive design can be summarized as follows: increasing power, improving efficiency, reducing space requirements, and enhancing reliability. For electric vehicles (EVs), efficiency is crucial in alleviating users' "range anxiety". Considering the various requirements of electric vehicles, we need to provide compact and lightweight power solutions for backup and auxiliary power sources. Smaller power supplies bring more challenges, including the need for greater isolation capabilities to prevent electrical breakdown between components with closer spacing and reduce electromagnetic interference (EMI).
Flyback power converters are commonly used in various low-power electric vehicle applications, including generating auxiliary power, battery management, and gate drive power. Its design is simpler, with fewer components, thereby reducing size, improving reliability, and lowering costs. The core of a flyback power supply is the flyback transformer, which is typically one of the largest components required to support high-voltage isolation.
This article introduces the working principle of flyback converters, the effects of parasitic inductance and capacitance, and the importance of component size and signal isolation. Then, Bourns' flyback transformer was introduced, and how it helped solve many automotive power supply problems was explained.
Flyback converter
The core of a flyback converter is a flyback transformer, which provides power transmission and isolation between the primary and secondary sides of the converter circuit (Figure 1, top). The converter can boost or buck the voltage of the DC power supply according to the configuration of the flyback transformer. In addition to a flyback transformer, the circuit also requires a primary side switch (SW) (usually MOSFET) and a secondary rectifier/filter.
Simplified schematic diagram of basic components of flyback converter
Figure 1: A simplified schematic diagram of the basic components (top figure) and important operating waveforms (bottom figure) of a flyback converter is shown. (Image source: Bourns Inc.)
By placing Vgs in a high level state (Figure 1, bottom), the duty cycle begins when SW is turned on. When the switch is closed, the voltage applied to the inductor is a step function. Inductors can counteract any instantaneous changes in current and integrate the applied step voltage. This creates a ramp function, where the current in the primary winding of the flyback transformer increases linearly due to the influence of the primary inductance. Due to the reverse bias of the rectifier diode (D), there is no current in the secondary of the transformer. The air gap in the core of the flyback transformer can prevent saturation when the transformer magnetic field increases.
When the switch is turned off (by restoring Vgs to a low state), the energy stored in the transformer's magnetic field is transferred to the secondary through the forward biased diode, charging the output capacitor (C2). The secondary current decreases linearly until the magnetic field energy is exhausted or the switch is opened again, starting the next cycle.
A typical transformer, such as a transformer in a linear power supply, continuously transfers energy from the primary winding to the secondary winding. The working principle of a flyback transformer is more similar to a pair of coupled inductors, as it does not continuously transmit energy during the working cycle. However, like transformers, the output voltage can also be adjusted by changing the turns ratio between the primary and secondary windings. The flyback transformer also provides electrical isolation between the primary and secondary windings. In addition, it also supports multiple secondary windings, allowing the converter to output multiple voltages.
Parasitic effects of flyback converters
As a typical electronic circuit, flyback converters are affected by parasitic inductance and capacitance (Figure 2).
Schematic picture of flyback converter
Figure 2: The schematic diagram of the flyback converter is shown, with the red highlighted parasitic capacitance and inductance related to the converter components. (Image source: Bourns Inc.)
Magnetized inductance (Lm) is the main inductive property that determines the energy storage of flyback transformers. Also related to transformers is parasitic leakage inductance (Llk) in series with switches. When the switch is disconnected, it will attempt to maintain the primary current and increase the voltage across the switch. Most flyback converters use clamp circuits or buffer circuits to protect switches from the effects of such transient voltages. This effect will also increase magnetic field radiation and affect electromagnetic interference. The circuit board routing inductance (Ltr) increases these effects.
Transformer designers will make every effort to minimize leakage inductance. The main method is to increase the coupling between the primary and secondary windings. To achieve this, it is necessary to minimize the spacing between windings and arrange them in a staggered manner.
The distributed capacitance includes primary capacitance (Cp), inter winding capacitance (Cps), secondary capacitance (Cs), field-effect transistor output capacitance (Co), and secondary diode capacitance (Cd). These capacitors interact with inductors, reducing the integrity of the converter signal waveform (Figure 3).
Schematic diagram of the influence of parasitic components such as capacitors and inductors on switch waveforms (click to enlarge)
Figure 3: The influence of parasitic components such as capacitors and inductors on the switching waveform is shown. (Image source: Bourns Inc.)
The switch waveform is preferably a rectangular pulse without overshoot or undershoot. The fast conversion time of this rectangular pulse ensures that the voltage waveform is at zero before the current increases. In fact, the effects of parasitic capacitance and inductance can slow down the conversion time and cause overshoot, undershoot, and instantaneous oscillation. In addition, due to the overlap of non-zero primary voltage and current waveforms, slower rise and fall times will increase the switching losses of the converter. This overlap will result in switching losses in FET switches, thereby reducing the efficiency of the converter. The significant decrease at the top of the pulse is caused by the load resistance and magnetizing inductance.
When designing a flyback transformer, efforts must be made to keep the self resonant frequency away from the switching frequency of the converter and to shorten the wiring between the switch and the flyback transformer as much as possible, which helps to minimize parasitic capacitance. In addition, the inter winding capacitance also provides a path for coupling the high-frequency components of the primary signal to the output. The larger the capacitance between windings, the greater the conducted EMI radiation of the converter. To achieve optimal performance, trade-offs need to be made in the design, as tighter winding coupling reduces leakage inductance but also increases inter winding capacitance. This is where the importance of transformer designers' experience lies.
Reduce size and isolate signals
Components used in automotive applications should be as small as possible. The physical dimensions of components are determined by the material properties and the physical characteristics of component functionality. For flyback transformers, the conductor spacing must be sufficient to withstand peak operating voltage and voltage testing required for standard certification. The key specifications related to voltage breakdown are gap and creepage distance (Figure 4).