Current mode flyback converter

Written by Andrew Levido. A simplified Flyback Converter circuit. I have omitted the control chip and main input circuitry for clarity. The key to understanding the circuit is to recognize that the transformer is not really a transformer at all — it is actually a pair of coupled inductors.

Energy is stored in the magnetic field of the core. The voltage reverses and the current switches to the secondary side, dumping the stored energy into the load. Here are the voltage and current waveforms associated with the circuit of Figure 1. This is the distinguishing feature of the discontinuous current mode converter. I real life these waveforms will be a little messier due to second order effects.

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Out of these cookies, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the website. We also use third-party cookies that help us analyze and understand how you use this website. The LTC has been designed to enhance the flexibility of the basic flyback converter, making it possible to optimize a single design for diverse applications.

The converter input and output voltage is limited only by the rating of external components such as the power MOSFET and the transformer. Figure 1 shows the LTC in a non-isolated flyback converter with an input voltage range of 36V to 72V and an output voltage of 3. The remainder of this section details the design decisions made in creating this converter and describes methods for altering the design for various applications.

An isolated version of the converter is described in the next section. Once the converter begins operation, a winding on the transformer provides a bias supply which turns off the NPN transistor to save power and increase efficiency. The FB pin monitors the output voltage by comparing it—via a resistive divider—to the 0. Since the FB pin is not connected directly to the output, the LTC can accommodate any output voltage down to 0. By changing this resistor, the operating frequency can be set anywhere between 70kHz and kHz.

High power designs tend to use lower frequencies while low power designs tend to use higher frequencies. The frequency programmability of the LTC allows selection of the optimum frequency for any given design. The rising threshold on the RUN pin is 1. Therefore the RUN pin accommodates designs with a wide range of input voltages and still has a high enough voltage rating to survive a transient overvoltage on V IN.

Generally, a larger output filter capacitor requires a slower soft-start to limit the inrush current caused by the charging filter capacitor. To get this right optimally several iteration or experimentation may be required for optimizing the core specification with reference to wire gauge and the number of turns. The following figure indicates the winding area for a given EE core. With reference to the calculated wire thickness and the number of turns for the individual winding, it may be possible to approximately estimate whether the winding will fit the available winding area w and h or not.

If the winding does not accommodate then one of the parameters out of number of turns, wire gauge or the core size, or more than 1 parameter may require some fine-tuning until the winding fits optimally. The winding layout is crucial since the working performance, and the reliability of the transformer, significantly depends on it. It is recommended to employ a sandwich layout or structure for the winding in order to restrict inductance leakage, as indicated in Fig5. Also in order to satisfy and conform with the international safety rules, the design must have sufficient range of insulation across the primary and secondary layers of winding.

This may be assured by employing margin-wound structure, or by using a secondary wire having triple insulated wire rating, as shown in the following respective figure. Employing triple insulated wire for the secondary winding becomes the easier option for quickly affirming the international safety laws concerning flyback SMPS designs. However such reinforced wires may have a bit higher thickness compared to the normal variant compelling the winding to occupy more space, and may require additional effort to accommodate within the selected bobbin.

To counter this a clamping circuit is usually configured across the primary winding, which instantly limits the generated spike to some safe lower value. You will find a couple of clamping circuit designs that may be incorporated for this purpose as shown in the following figure. In this clamp circuit we use a combination of a rectifier diode and a high voltage Zener diode such as a TVS transient voltage suppressor for clamping the surge spike.

The function of the Zener diode is to efficiently clip or limit the voltage spike until the leakage voltage is fully shunted through the Zener diode. The advantage of a diode Zener clamp is that the circuit activates and clamps only when the combined value of VR and Vspike exceeds the breakdown spec of the Zener diode, and conversely, as long as the spike is below the Zener breakdown or a safe level, the clamp may not trigger at all, not allowing any unnecessary power dissipation.

It is should always twice the value of the reflected voltage VR, or the assumed spike voltage. The rectifier diode should be ultra-fast recovery or a schottky type of diode having a rating higher than the maximum DC link voltage. Here the resistance parameter of the resistor becomes crucial while limiting the voltage spike. If a low value Rclamp is selected it would improve the spike protection but might increase dissipation and waste energy.

Conversely, if a higher value Rclamp is selected, that would help to minimize dissipation but might not be so effective in suppressing the spikes. In this case the capacitor Cclamp should be substantially large inhibit a rise in voltage during the absorption period of the leakage energy. The value of Cclamp may be selected between pF to 4. Preferably go for a schottky diode to minimize conduction losses. With a DCM circuit the Flyback peak current may be high, therefore try selecting a diode having a lower forward voltage and a relatively higher current specs, with regards to the desired efficiency level.

Selecting a correctly calculated output capacitor while designing a flyback can be extremely crucial, because in a flyback topology stored inductive energy is unavailable between the diode and the capacitor, which implies the capacitor value needs to be calculated by considering 3 important criteria:. The minimum possible value could be identified depending on the function of maximum acceptable peak to peak output ripple voltage, and may be identified through ghe following formula:.

Where Ncp signifies the number of primary side clock pulses required by the control feedback for controlling the duty from the specified maximum and minimum values. This may typically require around 10 to 20 switching cycles. For a specified high switching frequency of the flyback, the maximum peak current from the secondary side of the transformer will generate a correspondingly high ripple voltage, imposed across the equivalent ESR of the output capacitor.

Considering this it must be ensured that the ESRmax rating of the capacitor does not exceed the specified acceptable ripple current capability of the capacitor. The final design may fundamentally include the desired voltage rating, and ripple current capability of the capacitor, based on the actual ratio of the selected output voltage and current of the flyback.

Make sure that the ESR value is determined from the datasheet based on the frequency higher than 1kHz, which may be typically assumed to be between 10kHz to kHz. It would be interesting to note that a solitary capacitor with a low ESR spec may be enough to control the output ripple. You can try to include a small LC filter for higher peak currents, especially if the flyback is designed to work with a DCM mode, which might guarantee a reasonably good ripple voltage control at the output.

In this formula PF stands for power factor of the power supply, we can apply 0. For the voltage rating, it could be selected at V for a maximum V AC input specification.

It may be calculated with the following equation. The sensing resistor Rsense is incorporated to interpret the maximum power at the output of the flyback. Vcsth value could be determined by referring to the controller IC datasheet, Ip max signifies the primary current.

An optimal capacitance value is crucial for the input capacitor to render a proper startup period. Typically any value between 22uF to 47uF does the job nicely. On the contrary a larger capacitance value could result in an undesirable delaying of the startup time of the converter. Additionally, make sure this capacitor is of the best quality, having very good ESR and ripple current specifications, on par with the output capacitor specifications.

Feedback loop compensation becomes important to stop the generation of oscillation. As indicated the above figure a straightforward RC Rcomp, Ccomp mostly becomes just enough to maintain good stability across the loop.

In general Rcomp value may be selected anything between 1K and 20K, while Ccomp could be within the range of nF and pF. This concludes our elaborate discussion on how to design and calculate a flyback converter, if you have any suggestions or questions, you can put them forth in the following comment box, your questions will be replied ASAP.

Courtesy: Infineon. If you have any circuit related query, you may interact through comments, I'll be most happy to help! Your email:. Thanks for sharing your ferrite knowledge. That is the best article web from many web that I seek for many time. Hi Swag, you are a great electronics expert. And amazing smps expert too! I have a smps from Epson printer, output 42V 0,4A.

Can I modify it to output 12v only changing feedback Zener diodes? At this moment there are 7 Zener diodes of 6V to get 42V. Can I use only 2 Zener diodes of 6V to output 12V?

How can I send to you attached files? Hi Aldo, yes you can try replacing the 42 v zener with 12 v, I have already posted an article which addresses a similar topic, you can find it here:. Do you confirm to me that I only need to change Zener diodes to get wished voltage output?

Thanks very much! Hi Aldo, if the secondary winding is capable of generating more than 42V then the zener application should work, so make sure the secondary winding is designed to generate more than 42 V.

But I would like get 9V or 12V output. Why do I have to be sure if secondary winding is capable of generating more than 42V?