Design using flyback converter

Although it may be possible to design a flyback converter to work with DCM as well as CCM modes, one thing must be remembered that during the transition from DCM to CCM mode, this shifting function transforms into a 2-pole operation, giving rise to low impedance for the converter.

This situation makes it essential to incorporate additional design strategies, including various loop feedback and slope compensation with respect to the inner current loop system. Practically this implies that we have to make sure that the converter is primarily designed for a CCM mode, yet is able to work with DCM mode when lighter loads are used at the output. It may be interesting to know that by using advanced transformer models, it may become possible to enhance a CCM converter through cleaner and lighter load regulation, as well as high cross regulation over a wide range of load through a stepped-gap-transformer.

In such cases a small core gap is enforced by inserting a external element such as an insulation tape or paper, in order to induce high inductance initially, and also enable CCM operation with lighter loads. We will discuss this elaborately some other time my subsequent articles. Having such versatile DCM mode characteristics, no surprise this becomes the popular choice whenever a hassle free, efficient and low power SMPS is required to be designed. In the following we will learn the step by step instructions regarding how to design a DCM mode flyback converter.

Step 1: Assess and estimate your design requirements. All SMPS design must begin by assessing and determining the system specifications. You will need to define and allocate the following parameters:. The switching frequency denoted as. Fsw generally has to be compromised while getting the best of transformer size and losses incurred due to switching, and EMI. Which implies one may need to decide on a switching frequency at least below kHz. Typically this may be selected between a 50kHz and kHz range.

Furthermore, in case more than one output is required to be included for the design, the maximum power value Pout will need to be adjusted as the combined value of the two outputs. You may find interesting to know that until recent times the most popular conventional SMPS designs used to have the mosfet and the PWM switching controller as two different isolated stages, integrated together over a PCB layout, but nowadays in modern SMPS units these two stages can be found embedded inside one package and manufactured as single ICs.

Mainly, the parameters which are typically considered while designing a flyback SMPS converter are 1 The application or the load specifications, 2 Cost 3 Standby power, and 4 Additional protection features. When embedded ICs are used, usually things become a lot easier, as it only requires the transformer and a few external passive component to be calculated for designing an optimal flyback converter.

Depending on input voltage and power specifications, the standard rule for selecting Cin which is also referred to as a DC link capacitor can be learned from the following explanations:. In order to ensure a broad range of operation, a 2uF per watt or higher value may be chosen for a DC link capacitor, which will enable you to have a good quality range for this component.

Next, it may be required to determine the minimum DC input voltage which may be obtained by solving:. Where the discharge becomes the duty ratio of the DC link capacitor, which may be roughly around 0. In the figure above we can visualize the DC link capacitor voltage. As shown, the input voltage arises during maximum output power and minimum input AC voltage, whereas the maximum DC input voltage arises during minimum input power absence of load and during maximum input AC voltage.

During no load condition, we are able to see a maximum DC input voltage, during which the capacitor charges at the peak level of the AC input voltage, and these values can be expressed with the following equation:. The Flyback induced voltage VR could be understood as the voltage induced across the primary side of the transformer when the mosfet Q1 is in switched OFF condition.

The above function in turn impacts maximum VDS rating of the mosfet, which may be confirmed and identified by solving the following equation:. The following list tells us how much reflected voltage or induced voltage may be recommended for a V to V rated MOSFET, and having an initial limit value VR lower than V for an expected vast input voltage range.

Picking the right VR can be a bargain between the level of voltage stress over the secondary rectifier, and the primary side mosfet specifications. If VR is selected very high through an increased turn ratio, would give rise to a bigger VDSmax, but a lower level of voltage stress on the secondary side diode. And if VR is selected too small through a smaller turn ratio, would cause VDSmax to be smaller, but would result in an increase in the stress level on the secondary diode.

A bigger primary side VDSmax would assure not only lower stress level on the secondary side diode and reduction in primary current, but will also allow a cost effective design to be implemented. A maximum duty cycle can be expected at instances of VDCmin. In this case the duty cycle could be presented as:. Once the above is achieved we can go ahead and calculate the primary inductance using the following formula, within the maximum duty cycle boundaries.

Care must be taken regarding the flyback, it must not go into the CCM mode due to any form of excess loading conditions, and for this maximum power specification should be considered while calculating Poutmax in Equation 5.

The mentioned condition can also occur in case inductance is increased over the Lprimax value, so take a note of these. It might look quite intimidating while selecting the right core specification and structure if you are designing a flyback for the first time. Since this may involve a significant number of factors and variables to be considered.

A few of these that may be crucial are the core geometry e. TP4, 3F3 etc. If you are clueless regarding how to proceed with the above specs, an effective way to counter this problem could be to refer a standard core selection guide by the core manufacturer, or you can also take the help to the following table which roughly gives you the standard core dimensions while designing a 65kHz DCM flyback, with reference to the output power.

Once you are done with the selection of the core size, it is time to select the correct bobbin, which could be acquired as per the core datasheet.

Additional properties of the bobbin such as number of pins, PCB mount or SMD, horizontal or vertical positioning all these may also need to be considered as the preferred design. The core material is also crucial and must be selected based on the frequency, magnetic flux density, and core losses. To begin with you can try variants with the name 3F3, 3C96, or TP4A, remember the names of available core material may be different for identical types depending on the particular manufacture.

Where the term Bmax signifies the operating maximum flux density, Lpri tells you about the primary inductance, Ipri becomes the primary peak current, while Ae identifies the cross sectional area of the selected core type. It must remembered that the Bmax should never be allowed to exceed the saturating flux density Bsat as specified in the datasheet of the core material.

You may find slight variances in Bsat for ferrite cores depending on specifications such as material type and temperature; however a majority of these will have a value near to mT. If you find no detailed reference data, you may go with a Bmax of mT. Although selecting higher Bmax may assist in having reduced number of primary turns and lower conduction, core loss may significantly increase.

Try to optimize between the values of these parameters, such that core loss and copper loss both are kept within acceptable limits. In order to determine the secondary turns we first need to find the turn ratio n , which can be calculated using the following formula:.

Where Np is the primary turns, and Ns is the secondary number of turns, Vout signifies the output voltage, and VD tells us regarding the voltage drop across the secondary diode.

For calculating the turns for the auxiliary outputs for a desired Vcc value, the following formula can be used:.

An auxiliary winding becomes crucial in all flyback converters for supplying the initial start-up supply to the control IC. This supply VCC is normally used for powering the switching IC on the primary side and could be fixed as per the value given in the datasheet of the IC.

If the calculation gives a non-integer value, simply round it of by using the upper integer value just above this non integer number. In order to correctly calculate the wire sizes for the several winding, we first need to find out the RMS current specification for the individual winding. As a starting point, a current density of to circular mil per Ampere, could be utilized for determining the gauge of the wire.

It also shows you the diameter of the wire and the basic insulation for an assorted gauge of super enameled copper wires. After you have finished determining the above discussed transformer parameters, it becomes crucial to evaluate how to fit the wire dimension and the number of turns within the calculated transformer core size, and the specified bobbin.

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. Of the many converter topologies around today, the flyback topology is one of the most frequently used.

Although simple, this converter design offers great advantages for certain applications. New, more complex topologies have surfaced in recent years, but flyback converters remain a popular design choice. These switch-mode power converters offer competitive size, cost, and efficiency ratios in the low- to mid-power range about 2W to W.

The coupled inductor also enables multiple outputs, which makes flyback converters the standard for a wide variety of applications. The current from the inductor charges the output capacitor and powers the load. There are many important design decisions and tradeoffs involved in designing a flyback converter. The following sections will go through each step in the design process for a simple flyback converter. Figure 2 shows the design flow that will be followed.

Design inputs are either defined by the end application or selected by the designer. These parameters include, but are not limited to, the input and output voltages, power, ripple factor, and operation mode. Table 1 shows a summary of the design inputs for the circuit discussed in this article. Discontinuous conduction mode DCM was selected for this application due to its increased stability and higher efficiency. The switching frequency was chosen to be kHz. Given all these inputs, the designer must choose a controller IC that meets all of the initial requirements.

It also features primary-side regulation, which reduces the external component count. The first design calculation aims to find the maximum primary inductor value. There are many different design methods available, but the converter used for this example always operates in DCM. Calculate the primary inductor value L P with Equation 1 :. The worst-case scenario occurs when the converter works at full power with a minimum input voltage and maximum duty cycle.

Next, the required turn ratio nS1 is calculated. To do so, the same worst-case scenario is applied with a minimum VIN and maximum D. Estimate nS1 with Equation 2 :. To do so, calculate the maximum current and voltage that the switch will have to withstand. Calculate the maximum voltage with Equation 3 :. Estimate the maximum current with Equation 4 :.

This means that the controller IC can be used safely in this application. In this step, the rectifier diodes are assessed. As with the MOSFET, the aim is to ensure that the rectifier diode can handle the maximum voltage and current that it may encounter. Calculate the maximum voltage that the diode can withstand with Equation 5 :. Isolation is needed in order to reduce the noise level and for some safety-related activities.

The device comprises the flyback transformer, rectifier, filter, and switch. It even contains a controlling process to simulate the switch and provides regulation. This is the lower section count switching type of converter and correspondingly simple to construct. Also, a flyback converter is an isolated kind of switching converter which can be either step-up or step-down. In general, a transformer has the functionality of transmission of energy from the primary winding to secondary winding perfectly and in real-time.

Whereas a flyback transformer accumulates energy in its primary magnetic field and then after a specific period of time the energy gets transferred to the secondary winding. The main functionality of the switch is to flip around ON and OFF conditions that correspond to the ability of the primary circuit to either magnetize or demagnetize the flyback transformer. This switch is regulated by the PWM signal that is receiving from the chosen controller.

Here, the voltage rectification is done by the rectifier on the secondary winding of the transformer which makes a pulsating type of DC. The other functionality of either the diode or rectifier is to cut down and then connect the load section from the secondary winding. This rectified voltage is then filtered using the capacitor where augments the DC level and it can be applied for the desired application. In many scenarios, a flyback converter required a snubber circuit where this tries to eliminate the voltage across those are generated across diode or switch.

Before knowing the operation, let us know what components are involved in the construction of the flyback converter.

The operation of the flyback converter is mainly based on the switch operation. When the switch is in ON state, there will be the flow of current from Vin downwards to the primary and then to the ground.

This tends to the accumulation of energy into the primary winding. At the time of this, there will be no current flow in the secondary winding because the diode is in reverse biased condition. In this condition, the load demand will be delivered by the output capacitance which is Cout.