CONVERTISSEUR FLYBACK PDF

This is quite a specific requirement and the reader will probably find it impossible to find a transformer with such characteristics. All is not lost however, as magnetics companies are now introducing general purpose transformers with 6 windings on one bobbin that can be configured in any way. Since we have more current on the primary than secondary, it makes sense if our design has 3 windings in parallel for the primary and 3 windings in series for the secondary. This ensures each of our 3 windings shares the 1. The Wurth is a suitable device.

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Instead I use an AC adapter that fits into the mains wall plug. This page describes a simple boost converter and a more efficient flyback converter both of which can be used as a high voltage power supply for a 6 NIXIE tube display.

Frans Schoofs beautifully explained to me the working of the flyback converter and much of what he explained to me you find reflected on this page. I additionally explain the essentials of inductors and transformers that you need to know. This is just a practical guide to get you going, it is not a scientific treatise on the topic. What happens instead is the following. As the current increases with time, the magnetic flux through this loop proportional to this current increases.

The increasing flux induces an e. As the magnitude of the current increases, the rate of the increase lessens and hence the induced e. This opposing e.

The increase in current will finally stop when it becomes limited through the series resistance of the inductor. Figure 1 In words: the inductor does not allow for any abrupt changes in the current.

When a change in applied voltage occurs, the inductor will always generate an e. When the circuit is interrupted for instance, the inductor will still try to maintain the current flowing by generating a very high voltage over its terminals.

Usually this will result in a spark in which the magnetic energy stored in the inductor is released. This particular behavior of inductors is used in boost converters to boost the voltage to levels above the battery voltage. Materials like ferrites can be used to increase the magnetic flux in an inductor. When a magnetic field is applied to a ferrite the small magnetic domains in the ferrite will align with this field and increase its magnitude.

In this way inductors can be made smaller and with lesser turns and thus with smaller series resistances smaller losses. Note that the flipping of these domains costs some energy, but in good ferrites this can be very small.

With increasing magnetic flux more and more magnetic domains point into the direction of the field. At a certain point all the magnetic domains point into the direction of the field and at that point we say that the ferrite saturates.

Any further increase in current will only result in a small increase of flux, basically as if the ferrite was not present. Since most ferrites have a very high permeability, already small currents can result in a high magnetic flux. As a result the ferrite will saturate at a current which is not practical for power conversion applications Ferrite cores for inductors and transformers for power applications therefore have an air gap.

An air gap reduces the effective permeability and thus the magnetic flux. The larger the air gap, the stronger the reduction in flux an the higher the maximum current the inductor can handle.

We say that the magnetic energy is stored in the air gap. The inductor in front of the picture is the uH "reference" inductor I use. It uses a single inductor without the need for "difficult" transformers. Here the transistor is represented by an ideal switch and the control circuitry has been omitted.

A high voltage capacitor C is used to buffer the output voltage. Figure 2 Simplified circuit diagram of a boost converter. At a certain moment the switch is opened by the control circuit Fig. The current at that monent has reached a certain value Ipeak.

The switch is open, so the only way the inductor can achieve this is to forward bias diode D so that the current and thus the energy can be dumped in the buffer capacitor C.

Now remember that the capacitor was charged to V! So in order to forward bias the diode, the inductor has to generate an e. After a certain time the whole process repeats at a rate of f times per second.

So far so good. However, the boost converter has a serious disadvantage. To understand this we first have to consider the switch that we have been using. In a real circuit most likely a power MOS transistor will be used as the switching element. In the boost converter this transistor will have to handle both a high current when the switch is closed and a high blocking voltage when the switch is open! For the transistor this is a difficult combination. In order to make the transistor withstand high blocking voltages, the manufacturer of the transistor has to include regions in the transistor that will accommodate these voltages so that the intrinsic transistor will not breakdown.

However, when the switch is closed transistor conducting , these regions will result in additional parasitic series resistances and thus in an increased Ron.

This is the reason why transistors with a high breakdown voltage always have a higher Ron than transistors with a lower breakdown voltage. Since the currents can be quite high, this inevitably means losses in the form of dissipation in the transistor. As we will see in one of the next sections this problem is solved in the fly-back converter by the use of a transformer.

By balancing the amount of power stored in the inductor to the amount of power dissipated in the load it is possible the calculate the output voltage of the boost converter. But even if you think of building a real fly-back converter than it is a good idea to start with a simple boost converter. The boost converter only requires an "of the shelf" inductor and when you have it working it is easily converted into a fly-back converter by a few small modifications.

Figure 3 Simple V boost converter using the as controller. The circuit is very simple and closely follows the circuit topology of Fig. Equivalent or better types like the IRF will also perform well. The diode should be a fast switching type like the BYW95C or better. An old computer power supply will yield you most of these components. The most interesting aspect of the circuit is how an ordinary is used to regulate the output voltage. Now, there are hundreds of switched mode controllers ICs on the market which are all better suited for this job than the The problem with all these ICs is that if you build a nice NIXIE clock using them, and at one moment in the future the IC breaks down, it is more than likely that it is already obsolete and out of production.

The is very cheap, performs well enough and most likely will remain in production forever. To understand how the controller works it is best to first understand how the functions. On the internet you may find a number of excellent tutorials [1,2].

Without R3 and T1 the is configured as a normal astable multivibrator running at a frequency of: Without any feedback, the output voltage at this frequency will be well in excess of V.

However, the voltage divider formed by R4, R5 and R6 has been designed and adjusted in such a way that when the output voltage reaches V, T1 just starts to conduct. This is at a base-emitter voltage of ca. When T1 starts to conduct it will pull down the internal supply voltage of this network resulting in a smaller voltage swing and hence a higher frequency. From the last equation in the previous section we learn that a higher frequency smaller T will result in a lower output voltage.

In this way the output voltage will settle at a value determined by R5. For T1 I have used a high voltage type. There is really no need for that and any small signal npn transistor with a decent gain will work. A drawback of such a simple controller is that the circuit has no protection at all against short circuits or overload situations. An accidental short circuit of the output will therefore always result in a defect power transistor as I have experienced quite a number of times.

If you are in the testing phase, and do not want to connect the power supply to the NIXIEs yet, it is best to connect a dummy load to the output since the circuit is not designed to work without a load. I usually choose a value well below the operating condition specified in the datasheet. This will greatly extend the lifetime of the tubes. Using a high voltage supply I select the supply voltage and the load resistor in such a way that with a minimum of current the brightness of the tube is still good enough.

A few words about safety. Although the Volts are generated starting with an innocent 12 Volt an accidental contact with the charged buffer capacitor will be a painful, possibly a lethal experience. Always be very careful! I always place a small neon indicator lamp at the output of the converter even in the final clock to clearly indicate that a dangerous voltage is present at the output. Finally, the advice of my father who was from the radio tube area: always keep one hand in your pocket when touching the circuit when it is switched on.

In that way the current can never pass your heart. It is especially designed for SMP applications. The circuit depicted in Fig. Figure 5 Circuit diagram of the inductor test bench. The circuit is designed to test the inductor as closely as possible under conditions that occur in the boost converter presented in the last section or in the fly-back converter to be presented in one of the next sections.

Basically, the circuit is little more than the inductor which is connected to the 12V power supply by transistor T1. The current through the inductor is measured by the small series resistor R4. A voltage drop of mV over R4 corresponds to a current of approximately 1A. When the transistor is opened, the inductor can dump its energy in diode D3. Since the voltage drop over the diode is only 0. This is the reason why the gate of the transistor is driven with a highly asymmetric signal generated by the oscillator around N1-N6.

R2 is set so that the transistor on-time is equal to the transistor on-time in the converter under normal load.

The transistor off-time is determined by C1 and R3 and about a factor 20 longer than the on-time. Figure 6 The inductor test bench circuit left and a measurement off the reference inductor right.

In Fig. Not bad!

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