555 voltage boosters

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1/6/2015 Flyback Converters for Dummies http://www.dos4ever.com/flyback/flyback.html 1/18 Flyback Converters for Dummies A simple flyback converter high voltage power supply for NIXIE tubes. Ronald Dekker Special thanks to Frans Schoofs, who really understands how flyback converters work introduction What you need to know about inductors The boost converter A simple boost converter high voltage supply for NIXIEs An inductor test bench What you need to know about transformers The flyback converter A flyback converter high voltage supply for NIXIEs back to homepage If you are interested in Flyback Converters you might want to keep track of my present project: the µTracer : a miniature radio-tube curve-tracer Click here to read about my “low- noise” 6 to 90 V converter project which replaces the anode battery in battery tube receivers. introduction In the NIXIE clocks that I have built, I did not want to have the big and ugly mains transformer in the actual clock itself. Instead I use an AC adapter that fits into the mains wall plug. This means that I have to use some sort of an up-converter to generate the 180V anode supply for the NIXIEs.

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  • 1/6/2015 Flyback Converters for Dummies

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    Flyback Converters for Dummies

    A simple flyback converter high voltage power supply for NIXIE tubes.

    Ronald Dekker

    Special thanks to Frans Schoofs, who really understands how flyback converters work

    introductionWhat you need to know about inductorsThe boost converterA simple boost converter high voltage supply for NIXIEsAn inductor test benchWhat you need to know about transformersThe flyback converterA flyback converter high voltage supply for NIXIEsback to homepage

    If you are interested in FlybackConverters you might want to

    keep track of my present project: theTracer:

    a miniature radio-tube curve-tracer

    Click here to read about my low-noise 6 to 90 V converter project

    which replaces the anode battery inbattery tube receivers.

    introduction

    In the NIXIE clocks that I have built, I did not want to have the big and ugly mains transformer in theactual clock itself. Instead I use an AC adapter that fits into the mains wall plug. This means that Ihave to use some sort of an up-converter to generate the 180V anode supply for the NIXIEs.

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    This page describes a simple boost converter and a more efficient flyback converter both of which canbe used as a high voltage power supply for a 6 NIXIE tube display. Frans Schoofs beautifullyexplained to me the working of the flyback converter and much of what he explained to me you findreflected on this page. I additionally explain the essentials of inductors and transformers that you needto know. This is just a practical guide to get you going, it is not a scientific treatise on the topic.

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    What you need to know about inductors

    Consider the simple circuit consisting of a battery connected to an inductor with inductance L andresistance R (Fig. 1). When the battery is connected to the inductor, the current does not immediatelychange from zero to its maximum value V/R. The law of electromagnetic induction, Faraday's lawprevents this. What happens instead is the following. As the current increases with time, the magneticflux through this loop proportional to this current increases. The increasing flux induces an e.m.f. in thecircuit that opposes the change in magnetic flux. By Lenz's law, the induced electric field in the loopmust therefore be opposite to the direction of the current. As the magnitude of the current increases,the rate of the increase lessens and hence the induced e.m.f. decreases. This opposing e.m.f. results ina linear increase in current at a rate I=(V/L)*t. The increase in current will finally stop when itbecomes limited through the series resistance of the inductor. At that moment the amount of magneticenergy stored in the inductor amounts to E=0.5*L*I*I.

    Figure 1

    In words: the inductor does not allow for any abrupt changes in the current. When a change in appliedvoltage occurs, the inductor will always generate an e.m.f. that counteracts this change. When thecircuit is interrupted for instance, the inductor will still try to maintain the current flowing by generatinga very high voltage over its terminals. Usually this will result in a spark in which the magnetic energystored in the inductor is released. This particular behavior of inductors is used in boost converters toboost 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 isapplied to a ferrite the small magnetic domains in the ferrite will align with this field and increase its

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    magnitude. In this way inductors can be made smaller and with lesser turns and thus with smallerseries resistances (smaller losses). Note that the flipping of these domains costs some energy, but ingood ferrites this can be very small.

    With increasing magnetic flux more and more magnetic domains point into the direction of the field. Ata certain point all the magnetic domains point into the direction of the field and at that point we saythat the ferrite saturates. Any further increase in currentwill only result in a small increase of flux, basically as ifthe ferrite was not present. Since most ferrites have avery high permeability, already small currents can resultin a high magnetic flux. As a result the ferrite willsaturate at a current which is not practical for powerconversion applications

    Ferrite cores for inductors and transformers for powerapplications therefore have an air gap. An air gapreduces the effective permeability and thus themagnetic flux. The larger the air gap, the stronger thereduction in flux an the higher the maximum current the inductor can handle. We say that the magneticenergy is stored in the air gap. The photograph shows several inductors for DC/DC converterssalvaged from old PCBs from PCs, Laptops etc. If you consider playing with DC/DC converters it isbest to buy at least one decent inductor with a known inductance, series resistance and maximumcurrent. The inductor in front of the picture is the 100uH "reference" inductor I use.

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    The boost converter

    The boost converter is perhaps the simplest of all switched mode converters. It uses a single inductorwithout the need for "difficult" transformers. It's working can best be explained with the simplifiedcircuit diagram given in Fig. 2. Here the transistor is represented by an ideal switch and the controlcircuitry has been omitted. The dissipation by the NIXIE tubes is represented by the load resistorRload. A high voltage capacitor C is used to buffer the output voltage. In a typical configuration theinput voltage would be something like Vbat=12V and the output voltage Vout=180V.

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    Figure 2 Simplified circuit diagram of a boost converter.

    At t=0 the switch closes (Fig. 2A). As a result the current through the inductor will start to increaselinearly according to I=(Vbat/L)*t. At a certain moment the switch is opened by the control circuit (Fig.2B). The current at that monent has reached a certain value Ipeak. We have seen in the previoussection that the inductor wants to keep the current flowing through it's windings constant, whatever ittakes. The switch is open, so the only way the inductor can achieve this is to forward bias diode D sothat the current (and thus the energy) can be dumped in the buffer capacitor C. Now remember thatthe capacitor was charged to 180V! So in order to forward bias the diode, the inductor has to generatean e.m.f. (or induction voltage) of something like 180-12=168V., something like a "controlled spark.The current now quickly drops according to I=Ipeak-(Vout/L)*t. For Vbat=12V and Vout=180V thismeans that it will take only a fifteenth (180/12) of the time it took to reach Ipeak when the switch wasclosed, to drop again from Ipeak to 0 now the switch is open. After a certain time the whole processrepeats at a rate of f times per second.

    So far so good. However, the boost converter has a serious disadvantage. To understand this we firsthave to consider the switch that we have been using. In a real circuit most likely a power MOStransistor will be used as the switching element. In the boost converter this transistor will have tohandle both a high current when the switch is closed and a high blocking voltage when the switch isopen! For the transistor this is a difficult combination. In order to make the transistor withstand highblocking voltages, the manufacturer of the transistor has to include regions in the transistor that willaccommodate these voltages so that the intrinsic transistor will not breakdown. However, when theswitch is closed (transistor conducting), these regions will result in additional parasitic series resistancesand thus in an increased Ron. This is the reason why transistors with a high breakdown voltage alwayshave a higher Ron than transistors with a lower breakdown voltage. Since the currents can be quitehigh, this inevitably means losses in the form of dissipation in the transistor. As we will see in one ofthe 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 loadit is possible the calculate the output voltage of the boost converter.Every second the amount of power dissipated by the load is:

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    If T is the total cycle time, and x the fraction of T that the switch is closed, then the maximum currentin the inductor is:

    The energy per package delivered by the inductor is:

    In one second f=1/T packages are delivered so the amount of energy delivered per second is:

    Since in steady-state the amount of energy delivered should equal the amount of energy used [1]=[2]:

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    A simple boost converter high voltage supply for NIXIEs

    If you want to build a simple DC/DC converter to lighten up your NIXIEs and you don't care to muchabout the conversion efficiency, even if it means a (small) heatsink for the power transistor, then theboost converter is the best choice. But even if you think of building a real fly-back converter than it isa 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 smallmodifications.

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    Figure 3 Simple 12-180V boost converter using the 555 as controller.

    The circuit is very simple and closely follows the circuit topology of Fig. 2. For the transistor I haveused a BUZ41A. This transistor is rated at a maximum Vds=500V and an on resistance of 1.5ohm at4.5A. Equivalent or better types like the IRF730 will also perform well. The diode should be a fastswitching type like the BYW95C or better. An old (computer) power supply will yield you most ofthese components. The inductor I picked from a catalogue and is 100H with a few tenths of an ohmseries resistance capable of handling several Amps of current.

    The most interesting aspect of the circuit is how an ordinary 555 is used to regulate the output voltage.Now, there are hundreds of switched mode controllers ICs on the market which are all better suited forthis job than the 555. The problem with all these ICs is that if you build a nice NIXIE clock usingthem, and at one moment in the future the IC breaks down, it is more than likely that it is alreadyobsolete and out of production. The 555 is (very) cheap, performs well enough and most likely willremain in production forever.

    To understand how the controller works it is best to first understand how the 555 functions. On theinternet you may find a number of excellent 555 tutorials [1,2]. Without R3 and T1 the 555 isconfigured 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 200V. However,the voltage divider formed by R4, R5 and R6 has been designed and adjusted in such a way that whenthe output voltage reaches 180 V, T1 just starts to conduct. This is at a base-emitter voltage of ca.

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    0.8V. Now remember that the 555 works by charging and discharging the capacitor between 1/3Vccand 2/3Vcc as defined by an internal resistor ladder network. When T1 starts to conduct it will pulldown the internal supply voltage of this network resulting in a smaller voltage swing and hence a higherfrequency. 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 byR5. For T1 I have used a high voltage type. There is really no need for that and any small signal npntransistor with a decent gain will work. A drawback of such a simple controller is that the circuit has noprotection at all against short circuits or overload situations. An accidental short circuit of the outputwill therefore always result in a defect power transistor (as I have experienced quite a number oftimes).

    Figure 4 Testing the boost converter using a dummy load (and one NIXIE).

    If you are in the testing phase, and do not want to connect the power supply to the NIXIEs yet, it isbest to connect a dummy load to the output since the circuit is not designed to work without a load. Ialways first find out what the current is that I want to operate my NIXIEs on. I usually choose a valuewell below the operating condition specified in the datasheet. This will greatly extend the lifetime of thetubes. Using a high voltage supply I select the supply voltage and the load resistor in such a way thatwith a minimum of current the brightness of the tube is still good enough. Once the total current andthe voltage are known an equivalent load resistor can be calculated from Rload=Vout/Itotal. During thetesting phase this resistor connected to the output replaces the NIXIE tubes.

    A few words about safety. Although the 180 Volts are generated starting with an innocent 12 Volt anaccidental 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.Additionally during testing I permanently have a 20kohm/V multi-meter connected to the output so thatI always know the output voltage. 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 thecurrent can never pass your heart.

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    An inductor test bench

    When you want to start experimenting with boost or fly-back converters it is good idea to buy at leastone inductor with known parameters that may act as a kind of reference device for the inductors ortransformers that you make yourself. I use a 100H inductor with about 0.2ohm series resistancecapable of handling several Amps of current. It is especially designed for SMP applications. The circuitdepicted in Fig. 5 allows you to compare "an unknown" inductor (or transformer) with the referenceinductor.

    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 theboost converter presented in the last section or in the fly-back converter to be presented in one of thenext sections. Basically, the circuit is little more than the inductor which is connected to the 12V powersupply by transistor T1. The current through the inductor is measured by the small series resistor R4.A voltage drop of 100mV over R4 corresponds to a current of approximately 1A. When the transistoris opened, the inductor can dump its energy in diode D3. Since the voltage drop over the diode is only0.6V, it will take about 12/0.6=20 times as long for the current to drop to zero (remember I=(V.t)/L).This is the reason why the gate of the transistor is driven with a highly asymmetric signal generated bythe oscillator around N1-N6. The transistor on-time is determined by C1 and R1+R2. R2 is set so thatthe transistor on-time is equal to the transistor on-time in the converter under normal load. Thetransistor off-time is determined by C1 and R3 and about a factor 20 longer than the on-time.

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    Figure 6 The inductor test bench circuit (left) and a measurement off the reference inductor (right).

    In Fig. 6 (right) you find a measurement of the reference inductor. We find that with a supply voltageof 12V the current through the inductor reaches a value of I=V/R=0.361/0.11=3.28A in 27.1s. SinceI=(V/L)t we find for the inductance L=(V/I)t=(12/3.28)27.1=97.6H. Not bad! At a little bit highercurrent we observe a sharp increase in the current through the inductor. This is the point where theferrite saturates. The inductor should not be used beyond this point.

    You may now want to try different inductors e.g. inductors salvaged from old (computer) powersupplies. Switch S1 make it easy to compare these inductors with the reference inductor. Anotherimportant parameter to watch is the current consumption of the test-bench. An increase in switchinglosses in the inductor core is reflected by an increase in power consumption.

    An alternative simple and quick way to measure the inductanceof an unknown inductor can be found here.

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    What you need to know about transformers

    This section deals with a few basic things you need to know about transformers in order to understandfly-back converters. In Fig. 7 I have tried to sketch an elementary inductor and its schematicequivalent. Note that both windings have a certain direction and that equal directions are indicated by adot.

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    Figure 7 Basic transformer with open secondary windings

    In this example we assume that the primary side of the transformer has a certain number of turns withinductance L1. The secondary side of the transformer has ten times that number of turns. As a resultthe secondary side will have an inductance L2=10^2*L1=100*L1. First consider the case that thesecondary windings are not connected. When a voltage source is connected to the primary coil thecurrent through the primary winding will start to increase linearly at a rate I=(V/L1)*t. Since with openterminals at the secondary side no secondary current can flow, the transformer will behave as a normalinductor with inductance L1. The increasing primary current will generate a magnetic flux not onlythrough the primary windings, but the same flux will also flow through the secondary windings. It iseasy to see from reasons of symmetry that if the secondary coil would be identical to the primary coilthe voltage at the primary and secondary side would be equal. In this case we have 10 times thenumber of turns at the secondary side. This can be seen as a series connection of 10 coils eachcarrying a voltage of 10V so that in total 100V is induced at the primary side. The voltage of 100V atthe output remains as long as the current continues to increase linearly. In practice this means until thecurrent reaches its compliance or until the core saturates.

    Figure 8 Basic transformer with closed secondary windings

    Next the secondary winding is connected to some load which will allow for a current to flow (Fig. 8).If the primary winding is now connected to some voltage source, a current through the primarywinding will start to flow, resulting in magnetic flux in the direction as indicated by the arrow. Thismagnetic flux will obviously also flow through the secondary winding. We have seen that an inductorresists a change in magnetic flux. To counteract the increasing flux, a current flowing in oppositedirection through the secondary winding will start to flow as indicated in Fig. 8 Resulting in a voltagedrop over the load as indicated.

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    Figure 9 The transformer in flyback

    Finally the voltage source at the primary side is suddenly removed (Fig. 9). The only way thesecondary winding can prevent a sudden collapse of flux is to reverse the direction of the currentflowing through the secondary winding. As a result alsothe voltage drop over the load will reverse.Note that the voltage over the load will increase to any value that is needed in order to maintain aconstant flux. The magnetic energy stored in the inductor is dumped into the load and the secondarycurrent decreases at a rate Vout/L2

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    The flyback converter

    Figure 10 depicts the basic elements from the flyback converter. Again all control circuitry is omitted,and the switching MOSFET is represented by an ideal switch.

    Figure 10 Phase one, storing energy in the transformer.

    For the moment we assume that at t=0 the buffer capacitor is charged to the nominal output voltageVout and that the current through the primary windings of the transformer is zero. At t=0 the switchcloses and a current will start to flow through the primary winding. This will induce a voltage over thesecondary winding with a polarity as indicated (see previous section). Since the diode is reverse biased

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    no secondary current will flow, so basically the secondary winding is "not connected". In other wordsat the primary side of the transformer we "just see an inductor". As a result the primary current willstart to increase lineary according to I=(12/L1)*t. During the time the switch is closed the voltageinduced over the secondary windings will be n*12V. This means that the diode minimally has to blocka reverse voltage of n*12+Vout

    Figure 11 Phase two, dumping the energy from the transformer into the buffer capacitor.

    At a certain moment the switch will open (Fig. 11). Lets call the current that was flowing through theprimary winding at the moment just before the switch was opened Ipeak. The energy then stored atthe moment of opening is 0.5*L1*(Ipeak*Ipeak). The transformer wants to sustain the magnetic flux.Since the circuit at primary side is open the only way the inductor can do this is by inducing a voltageat the secondary side high enough (>Vout) to forward bias the diode. The initial value of the currentwill be I2=Ipeak/n. During the time that the diode is forward biased, the voltage over the secondarywinding will equal Vout+0.8V. The 0.8V is the voltage drop over the diode and can for a high outputvoltage like in a NIXIE converter be neglected. The transformer will transform this voltage down toVout/n. So the total voltage that the switch has to block in open position is 12+(Vout/n).

    Actually this is the big advantage of a flyback converter over a boost converter. In a boost converterthe switch (MOSFET) has to carry a large current during the on phase and a high voltage during theoff phase. In the flyback converter the voltage during the off phase is transformed down to a valuedetermined by the ratio in turns. This means that a MOSFET with a much lower Ron can be used (seesection on the boostconverter). Similarly, in the boost converter the diode has to carry both the high oncurrent and a high reverse voltage. In the flyback converter the diode at the secondary side only has toblock a high voltage while the current is low (Ipeak/n). This makes it possible to select a diode withsmaller capacitances and hence higher switching speed. All this results in reduced losses and anincreased efficiency.

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    Figure 12 Phase three, energy dump completed discharge of drain-source capacitor

    This continues until all energy stored in the transformer is dumped in the buffer capacitor. At thatmoment I2 becomes zero (Fig. 12). At that moment the e.m.f induced at the primary side (Vout/n) willvanish. However, the parasitic capacitance of the switch (source-drain capacitance of the MOSFET)will be charged to (Vout/n)+12 V. At the primary side now a series resonant tank is formed with acharged capacitor (Fig. 12 right). This will cause a dampened oscillation.

    Figure 13 Voltage over the switch during all three phases

    Figure 13 schematically shows the drain-source voltage (the voltage over the switch) during all thephases of the converter just described. During phase the switch is closed. What we see is the voltagedrop over the switch caused by the non-zero on resistance. During this phase the current will increaselinearly, so also the voltage drop over Ron will increase linearly. At point b the switch opens. Thesecondary current will start to flow and the output voltage wil appear down transformed over theprimary winding. The total blocking voltage over the switch will be 12+(Vout/n) (Fig 13c). At point dall the energy is dumped in the capacitor and the secondary current drops to zero causing the inducede.m.f. at primary side to vanish. The charged drain-source capacitor, now suddenly connected in serieswith the inductance of the primary winding will result in a dampened oscillation (Fig. 12e). At point fthe switch closes again, and any remaining energy in the LC tank will be dissipated in the transistor.

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    Figure 14 Stray inductance.

    This leaves just one small phenomenon to be explained. No transformer is ideal. There will always bemagnetic field lines generated by the primary windings which are not (fully) enclosed by the secondarywindings. This will cause a stray inductance that can be modeled as a small inductor in series with theprimary winding of the transformer (Fig. 14). We have seen that all the energy that is stored in thetransformer is dumped in the buffer capacitor. This does not hold for the (small) amount of energystored in the stray inductance. So the sudden opening of the switch will cause a sharp voltage peak,just as with any inductor which is suddenly disconnected from a DC current. The small stray inductorin series with the source-drain capacitance will cause a dampened high frequency oscillation (Fig. 15).

    Figure 15 High frequency oscillations due to energy stored in the stray inductance.

    If needed the switching transistor can be protected from the high voltage peak by an RC snubbernetwork or a zenerdiode which limits the maximum source-drain voltage.

    Finally you can check for yourself the equation derived for the output voltage of the boost converteralso holds for the flyback converter. This is not really surprising, like in the boost converter the flybackconverter is based on the dumping of the energy from an inductor or the primary winding of a

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    tranformer in the load. The transformer just serves to lower the voltage over the switch.

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    A flyback converter high voltage supply for NIXIEs.

    After all what has been said so far, the circuit diagram of the flyback converter will hold no surprises(Fig.16). Literally the only difference with the boost converter is that the inductor is replaced by atransformer, and that the transistor has been replaced for a BUZ21. The BUZ21 has a much lower onresistance (Ron=0.085 ohm) as compared to the BUZ41A (Ron=1.5 ohm) but also a lower drain-source breakdown voltage (100V versus 500V).

    Figure 16 Circuit diagram of the Flyback converter.

    The difficult part of the circuit is the transformer. Well it is not exactly difficult, but the problem is thatyou have to make it yourself. What makes things worse is that finding a suitable ferrite core can sometimes be difficult since component vendors often only have a few types on stock. The E-shape ferritecore that I use measures 20x20x5 mm (Fig. 16) I got them from Paul van de Broek who always helpsme when I need something special.

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    Figure 17 The ferrite core that I use (20x20x5 mm).

    So what is the strategy for finding the number of turns you need on the ferrite core that you have?Well first of all I always start with my inductor test-bench so that I can compare what I have madewith the reference 100 H inductor. If this is your first fly-back converter it might be illustrative to firsttry the ferrite core without an airgap. Mind everybody always says airgap, but what they actually meanis a spacer, often made from plastic (cellotape). So start with say 10 or 20 windings without an airgap.What you probably will see in the test-bench is a too high inductance (slower increase of current ascompared to the 100 H inductor). At the same time you will find the ferrite saturating at a lowcurrent. It is now time to include the spacer. Attach a peace of cello tape and cut the excess amount oftape with a razor blade so that only the touching surfaces of the ferrite are covered with tape. If youtry the inductor now you will find a much lower inductance and a higher saturation current. Probablyyou will need to add or remove some turns to get an inductance of 100 H (same slope). For theprimary winding I use 0.4 (or 0.5) mm diameter insulated copper wire. When you have determined theproper number of primary turns, the secondary winding consists of ten times that number of turns. Forthe secondary windings I use something like 0.1-0.15 mm diameter wire. I always include a layer oftape in between two layers of secondary windings to prevent arcing. The transformers that I use have22 primary turns and 220 secondary turns.

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    Figure 18 Two examples of the Flyback converter built on a peace of breadboard.

    Figure 19 shows the drain-source voltage of power MOSFET measured with a 1:10 reduction probe.The 1- on the left axis marks the 0 V input level. The image is not very sharp due to some trigger jittercaused by a 50Hz ripple on the power supply. Nevertheless, several features from Fig. 15 can berecognized. The repetition frequency is 32 kHz and the maximum blocking voltage of the transistor isabout 31 V according to theory. The voltage over the transistor almost swings for two full periods untilthe transistor switches on again. The high frequency oscillations due to the stray inductance are there,but difficult to see on the photograph. The increasing voltage drop over Ron during the on phase isclearly visible.

    Figure 19 Drain-source voltage of power MOSFET measured with a 1:10 reduction probe.

    The total converter can easily be built in an area of less than 4x4 cm. To increase the lifetime of my

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    tubes I usually run them on as low as current as possible. Typically 1-1.5 mA. This means that theconverter has to generate for 6 digits about 6 to 7 watts. The efficiency is ca. 80%. This is notspectacular but good enough for such a simple circuit. If you decide to built one: have fun, be carefuland good luck!

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    Web links

    [1] http://www.williamson-labs.com/555-tutorial.htm [2] http://www.uoguelph.ca/~antoon/gadgets/555/555.html

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