niversal IC 555 Buck-Boost Circuit Posted by Swagatam Majumdar The post explains an universal IC 555 based buck-boost circuit which can be used for various different applications involving efficient power processing requirements.
This highly efficient and effective buck-boost circuit using the work horse IC 555 would allow you to convert an input source voltage to to any required degree, either bucked or boosted, as desired. We have already learned the concept comprehensively through one of my previous articles where we discussed the versatility of this buck-boost type of topolo gy. As shown in the circuit diagram below (click to enlarge) the configuration is basically a combination of two distinct stages, viz the up per buck-boost converter stage and the lower IC 555 PWM controller stage. The buck-boost stage consists of a mosfet mo sfet which acts like a switch, the inductor which is the main power converting component, the diode which just like the mosfet forms a complementary switch, and the capacitor quite like the inductor forms a complementary power converter conv erter device. The mosfet needs to operate through pulsed triggering so that it alternately switches the input voltage ON and OFF across acro ss the inductor in response to its gate voltage. Therefore the gate voltage should also be in a pulsed form which is accomplished through a IC555 PWM generator stage. The associated IC555 PWM generator is integrated to the mosfet for accomplishing the above discussed operation. During the ON time of the mosfet, the input inpu t voltage is allowed to pass through the mosfet and is applied right across the inductor. The inductor owing to its inherent property tries to co unter this sudden infliction of current by absorbing and storing the power in it. During the subsequent OFF period of the mosfet, the input voltage is shut off o ff by the mosfet, the inductor now experiences a sudden change in current from peak to zero. In response, the inductor counters this by reversing its stored power across the output terminals via the diode which now acts in the forward biased condition. The above power from the inductor indu ctor appears with opposite polarity across the output where the intended load is connected. The capacitor is positioned to store a portion o f the power in it, so that it can be used by b y the load
during the ON time of the mosfet when the diode is reverse biased and power cut off across the load. This heps to maintain a steady and stable voltage across the load during both ON and OFF cycles of the mosfet. The level of voltage, whether it's a boosted voltage or bucked voltage depends upon how the mosfet is controlled by the PWM generator. If the mosfet is optimized with higher ON time than the OFF time then the output would generate boosted voltage and vice versa. However there may be a limit to this, care must be taken not to exceed the ON time beyond the full saturation time of the inductor, and the OFF time must not be below the minimum saturation time of the inductor. For instance suppose it takes 3ms for the inductor to become fully saturated, the ON time in this case can be set within 0 - 3ms, and not beyond that, This will result in a boost from minimum to maximum depending upon the value of the chosen inductor. The associated pot wit the IC555 PWM generator can be effectively tweaked for acquiring any desired buck-boost voltage at the output. The inductor value is a matter of trial and error, try to incorporate as many winding as possible for acquiring better and efficient results and diverse range.
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 w ith the boost converter is that the inductor is replaced by a transformer, and that the transistor has been replaced for a BUZ21. The BUZ21 has a much lower on resistance (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 that you have to make it yourself. What makes things worse is that finding a suitable ferrite core can some times be difficult since component vendors often only have a few types on stock. The E-shape ferrite core that I use measures 20x20x5 mm (Fig. 16) I got them from Paul van de Broek who always helps me when I need something special.
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 made with the reference 100 µH inductor. If this is your first fly back converter it might be illustrative to first try the ferrite core without an airgap. Mind everybody always says airgap, but what they actually mean is 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 as compared to the 100 µH inductor). At the same time you will find the ferrite saturating at a low current. It is now time to include the spacer. Attach a peace of cello tape and cut the excess amount of tape with a razor blade so that only the touching surfaces of the ferrite are covered with tape. If you try the inductor now you will find a much lower inductance and a higher saturation current. Probably you will need to add or remove some turns to get an inductance of 100 µH (same slope). For the primary winding I use 0.4 (or 0.5) mm diameter insulated copper wire. When you have determined the proper number of primary turns, the secondary winding consists of ten times that number of turns. For the secondary windings I use something like 0.1-0.15 mm diameter wire. I always include a layer of tape in between two layers of secondary windings to prevent arcing. The transformers that I use have 22 primary turns and 220 secondary turns.
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 jitter caused by a 50Hz ripple on the power supply. Nevertheless, several features from Fig. 15 can be recognized. The repetition frequency is 32 kHz and the maximum blocking voltage of the transistor is about 31 V according to theory. The voltage over the transistor almost swings for two full periods until the 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 is clearly 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 tubes I usually run them on as low as current as possible. Typically 11.5 mA. This means that the converter has to generate for 6 digits about 6 to 7 watts. The efficiency is ca. 80%. This is not spectacular but good enough for such a simple circuit. If you decide to built one: have fun, be careful and good luck!