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A 5 volt to +24V flyback output DC to DC Converter

A simple regulated +24 volts power supply for low current applications.

The circuit was built on a small piece of phenolic board
about 2 cm x 5.5 cm. It will be placed "piggyback" on a larger assembly.

I needed a few milliamps at +24 volts for a varactor bias circuit. Since my systems approach is still to run all of the power though a 5 volt regulator, this meant that I had to come up with a 5VDC-to-24VDC power supply. The resulting DC to DC converter and regulation circuit are described here.

The intended use of the 24 volts is to power an amplifier that will supply wave forms of up to a couple hundred Hz to bias a varactor or pair of varactors in an RF circuit. This meant that the current requirements would be low - probably a little over a milliamp. It also made me worry about recovery spikes from the rectifier, and harmonics from the switching itself, as they might affect the operation of the circuit the power supply is meant to power. The circuit here only has minimal filtering. Filtering to remove filter glitches will be added to another assembly.

I first considered a switched capacitor power supply since this could be made to switch "softly" at a low frequency, but the parts count started looking pretty high considering I had to go from a regulated +5 volt power supply up to a regulated +24 volt output.  This would have taken five half wave multiplier stages.

I also looked at inductor based blocking oscillators, such as that used in the 1.5 volt  LED flashlight. The problem there was that I did not have a suitable ferrite core. I also considered a forward converter made with a pair of transistors connected as a multivibrator, driving a center tapped coil with a center tapped base drive winding and a secondary for output, but I didn't have an appropriate size core for this either. My choice at the time was between cores that are way too large and cores that are way too small.

Happily, I have a selection of molded chokes I picked up at a surplus store and using a reliable old oscillator circuit, was able to produce 24 volts DC at the required current levels, in a size that seems reasonable. It is small enough that  I can easily enclose the entire circuit inside a copper shield.

Block Diagram

(Above) The circuit can be seen as an analog regulator with the voltage converter circuit in series with the feedback path.

Basic Step-Up Circuit

(Above) The basic converter was once a popular code practice oscillator circuit.

The basic converter provides an output voltage that is about 10 times the input voltage. The voltage boost occurs when the 2N4401 switches off, and the magnetic flux that had been supported by current flowing in the inductor, collapses. This rapid change in flux is accompanied by the voltage on the collector of the 2N4401  rapidly rising, limited for the most part by the turnoff time of the transistor, but ultimately being clamped to a diode drop above the voltage across the output capacitor. This type of converter is often referred to as a boost converter or a flyback converter, the later term referring to the high voltage power supply that was integrated with the horizontal deflection circuits in cathode ray tube television sets. High voltages would be developed when the horizontal scan circuit rapidly made the luminous spot return (or "fly back") to the left side of the screen.

Oscillation frequency is a little under 300 kHz when under load. I had first used a 2N2222 as the NPN because they are pretty common parts, but I could not resist the improved efficiency of the faster switching 2N4401.

After measuring the input voltage required to maintain the output voltage at +24 volts as the load was varied, the basic circuit was then treated as a building block and placed inside an analog regulation loop that could supply the range of voltages needed. From the graph (further below) , the analog regulator's output, the emitter of the 2N2222, needs to be able to cover the range from 2.2 volts to about 3.5 volts in order to supply +24 Volts from no load to a 5 milliamp load.

After some refinement to the basic converter, I determined that the converter's analog section should also be able to provide about 40 milliamps to the converter section when supplying a load current of 5 milliamps.

The completed circuit

(Above) The Analog regulator is made from a TL431 and a 2N222. Q2 and Q3
for an oscillator that develops +24 volts on the cathode of D3.

The completed circuit is shown above.  The analog regulator is a TL431 shunt regulator chip. It is followed by Q1, a 2N2222 which both provides more output current, and shifts the voltage one base-emitter drop lower. The TL431 adjusts the current through its cathode, and thus the 500 Ohm resistor to maintain 2.5 volts on the reference input. The voltage divider on the output of the supply divides the +24 volts down to +2.5 volts. I added a pot to adjust the output voltage since the build up of the tolerances could result in more than a volt of error. 

The voltage at the cathode of the TL431 is a function of the current drawn by the TL431, and this voltage is buffered by the 2N2222. The 10 Ohm resistor in series with the 2N2222 limits the peak emitter current that is available to change C1 during turn-on and in case there are large swings in the load current. C4 and R10 reduce the closed loop gain at high frequencies to keep the TL431 from oscillating.

(Above) Input voltage, in volts, applied to the converter stage to maintain +24 volts at the output
varied as a function of the load current in amps. Note that for this test, the input to the regulator circuit was +9VDC. When powered from a +5VDC supply, the maximum load current before dropping out of regulation is 6.8 milliamps.

(Above) Input current, in amps, drawn by the converter stage while maintaining +24 VDC on the output, as a function of load, in amps. The current determined by measuring the voltage drop across the 10 Ohm resistor in series with the collector of the 2N2222.

Since the TL431 cathode is designed to operate at +2.5 VDC, the base emitter drop of the 2N2222 assured that the voltage supplied to the converter could be lower than 2.0 volts, to accommodate no load. The value of R1 was chosen so as to provide the minimum required current through the TL431 when operating at the maximum output voltage, and still provide sufficient base current to supply over 40 milliamps of emitter current.

A casual glance will show some differences between the basic converter stage and the one in the completed circuit.

The addition of D2 prevents the flyback pulse from avalanching the base-emitter junction with the collector pulse from Q3. This might have degraded the beta of Q2 over the long run. A significant amount of power was also being dissipated in the avalanched base-emitter junction.  R4 helps Q2 turn off more quickly than it would if R4 were not in the circuit.

A modification to the base drive circuit for Q3 helps Q3 turn off more quickly, thus saving more power. When Q3 is driven by current from Q2 through R3, inductor L1 draws some of that base current. When Q2 turns off, the current in L1 causes the base of Q3 to swing negative, thus turning it off more quickly. R6, the 330 Ohm resistor across L1, absorbs some of the energy from L1 so that the voltage on the base of Q3 does not ring positive during the flyback pulse, thus saving several milliamps of input current. In  a higher power circuit, Q3 turning on during the flyback pulse might well mean destruction of Q3.

The base drive enhancements are not required, as they only add to the efficiency of the circuit. D2, R4, D1, and L1 can all be omitted, and the original base drive circuits shown in the basic converter circuit can be used instead, only the current drain will be a lot higher. The input voltage and input current curves on this web page were measured on the completed, improved circuit, inside the regulation loop.

The value for C3 was calculated to limit the maximum positive-going voltage transient on the output in case the load became disconnected. If all of the energy stored in L2 was suddenly dumped into C3, how large would C3 have to be to keep the output voltage from rising more than 1 volt?

A safe and practical answer is found by quickly by ignoring the resistance of the circuit and noting that the energy stored in an inductor is 1/2 * L*I^2, and the energy stored in a capacitor is 1/2 * C * V^2. Using 220 microhenries and taking the inductor's saturation current of 300 milliamps as the maximum current, C = (L*I^2)/V^2 = 20 uf.

I should mention that the 1N4141 is not normally my first choice as a rectifier, but given the light usage, about 50 milliamps peak current, and low demands on efficiency, it works very well in this application.

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First posted in October, 2005. Revised November 2005, September 2006.

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