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Ultra Simple 5V Switchmode Regulator
Rod Elliott (ESP)

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5V Supplies
There are numerous switchmode regulators available, most based on ICs. While convenient (if you can get the IC), they are not always readily available, and like all ICs, exclude the user from understanding their operation for the most part. This appnote is based on AN-003 - a constant current version of an otherwise almost identical circuit.

Instead of operating in constant current mode, the circuit shown uses a shunt zener diode to set the output voltage. This does have some limitations, the main one being that the output voltages available are limited to the zener voltages available, and the output voltage is about 0.65V above the zener voltage. As shown, the circuit will regulate at around 5.35 - 5.4V, but this is well within the recommended maximum rating for 5V logic circuits - typically 5.5V. In fact, it is quite common to set (adjustable) 5V regulators a little high to account for resistive losses in PCB tracks and other wiring.

PWM (Switchmode) Regulators
Although there are many ICs available, ranging from simple linear regulators such as the 7805, buck regulator chips and complete encapsulated regulator modules, most are not available in hobbyist quantities, and/or are relatively expensive. Not so the circuit shown here. All parts are cheap, nothing is especially critical, but (of course) efficiency is not as good as the dedicated circuits.

Linear regulators are very inefficient. The full output current is drawn at all supply voltages, so with 12V input and (say) 500mA output, the total circuit dissipation is 3.5W. While this is not a great deal, where efficiency is paramount such as with battery operation or where limited space is available for heatsinking, this is not a good solution.

Figure 1
Figure 1 - Ultra-Simple Switchmode Regulator

Like the original in AN-003, it uses only three cheap transistors, and works remarkably well. It is less efficient than most of the dedicated ICs, but is still more efficient than a linear regulator. It has the great advantage that you can actually see what it does and how it does it. From the experimenters' perspective, this is probably one of its major benefits.

Again, like the LED current regulator, it will change from switchmode to linear as the input voltage falls. It still remains a voltage limited supply, and the design voltage (set by D2) does not change appreciably as the operation changes from linear to switchmode or vice versa.

How Does It Work?
Operation is quite simple - Q1 monitors the voltage across R1, and turns on as soon as it reaches about 0.7V. This turns off Q2, which then turns off Q3 by removing base current. If the voltage is low, a state of equilibrium is reached where the voltage across R1 remains constant, and so therefore does the current through it (and likewise through the zener).

The value of D2 can be changed to provide the output voltage required. Although in theory the output voltage should be ...

V = VD2 + 0.65V   (approx.)

... in reality it will be less. A typical 4.7V zener will normally operate at around 150mA, but this circuit operates the zener at a lot less (around 15mA), and this will cause the output voltage to be lower than expected. R1 can be reduced to increase zener current, but at the expense of efficiency.

Operation is almost identical to that described in AN-003. At higher input voltages (typically about 1.5-2V above the output voltage), the circuit will over-react. Because of the delay caused by the inductor, the voltage across R1 will manage to get above the threshold voltage by a small amount. Q3 will get to turn on hard, current flows through the inductor and into C1, the load and through D2. By this time, the transistors will have reacted to the high voltage across R1, so Q1 turns on, turning off Q2 and Q3. The magnetic field in L1 collapses, and the reverse voltage created causes current to flow through D1 and into C2. The cap now discharges through the LED and R1, until the voltage across R1 is such that Q1 turns off again. Q2 and Q3 then turn back on.

This cycle repeats for as long as power is applied at above the threshold needed for oscillation. The circuit changes its operating frequency as its method of changing the pulse width. This is not uncommon with self-oscillating switchmode supplies.

For testing, I only had a 3.9V zener available, so I used that. In theory, the output voltage should have been around 4.6V, but the zener current was much too low to get good voltage stability. This will also apply for most other zener voltages in the range this supply will be used, so ...

Table 1 - Operating Characteristics

The table above shows the operating characteristics of the prototype. The output voltage remained stable at 3.93V (±0.01V) with loads ranging from infinity down to 10 Ohms (393mA output). Although the switching waveform becomes chaotic with no load (the frequency and waveform are rather unpredictable), the voltage remains stable.

The efficiency is not as high as you would get from a dedicated IC, because the switching losses are higher due to relatively slow transitions. At best it manages 68%, which is not bad for such a simple circuit. Input voltage can range from just above the zener voltage up to about 16V or so. Maximum efficiency is provided with an input voltage of between 7 and 12V. It is still much better than a linear regulator though - at 7V input, a linear regulator will manage 55%, and this falls as input voltage is increased - about 32% at 12V and 24% at 16V. All wasted power is dissipated as heat.

Construction is not critical, but a compact layout is recommended. L1 needs to be rated for the full load continuous current, Q1 may or may not need a heatsink, depending on the input voltage and output load. The ripple current rating for C2 needs to be at least equal to the load current, so a higher voltage cap than you think you need should be used. I recommend that a minimum voltage rating of 25V be used for both C1 and C2.

Q1 and Q2 can be any low power NPN transistor. BC549s are shown in the circuit, but most are quite fast enough in this application. Q3 needs to be a medium power device, and the BD140 as shown works well in practice. D1 should be a high speed diode, and a Schottky device will improve efficiency over a standard high speed silicon diode. D1 needs to be rated at a minimum of 1A. L1 is a 100uH choke, and will typically be either a small 'drum' core or a powdered iron toroid. The current rating of L1 must be at least the expected output load current to minimise losses (and heat). All resistors can be 0.25 or 0.5W.


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Copyright Notice.This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright © 2004. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro-mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference while constructing the project. Commercial use is prohibited without express written authorisation from Rod Elliott.
Page Created and Copyright © Rod Elliott 29 Jun 2005