vrijdag 2 november 2012

A 555 based simple low power boost converter

I'm experimenting with boost converters to increase the voltage delivered by NiMH or solar cells.

I played around with the TL499A which is a moderately priced IC, about 3€. It can convert from 1.1V upwards. A pricier modern alternative is the LT1073 for about 8€. If you just need to convert from 3V upwards then you're lucky, the MC34063A is available for 50 cents! Finally, an interesting 1.0V to 3.3V (or 5V) ready to use breakout board.

When you just need to double the voltage and have modest current requirements (e.g. less than 10mA) then a charge pump voltage doubler is an option. The Intersil ICL7660S is a modern and efficient solution for 1.75€. You can also build one yourself, e.g. with a 555 timer IC, there exist plenty of references on the internet. Note that you loose about 0.8 volt, so 2.4V fed to the doubler, will result in about 4 volts. I used these doublers to trickle charge super capacitors or NiMHs from solar cells at dark locations. The cells deliver e.g. 0.5 mA at 2V. That is insufficient voltage to light a led, but by using a doubler you can charge a (super)capacitor/battery with 0.25mA to 3V, and use it to power your (AtTiny) micro controller, so it can flash the led. Note that if you build one yourself that you should use the CMOS 555 versions. The normal 555s will not run at these low voltages and use far too much power to be useful in low power applications.

Ok, so you decided to build your own boost converter. The schema shown below is quite versatile. It will accept an input in the 2V to 12V range and the output can be in the 2V to 24V range. Note that you NEED a CMOS 555 for input voltages below 4.5V! Output currents in the 20-100 mA range should be no problem with the schema as shown.

How does it work? The concept is simple, the 555 generates an on/off signal in the 100kHz range which is used to switch the Darlington transistor pair.

As soon as the required output voltage is reached the 555 stops oscillating for a while because the BC547 pulls the 555 reset pin to the ground.

The values shown in the schema allow you to triple the input voltage, or even more if you have low output current requirements, e.g. under 5mA. If you want more then you should experiment with changing the duty factor of the 555. Replacing R2 with a 100k resistor will increase the duty factor from the default 50%, so that the inductor will store more energy. Increasing the value (inductance and max current specification) of the inductor might be needed to prevent saturation.

I used a 10p timing capacitor on the bread board which will result in a 100kHz (due to stray capacitance) and near 50% duty cycle with low energy consumption. If you want more exact timings for higher duty cycles then you could use eg 100pF, 10k and 33k for the 555 timing network.

Converting 2.4V input to 5.0V output and loading the output with a 100ohm resistor (50mA) results in a 62% efficiency (170mA input) in the R2=100k configuration. Note that the converter is most efficient when it can operate in continuous mode, this means that the switching frequency, duty cycle, inductor values and output voltage and current are matched. Use the default values for e.g. 50% efficiency at a large range of input/output voltages.

The Darlington can be replaced by an N-Channel (Power) MosFET, which will improve efficiency a lot. If you use input voltages over 4V then the IRLZ34 Logic Level power MosFET will allow you to scale up your power requirements. The Darlington has a 0.8V collector emitter drop at moderate currents, so at 2.4V input we will loose about 33% efficiency. The IRLZ34 has a milli-Ohm resistance for gate-source voltages over 4V. The best MosFETs I could find which should switch efficiently at 2.4V are the (SMD sot23) IRLML2502 and the FDN339AN which should be fine at 2.0V or even a bit lower!

Note that the output Zener should not be used as a regulator when you want high efficiency. You should calculate the duty factor of the 555 and the Zener should only become active when the output voltage rises over the desired upper output voltage limit because of a very low output load.

You can replace the Zener diode with a TL431 programmable voltage regulator as shown in the next schema if you want to experiment:

Interesting links:

A very clear explanation about the concepts and the calculations for converters. You really should study this if you want to adapt my example converter for optimal efficiency.

If you want more power, an interesting schema for a 6 to 12V converter.

You don't need a precise output voltage and have a square wave signal from eg a micro-controller? Look here.

The MC34063A is your preferred solution? A handy calculator aid.

Another simple converter circuit.

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