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12V 100W Converter With Adjustable 15-35V DC Output

Did you build the 12/24V 3-stage MPPT solar charge controller published in the February 2011 issue? Then you will probably want this companion 12V 100W inverter as well. It will power laptops and other devices which require a DC voltage between 15V & 35V.

By John Clarke

Main Features

• Steps up a 12V DC input to between 15V and 35V DC

• Maximum input current 10A

• Efficient switchmode design

• Fuse & reverse polarity protection

• Power switch and indicator

Small solar systems are growing increasingly popular, whether it is for mains power, battery charging on boats, recreational vehicles and remote homesteads. But was well as needing solar panels to charge 12V or 24V batteries, you also need DC-DC converters to obtain supplies than cannot be run direct from batteries. Laptop computers are just one example.

We last published an adjustable DC-DC converter in the June 2003 issue of SILICON CHIP. This unit was powered by a 12V battery and could deliver an output voltage anywhere between 13.8V to 24V. The maximum output current that it could deliver was 2A at 16V, falling to 1.1A at 24V.

Unfortunately, this output current is often not sufficient to run a laptop computer or similar equipment. Many recent laptops require a supply voltage of about 19V and a current of 4A or more possibly more. So the June 2003 unit is simply not up to the job.

By contrast, this new design has a much higher output current capability and is suitable not only for powering most laptops from a 12V supply but for powering higher voltage equipment as well.

Fig.1 shows the output current capability of the new converter. The graph follows a nominal 100W power curve and as indicated, the circuit can supply just over 4A at 25V and 5A at 20V.

As well as its enhanced current capabilities, the new converter also boasts excellent voltage regulation at better than 99%. This means that the output voltage is maintained to within ±0.35V of its open-circuit voltage. However, some additional voltage drop can be expected in the leads running from the DC-DC Converter to the unit being powered.

Another excellent specification is the output ripple which is less than 200mV peak-to-peak at full power. However, once the input current exceeds 10A, the unit’s output voltage begins to droop.

High efficiency

As with any such circuit, there are some power losses involved in converting from 12V to a higher output voltage. For this DC-DC Converter, the efficiency is well over 80% when supplying full power. This means that the unit only runs warm at full power, with any heat dissipated by the diecast box that houses it.

This box measures 111 x 94 x 54mm and is fitted with a power switch and power-indicator LED at one end. Two cable glands are fitted at the opposite end and these clamp the figure-8 power input and output leads.

A cigarette lighter socket can be used to make the connection to a car’s battery. Alternatively, the unit can be connected via 10A-rated wiring in a solar-powered system.

Click for larger image
Fig.1: this graph plots the output current capability of the 100W DC-DC Converter. It can supply just over 4A at 25V and 5A at 20V.
Click for larger image
Fig.2: how the DC-DC Converter operates. When Q1 closes, current i1 flows and stores energy in inductor L1. When Q1 opens, this energy is dumped via D2 into the capacitor C2 (via current path i2).

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