Time for the first actual blog. I’m working on a lab power supply at the moment and I’d like it to be efficient but still low noise. The best way to make an efficient power supply is a switch mode power supply or SMPS. Making one with a variable output voltage is not too hard. The big downside is noise. If the SMPS uses a switching frequency of 1 MHz, something quite common with efficient SMPS’s, making it low noise is difficult. Not only you need a fairly big capacitor to get the ripple low, the capacitor needs to have a very low ESR, if the ESR is too high the capacitor won’t be able to charge in the short time it has with the 1 MHz switching frequency. A lower frequency helps a bit but achieving a low noise of something around a 1mV is almost impossible.
Cheaper power supplies use a linear regulator like the LM317. These devices have a low ripple and noise output without using expensive low ERS capacitors. The downside is the efficiency, with a 20V input and a 5V output at 1A a loss of 15 Watt is happening, meaning a big heatsink is required.
Why not combine these two, giving the low ripple of a linear regulator but the efficiency of a SMPS. making an SMPS that has an output voltage of 2 to 3 Volt higher than the requested output voltage would be great, this would only mean a 2 to 3 Watt loss in the linear regulator.
This kind of circuit is possible and called a tracking pre-regulator. An example of this is mentioned in the datasheet of the lt3080 or lt3083 device from Linear Technology.
Using the 10K and 200K resistor the output voltage is set to roughly 16.5V. But they also put a MOSFET in there. This schematic actually uses the threshold voltage of the MOSFET to set the output of the SMPS. The output of the SMPS is roughly Voutlinear regulator + Vthreshold. Using this schematic there will be a loss of 1.5 to 3V depending on the used MOSFET, much better then with just the linear regulator. The downside is the extra costs and more complex schematic because of the extra parts. The noise will be more then with just the linear regulator but much better then with only a SMPS.
It also works wonders in LTSpice, the LTSpice can be found here: http://sdrv.ms/13vRUDb
The SMPS in this schematic is an LT part as it’s from an LT datasheet. Linear Technology makes great IC’s but most of them are a bit expensive, the LT3680 is not different at almost 10 euro’s in singles. The schematic should work with quite some SMPS IC’s like the cheaper LM2670 from Texas Instruments. The LM2670 is also available in a TO220 package, great for a hobbyist. Time to order one and build it up!