Paul posed a question about the base resistor and how it affects efficiency. This is a point that was apparently addressed by the low Vce(sat), high gain transistors that were made especially for DC to DC converters. Since the BJT (bipolar junction transistor) is a current operated device, the current that it needs to turn fully on is a fraction of the total power and if it can be minimized, it will help increase the circuit’s efficiency.
The graphs for the BC337 and similar transistors show a graph of collector current versus the Vce(sat). Typically in the corner there is text that says “Ic / Ib = 10″ or something similar. This means that the base current, which is normally 1/100 or less times the collector current, has been forced to 1/10 of the collector current in order to drive the transistor out of the linear region and into saturation. The saturation measurements are made at this ratio.
But in the datasheet for transistors that are made for high current switching, the graph may say “Ic / Ib = 30″ or even a higher ratio. I assume that the transistor has such a high gain and low Vce(sat) that forcing the base current down to 1/10 of the collector current has no more benefit than forcing it down to 1/30.
What I see is that with the BC337, the base current is ten percent of the collector current and is using up to ten percent of the total power. But in the case of the special transistors, the current is only 3.33 percent of the total, and wastes 1/3 as much power as the BC337. The less power is wasted, the greater the efficiency.
What is also important is that during saturation the special transistor takes much less to get the voltage drop to less than a quarter volt, and in some cases less than a tenth of a volt. This gives as much of the energy as possible to the coil, since almost all of the battery voltage is across the primary winding.
Some of the special transistors are:
2SD965 – Panasonic – for an example see page 2 of the .PDF datasheet here.
2SC2500D – Toshiba – for an example see page 3 of the .PDF datasheet here.
2SD5041 or KSD5041 – Fairchild – see page 2 of the .PDF datasheet here.
ZTX651 – Zetex – .PDF here
ZTX1048A – Zetex – .PDF here
NTE11 – NTE nteinc.com
There are several more that are older and no longer used, and there are several common ones that are in surface mount packages. All of the above are for through hole mounting, typically in the TO-92 package.
I guess the question in my mind is. If you dim the LED by increasing the base resistor in a given JT, does it behave like a linear power supply, reduce power overall BUT also reduce efficiency of light output or is it like a switch mode power supply turned low i.e. it keeps the efficiency fairly high and fairly constant.