'William Chops" Westfield ' By the time you pay $3 for the specialty controller chip (Q100 from > digikey), and $4 for the specialty transformer (Q100 from mouser; DK's > MOQ is 200; neither actually stocks the lower power transformers), the > whole thing isn't looking very attractive... You are making this too complicated and needlessly expensive. One of my side projects right now is experimenting to make a reasonably affordable isolated supply that takes a fairly wide input power range. I want something I can plunk down for isolated power from a 48V DC bus, but that can also work just as well with as low as 12V DC in minimum, preferably less. This is work in progress. I put the current snapshot at http://www.embedinc.com/temp/isol1.pdf. The transformer is from CoilCraft, and is meant for flyback operation for POE applications. It costs well under $1 in quantities. The PIC 16F616 is only because that's what I had in stock of that family of PICs that contains the power supply PWM module with shutdown. Eventually I expect to use a 12F615, or maybe a 12HV615. Samples are on order, but for now I can test just fine with the 16F616 because it's the same thing with 14 pins instead of 8. The 12F615-I/SN costs $.57 each for 1000. As I said, this is work in progress and a testbed. Even though the point is to create a isolated supply, I have both grounds connected together to make probing around easier. It should be obvious that the two grounds could be separated. This is built on a ReadyBoard-01, so I'm using the convenience of having known regulated 5V available. Vout is intended to be one diode drop above the regulated output. It would normally be the input to a LDO, with the right side of R4 connected to the output. In this case I used the existing 5V line instead of putting the LDO on the test board. D1 does nothing in the current circuit. It is left over from earlier experiments. I left it on the schematic because it is still physically on the test board. The firmware strategy is to use the hardware PWM with shutdown to generate the transformer pulses. The PWM duty cycle will be optimum for the particular input voltage. The NSHUT line is driven low by the opto when the output is above its regulation threshold. This shuts down the PWM output automatically without firmware intervention. The foreground code periodically polls the A/D and adjusts the PWM duty cycle according to the input voltage. This all works fine so far, although the best relationship between input voltage and duty cycle still has to be determined. It's easy to change the duty cycle on the fly, but not the PWM period. Right now the PWM is set to a fixed 100KHz, so at low voltages the raw pulse width based on transformer saturation alone is too long without the duty cycle fraction being considered. That consideration isn't in the code yet. Eventually this needs to create a 5V supply on the input side too. I plan on using interrupts and the comparator input and internal absolute voltage reference for that. The bias winding of the transformer is tempting, but I'm not sure it can be used without assuming a minimum load on the output, which I don't want to do in this application. I may use a separate inductor with a simple PNP high side switch. So in brief, the hardware PWM will generate and regulate the isolated supply, interrupts with comparator and internal voltage reference the local supply, and the foreground code will read the input voltage and adjust the PWM duty cycle. It will probably also leave information around for the local supply pulse width. ******************************************************************** Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products (978) 742-9014. Gold level PIC consultants since 2000. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist