Richard, I find your solution interesting.
But I'm not convinced that it is better/cheaper than Daves version with a single LT3080. The 2n2907A is $0.80, a LT3080 is $4 so the price is about the same. But I'd feel more comfortable with the TO220 LT3080 screwed to a heatsink dissipating 25W than the same spread over 9 TO-18 devices with their flimsy heat-sinks.
The 25 W is probably about the worst case when drawing a full amp at low voltage.
The 2N2907A is below 10c - if not you are being ripped off. In quantity, the prices get very low. The 2N2907A and the 2N2222A are exactly the kind of transistors it is great to stock in the lab anyway, so you can go for the 100 off prices without straining the wallet too much. I would use the TO92 devices, not the TO18 ones. The exact part number varies from manufacturer to manufacturer. The through hole version of the circuit relies on air cooling of the TO92 packaged transistors - no heatsink is used. The PZT2907A and PZT2222A can dissipate over 1W, so they are pretty nice surface mount variations to have. I may have to increase the transistors to 10 for the through hole board.
My solution is fine, as long as I can limit the maximum dissipation on the board to 5W which amounts to about 3.6W peak in the transistors. My job is to see that the regulator board does not dissipate more then 5W continuous under any condition.
But why not just use Dave's LT3080? It is an extremely nice chip, but not perfect. Some of the problems are:
- The supply current for the chip comes from the load, so there is a minimum load the chip needs to regulate. Dave's solution was to add a LM334 temperature dependent current source, but this has issues below 1V output, and the temperature dependency makes it hard to properly compensate for it in the current measurement circuit. I just like supplies that work properly down to zero volts - I do not design lab supplies that do not. It is easy to overcome this limitation by adding an extra resistive load to Dave's supply when workng at low voltages, but my choice is to avoid that.
- The LT3080 only regulates to the output pin of the chip, not the output terminal of the power supply. If there is a total of 0.1 ohms resistance between the LT3080 and the output terminal (wiring resistance and maybe an output ON/OFF switch), then you get 0.1V regulation only as you go from 0 to 1A.
- You cannot add a fuse on the LT3080 output as the resistance will wreck the regulation. This limits the ability to protect the supply from something like a SLA battery you accidentally connect in reverse. I can add a fuse to my board with no effect on the regulation and a fuse or breaker is the only protection against excessive high energy positive of negative voltages from a connected load.
- The LT3080 has a reasonably high thermal resistance from junction to case which limits how much supply you can get in a small case. That is why Dave's original supply was going to be limited to about 10V maximum, and to boost that to 20V, he had to add a switching pre-regulator. As discussed in this thread, there are problems with the switching pre-regulator in that a specially crafted pulsed load can end up cooking my regulator board or in Dave's case, the LT3080. The LT3080 is thermally protected right? Well only you a point. In thermal limiting mode, the LT3080 chip is at 160 degC. The Absolute Maximum operating temperature of the chip is 150 deg C. Translated into english, that means Do Not Rely On Thermal Limiting for Normal Operational Modes. In a Lab power supply, difficult loads are a normal operating mode. The whole idea of a Lab supply is it is meant to be able to cope with difficult loads without failing. The thermal limiting of the LM317 chip has the same concerns as the LT3080
- The LT3080 requires a reverse bias bypass diode across the LT3080 to prevent damage from voltages coming from the load. I like the rather unique property of the LM324 in that is can take over 30V on the input pins even when the LM324 is unpowered, so I can avoid the need for this diode. With the reverse diode solution, I do not like the fact that attaching a 12V SLA battery to the supply while it is off basically powers up the power supply. But this is my preference - not really an issue I want to get into a big discussion about. People choose different colours to paint their house - it does not mean that one colour is right and another colour is wrong - they are just different colours
That list will do for the moment. The LT3080 is a brilliant chip, and Dave chose it for very good and sound reasons. My board is a different solution that will have its own positives and negatives.
Just to go back to the start, I was frustrated with the number of times people had to go to the LM317 IC to make a cheap power supply as it does not make a great lab supply. I initially wanted to see if I could come up with some kind of general purpose regulator board that could be used to make different supplies with different output ranges and current ranges without needing to design a new compensation circuit each time.
It looks like it just cannot be done - change a voltage divider or a regulator power transistor and you need to redesign the compensation. A really well designed power supply compensation circuit is very hard to do for any power supply as all lab power supplies relying on a feedback stabilization operate very close to instability. It is something impossible for beginners to get right, and it can be very time consuming to get right for experienced designers.
The reason why devices like the LM317 are so popular is they have built in compensation. Not perfect for all capacitive loads, but pretty good. The LT3080 also has excellent built in compensation.
There are IC's available that can drive an external MOSFET to make a linear supply, but I really do not like lab supplies using MOSFETS. The thing I just do not like is that if you switch a sudden overload on to a MOSFET supply, the Miller effect causes turn the MOSFET hard on causing peak current in the 10s to 100s of Amps regardless of your current limit setting. This big current spike is something that can easily be eliminated in transistor-based supplies, but it is extremely hard to prevent in MOSFET based linear supplies. I have chosen to stick to transistor-based supply designs.
Inspired by some ideas in Dave's design - in particular the fact that the regulator was always adjusting for zero difference between the reference voltage in and the out voltage (no voltage dividers, so no loop gain change for different output voltage ranges) and the fact that the the supply to the reference circuit was allowed to change with the required output voltage - I started looking at a regulators board made from common parts that are extremely cheap to make a fully compensated module, and then work out how to add extra external parts to extend the range without affecting the regulation compensation.
Compensation ends up being much harder then looking at Bodes plots and getting the zero's and poles right. Things change for different currents, different voltages, different load capacitances, and even if you get stability with all those variations, then you have to look at conditional instability. If you tailor a load to force the regulating opamp into output slew rate limiting, you have changed the loop gain and opamp speed, and this can cause an instability that is not there with static loads. If you can force the opamp to saturate - more problems. Power supply compensation circuits can look simple when completed, but it can be a total nightmare finding that right solution. I bet there are many commercial lab supplies that are conditionally unstable and this is one of the things that separates the $1000+ quality designs from the $100 designs.
My board can be used directly to make a 0 - 22V 210mA supply or a 0-6V 800mA supply , and with an NPN darlington preregulator transistor, it could be extended to 0-25V at 1A. My latest idea would allow it to be extended further to almost anything - in theory you could make a 0-1000V 1A supply. I will probably limit any designs to 60V maximum myself, but that will not be limited by my board.
I would be prepared to use my board to make an internal HV supply inside another project, but a HV lab supply is extremely dangerous - I really caution against the idea.
If I can get the module paralleling to work, then you can get 2, 3, 4 amps out and the modules will be cheap enough to make this economical.
All these supplies will regulate fully down to 0V.
At the moment, it is really hard to find any kind of generalized design that allows you to make a supply with exactly the voltage and current specs you need. Hopefully, this design will mean that there is at least one option available.
Richard.
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