Another Power Supply

[braddrew0]

Hi Guys,


Using Dave's uSupply as inspiration, I've designed a 20V, 2A variable power supply. This is my biggest project to date, and I'd appreciate any feedback at all you can offer. Even if the design is completely messed up, I've had a lot of fun and learnt a lot so far

The design is broken into a bunch of blocks. I'll go over each one in turn with justification over why I picked certain components over others (which a lot of the time was because I had them on hand...)

The design will be fed from mains via a transformer with an 18VAC output. This passes through the fuse, bridge recitifier and is filtered, giving around 26V output. The ripple is load dependent but I calculated max of around 1V. I used four 10,000uF caps vice one single big one for two reasons. Firstly, it was cheaper, and secondly I thought it might give me a bit of redundancy if one fails.

There are two secondary chip regulated supplies, being 5V and 3.3V. The 5V is fed from 26V as I wanted all the low power devices to power up before the main supply (more on that later). The 3.3V is used to drive the uC and peripherals.

Along with the 3.3V supply, I've made a simple linear 4.096V supply. The ADR4540 is a 4.096V reference that I have excess of here. C8 and C15 provide supply buffering. D1/Q1/R2/R3 is a simple 100mA constant current source. This is fed into the gate of the MOSFET. The AD818 compares the output voltage to the reference, and D2 allows it to sink current only. So the basic idea is the constant current supply is fed into the MOSFET, then sunk when not required. I think this is more complicated than it needs to be (my original design ran the ADR4540 into a unity gain amp) but I'm also interested to test out the principle of this circuit (it's based on something similar by Walter Jung).

Most of the uC circuitry is standard. It will be run on 3.3V, including the i2c line. P2 is a NewHaven display which is controlled via i2c. There are two rotary encoders (current and voltage) and two switches (reset and enable). The capacitor bank on the RHS is for filtering the 4.096V IC's (ADC, DACs, OpAmps).

U8 and U9 are two i2c DACs. I've got excess of these at home, hence the choice. One is to set the reference voltage for current (0 - 4V => 0 - 2A) and the other for the output voltage (0 - 4V => 0 - 20V). The ADC is to return the signals back to the uC so I can display set and actual voltage/current. It's a 4 channel i2c because that's what I had lying around.

The bulk of the power supply is in the top right. It's a three stage topology, with a pre-regulator, current control and MOSFET voltage control. I wanted to use a pre-regulator for two reasons, firstly to remove the ripple from the 26V line and secondly so I could use the enable port to switch the supply on/off. This means the unit will always start/restart with enable low, hopefully stopping initial voltage spikes. The output of the pre-regulator should be around 23.5V.

The current control is achieved with a simple 0.01ohm shunt and a differential amplifier. It has a high gain on it (50) to convert the shunt current into the 0 - 4V range. Like Dave's design, this sinks the MOSFET gate when the current is higher than the set value.

Voltage control is again reasonably standard, with a resistor divider converting 0-20V into 0-4V. This is also tapped off to be read by the ADC.


That's all I can think of at the moment - I'm holding off on the board layout until I'm convinced that the design is workable in it's current state. I'd love to hear any feedback or criticism that any of you have. Like I said this is my biggest project to date, and I don't know what I don't know... so help me learn


Regards,



Brad

[braddrew0]

Bump.... any feedback?

[dadler]

The ADA4177 is not a single supply op amp. The current shunt diff amp seems to be relying on voltage near ground, but it can only read Vss+1.5V.

[viperidae]

A 5v linear regulator powered from a 26v source is going to have to drop 21v. That's going to be 2.1w for every 100ma.

[Kleinstein]

The voltage regulating OP likely needs something to slow it down to prevent oscillations.

The current regulation part does not look good to. 

The Sourcefollower output stage is tricky: ouput is limited to something like 2 or 3 volt below the driving voltage. With the OP supply at only 4 V this is not much.  Also there is a danger of having two much of a gate voltage if the output is still high and the driver goes low very fast.

[braddrew0]

Thanks guys,

dadler - Good pickup, thanks! I've replaced with ADA4084.

viperidae - One of my aims for this was to work on cooling with SMD components. The D2PAK can handle 3W power dissipation with a 30mm x 30mm plane under ground. I think I will be right on the limit of this - my plan was to place the regulator on the bottom of the board with a solid ground plane, and no vias within 30mm of the device. Do you think this is suitable? If not, in terms of alternatives, would the best option be a pre-regulator (say a 7812) which will spread the power dissipation over two devices? Thanks!

Kleinstein - Thanks, this is great! On oscillations, is there a good rule of thumb on how much capacitance to add? I tried to read up on analysing the control system to look at stability, but most references go straight into adding poles/zeros to the transfer function. I did control theory in university about 15 years ago (aeronautical engineering) but I remember having nightmares over this stuff and I think it will take a good couple of months to get my head back around it.
On the source follower - with a rail to rail amp like ADA4084, will this still be an issue?
On the gate voltage - I'm not 100% sure what you mean here. Wouldn't the gate voltage in the absolute worst case be limited to the OP supply?


Thanks guys - I really appreciate the help 

[motocoder]

Hi braddrew0 -

One comment I had is about using the MOSFET as the main series/pass regulator. I have always been told that the typical power MOSFET is suitable for switching applications, but may not be a good choice for a linear regulator. The reason for this is that in the typical switching application, the MOSFET is either off and not conducting, and hence not drawing any power, or in saturation with low on resistance, and thus not dropping much power. The primary power dissipation is due to the period where it switches from off to on or vice-versa. The MOSFETs are optimized to minimize switching time, and not to operate in linear mode. If you try and operate them in linear mode, for example in your circuit, you may end up dropping significant voltage across the MOSFET, and hence dissipating significant power. There is an effect called the "Spirito Effect" that can in these situations lead to a failure of the MOSFET. You can read more about it in this app note, or just search for "Spirito Effect"

https://www.fairchildsemi.com/application-notes/AN/AN-4161.pdf

So I'm not saying it's bad to use a MOSFET, but read that article and make sure you are operating the MOSFET in the Safe Operating Area (which might be more restrictive than you think due to the Spirito effect).

For that reason, you will often see series voltage regulator designs using some sort of BJT device rather than the MOSFET, or perhaps an IGBT for very high voltage designs.

I'm certainly not an expert in this area - so please confirm what I'm saying independently.

And kudos on expanding your knowledge with this project; I think it's a great idea.

[dadler]

The SOA graph may or may not show a mosfet as being DC rated.

There are mosfets designed for linear operation, such as the IXYS Linear2 series:

http://www.mouser.com/ProductDetail/IXYS/IXTQ60N20L2/?qs=%2fha2pyFaduh78JBcne8bXIOtGyZxvW3LDkwxtjcUKhF9DBxmlqF3%252bg%3d%3d

[mij59]

U10 is powered by 4.096V you won't get much output voltage, connect it to +26V.
The common mode voltage of U10 and U7 need to include zero Volt.
Using 40.000 uF as filter caps is a bit over the top, what's wrong with 4700 uF ?
The current sense amp has an attenuation of 50, you need to swap the resistors 10k / 500k, check if U6 can handle the high common mode voltage.

[braddrew0]

motocoder - That's something I didn't even think of... I must have read through the app note a dozen times without seeing a big graph saying "safe operating area"...  I've switched over to MJB44H11T4-A from STM (http://www.digikey.com.au/product-detail/en/MJB44H11T4-A/497-15455-1-ND/5244680). The 20V/2A is right on the limits of safe DC operation. One other question I do have though is the parameter "VEBO - Emitter-Base Voltage" of 5V. Does this mean that if I want to pass 20V from the emitter I need 15V on the base? Thanks!

dadler - Thanks again - I've switched over to a BJT but I've bookmarked these. I've got another project in mind that uses PWM which looks perfect for these.

mij59 - I've replaced U10 and U7 with the same ADA4084. Both are fed from the 26V supply now. My initial concern on using the 26V is that it wouldn't be a "stable" 26V... so wouldn't that just add ripple to the output? If I need 15V to drive 20V though, it's obviously not going to work with 4.096V, so thanks!
On the caps - the MIC29302 had a maximum input voltage of 26V, and I've configured it for about 23.5V output. I initially had 4700uF but at maximum draw (I gave it around 2.5A to be safe) the ripple was too high and would drop below 23.5V. With 40,000uF, I calculated around 1V maximum ripple, which gives me around 1.5V for the dropout in the regulator. Is this the correct process?
On common mode voltage - I'm finding it difficult to interpret this from the datasheet (http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4084-1_4084-2_4084-4.pdf). I'm assuming +26V would give similar data to +/- 15V except without the ability to exactly hit the 0V rail. The graph on page 24 - the way I interpret that, over most ambient temperature conditions, the behavior of the differential amp will be non-linear inside about 100uV - am I reading that correctly? Thanks for your help!

Revision A2 schematic attached - thanks again all!

Brad

[mij59]

mij59 - I've replaced U10 and U7 with the same ADA4084. Both are fed from the 26V supply now. My initial concern on using the 26V is that it wouldn't be a "stable" 26V... so wouldn't that just add ripple to the output? If I need 15V to drive 20V though, it's obviously not going to work with 4.096V, so thanks!
On the caps - the MIC29302 had a maximum input voltage of 26V, and I've configured it for about 23.5V output. I initially had 4700uF but at maximum draw (I gave it around 2.5A to be safe) the ripple was too high and would drop below 23.5V. With 40,000uF, I calculated around 1V maximum ripple, which gives me around 1.5V for the dropout in the regulator. Is this the correct process?
On common mode voltage - I'm finding it difficult to interpret this from the datasheet (http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4084-1_4084-2_4084-4.pdf). I'm assuming +26V would give similar data to +/- 15V except without the ability to exactly hit the 0V rail. The graph on page 24 - the way I interpret that, over most ambient temperature conditions, the behavior of the differential amp will be non-linear inside about 100uV - am I reading that correctly? Thanks for your help!
Revision A2 schematic attached - thanks again all!

Brad

With 18V AC input is not much room for regulation, if you want an output voltage of 20V get a higher input voltage, ( rule of thumb AV voltage in is DC voltage out )
The pre-regulator is not necessary, its only decreases the room for regulation.

The data sheet of the AD4084 states that input and output are rail to rail, so it shouldn’t be a problem.
The input stage of most opamp's won't work properly when the input voltage comes within a few volts of the one of the power rails, e.g. a LM324 with a single power supply will work with input voltage form zero  to V+ minus 1.5V.
 
 

[motocoder]

motocoder - That's something I didn't even think of... I must have read through the app note a dozen times without seeing a big graph saying "safe operating area"...  I've switched over to MJB44H11T4-A from STM (http://www.digikey.com.au/product-detail/en/MJB44H11T4-A/497-15455-1-ND/5244680). The 20V/2A is right on the limits of safe DC operation. One other question I do have though is the parameter "VEBO - Emitter-Base Voltage" of 5V. Does this mean that if I want to pass 20V from the emitter I need 15V on the base?

The VEBO Rating of a BJT is the maximum allowable voltage that the emitter-base junction of a transistor can handle before it becomes damaged or destroyed.

Also, see dadler's comment; if you want,  it's OK to stick with a MOSFET, just make sure you use one rated for linear operation, and check the SOA graph in the datasheet to make sure you're OK there.

Also, when checking specs, you want to check the amperage through the transistor and the voltage drop across it - not the output voltage. For example, if the rectified unregulated voltage is 30V at full current, and someone shorts the output or programs the output voltage to be near zero volts, then the voltage drop across the transistor might be as high as 30V. If you are having trouble finding a device that can support this, then you can parallel two or more devices, but be sure to add a small value emitter or source resistor to each to ensure the current across the devices is distributed fairly evenly. I think that you should be able to find a single transistor that can easily to support this though, but you will need to heatsink it.

You can also also add a current limiting feature if you want. There are ICs like the LM723 that you could use, instead of the op-amp, which can still drive the external transistor but which have a current limit feature built in. You just need to add a current sense resistor and connect things appropriately to enable that - see the data sheet. You can also add a current limit feature to your existing, op-amp based, circuit - let me know if you want an example of that. BTW, the LM723 is what is used in those Astron linear supplies that are popular with HAM radio folks. It's kind of an old IC, but it's cheap and works.

[braddrew0]

With 18V AC input is not much room for regulation, if you want an output voltage of 20V get a higher input voltage, ( rule of thumb AV voltage in is DC voltage out )
The pre-regulator is not necessary, its only decreases the room for regulation.

The data sheet of the AD4084 states that input and output are rail to rail, so it shouldn’t be a problem.
The input stage of most opamp's won't work properly when the input voltage comes within a few volts of the one of the power rails, e.g. a LM324 with a single power supply will work with input voltage form zero  to V+ minus 1.5V.
 
 

I only had two considerations for the pre-regulator - reducing ripple and giving me an "enable" switch. I guess for the type of supply I'm trying to build, the levels of ripple I'll get from the op amp shouldn't be prohibitive. I like your rule of thumb - that's handy to know Do you think there are any issues if I replace the pre-reg with a similar BJT to the main pass transistor and use that as an enable/disable? I was trying to avoid using a relay if I could but I'd love to have the ability to isolate... Thanks again!

The VEBO Rating of a BJT is the maximum allowable voltage that the emitter-base junction of a transistor can handle before it becomes damaged or destroyed.

Also, see dadler's comment; if you want,  it's OK to stick with a MOSFET, just make sure you use one rated for linear operation, and check the SOA graph in the datasheet to make sure you're OK there.

Also, when checking specs, you want to check the amperage through the transistor and the voltage drop across it - not the output voltage. For example, if the rectified unregulated voltage is 30V at full current, and someone shorts the output or programs the output voltage to be near zero volts, then the voltage drop across the transistor might be as high as 30V. If you are having trouble finding a device that can support this, then you can parallel two or more devices, but be sure to add a small value emitter or source resistor to each to ensure the current across the devices is distributed fairly evenly. I think that you should be able to find a single transistor that can easily to support this though, but you will need to heatsink it.

You can also also add a current limiting feature if you want. There are ICs like the LM723 that you could use, instead of the op-amp, which can still drive the external transistor but which have a current limit feature built in. You just need to add a current sense resistor and connect things appropriately to enable that - see the data sheet. You can also add a current limit feature to your existing, op-amp based, circuit - let me know if you want an example of that. BTW, the LM723 is what is used in those Astron linear supplies that are popular with HAM radio folks. It's kind of an old IC, but it's cheap and works.

I think I'm thinking about VEBO wrong... so it's basically saying the base should never be more than 5V higher than the emitter? I'm just trying to work out if I'm going to damage something here...

On the MOSFET, to be honest most linear regulators I've seen use a BJT (and I think I remember Dave mentioning it in the uSupply videos?) and really the only reason I went that way was because "Oh cool, MOSFET!" rather than any real requirement... I'm happy to stick with BJT for anything that doesn't need switching, and if I ever find myself in a situation where that doesn't work, I'll know where to look

I meet the maximum continuous current and maximum DC power ratings, but the only other voltage information in the sheet (http://www.onsemi.com/pub_link/Collateral/MJB44H11-D.PDF) I can see is saturation voltage - is that the same thing?

With respect to current limiting, this has probably been the aspect of this project which is most dis-similar from any I've done before. Every one seems to current limit a different way - I wanted to try and get a "minimalistic" design that works reasonably well, hence I didn't buffer any of the outputs or anything like that. I've tried to stay away from using the all in one IC's because I'm trying to learn how these things work - from what I've read, there is a lot of "tuning" that needs to be done of your control circuit, and thats the bit I'm trying to figure out (which parts matter, and what specs on those parts matter) if that makes sense? More than happy to see any other designs that you can recommend if you're happy to share. Thanks again!

Brad

[motocoder]

I think I'm thinking about VEBO wrong... so it's basically saying the base should never be more than 5V higher than the emitter? I'm just trying to work out if I'm going to damage something here...

The notation means voltage (V) from emitter (E) to base (B) with collector open (O). So this is in effect the reverse breakdown voltage of the base emitter junction. I don't think that should be an issue for you unless you've got something outside the supply feeding voltage back in.

On the MOSFET, to be honest most linear regulators I've seen use a BJT (and I think I remember Dave mentioning it in the uSupply videos?) and really the only reason I went that way was because "Oh cool, MOSFET!" rather than any real requirement... I'm happy to stick with BJT for anything that doesn't need switching, and if I ever find myself in a situation where that doesn't work, I'll know where to look

Sounds like a good idea. You'll certainly have an easier time finding a suitable device that way.

I meet the maximum continuous current and maximum DC power ratings, but the only other voltage information in the sheet (http://www.onsemi.com/pub_link/Collateral/MJB44H11-D.PDF) I can see is saturation voltage - is that the same thing?

The saturation voltage is the voltage across the collector-emitter junction when the transistor is completely turned on. Lower is better as it means your wasting less power in the transistor.

With respect to current limiting, this has probably been the aspect of this project which is most dis-similar from any I've done before. Every one seems to current limit a different way - I wanted to try and get a "minimalistic" design that works reasonably well, hence I didn't buffer any of the outputs or anything like that. I've tried to stay away from using the all in one IC's because I'm trying to learn how these things work - from what I've read, there is a lot of "tuning" that needs to be done of your control circuit, and thats the bit I'm trying to figure out (which parts matter, and what specs on those parts matter) if that makes sense? More than happy to see any other designs that you can recommend if you're happy to share. Thanks again!

Makes total sense. Have some fun with the circuit, and once you have it working you can always go back and make a rev 2. I'd do the same thing. Please check back in and let us all know how it goes.

BTW - I think a really great way to learn about this topic is to download the manual for some of the older linear power supplies, and read the "Theory of Operation" section. I am particularly fond of the old HP 6114a/6115a supplies, which have a really good section like that in their manual. The schematic is also worth looking through.

[braddrew0]

Great suggestion - I'll definitely post here when built. Thanks for all your help! 

[mij59]

I only had two considerations for the pre-regulator - reducing ripple and giving me an "enable" switch. I guess for the type of supply I'm trying to build, the levels of ripple I'll get from the op amp shouldn't be prohibitive. I like your rule of thumb - that's handy to know Do you think there are any issues if I replace the pre-reg with a similar BJT to the main pass transistor and use that as an enable/disable? I was trying to avoid using a relay if I could but I'd love to have the ability to isolate... Thanks again!

You could use the dac which controls the output voltage  as an enable switch, no need for an extra switching element in the main power path.
Or use then circuit in the attached file PS2803_V3_main.pdf using the  mosfet T4, for the control circuit see PS2803_V3.pdf.

When using BJT's keep in mind that the output of an opamp has a  limited current source and sink capability. 

[Kleinstein]

Using a MOSFET instead of a BJT as the power device gives extra choices in where to put the current sensing resistor, because there is essentially not control current, so source and drain side current are essentially the same. The main advantages with BJTs is that it's easier to have pwoer divices in parallel, usually lower cost, lower control voltage and less current dependent properties.

The choice of where to have the current sensing rsistor is a main decission in the design.  The other important point is whether the ouput stage is controlling the current (like a BJT in emitter circuit) or the voltage (like an emitter follower).
The Basic regulator circuit uses an emitter follower and a current sensing resistor on the low side. This works good with BJTs  up to about 30 V - the voltage swing a normal OP can provide.

The second major type uses a floating second supply for the regulator circuit itself. The shunt is at the emitter/source of the power device, thus usally the positive output. This version is very flexible (choice of BJT or MOSFET and easy to adapt for low and high voltages) but needs a second isolated supply / transformer winding.

[motocoder]

The second major type uses a floating second supply for the regulator circuit itself. The shunt is at the emitter/source of the power device, thus usally the positive output. This version is very flexible (choice of BJT or MOSFET and easy to adapt for low and high voltages) but needs a second isolated supply / transformer winding.

You can use a shunt regulator instead of a second transformer winding, can be as simple as a zener diode, to provide the floating supply for that. I played around with this circuit on the current source I am working on currently, and it works quite well.

Also, Linear and TI make high side current sense amplifiers, for example the LTC6101 or LTC6101HV. I think these are effectively just the same circuit (with the shunt regulator and a MOSFET to reflect the current from high side to low side) all in one package.

[braddrew0]

You could use the dac which controls the output voltage  as an enable switch, no need for an extra switching element in the main power path.
Or use then circuit in the attached file PS2803_V3_main.pdf using the  mosfet T4, for the control circuit see PS2803_V3.pdf.

When using BJT's keep in mind that the output of an opamp has a  limited current source and sink capability.

The only thing that stopped me from going down that route is that I was worried if I just set the DAC to zero that there would still be current flow through the amp (DC bias), which would cause a small amount of current flow through main line.... your solution is much more elegant though!

Is this your design? I've got to say, I really wish I'd seen this before I started because it answers a lot of my questions! It took about an hour of staring at it last night before I could make sense of what every functional block did, but I think I have a reasonable understanding now, although the pre-regulator (specifically the use of the NAND gates and the 100Hz signal) will take a bit more work. In general, it looks like the ideas you used are similar to where I'm trying to go, but the execution is light years ahead.

The two big take away points for me I think are that you seem to use a lot of protection circuits (isolation, clamping diodes, the ADuMs) which are absent in mine, and your 'switches' (BJT and MOSFET) have much more complex driver circuitry - mine is more a case of "put current in and hope it works", whereas yours have levels upon levels (which I assume keeps them well within their safe operating area). This is an area I definitely need to work on!

Using T4 to sink the output of the voltage comparator op amp is exactly what I was after for mine design - that removes any bias and gives me a specific enable/disable line from the uC. I'll add it to my next revision!

I also love the 15V supply you've made from R1/D3. I'm going to have a play around with this idea and see if I can limit the voltage entering my 5V regulator. This will save me from having to use a second regulator.

I'll probably have a bunch more questions on this design as I absorb it a bit better (if you're happy to answer) but I'll start with one - I can't get my head around the direction of polarisation of C1... the way I figure, you should have a 'high' voltage (you've got 50V, I'm guessing maybe this is a rectified ~48VAC input that you later constrain in the 0-50V bracket?) from BR1 passing through the junction between the two bridges to the negative terminal of C1, and D1/D4 should constrain the output of BR2 to 15V onto the pre-regulator FET T1. This would mean the voltage is higher on negative terminal of the cap than the positive, which is back to front right?

Thanks once again - this design is great inspiration for me!

Using a MOSFET instead of a BJT as the power device gives extra choices in where to put the current sensing resistor, because there is essentially not control current, so source and drain side current are essentially the same. The main advantages with BJTs is that it's easier to have pwoer divices in parallel, usually lower cost, lower control voltage and less current dependent properties.

The choice of where to have the current sensing rsistor is a main decission in the design.  The other important point is whether the ouput stage is controlling the current (like a BJT in emitter circuit) or the voltage (like an emitter follower).
The Basic regulator circuit uses an emitter follower and a current sensing resistor on the low side. This works good with BJTs  up to about 30 V - the voltage swing a normal OP can provide.

The second major type uses a floating second supply for the regulator circuit itself. The shunt is at the emitter/source of the power device, thus usally the positive output. This version is very flexible (choice of BJT or MOSFET and easy to adapt for low and high voltages) but needs a second isolated supply / transformer winding.

You can use a shunt regulator instead of a second transformer winding, can be as simple as a zener diode, to provide the floating supply for that. I played around with this circuit on the current source I am working on currently, and it works quite well.

Also, Linear and TI make high side current sense amplifiers, for example the LTC6101 or LTC6101HV. I think these are effectively just the same circuit (with the shunt regulator and a MOSFET to reflect the current from high side to low side) all in one package.

I like the idea of using the BJT for this design (for cheap, simple, low power), and I can see I'll probably want to have a go a second design when I want something slightly more complex. On the current sense resistor - this is probably another misunderstanding I have on how transistors work, but why is having the sense resistor on the high side more complex?

Thanks!

[mij59]

I'll probably have a bunch more questions on this design as I absorb it a bit better (if you're happy to answer) but I'll start with one - I can't get my head around the direction of polarisation of C1... the way I figure, you should have a 'high' voltage (you've got 50V, I'm guessing maybe this is a rectified ~48VAC input that you later constrain in the 0-50V bracket?) from BR1 passing through the junction between the two bridges to the negative terminal of C1, and D1/D4 should constrain the output of BR2 to 15V onto the pre-regulator FET T1. This would mean the voltage is higher on negative terminal of the cap than the positive, which is back to front right?

The schematic I posted is version 3,  don't use the pre-regulator,  now several dead mosfet's later I have arrived at version 6.
Controlling the output voltage of the pre-regulator turned out to be the most challenging.

The main power path is through Br1, T1 and  L1.
C5, C6 and Br2 are used as a voltage multiplier / level shifter, the voltage on + Br2 will be 15V higher than on +Br1.
The opto coupler U2 is used to switch T1 on, when T1 is on, the voltage on the gate of T1 will be about 15V higher than on the source.

[Kleinstein]

Having the current sensing resistor high side is more complicated if one needs to transfer the current signal to the low side to combine it with the voltage sensing. There are high side current sensor chip, but many of them are rather slow which causes trouble in the regulation loop. The second problem is that they usually need a minimum voltage to ground - so they may not work at low output voltages (e.g. < 2 V).  Having the shunt high side, directly at the supply is a little easier ( no low voltage there, and less voltage swing), but the controlling current is not measured in that case - so it will not be precise with BJTs.

The floating voltage regulator usually needs a real second winding. One could get away with current sources, but this is tricky as higher voltage and negative supply is than helpful or needed. A possible alternative would be a small DCDC converter or royer converter if the regulator and display don't need much current.
It is possible to use a shunt regulation for a floating current regulation on a high side shunt, especially with analog setting via pot. With a digital setting one needs a transfer of the setpoint, that needs quite some effort.

P.s.:
The schematic PS2803 shows the problems with high side current sensing and rather poor solution to that. divides down the shunt voltage to bring it to convenient levels and than need high end OPs to bring it back to workable levels. So high effort (precision, e.g. < 0.1 % resistors and higher end effort) and still poor performance.
With such a low power version one can get away without preregulator.

[mij59]

The schematic PS2803 shows the problems with high side current sensing and rather poor solution to that. divides down the shunt voltage to bring it to convenient levels and than need high end OPs to bring it back to workable levels. So high effort (precision, e.g. < 0.1 % resistors and higher end effort) and still poor performance.
With such a low power version one can get away without preregulator.

The current sensing is reference to 0B, and is floating with respect to the voltage sensing reference 0A.

[Kleinstein]

The current sensing is reference to 0B, and is floating with respect to the voltage sensing reference 0A.

Sorry, I have not seen the the voltage regulator is floating. It was not that obvious as the floating supply is somewhere hidden. So no high precision resistors needed and performance can be good. But still there is a lot of effort with a floating supply and isolation for current setting DAC.  If you already have a floating supply one could have chosen the classic fully floating regulator: this would have saved the isolation and the special high voltage OP. I still think the circuit might have problems with low output voltages, as the current sink is not working all the way down to 0.

[SteveP]

Hi,

I think you're just a bit behind where I am with my power supply design, so I offer the following general comments based on my experience to date:

1) I am guessing that you're not (yet) running any simulations. I *strongly* suggest you start doing so. You'll find more problems than you'd ever think existed once you do. Much easier to fix in simulation than on a breadboard and, believe me, there are lots of ways to get a power supply to misbehave. I'm using LTSpice and while it has a lot of frustrating aspects to it, it is free, easy to install, and is saving me a boatload of time and parts.

2) You don't mention any targets for performance--like how you expect the supply to respond to a pulse (sudden demand for more current). If you don't have at least some idea of what you want, you'll never know when you're done.

3) Are you planning short-circuit protection? If not, what do you think is going to happen when you accidentally cross your output cables (and someday, you will)?

4) Without simulation, you're going to have to find out the hard way whether any (and any combination) of the following will cause problems:
a) large output (load) capacitance
b) low esr output capacitance (say 1-10uF of 0 esr, just for grins).
c) fast output pulses  (di/dt of 1A/usec)
d) interactions between current limiting and voltage limiting, especially during startup and during output disable/enable.
e) probably a few more I can't recall off the top of my head
Problems can be anything from unwelcome spikes, to long settling times, to outright oscillation.

I don't mean to discourage you, but based on the schematic, you are quite a ways from buying parts/making boards. Power supplies are hard (as I found out). You'll learn a ton (and I'm glad to hear that's why you're doing it), but you haven't gotten to "compensation" yet, and that's a beast all by itself. And I went back and forth on my pass device (from mosfet to BJT, PNP vs NPN, N-Fet, P-Fet,  and back a dozen times or so).  Looked at quite  a number of mosfets in excruciating detail before I settled on one. I'd be surprised if you're done with that. But my performance goals were pretty high--to be as good as an LT3083 but with higher current. If yours aren't that ambitious, you'll have an easier time of it.

--Steve

[mij59]

If you already have a floating supply one could have chosen the classic fully floating regulator: this would have saved the isolation and the special high voltage OP. I still think the circuit might have problems with low output voltages, as the current sink is not working all the way down to 0.

My first design was based on the fully floating regulator, found it more susceptible to mains noise.
The circuits works also without the current sink, I added it to get a slightly better step respond at low output currents.