DIY Scalable Bench Power Supply Design

[void_error]

Anyway, If it's possible to get a PSU output swing down to 0V without a negative rail it's even better.

Out of interest I am using OP295 op-amps (single supply rail to rail) in my design without any negative supply and I get down to about 50mV minimum with a 25v span.

PS. How are you getting on with the LM2956-ADJ..........are you noticing any noise (on load) on the output. Albeit, less than 10mV at the switching freq?

Ian.

I used an LM324 in my first tests since I haven't ordered any of the parts yet, so I'm experimenting with what I've got lying around.

That also means I didn't get the chance to play with the LM2596 either, using a LM317/TL431 temporarily in place of the pre-regulator set to track the same way as the LM2596 would do.

What I don't like about the current design is that its maximum output voltage depends on the opamp supply voltage so if I want a higher voltage version (like 0-50V) I'm screwed. I've been working on a version today that eliminates the opamp supply voltage dependency meaning that realtively low voltage opamps could be used and no, it's not going to be a LDO topology as they're really hard to be made stable.

A schematic of the new version will be available as soon as testing/tweaking is finished.

[dannyf]

:realtively low voltage opamps could be used and no,:

you would need to use some dark force to be able to do that.

[akis]

Here is a table of op-amps for your perusal

Code: [Select]

Op-amps
Max Supply Input resistance (Mohm) GBWP MHz Slew Rate (V/us) Input noise (nV/Hz) THD + N Common Mode Voltage Range Output Voltage Swing Supply Current (mA) common mode ground common mode Vdd Notes Package Price £
Common Differential +/- 15V 2mA 10mA
LT1361 36 50 5 800 Vdd-2, Vee+2 4 no no HF DIP8 6.77
LM6172 36 40 4.9 3000 Vdd-1.5,Vee+1.5 2.5 no no HF DIP8 3.47
MC33072 44 150 4.5 13 32 0.02 Vdd-2, Vee Vdd-1,Vee+0.3 Vdd-1.6,Vee+1.6 3.8 yes no Single supply, low side DIP8 0.6
TLE2141 TLE2142 44 65 5.8 42 10.5 @ 1KHz 0.01 Vdd-1.8, Vee-0.3 Vdd-1.3, Vee+1.3 2 yes no Audio, Buffers, low side DIP8 2.088
TLE2072CP 38 infinite 10 40-45 12 0.008 Vdd, Vee+4 Vdd-1.1,Vee+1 3.1 no yes Audio, Buffers, high side DIP8 1.668
LM4562 34 1000 30 Kohm 20 2.7 @ 1KHz 0.00003 Vdd-2, Vee+2 Vdd-1,Vee+1 10 no no Audio, Buffers DIP8 2.23
LT1632 36 45 0.001 Vdd, Vee Vdd-0.2,Vee+0.3 9.2 yes yes RtoR DIP8 6.456
LT1490ACN8 44 15000 17 0.2 0.07 50 Vdd, Vee Vdd-0.25,Vee+0.25 0.1 yes yes RtoR, DC, low power DIP8 3.432
LM7332 35 15000 21 15.2 15.5 0.003 Vdd+0.3,Vee-0.3 Vdd-0.2,Vee+0.025 2 yes yes RtoR, DC 8SOIC 4.104
LM7322 32 15000 20 18 15 0.005 Vdd+0.3,Vee-0.3 Vdd-0.25,Vee+0.06 2.5 yes yes RtoR, DC VSSOP-8 1.212
OP284 36 4.25 4 3.9 Vdd, Vee Vdd-0.2,Vee+0.125 2 yes yes RtoR DIP8 10.092
TL072IP 36 infinite 4 13 18 0.003 Vdd,Vee+3 Vdd-1.5,Vee+1.5 Vdd-3,Vee+3 2.8 no yes Audio, low nse, high side DIP8 0.84
LM324 32 1600 1.2 0.5 35 Vdd-1.5,Vee Vdd-4,Vee 1.4 yes no Single supply, low side
LM6132 24 210 11 14 0.0015 Vdd,Vee Vdd-0.2,Vee+0.15 1 yes yes RtoR, low power DIP8 2.99
OPA251 36 4500 0.035 0.01 45 Vdd-0.8,Vee-0.2 Vdd-0.05,Vee+0.05 0.05 yes no Single supply, low side, micropower DIP8 2.98
TSH22 36 100 25 15 14 0.003 Vdd-1.8,Vee Vdd-1,Vee+0.3 4.3 yes no Single supply, low side DIP8 1.44




[void_error]

Here is a table of op-amps for your perusal

Thanks akis but I managed to get the circuit to work with 5V powered opamps.

you would need to use some dark force to be able to do that.

It seems I have found some dark force and put it to use. 

Jokes aside, here's the idea:



Had a LM317/TL431 tracking pre-regulator instead of the LM2596 tracking pre-regulator for testing purposes. Won't order any parts until every detail is sorted out.
Also, not all the components have the correct values, too lazy to do some math, I'll fix that soon though.

For keeping the whole power supply module from falling apart I'll use a bunch of these:



I think it's a simple and elegant solution.

On a side note (rant): Why the heck did they make the LCD mounting holes M2.5 instead of M3?! They had enough space to use that but no, now I need to order two bloody spacer sizes and drill twice the amount of mounting holes on the digital board. 

[dannyf]

I have found some dark force and put it to use

the world of physics would crumble down then,

[void_error]

All the tweaking has been completed
This is the version I'm actually going to build. 

You may notice that I've added a preload resistor to the output - R6, with it's own current sense resistor - R9. Now the output goes down to 50mV. U5 removes the output current offset caused by R6+R9. I could have done this in software but for the sake of versatility I decided to leave a hardware zero adjustment (via R8) for the current in case one wants to use this without MCU control.

Opamp choice was based on price/offset/bandwidth that's why I ended up using MCP6021 where offset voltage needed to be low (single opamps because I found it easier to route a single-sided PCB using SOT23-5 as opposed to SO-8, on top fo that I might need single opamps in pther projects). The current sink (U4/Q7) could have been made simpler but at the expense of higher voltage drop on R21 which would have caused V_OUT to be higher with no load and maybe unstable operation at low output voltages.



I've also made changes to the digital part (sorry, no schematic/code yet as it's still in the early design stages).
Found some nice and extremely reliable pushbuttons so I'm ditching the rotary encoders.

Planned features for the user interface (button part) will be, apart from setting the voltage/current and switching the output ON/OFF are the ability to lock the settings to prevent accidental button pushing (idiot-proof feature), setting the limits between which the voltage/current can be set and a preset button which cycles through some common voltage/current settings (just because it's convenient).

Moved the LCD to a SPI I/O Expander (MCP23S17) and swapped the PIC16F887 for a PIC16F886 for two reasons: not in stock (at the moment from the supplier I'm using and I don't plan on paying shipping twice) and SO-28 is easier to route on a single-sided board than a TQFP44 (I hate using lots of jumper wires).

Another planned feature is to add some form of self-calibration to minimize the errors introduced by the (reference) DACs and storing the offsets in an external memory (most likely an EEPROM). This relies on the fact that the ADCs used are 22bit delta-sigma type and can be used to measure the DAC error for each voltage step and compensate to a certain extent.

[dannyf]

why I ended up using MCP6021

I thought you were looking for an opamp that can output above its rail. Does this do that?

Moved the LCD to a SPI I/O Expander (MCP23S17)

A shift register, if you already have one, would do that.

an external memory (most likely an EEPROM).

The internal one doesn't do it? The chip is capable of self programming so that's another route.

[void_error]

why I ended up using MCP6021

I thought you were looking for an opamp that can output above its rail. Does this do that?

It doesn't need to. I added an extra gain stage with Q5/Q6 - Q4 - Q3 - Q7. It has local negative feedback via R13/R15 (basically gain limiting). The opamp handles the overall feedback.

Moved the LCD to a SPI I/O Expander (MCP23S17)

A shift register, if you already have one, would do that.

It will need to be able to tri-state its outputs for when I put the LCD into read mode to check the BUSY flag. A 4094 would probably do (two of them, one for data, one for commands). I'll consider the shift register option as well although hardware SPI is much easier to use than bit-banging a shift register.

an external memory (most likely an EEPROM).

The internal one doesn't do it? The chip is capable of self programming so that's another route.

I suppose you mean I can use the program memory to store the values...?

I'll have to do the math to see how much internal EEPROM space I need for the calibration data (400 points x 8 bits for 10mV steps (4.096V DAC reference) I guess? maybe half if I can use one byte for two calibration values). If that's the case 256 bytes will be enough. Haven't used the internal EEPROM before though, so I'll have too look at the datasheet.

[dannyf]

hardware SPI is much easier to use than bit-banging a shift register.

You can use hardware spi on a shift register too.

I suppose you mean I can use the program memory to store the values...?

No.

[dannyf]

It doesn't need to.

Too bad. Finding an opamp that's capable of outputing beyond its rails has a lot of practical value.

[void_error]

You can use hardware spi on a shift register too.

I realized that just after I hit the post button. But I won't bother since I/O expanders don't cost much and have more features than shift registers.

I suppose you mean I can use the program memory to store the values...?

No.

Thought so. I'll go for an external EEPROM then, internal EEPROM is too small for storing everything I need to store plus they're dirt cheap.

Too bad. Finding an opamp that's capable of outputing beyond its rails has a lot of practical value.

Too bad you can only find those opamps as dodgy (maybe) SPICE models. Had it happen. I guess leaving the output voltage swing out of the simulation model requires less computing power. It's even funnier when EE students wonder why an opamp's output doesn't go higher than the supply voltage after they do the math for a higher gain circuit 

[void_error]

I've been recently working on the PCB (not done yet) for the main PSU board and made some improvements along the way. Here's the result:



Then I had another idea - to add an option to disable the pre-regulator (the LM2596 can go up to 100% duty cycle - internal switch on all the time) while limiting the output current to a value that does not cause excessive heating of the series pass transistor. It cand be done by shorting the feedback pin to ground. This will basically result in a cleaner output which might be handy sometimes.

[TonyStewart]

  I just use any Universal Laptop Charger either with a dozen switched voltages from 12 to 24V 65W to 85W or use the kind with 4 pin adapters to many plug styles. They use remote sensing to the plug, which uses a 2.5V reference so a pot can be used to vary the output in a linear mode. I use to power 21V 65W LED arrays dimmable and only costs $30 at Walmart.

[void_error]

Any Universal Laptop Charger I can get over here is a worthless badly engineered chinese piece of junk so it was out of question from the start. I've seen some break within minutes of being plugged in.

which uses a 2.5V reference

Most likely a TL431.

[SharpEars]

I just use any Universal Laptop Charger either with a dozen switched voltages from 12 to 24V 65W to 85W or use the kind with 4 pin adapters to many plug styles. They use remote sensing to the plug, which uses a 2.5V reference so a pot can be used to vary the output in a linear mode. I use to power 21V 65W LED arrays dimmable and only costs $30 at Walmart.

• Where is the fun in that?
• What are its ripple characteristics?

[prasimix]

Hi, this is my first post and I'm came here attracted with this interesting project. I'm working on some prototyping of PSU and would like to share some thoughts, questions and problems.

Then I had another idea - to add an option to disable the pre-regulator (the LM2596 can go up to 100% duty cycle - internal switch on all the time) while limiting the output current to a value that does not cause excessive heating of the series pass transistor. It cand be done by shorting the feedback pin to ground. This will basically result in a cleaner output which might be handy sometimes.

Yes, that is something that I'd like to implement. It especially make sense when you are using MCU so you can limit a total output power that you're not limited only by voltage or by current.
I didn't know that switching regulators such as LM2596 has possibility to go up 100% duty cycle. That's good news since I was prepared to disable pre-regulator and use extra relay to bypass it.

I have one question. What is happening with output voltage when you are simply switch off the PS? I spent some time to create TINA9 simulation of only output transistor driver section and see what will happen if voltage and current regulation signals are removed (both switch are in off position). In that case I have 22.72V on output. So if have device that require i.e. +5V when I switch off PS and depending of input capacitor some dangerous voltage will be present on output. Ok, this is just an simulation. What's happening in practice? Please find in attachment screenshot and simulation file (zipped since attachment uploader do not permit .tsc file type).

[void_error]

prasimix - the circuit you posted has no overall negative feedback, only local feedback for the extra gain stage - R4 & R5.

Also I forgot to mention SENSE_OUT_P connects to V_OUT at the positive output jack while SENSE_OUT_N connects to PGND at the negative output jack so there will always be overall negative feedback.

This is to compensate for the voltage drop across the wires going from the board to the output jacks.

Take a closer look at the schematic. Voltage regulation is handled by U9 & U10. U9 is the error amp while U10 is a differential amp (more like an attenuator here) with a gain of 0.2 (R37/R36 if R37=R44 & R36=R46 which is the case).

D11 & D12 are for input protection so the opamp inputs don't get damaged in case there's a voltage spike coming from a load like a brushed DC motor. With that said I'll have to replace C17 with a 100nF-1uF ceramic in parallel with a 10uF electrolythic, both 100V rated. D5 protects Q2 from reverse voltages.

The purpose of R28 & R30 in the local feedback loop is to limit the gain of the discrete gain stage to prevent oscillations and it's set higher than the minimmum required value.

I didn't know that switching regulators such as LM2596 has possibility to go up 100% duty cycle. That's good news since I was prepared to disable pre-regulator and use extra relay to bypass it.

It's in the datasheet. Only the ones which use a bipolar transistor (usually a NPN) can go up to 100%. The ones using a MOSFET (most likely an N-channel) will use an internal charge pump to drive the gate above the supply voltage. One example is the LM2676.

[prasimix]

Thanks void_error for quick response! Yes, I'm aware of global neg. fb but I'm talking about an extreme case. I'm wondering does everything still works when you simply remove Vin. If, yes that great, just checking

Some more question:

1. Does Q7, U4 works as constant current source or they are define some operating point?
2. What do you think about current monitor such as INA282? In that case you don't need to spent one more opamp for buffering (U7)?
3. Thanks to mentioning protection diodes on differential amp (U10). Do you think that TVS (Transient voltage suppressor) could be a better choice then zener diode? I believe that TVS reaction time should be better (of course if reverse current and parasitic capacitance do not influence overall stability).

Thanks, once again. 

[void_error]

Thanks void_error for quick response! Yes, I'm aware of global neg. fb but I'm talking about an extreme case. I'm wondering does everything still works when you simply remove Vin. If, yes that great, just checking

Yes, it will work.

1. Does Q7, U4 works as constant current source or they are define some operating point?

It's just a constant current sink to pull the base of Q2 to ground. Tried using a resistor but the output voltage wouldn't swing low enough.

2. What do you think about current monitor such as INA282? In that case you don't need to spent one more opamp for buffering (U7)?

I chose the INA138/168 for a reason: bandwidth. If you look at the datasheet of the INA282, page 4, youll notice that the bandwidth is only 10kHz. The gain of the INA138/168 the gain is set by a resistor. With a 5k resistor the bandwidth is 800kHz, with the 15k resistor it's still over 250kHz. Just below that there's the step response, 1.8us for 5k, less than 6us for 15k which means the current limiting will kick in faster. I'll do some measurements after I build it on a proper PCB and design a simple constant current electronic load.

3. Thanks to mentioning protection diodes on differential amp (U10). Do you think that TVS (Transient voltage suppressor) could be a better choice then zener diode? I believe that TVS reaction time should be better (of course if reverse current and parasitic capacitance do not influence overall stability).

The MCP6021 opamps used have ESD protection diodes on the inputs so theoretically the zeners are not needed but it's a good idea to have them there. If there's a positive voltage spike exceeding about 23V the zeners will clamp the opamp input voltage to less than 5V - that way the upper internal ESD protection diode won't conduct - less chance of damaging the opamp.

Thanks, once again. 

I'm glad people are interested in my project

[TonyStewart]

I just use any Universal Laptop Charger either with a dozen switched voltages from 12 to 24V 65W to 85W or use the kind with 4 pin adapters to many plug styles. They use remote sensing to the plug, which uses a 2.5V reference so a pot can be used to vary the output in a linear mode. I use to power 21V 65W LED arrays dimmable and only costs $30 at Walmart.

• Where is the fun in that?
• What are its ripple characteristics?

The fun was using a surplus fixed charger into a variable supply by adding a pot and a couple passives to make a useful cheap dimmable 65W LED ceiling lights by the bay window brighter than sunlight, because my wife hated the shadow cast by the mighty Maple tree. 

Another application was an LED powered patio table backlight.  No regulator needed with 19.2V fixed into 6x3W leds on surplus MCPCB lights scrapped from Solectron intended for ambulance lights.

https://www.dropbox.com/s/uusqgoqytkhho8m/2014-06-28%2023.02.29.jpg

Ripple was not a factor with lighting low ESR load.

[macboy]

I was just looking at your most recent schematic. One thing jumps out at me... you chose to ground-reference your control circuitry (voltage and current error amps) to the supply negative output terminal, rather than the positive terminal. If you look at virtually any commercial design, those circuits are referenced to the output +, which makes sense, since the current sense resistor is right there, and the regulator pass transistors are there too. This makes interfacing the error amps to those things very simple. So I wonder why you chose to use the - output terminal? This necessitates fancy high-side current sense amplifiers and level shifters and all sorts of other compilcations.

[void_error]

The DACs used for the reference voltages are ground (not mains earth) referenced. Also, if you noticed, the 'level shifting' (I think you mean the discrete gain stage) is needed since the opamps are powered from 5V. The only 'complication' is using an INA138 as the current sensor, plus a few opamps to increase bandwidth.

[prasimix]

2. What do you think about current monitor such as INA282? In that case you don't need to spent one more opamp for buffering (U7)?

I chose the INA138/168 for a reason: bandwidth. If you look at the datasheet of the INA282, page 4, youll notice that the bandwidth is only 10kHz. The gain of the INA138/168 the gain is set by a resistor. With a 5k resistor the bandwidth is 800kHz, with the 15k resistor it's still over 250kHz. Just below that there's the step response, 1.8us for 5k, less than 6us for 15k which means the current limiting will kick in faster. I'll do some measurements after I build it on a proper PCB and design a simple constant current electronic load.

Oh, that possibly could explain some of my misery when PS enters CC (Constant Current) mode. I'm playing with TL074 (and OPA4227) opamp that is connected to INA282 and current control loop is not so stable as voltage one. End result is output ripple that is 10 times higher than in CV (Constant Voltage) mode. Also it comes completely unstable when Iout goes over 2A with short circuit condition on output. Before starting to think about INA16x how you resolve a problem with monitoring current when Vout is below 2.7V?

[void_error]

Before starting to think about INA16x how you resolve a problem with monitoring current when Vout is below 2.7V?

The collector of Q2 will always be about 3V higher than the emitter, that's how the pre-regulator is set up.

[prasimix]

Oh, I completely forgot that shunt is BEFORE serial transistor Sorry.