Triple Power Supply Project: Just a proposal / prototype for now

Introduction

Here are my design notes for a small and simple, triple, lab power supply. The design uses an off-the-shelf and low-cost switching  +5V, +/- 15V power supply. The outputs are adjusted a bit high  (+/- 16V, +5.8V) to provide headroom for the pass transistors and the current shunt resistors: +6V, +/- 16V. Then the three regulators are designed for low drop-out (LDO) to supply 0 to +5V and +/- 15V. They would share a common ground. But the whole supply should be isolated.

Basic requirements:

Current status

I need a good LDO regulator design to make this happen. I worked on a design using PNP transistor for the + regulators, NPN for the - side. It simulated and tested nice. I did some initial tests using a MeanWell power supply, it works OK. I began the board schematic and PCB layouts, which are about 80% complete. It shows a Blue Pill CPU which I like for the performance and cost, but has development issues. R-PI Pico is another good candidate, depending on its ADC performance. I can always fall back to a Teensy LC.

triple3d

Here is the preliminary schematic.

sch

Design Notes: LDO power stage

I worked on the LDO power stage for a while. I haven't seen a similar design anywhere. Most linear positive supplies use NPN transistors. These require a transistor voltage drop in addition to some biasing drop, so usually 1.5 to 2V minimum, not counting the current shunt drop. Using a PNP transistor is appealing because it can be driven almost to saturation, so less than 0.5V drop. And its base drive doesn't require more voltage, the base drive comes from ground.

One problem with a PNP output stage is that its voltage gain is load dependent. So stability and step response over a wide range of loads is tricky.

Another problem with PNP output stages is that when driving a capacitive load, the PNP's collector looks like a current source, and when driving a capacitive load, a large pole in the control loop is introduced. So the step response can be sluggish. I came up with a way to lower the PNP's output impedance and stabilize the voltage gain by adding local voltage feedback. This simple circuit uses a second, lower power transistor in a common emitter configuration to drive the PNP. But, you say, doesn't two cascaded common-emitter stages make for very high and poorly controlled voltage gain? Yes, but the local voltage feedback lowers the voltage gain to a reasonable and well controlled x3 to x5.

Check out Q4 driving Q3, with voltage feedback provided by R58 and emitter resistor R57. Loop gain is 1+ (R58/R17) = 1+ 1K/330 = 4.0.  It has the added advantage that R58/R57 also act as a load to discharge the output capacitors and bring the power supply voltage down faster. The base of Q4 is a convenient place to drive the ouptut stage from an op-amp. to get 0 to +15V, you drive Q4 with 15V / 4 or about 0 to +5V.

To make the negative supply, the polarity of the transistors are reversed.

Design Notes: Cost

The MeanWell power supply is about $25, the board and parts about $40, and the case would be about $30 for a ~$100 BOM.

Debbie Downer: Reality check

I hate to be a downer, but designing and building a DIY Lab power supply is hard work and for not much benefit. There are excellent power supplies and modules available for cheap. But I need to design and get right:
 And when you get everything done and done right, you have yet another small Lab power supply: not too exciting. That does not mean I won't do it, it's just not currently a priority with everything else going on....

Random Design Notes

Common Ground, uses +5V, +/- 15V CUI, MeanWell, orTDK-Lambda CUT35 Single board has 3 channels, 3 TO-220 pass transistors: TIP41/42

Controller board module mounted on Main Board
 Current sense for +5V and +15V
 3x V and I, 6x 12b DACs or S/H?
 CPU
See lab notebook P50... for notes on Meanwell, Blue pill...

When I started this in 2019, BluePill seemed like a good choice. I bought a few, and there was no cheaper Arduino compatible ARM processor. Then reality set in. There is no good USB programming for it, and no USB serial port or EEPROM. These are pretty fundamental neeeds.
    Use Black pill instead?
    Maybe ESP32? Free WiFi and Bluetooth. Not great ADC though
    Or Teensy LC for $12, kind-of expensive.
    Isolated CPU: probably isolate one serial port for SCPI and Debug, and be careful during programming via USB
    Needs a decent 6 channel, 12b ADC
    Use I2C for OLED
    Use SPI for DACs
    Preferably ~$5-10 for a module with 30-40 pins


 *   Designed buffer inverter, with feedback. Feedback helps.
*   Simulated V loop with PNP, various loads.
*   Needs R-C on integrator
    Needs a load for no-load condition
*   Proto OLED, use I2C: less wires, fast enough (30mS update)

  
Meanwell RT65C power supply.
    Looks OK
    +5V adjustable up to 5.80V, Could use 6.0V
    +15V tracks +5V well: increasing +5V increases +15V
    -15V is -15.02V Uses 7815 linear regulator.
        Removed regulator, connected pins 1-2 -17V
        -15 is good for about 1A. Nice
    Need to change V resistor if > 5.8V is needed
    Common mode V is pretty terrible: 8V p-p into 10 ohms !?! Maybe add SMT cap
    Bought RT50C, lower power, same size as RT65C
    Look at Lambda/TDK CUT35. Lower power, open frame, $43, may be cleaner and require less mods

Built proto
    Measured NCS213R: 109dB CMRR. Spec is 120dB typ, 100dB min. Pretty consistent but drifts with temp.
    Good CMRR is real important for high-side current monitoring
    Looks like a CMRR I offset correction as a function of V would be pretty easy to do in SW
        No load, set V low, set V high, pretty linear
        BUT: Need to correct ISet on the fly as well. Kinda ugly
    Need low-Z +0.02V reference for offsets
    Need a design for V- monitoring
    Gain of output amp with NPN driver, PNP TIP42 output works well.
        1-5V input for 0-15V output, 2A

Simulation:
    I and V works
    Did V- supply simulation, works.
   
Ordered Blue Pill STM32F103 board from Amazon
    Have 2 FT232 serial boards for programming, but just using for serial port.
    ST-Link download working, no serial boot-loader yet
    ADCs work well >12b
    Need SPI and SPI OLED
    Had trouble with SPI oled, and it uses many wires. Using I2C and it works well.


Blue pill links
Has OLED working:
https://squonk42.wordpress.com/2016/11/12/stm32f103c8t6-boards-blue-pill-or-red-pill/

https://os.mbed.com/users/hudakz/code/STM32F103C8T6_Hello/

Nice little breadboard power supply:
https://hackaday.com/2019/04/20/a-breadboard-power-supply-thats-more-universal-than-most/

Enclosures
Hammond 1402F 7 x 10 x 3, $66 q1, kind of expensive, but metal sides help with heat sinking
Needs front and rear panels: $7 each from PcbWay

Maybe get a 7" long one
Cut on table saw, drill 4 holes for side screws

Build blue pill Leo board??
No, Blue Pill is the devil. Consider Black Pill ($6), Teensy LC ($12) or other CPUs.

7 x 3 FP
Design for ~4" wide? 1/4 Rack is 4.25"
No,  board less heat sink is already 4" wide

------------------------
|    --------          |
|    | OLED |          |
|    |      |  O  o o  | Encoder
|    --------          |   
|  o   o   o   o   o   | Buttons
|      o   o       o   | Status LEDs
|  O   O   O   O   O   | 5-way Binding, 0.75" spacing
-----------------------


Front Panel
    128 x 64 OLED, 1.3"
    3 or 4 ON/OFF buttons
    Encoder w/SW
    Digit select buttons (2)
    3 On/Off LEDs
    3 Current limit LEDs?
    Or GRN = on V, RED = on + I Limit



UI for Triple PS
1.3" OLED is a bit small for 3x current and voltage readings
    Encoder switch goes to next screen:

Main Screen: Status of all:
    5.123V 1.234A OFF V
    +12.345V 1.234A ON  V
    -12.345V 1.234A ON  V
    Encoder switch sets screen
    <> selects digits. Field Select?

Ch 1 Set
+12.345V 1.234A Set
+12.347V 1.232A ON  V

Ch2 Set
-same-

CH3 Set
-same-

< > selects digit
up.down selects channel and I/V

ON/OFF per channel + ALL
Can timeout to main screen
One screen:
 12.345 A
 42.123 V
 ON/OFF
 
ON/OFF button

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Last Updated: 8/4/2023