Linear lab power supply

[xavier60]

This is how the PCB is doing. I have changed the SOIC footprints to DIP so it's easier to test different op-amps or whatever. I also added the current amplifier right next to the shunt and the voltage divider that monitors the CV CC modes. I have moved things around a bit and added two 5 V current paths, one for a connector to the displays and another one just for the arduino itself. PWM filtering is still pending.
    Juan

Yes, that should help. Do you know the display current per segment?

[JuanGg]

Yes, that should help. Do you know the display current per segment? 

I have 2x 4-digit 7 segment displays. Each draws about 35-40 mA with all segments lit at full brightness. So together about 80 mA maximum. That is ≈1.5 mA /segment.

That would be a TL431 just for the micro, to keep input voltage stable, and keep the 7805 for the displays. I have that regulator, I'll do some testing.

I can lower PWM resolution (I don't need 12 bits when I can only measure with 10 bits) and thus increase frequency, which would help. I don't need fast settling time for the filters anyway.

    Juan

[xavier60]

Because the current draw of the MC should be low, a TL431 could be used to shunt regulate 5V from the 8V rail.

[xavier60]

Because I used a MOSFET in my project for the 5V regulator, it has no current limiting.
 I haven't tested the circuit below. Using an LM317 instead of a MOSFET provides current limiting.
The main advantage of this regulator is that the regulated rail can be remote sensed.

[JuanGg]

Now that I think about it, the Arduino itself has an on-board 5 V regulator, which I could feed from either the 12 V or the 8 V rail. 12 V rail is going to be noisy. I don't know if the Arduino is going to upset the 8 V rail.
    Juan

[JuanGg]

I have done some testing with the PWM, I just built up a two pole RC passive filter, with 1k and 10k resistors and 10uF capacitors (just what I had around). PWM is 4 kHz in the end.  Output is not perfectly clean, I'll have to do some measurements. I had the arduino read a potentiometer and generate PWM modifying duty cycle acordingly.

I have measured the 5V derived by the Arduino from the 8V. There is a lot of noise when the Arduino is on (off when pressing reset), see screenshots. I have connected the filtered PWM to the PSU on the perfboard, see some measurements below, that is with the 12V fan on (separate 12V rail, as I measured, better to keep that separated) , PSU and Arduino running from the 8V rail. I have to do more measurements and improve the test setup, which is not the best with wires running all over the place.

    Juan

[xavier60]

High frequency noise on the 5V shouldn't be much of a problem, it would mostly get filtered out by the PWM filter.
When measuring noise, also check how much noise there is when the probe is connected to where the probe ground clip is connected.
The problem with a low value first filter resistor is the high current that can induce spikes across the capacitor's ESR and ground paths.
I used a 5.6K and a 1uF MLCC for the first stage. The second resistor doesn't have to be 10x the value of the first resistor, 5x should be ok, it doesn't have to be a precision filter.
 The second TIP35C order arrived. Transistors in both orders are ST and TO-247. The current gain of the second lot is 60, the first is 100.
I'll be using the 60's for future testing.

[JuanGg]

High frequency noise on the 5V shouldn't be much of a problem, it would mostly get filtered out by the PWM filter.
When measuring noise, also check how much noise there is when the probe is connected to where the probe ground clip is connected.
The problem with a low value first filter resistor is the high current that can induce spikes across the capacitor's ESR and ground paths.
I used a 5.6K and a 1uF MLCC for the first stage. The second resistor doesn't have to be 10x the value of the first resistor, 5x should be ok, it doesn't have to be a precision filter.
 The second TIP35C order arrived. Transistors in both orders are ST and TO-247. The current gain of the second lot is 60, the first is 100.
I'll be using the 60's for future testing.

How do you check for noise where the ground clip is connected? Floating the probe?
I did some measurements with my old setup, and I found noise everywhere, even more when the fan was on. I got sick of the mess of wires and soldered the arduino, voltage regulators and filter on the perfboard. Needless to say, much better results. I also found that the transformer that I'm using drops a bit too much when the fan is on, so the 12V reg cannot regulate any more and the fan gets 10V, still runs fine though. I'll probably put in a 9 or 10 V reg instead.

I'll also connected the displays and the encoder, so I can set the PWM, from 0 to 4096. It seems to work fine now. I am just using a 10k and 10uF RC filter for now.

See an scope screenshot of the output attached. That is with everything connected, fan and displays, and drawing 200 mA with my electronic load. BW limit is not enabled.

I'll do further testing.

    Juan

[xavier60]

Probing the ground clip is just to see how much of the noise is because of ground loops.
Use a faster time base so we can see what the noise origin is.

I know that the regulation loop is very quiet. I see 2mV pp on the output with 1A load.
The noise level stayed the same after I turned off the input supply. So most of the noise I am
seeing is stray pickup.

I have made another PCB for low profile mounting and to test 2 power transistors. All is good so far.

[JuanGg]

I have made a couple more measurements, with a faster timebase. Noise on the ground clip is very similar to the noise on the output, that is with the PSU switched on or completely disconected. It does not change much with output voltage or current. Noise drops a tiny amount when disabling the fan, which was left on together with the displays for these measurements. See screenshots below.
I suppose that with a circuit built on a proper pcb with a groundplane, and inside an earthed enclosure, will perform better.

I also found that when switching on and off the mains switch, i see spikes on the oscilloscope, with the probe unconnected. I assume that is normal.
    Juan

[Wolfgang]

Do you have PLC (power line communications) or LED lights in your vicinity ? That could be the reasons for this type of noise ?

[xavier60]

Sometimes measuring the frequency of the interference can give a clue, sometimes not.
It took me years to realize that the hash that was messing with my DSO's triggering was originating from its own SMPS!

[JuanGg]

I do have a led light bulb less than 1 m away from the setup. I should have tried measuring with it powered off. I'll try just in case.
    Juan

[JuanGg]

I just made a wire loop with one of may probes and moved it around my bench. Switching lights on and off makes no difference. I had everything but the scope unplugged, and about the same amount of noise is present everywhere Bringing the probe closer to the scope's screen increases the noise, as expected. It maybe that. Or even radio signals pickup.
    Juan

[xavier60]

I just made a wire loop with one of may probes and moved it around my bench. Switching lights on and off makes no difference. I had everything but the scope unplugged, and about the same amount of noise is present everywhere Bringing the probe closer to the scope's screen increases the noise, as expected. It maybe that. Or even radio signals pickup.
    Juan

Does the interference signal keep the same characteristics when the loop is near the screen?
The next best thing to do is to take the DSO to another location to see what difference it makes.

[JuanGg]

Attached are some scope screenshots with a 50 Ohm terminator on the BNC no probe, probe pickup on my bench, near the scope screen and in another room downstairs from my lab. There is some difference but not much.

I'll finish off the PCB. I think I'll stick with the simple RC low-pass filters, the seem to work quite well. I'll upload some pdfs/screenshots should anyone want to take a look. Needless to say the files will be made public/open.

Should a reverse-biased diode be added across the main power transistor, for protection?, and also a reverse protection diode on the output?

    Juan

[xavier60]

Looks like there is a chance of damage if another power source is connected to the output while power supply is powered down.
A reverse diode from C to E of the power TIP35C should give some protection.
I think that the most likely accident is a battery being connected with reversed polarity to the power supply's output while powered up.
My bench supply tolerates this, it simply goes into CC mode. I had to add a diode and polyswitch to protect the preload circuit.
I later added a transistor that detects the reverse polarity and clamps the Gate of the MOSFET after a short time. The output capacitor remains unprotected.
I have made provision for this protection on the recent PCB for this project but have not tested anything yet.

[JuanGg]

Wouldn't just a reverse protection diode across the output protect both the circuit and the output capacitors?
I'll add a reverse protection diode and a diode on the power transistor to the circuit.
Any more protection measures I should look at?
    Juan

[Wolfgang]

Juan, are there other switching PSUs around your bench (wall warts, e.g.) ?

[xavier60]

Wouldn't just a reverse protection diode across the output protect both the circuit and the output capacitors?
I'll add a reverse protection diode and a diode on the power transistor to the circuit.
Any more protection measures I should look at?
    Juan

It depends on what you are protecting for. If a high current battery is connected in reverse across the output which is protected by a diode, there is going to be a high fault current. Then you will need a fuse which will add to the power supply's output resistance.
I started doing some tests and got sidetracked by another problem.
I got as far a momentarily applying reverse 9V to it and everything survived. Electrolytic capacitors can tolerate reverse voltage for some seconds before they explode.
Some power supplies have an over voltage protection crowbar circuit on the output. I'm not going to bother with that.
I don't yet fully understand what effect reverse voltage has on the preload circuit.

[xavier60]

This is the schematic of what I'm currently testing, mainly to show how I will be protecting the preload circuit. I have ordered 100mA Polyswitches.
Nothing much has changed except for having 2 power transistors.

Extra: There is supposed to be a 47uF across the 8V rail.

[JuanGg]

Juan, are there other switching PSUs around your bench (wall warts, e.g.) ?

I unplugged all of them and it made no difference. I also moved to another room and noise was a bit lower, but pretty much the same.

[JuanGg]

It depends on what you are protecting for. If a high current battery is connected in reverse across the output which is protected by a diode, there is going to be a high fault current. Then you will need a fuse which will add to the power supply's output resistance. 

I am not planning on using this to charge batteries, and if I do, I can always add a fuse later on. I'll stick with the diode.

This is the schematic of what I'm currently testing, mainly to show how I will be protecting the preload circuit. I have ordered 100mA Polyswitches.
Nothing much has changed except for having 2 power transistors.

That looks neat. Although the main PSU circuitry remains unprotected right?
    Juan

[xavier60]

The main regulator circuitry doesn't care much about momentarily  applied reverse polarity, it simply goes into CC mode. Dissipation will be high.
A transistor can be added to detect reverse polarity and set the output current to zero.

[xavier60]

Just thought of a neat idea for my project.
A diode in series with a piezo buzzer across the output terminals will give immediate indication that reverse polarity has been applied.