Electronics > Projects, Designs, and Technical Stuff

fast CC/CV power supply... again

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exe:
Hello my friends!

This is my another attempt to make a fast and precise power supply. Previous two attempts are here:
1. https://www.eevblog.com/forum/projects/fast-cccv-power-supply-take-two/msg4988506/#msg4988506
2. https://www.eevblog.com/forum/projects/how-to-design-fast-bench-supply-with-cc-and-cv/

In my third attempt (not really true, I've built 20+ prototypes) I'll try to combine the two (kinda). Harrison/HP topology is good for both CV and CC modes of operation because in CV mode it's a voltage follower (i.e., low output impedance), and in CC mode it works as collector current source (high output impedance, good for CC). The problems are:
Problem 1: switching between CC and CV modes
Problem 2: shunt and output capacitor lower bandwith in CV mode, but increase bandwidth in CC mode. You can't have high bandwidth in both CC and CV modes. So, pick your poison.

## The idea

Since I want fast CC mode to protect DUTs, I can insert imprecise but fast current limiter. It doesn't have to be accurate, all it needs to do is to limit current until CC-circuitry engages. Here is what I'm trying with a simple bjt current limiter (see screenshot). I quickly checked it on breadboard, it works fine.

I tried to use jfet to limit current, but it's hard to find a jfet with any high Idss. I also tried depletion mode mosfets and normal mosfets for current limiting, but they need quite some voltage drop to work reliably due to low transconductance comparing to bjt. So I ended up with bjt. Also, bjt inherently limiting current due to finite beta and limited base current.

## Practical tests

I used a red LED that I connected to 32V supply with current limit of a few mA, but with ~2.5uF output capacitance (two 16V PSUs in series, each with 4.7u MLCC output capacitor) . Without the limiter the LED dies pretty quickly, just after two or three connects to power supply. With the current limiting circuit I wasn't able to kill it. Success!

Now not so great thing about this current limiter. It seems I can just use 24 ohm resistor to limit the peak current and LED still survives. So, my LEDs survive peak currents of more than 1A without problem. Also can pass 50mA no problem, at least for a short period of time.

So I need to make a better test.

## Downsides

1. Protection circuit has some voltage drop across it. So, not ideal, probably works well only for small currents. I don't think this is the problem as I only need protection for low currents anyway. I plan to make circuit bypassable when higher current range is selected.

## Further works

Okay, red LED is good, but I need a more scientific way of measuring stuff. So, I plan to measure how quickly voltage drops across a resistor when current limiter engages. I can capture that with o'scope and even calculate peak current and energy dumped. With this I can benchmark power supplies and different protection circuits.

## Why previous attempts fail.

The first design had Harrison topology ([1]) heavily inspired by @blackdog 's PSU ([2]). While it worked, I had troubles making it fast enough under all operating conditions, particularly at low currents (more on that later). Probably that was due to my lack of skills. So I had to try something else.

The second attempt was a radical way to simplify things: I just made fast current source (easy), and added a voltage clamp (not so easy, but was working). That's how design 2 was born. I've built the circuit and it worked really well from first attempt. There was only one problem: it couldn't measure current accurately. That's because some current goes through the load, and some through the BJT shunt. That base current error is quite significant. So I tried to fix that with adding a transimpedance amplifier to measure load current, but with that the topology is no longer simple and fast.


References:
[1] http://hparchive.com/Journals/HPJ-1962-07.pdf
[2] https://www.eevblog.com/forum/beginners/how-does-blackdog_s-psu-work/

exe:
What is safe current to pass through sensitive components? Here are much thoughts.

Most ICs specify they can pass up to 10mA through their pins continuously (with some rare exceptions, like LMC662 can only pass 5mA). Also, most ICs can sustain ESD discharge of 2kV with Human Body Model (HBM). Since HBM is a 200p capacitor with 1.5k series resistor, we can calculate peak current of 1.33A.

With some hand-waving, I assume that as long as we keep peak discharge current below an amp or two, and quickly limit to some tens of milliamps in less than a few microseconds, most sensitive parts like small bjts, opamps, microcontrollers etc should be fine.

So, limiting instantaneous current to below 1A, and more prolonged current to 30-60 mA (until CC kicks in) looks like a safe choice.

Kleinstein:
A lab supply is not made to have a super fast current limit. Even if the electronic part is fast the output capacitor could kill a sensitive device when starting from a higher voltage.
Another point is that lab cables have inductance and even of the voltage is stable at the supply the voltage at the load may still drop from the cable inductance. So a load that need a stable voltage even with current spikes kind of wants a local buffer capacitor. Not many needs for really fast regulation. The more important point is making shure that even tricky loads don't cause oscillation.


A point why one may want fast regulation is that the faster the regulator, the smaller the output capacitor can be. Commercial lab supplies may have 100 or even 1000 µF of capacitance at the output. At least in the simulation one can use rel. fast regulation and get a design with maybe some 1 µF or so and still not have much overshoot for the CC to CV transition.

A point for a fast CC-CV and CV-CC transition is to look at methods to limit integrator wind-up. This can be somewhat tricky if high precision is needed, as diodes have leakage.
A radical way is to have only 1 integrator for the CC and CV mode and this the diode "AND" function before the regulator / integrator part. Many SMUs seem to work this way, doing the min/max with the raw error signal and the integrator part of the regulator only after this.

Terry Bites:
An addon current source will limit the max current in your load irrspective of the PSU behaviour.
Thats a trivial circuit. eg LM317. That'll respond to a transient in 10uS or so for a resistive load.
See data sheet.

xavier60:
I did some experimenting with an idea that seems to work well in the other thread, https://www.eevblog.com/forum/projects/fast-cccv-power-supply-take-two/msg5093919/#msg5093919

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