Any idea how well it will reject the (relatively) high frequency switching noise from the pre-regulator? Would you expect similar performance to the traditional lab supplies with huge heatsinks or do you trade off noise for efficiency? Most three terminal regulators have a fairly limited ripple rejection once the frequency gets much higher than 120 Hz.
I'm aware of one well-designed universal variable power supply, a design by German electronics maganize ELV. There's a link to a PDF of the article on the product page, unfortunately the text is in German. This design is quite flexible in input voltage and current range, and I haven't heard any complaints about stability. It is a more traditional design intended to use with transformers, however.
Interesting that you were able to get the MJE3055 to oscillate at 108 MHz, just make C3 switchable and you have a portable power supply and radio transmitter in one .
Intuitively I would be worried about the effect of D5 on the stability at low currents, since you're introducing a component with a dynamic resistance inside the feedback loop, but I don't have time to go over the circuit in detail, and I trust you've done your homework correctly.
I agree about replacing R3 with an active down programming circuit. How about driving M1 in linear mode, to represent a moderate resistance to keep dissipation in check, if D5 is not forward-biased?
It is an issue with the LT3080. It has 50dB rejection at 100KHz but it falls off like a stone after that. Most of the modern switching regulators work at 200KHz or higher. The one Dave used in has recent LTSpice simulation of a tracking regulator runs at over 1MHz.
1. a low dropout design
2. single input supply
3. supply works to full specs with a wide range of input voltages
4. the regulation of the supply sets the output voltage to be the same as the opamp sense input (ie no voltage divider or voltage references involved in the supply regulation).
The result of #3 and #4 is the result is super flexible as you can feed the supply with whatever source supply voltage you like, and it will output whatever voltage you give it on the voltage control input. So a fully stabilized supply can be designed, and you can then make it output any voltages you like between about 0 and 30V without any change to the supply design at all. I do not have to redesign the control loop just because the voltage divider ratio has been changed.
So on to my design. Here were my design specs:
3V to 30V input source voltage range
1.5V dropout voltage
True regulation down to zero volts
1A output current with 0 to 1A current limit
High level of protection from loads.
Transient performance similar to the LT3080
No-load to full load output voltage regulation better then 0.001% (Wiring and track resistance is the biggest problem)
Loads like a battery can be connected to the supply while the supply is off without causing any problems to the supply or the battery.
Design based on "garden' components - potential to make it very cheap.
Possibility of a wide range 1A supply using only board mounted devices and no heatsink
That circuit goes way over my head, but it sure looks pretty
On the pre-regulator aspect, what are the possibilities of making a switching pre-regulator that works at a low frequency like 100 Hz to avoid switching noise getting into the output? I'm thinking along the lines of a big filter cap as found in a normal linear supply, but switching current into it to top it up to just the right voltage. What size inductor in the switching circuit would adequately handle the low frequency, high current pulses?
I have seen an example in some application notes where an active SCR switching rectifier/pre-regulator was used, but I couldn't clearly follow how the circuit worked. It seemed to switch just enough of each AC half cycle to top up the reservoir capacitor to the desired voltage. What puzzled me is that this is clearly a very non-linear system and I was interested in how it was made to work in a stable and effective manner.
Yes, I agree, a high frequency switcher to pre-regulate is the way to go. We probably should leave 2V for the linear regulator, this limits its power dissipation to 2W/Amp, something we can live with.
Definitely can make a switching pre-regulator at 100Hz like the SCR one in the Linear Technology app note, but the high frequency switchers are just so small and efficient, and the ripple can be pretty good - you tend to get a fair amplitude of ripple at 100Hz unless you go to huge capacitors.
For me, the key to low power is to reduce the ripple amplitude from the switching regulator to minimize the switcher to linear regulator differential, and for that the high frequency switcher is going to be the way to go. 100Hz switching designs were ideas from the 1970's electronics mags. Time to move on.
In Daves latest video he looks at a switching regulator in series with a LT3080. The switching regulator in question runs at 750kHz and this is in the official LT3080 data sheet. The TL3080 seems to have no problem with the ripple of the switching regulator there.
I seem to read data sheets too rapidly, at leat I overlooked that part before, looks very interesting !
If I´m understanding correctly, you are feeding the pass transistor (Q1) with a current of 10 mA generated by a constant current source (Q3, D1, D2, R1, R2 and C3) and you regulate the output voltage by bleeding this current with Q2/R2, right?
A couple questions.
1. Shouldn't the dropout voltage be about 2.5V, as you need 1.4V for the current source feeding the base, another 0.7V for the Vbe of Q1 and 0.4V for D5?
2. With a hfe of 20-70, there is no way of getting 1A with an MJE3055 without increasing the current from Q3 (decreasing R1 to 12 ohms), right?
Design based on "garden' components - potential to make it very cheap.
I will get onto the design in the next post. This design exercise is not to make something similar to Dave's supply. I have left out any attempt at this stage in a uCurrent type device, and I am only looking a very low cost high performance regulator that could be controlled by just plain potentiometers.
Design based on "garden' components - potential to make it very cheap.
I will get onto the design in the next post. This design exercise is not to make something similar to Dave's supply. I have left out any attempt at this stage in a uCurrent type device, and I am only looking a very low cost high performance regulator that could be controlled by just plain potentiometers.
Richard, thanks for starting this initiative especially on not choosing exotic part, really appreciate it.
Love the idea on using "garden" components, please keep it this way should this design evolved into more complex circuit. Eagerly waiting to see the pre-regulator section, hopefully you will use those "garden" switcher as well.
Btw, how does this design scale at higher current rating ?
For the discreet componect supply designer, you usually want to use an opamp that has the 90 degrees RC time constant built in, so you have to do tricks with external RC networks to cause a bit of phase lead to cancel out the excessive phase lag. It is not easy which is why Dave went for the LT3080 solution. If he did design a discrete supply, he would probably be on video blog number 23 by now - still fiddling with the compensation.
I am close to a simpler MOSFET design that does scale really well. The problem I have now is picking some MOSFETs to focus on. There once was a time when there were only about 20 common MOSFETs and life was extremely simple. Now the numbers are ridiculous.
I think people may not get how brilliant and revolutionary Dave's concept is. We are totally used to hand held meters, but we are still stuck to the idea that a power supply has to be anchored to a wall socket on the bench, and the idea that you do development on the bench, rather then the place the design is actually going to be used - whether that is on the roof, half way down a canyon, in the car, on a boat or wherever.
If a supply efficiently uses the energy in a lithium battery, the supply can easily last for days powering a typical modern low power circuit. Given that, why would you want a power cord at all?