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Smart bench supply

Digitally controlled switch mode supply with LDO post-regulation for good efficiency and low noise.

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This is a continuation of the work from my project: "Adjustable Linear Bench Supply in 1k".

I attempt to improve its output current, range and efficiency by using switch mode supply with LDO post regulation. The voltage accuracy and resolution is only limited by the ADC.

One thing to take away from this project is learning to read between the lines in the datasheet and understand the limitations of parts in the fine prints. Simulations are useful to find potential issues before building or even committing to the parts.

Until very recently, I wasn't going be building one.  I found that I need a low noise supply with very fine and precise voltage adjustments that can go to tens of mV in part of my  Boost converter for low voltages project.  My usual 300W bench supply just wasn't good enough for this type of work.

Analog Simulation

Here it is the block diagram Microchip/Micrel MIC29xxx series LDO that span the range of 1.5A  to 7.5A.

There are a lot of features built into a LDO like this: Thermal shutdown, Enable pin, Over-voltage protections etc that are messy to replicate in a discrete design.  You can get higher output current by simply upgrading the chip.

This can be simplified to the figure below.  It is simply an opamp + output stage circuit connected to an internal reference. 

Minimum load requirement 

The LDO has a minimum load of about 7mA required.   

Normally this is factored in the feedback circuit, but I have decided to separate this out with a constant current sink at about 7.5mA. This removes the thermal and high voltage rating away from the feedback circuits.

Here is the simulation result.  The circuit sinks about 7.5mA (Cyan) over most of the output range except when the voltage drops below 0.7V.  It is effectively a 82R load below 0.7V.  An opamp based circuit could further reduce this voltage.

It is not a LDO if you don't intend to use it as such

The big problem with designing a linear supply is the output current and range is limited by the thermal budget.  A switch mode supply can be very flexible and efficiency.  It has a bad reputation of being a bit more noisy, but the noise can be reduced with filtering and post regulation.

For the MIC29302, the dropout voltage over the full temperature range is about 0.5V.  The power dissipation is only 1.5W at 3A load at 0.5V drop.  It gets a bit higher for the 7.5A part, but it is still easily manageable with a heat sink.

The following simulation waveforms shows the output of the LDO (Red) and the switch mode supply feeding it (Blue).  The green traces show the voltage difference which is normally about 0.6V.  The switch mode output does not go all the way to 0V (even though it can) to provide a minimal operating voltage for the LDO. The two regulators have slightly different reference voltages, so there is a slight voltage variation over the entire range.

So the big question is how...

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  • AC/DC front end noise considerations

    K.C. Lee4 days ago 0 comments

    Over the years I have seen a lot of DIY bench supplying using modified PC PSU or laptop supply as their AC/DC frontend.  Instead of the blanket statement of "SMPS is bad news", I am going to sit down and explain why I won't be doing that and rather be using a big heavy hunk of copper and iron for my front end AC/DC conversion.

    This is the block diagram of a common AC/DC converter.  There are two type of high frequency noise: Differential and Common Mode.  The differential noise is filtered with an inductor (or leakage inductance of a common mode choke) and a "X-cap". The common mode noise is filtered with a common mode choke and forces to go to "Earth" ground a pair of "Y-caps".  (see pdf for filtering)  The amount of conductive and radiated noise is governed by regulatory standards such as FCC part 15.

    The story doesn't end there as some of the high frequency common mode noise can reach the second side by means of parasitic capacitive coupling.  The problem is when the secondary side is not grounded. 

    Those old boat anchor 50/60Hz transformers don't have a high enough frequency for the capacitive coupling to work.

    The following picture shows such a power supply.  There is a yellow Y-cap sitting between the Primary and Secondary side.  Usually it is right next to an opto-isolator that is part of the feedback loop.  The Y-cap is connected  between the low voltage 0V and the "Earth" ground.  It allows the Common mode noise to flow back to the "Earth" ground.  The AC grounding isn't perfect as the capacitor has a non-zero impedance.  Some of the noise still persists on the secondary side.  It'll try to go to the "Earth" ground through whatever least impedance path e.g.  I/O in your device.

    It gets even worse for those laptop supplies without a "Earth" ground connection. Let's say that tingling sensations when you touch the metal part of a laptop are not your spider senses.  It is small enough to not hurt you too much but will cause enough of a noise problem for sensitive analog circuit.

    Reference:

    Block diagrams are from:

    Journal of Engineering and Development, Vol. 16, No.1, March 2012  ISSN 1813- 7822 , "Practical Approach in Designing Conducted EMI Filter to Mitigate Common Mode and Differential Mode Noises in SMPS", By: Nidhal Y. Nasser Assistant Lecturer, Electromechanical Eng. Dept.,University of Technology
    (pdf)

  • Current sensor - rediscovered

    K.C. Lee5 days ago 0 comments

    While looking through my own project directory under Current sensing, I rediscovered a part: MAX44284  So far I have not found any flaws yet. It is worth looking at if you require accurate current sensing.  

    • Very Low 2μV Input Offset Voltage (MAX44284F/H)
    • Extremely Low 50nV/°C Input Offset Tempco
    • Low 0.05% Gain Error
    • -0.1V to +36V Wide Input Common-Mode Range - covers full range without needing a work around.

    The only reason why I might not be using this is that I don't own any of these parts. It is much hard to get free samples these days.

    I think I can get by with my Linear Tech part that is sitting on a protoboard.

  • Chinese current sensing modules

    K.C. Lee07/13/2018 at 15:07 0 comments

    I guess I forgot to look at those popular Hall Effect current sensors from China using ACS712.  

    The parameters Noise and Total Output Error are the ones of interest.  The total output error isn't too bad and in some cases can be calibrated out.  

    There is a tradeoff between the amount of noise vs bandwidth.

    Resolution: 21[mV] / 185 [mV/A] = 0.114A (typ) for a 47nF cap. This is the part that fails this application.

    I guess that's why those Chinese ACS712 modules are cheap.  The ACS723 based modules are available elsewhere, but not yet from China until the existing cheaper stocks of ACS712 runs out.

    The new replacement part seems to be improvement on the noise, but worse on the accuracy.


    The other popula modules are MAX471 (pdf)  which is also designated ''Not for New Design''.

    Here are the gotchas: minimum input voltage and large error at current below 0.1A. 

  • High side current sensing revisited

    K.C. Lee07/07/2018 at 14:25 0 comments

    I have done more thinking.  Basically it boils down to this:

    1. Roll my own.  This has the highest complexity, but use more commonly available parts and is the cheapest.  There is an issue with the opamp Vos interfering with low current reading. LTSpice thinks it is about 20mA, but it'll be better or worse depending on luck. 

    Note: the constant current sink, level translating circuits are left out.  Revise circuit here.

    This is not easily solvable with common parts as the common mode input of opamp needs to include the positive supply rail.  A precision opamp with low dc offset does not usually reach the positive rail.  A rail to rail opamp compromises by using two complementary input stages to cover the entire input range resulting in the following type of performance - good in the middle but worse off on either ends of the rail.  If we are talking about trimmed parts, but those are unicorns.

    Fixes: 

    The value of sampling resistor can be increased so that this nonlinearity will only show up for a lower current. The minimum LDO load can be used to move the operating point above this.

    2. TI INA201
    As mentioned before, this requires a 20mV offset to be inserted into input and 1V removed from the output.  A precision voltage reference, a voltage divider, a floating supply and opamp for subtracting the output are needed.  I do have some samples of this.

    3. Analog/LinearTech LT6100
    This requires operating from a negative supply to meet the common mode requirement and an opamp to convert the output back to ground reference. This approach has the least complexity with less gotcha.  I only have one sample.

    There are some non-linearity below 8.6mA.  We can operate above that by increasing minimum LDO load slightly.

    The -5V rail can be provided by a switched cap converter.  A RC filter is needed to clean up the output.

    As the negative rail is initially ramped up, there is a glitch at the output.  It is not something that would cause any issues.

  • MIC29302A graph makeover

    K.C. Lee07/06/2018 at 18:35 0 comments

    I was trying to find the ripple rejection spec for the MIC29302, but found a pre-Microchip MIC29302A datasheet: here  It seems to be a slightly different version (low cost) of the chip for 20V operation.  While it is a different chip, it would be from the same base design and can give some rough idea what to expect.

    It is hard to read off the log scale.  A bit of messing around with Excel graphing option, I got myself a semi-log graph.

    After some image editing and fooling around with transparency and scaling, I superimposed the grid onto the graph.  

    The ripple rejection drop to about 5dB at around 250kHz.  Above that, it is mostly the bulk decoupling cap doing most of the filtering work.  

    XL6009 Buck-Boost converter switches at 400kHz, the ripple rejection is ~15dB. What this means is that if the ripple at the LDO input is 10mV p-p, the amount of ripple at the output: 

    10mV * 10^(-15/20) = 1.77mV p-p

    Chances are that the ripple from cheap Chinese modules could be pretty bad.  They have a small LC filter using a small inductor and a MLCC.   I don't know the extend of its performance until it arrives.

  • Proof of concept - low voltage supply

    K.C. Lee07/06/2018 at 16:17 0 comments

    I need an analog supply for testing my energy harvesting project as my switch mode bench supply can be a bit too noisy.  This is what got me thinking about building an analog supply.

    I have used a resistor divider up to this point, but needed something that can source a bit more current.  

    This is a quick and dirty proof of concept hack of a MIC29152 (1.5A Adjustable) to show that it is possible for its output to go below the 1.24V reference with the help of feedback magic.  

    It is wired as an inverting amplifying of gain -1 and reference point at 1.24V (instead of usual 0V)  The 10 ohms provides a minimum load (>5mA) for the LDO and needs to be reduced for output below 50mV.

    This is just a quick hack.  The voltage adjustments are bit finicky.  Without properly closing the feedback loop to a more stable reference, it isn't all that great.  It is already an improvement over a resistor divider.

    I have notice that the LDO needs at least 1.6V to operate correctly.  Reading between the lines of the following dropout characteristics points to a similar value.  Officially they don't list the minimum operating voltage spec as they expect its output to be >=1.24V, so by the time you add in the minimum dropout for the load, you would have meet the requirement.

    This has been accounted for in the control voltage design.  I might need to increase the voltage slightly.

  • High side current sense

    K.C. Lee07/05/2018 at 15:38 0 comments

      I have decided to move this topic outside of my Detail page as I am still not happy with what I have.  Some of you might question my sanity of not using a proper part.   They all look fine at first glance until you go and read the datasheet.  That's why I still have some INA201 unused.

      7.4.2.4 Low V SENSE Case 2: V SENSE < 20 mV, 0 V ≤ V CM ≤ V S

      For my application, it can fall within this condition.  One way to get around this is to add 20mV offset to the input.  

      For the INA201, this is a 50V/V * 20mV = 1V offset at the output that I have to remove. 
      It is ~1/3 of the input range of my ADC.

      LT6100:

      The common mode range is Vcc + 1.4V to 48V.  This means that the part has to be sitting on a negative rail and the output needs to be level shifted.  Unfortunately I only have one of these in questionable condition.

      Old details page archive as follows:

      ---------------------------------------------------------

      High Side Current Sense

      The high side current sense circuit is a lot more complicated because of requirement 2.

      1. The load current can be sensed at the output of the LDO as it draws different amount of currents based on supply voltage, temperature and load requirements. The feedback resistor needs to be connected after the sense resistor to compensate for the drop.
      2. VCC rail can reach 0V.

      R3 is the current sampling resistor.  This adds up to 0.05R x 3A = 0.15V drop at full load.

      The I*R voltage drop is level shifted to a negative rail via a voltage controlled current source (R5, R2, U3 and M2).  The negative rail provides a DC offset for the circuits to work as V2 can sometime drops to 0V.  The voltage gain is set by resistor ratio of R2/R1. A gain of 20 gives 1V/A.

      Voltage across R2 drive a voltage controlled current sink (U2, R1 and M1). This current is converted back to a ground reference voltage using a transimpedance amplifier (U1 and R4). The ratio of R4/R1 determines the voltage gain.

      The constant sink on the -5V rail along with a zener diode is used to provide a floating supply for the current sense opamp U3. This current contributes towards the LDO minimum load.

      ---------------------------------------------------------

      This is a revised version.  The constant current circuit has been replaced. The 20K resistor is on a DC offset as the opamp buffer U1 cannot swing to 0V. The offset is subtracted off with the different amplifier.

  • Parts ordered - expect a long wait

    K.C. Lee07/05/2018 at 00:35 6 comments

    Order a whole bunch of PSU related parts from China.  As usual, these things get shipped reasonably fast but ended up spending a very long time in the trailers at Canada Post and Canada Custom.  Based on history, I'll be expecting them to hit the 60+ days limit.

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