Smart bench supply

Projects / Smart Bench Supply

This is a work in progress.  It is a bit on the low priority side as I already have 2 60V 5A bench supplies.  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 bench supply just wasn't good enough for this type of work.

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.

Simulation

Simulation is a good way to try to build something without building it.  You can measure things in circuits that are difficult to do so in real life.

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.

The ripple rejection performance is probably similar to this.

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 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.

So the big question is how to servo the switch mode supply?

Both linear and switch mode supplies can be view at as an inverting amplifier at the DC level.  The usual opamp summing circuits works here. There are no need for haphazard control circuits like this.

This can be simplified to the figure below.  It is simply an opamp + output stage circuit connected to an internal reference.  I have made a proof of concept supply that shows the output can be set close to 0V.

Similarly a switch mode power supply at steady state can be simplified to something like that too.

LDO

U1 and V2 simulates the LDO.  The control voltage is set at R8, R1 is the voltage feedback from the output. In this case, I subtracted ~0.6V to account for the dropout voltage by using R9.

Switch Mode Supply

U3, R3, R5 and V5 simulates the switch mode supply.  There is a bit of a twist as the voltage voltage is not allowed to drop all the way to 0V.

Since U3 is wired in an inverting amplifier, it means that I have to stop the control voltage from going above a certain point.  I wired U4 as a regulator with the output set by voltage divider R4 and R12.  The output follows the input untils it tries to rise above the threshold.  U4 counteracts and maintain the voltage by reducing the bias to M2.

These are buck converters based on XL Semi XL4015 (180kHz 5A) pdf

A buck-boost converter allows the output to have a wider range than the input voltage source. Common ones based on a fake "XL Semi XL6009" are fairly cheap. It is probably a remarked XL6019 (180kHz 5A) instead of the "400kHz" because I can see the 180kHz switching waveform.  As a result, the ripples are a lot higher than advertised.

I have ordered both of these and they are going to take a very long time.

This is how everything fits together.  U2, U4 are the opamp for the control circuits. Blocks with U1 and U3  simulates the power supplies.

Control voltage

Each of the supplies regulates its voltages independently but are controlled and trimmed by U2.  U2 will be replaced by an integrator and a microcontroller feedback control similar to this.

The integrator approach has a lot of advantages.

• It is cheaper than a DAC as it uses a cheap opamp and a capacitor.
• Its output voltage resolution is only limited by the ADC.  Both the ADC and DAC resolution can probably be increased to 13 or 14-bit (if needed) using Over-sampling.  External ADC e.g. ADS1115 Delta Sigma 16-bit ADC (available as $2 modules) can also be used.
• It is not a noisy reference like PWM. The change to the output voltage is very gradual and quiet.

Because the control voltage is from a inverting input (vs non-inverting in the original design), this doesn't suffer from the power up glitch as the integrator cap charges up during power on.

The two regulators are decoupled from each other by design.  The changes to the feedback circuit does not affect the AC load transient response of the regulators. The control voltage is merely a low frequency component applying a DC offset to the output.

 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.

High Side Current Sense

High side current sense
Chinese current sensing modules - tl;dr They are all useless for this application

Overcurrent limit

The overcurrent limit is set by using a PWM as a cheap DAC.  For a 3A full scale output, 10-bit resolution corresponds to 3A/1023= 2.93mA

With a 48MHz timer clock, 10-bit PWM produces a 46.9kHz output signal which can be filtered easily.

The opamp compares the current limit threshold vs the current sense signal.  Its output overdrives and raises the control signal to decrease the output voltage. The opamp output level will be above 2 diode drops which guarantees the NPN transistor will be able to turned on to drive a LED. The diodes prevent the circuits from interfering with normal operation.

AC/DC front end noise considerations

Links:
Analog Device/LinearTech AN105: Current Sense Circuit Collection
Diode Inc: AN39 Current measurement applications handbook
Current sensing