Issue 149 December 2002
Quad Bench Power Supply

---

Start • The Analog Core • The Zetex ZXCT1009 • An Ideal Isolator • MCU and User Interface • Firmware • Sources and PDF

THE ANALOG CORE

Although there certainly is a digital component to this project, the basic power supply core is a standard analog series-pass regulator design. I borrowed a bit of this design from Robert’s lab supply circuit.

Basically, all three power supplies share the same design. The ground-referenced power supply provides less voltage and more current than the floating supplies. Thus, it uses a different transformer than the two floating supplies. The ground-referenced supply’s digital circuitry (for control of the digital potentiometer and ADC) can be connected directly to the MCU port lines. The two floating supplies, in addition to the different power transformer, also need isolation circuitry to connect to the MCU.

Figure 1 is the schematic for the ground-referenced supply. As you can see, a 24VCT PCB-mounted transformer provides all four necessary voltage sources. A full wave rectifier comprised of D4, D5, and C5 provides the 16 V that’s regulated down to the actual power supply output. Diodes D6, R10, C8, and Zener diode D7 provide the negative power supply needed by the op-amps.

(Click here to enlarge)

Figure 1—The ground-referenced power supply includes an independent 5-V supply to run the microcontroller module.

A UA7805 regulator is used to drop the 16-V supply down to the 5 V needed for the digital potentiometer and ADC. Finally, an independent 5-V power supply for the MCU is provided by D3, C4, and U4, another UA7805 three-terminal regulator. Because I eventually added a 5-V, 3-A commercial power supply to the unit, I think it would have made more sense to run the MCU from that supply instead.

The series-pass element is an IRL520 power MOSFET that’s driven by U1, which is configured as a unity-gain buffer. I had the IRL520 devices on hand, but I suspect that NPN Darlington transistors could have been used in their place with the advantage of a lower base drive voltage requirement.

Voltage regulation is performed by comparing a portion of the power supply output voltage with the B-section output of the digital potentiometer U6. A TL082, U3-B acts as a comparator for this purpose. The full-scale output of the digital potentiometer is 5 V, and the power supply output voltage is scaled down to this level by R5 and the potentiometer R10. Without any initialization from the MCU, the digital potentiometer presets itself to half scale, or 2.5 V at power-up. When testing this power supply, prior to connecting it to the MCU, potentiometer R10 is adjusted to provide an output voltage of 6.4 V at power-up. This gives a resolution of 50 mV per step of the digital potentiometer.

Current limiting is provided by comparator U3A and the A section of the digital potentiometer. Current monitor IC U2, which you’ll learn more about later, provides a voltage that’s proportional to the output current. Basically, comparator U3A compares a voltage proportional to current draw, with the current limit set point value programmed into the digital potentiometer, and throttles back the drive to the pass regulator when necessary.

The two sections of the TL082, acting as comparators, have their outputs connected to buffer U1’s input via diodes D1 and D2. In combination with R1, these components provide a NOR function. To be precise, if either comparator’s output goes low, the drive to the pass regulator (provided by R1) will be reduced until the over-voltage/current condition ceases.

Apart from the digital potentiometers replacing mechanical ones, this circuit is somewhat similar to that used by Robert in his lab power supply. You’ll learn more about how I used the high-side current monitor circuit a little later.

Although Robert didn’t mention any instability problems in his article, I experienced them myself as I was building this circuit. I found it necessary to use 0.01 capacitors (C2, C3) for feedback compensation on both comparators in order to eliminate RF oscillation on the power supply output under varying load conditions. I thought I could eliminate buffer U1 in my design because of the low current requirements of the MOSFET pass element; however, the diode NOR circuit seemed to produce RF oscillations on the power supply’s output without this buffer in place.

The final part of the circuit is the metering portion. In place of the DPMs, I used a Microchip MCP3202, which is a dual 12-bit ADC. This ADC is inexpensive (it costs less than $3) and doesn’t need an external reference. The fact that it uses an SPI interface really simplifies the isolation circuitry needed in the floating supplies.

Even though the MCP3202 can operate from 2.7 to 5.5 V, I chose to operate it from 5 V, because that regulated voltage was easy to provide with a UA7805. The disadvantage to this power supply voltage was that the ADC’s full-scale input is also 5 V. Though the power supply’s output voltage is scaled down to this range for the regulation circuitry, the current-monitoring circuitry converts current to a somewhat lower voltage. Despite the fact that the actual scaling differs between the floating and non-floating power supplies, the net result is that current resolution is only about 9 bits. This current resolution was sufficient for my purposes, however.

The MCP42010 dual digital potentiometer has a neat feature: it contains a Serial Out terminal. Using this feature, you can daisy-chain these devices and load many of them simultaneously, using only three control lines—CS, SCK, and SI (with the daisy-chained devices being fed from the previous device’s SO line).

Although I needed only one dual potentiometer for each power supply, I used this feature to daisy-chain the MC3202 ADC device to the digital potentiometer, thereby eliminating one—the CS control line—for the nonessential ground-referenced supply. For the floating supplies, it allowed me to provide all of the necessary isolation in one device package, which was beneficial.

To protect against short circuits, I added a Raychem PolySwitch RXE075 resettable fuse, which limits short-circuit current to 750 mA. I did this because the Zetex high-side current monitors need at least 2.5 V to operate properly. A direct short circuit would not provide this, and the current-limiting action would not work. The PolySwitch fuses more than function: they act as fuses and provide enough voltage drop during short-circuit conditions to allow the Zetex current monitors to operate.

Although it isn’t obvious from the schematic, I designed this power supply’s PCB to include a 30-pin SIMM connector. The MCU module is a daughterboard on this PCB. Also, the two isolation chips that interface the MCU to the two floating power supplies are contained on this PCB. Photo 1 depicts the PCB and the backside of the MCU module. I’ll describe both the MCU module and the isolation circuits later.

(Click here to enlarge)

Photo 1—The ground-referenced power supply PCB also contains the SIMM100 MCU daughterboard. The IsoLoop isolators, being SMD components, are mounted on the bottom of the PCB and aren’t in view.

SEE IF IT FLOATS

I’ve explained in detail the ground-referenced power supply. There are only a few differences between it and the two floating power supplies; however, I’ve provided Figure 2 to show you these differences.

(Click here to enlarge)

Figure 2—The floating supplies are almost identical to Figure 1, but there are different component values. Note that the ground symbols in this figure are local to this board alone (i.e., they are not connected to ground on any other boards shown in the other figures).

Where the ground-referenced supply was meant to provide 8 V at about 500 mA, the floating supplies were meant to provide higher voltages for powering analog circuits such as op-amps. I wanted at least a 15-V output, but a current capacity of 300 mA was deemed sufficient for my needs. I substituted a 34-V transformer for T1. It’s the same size as the 24-V device used in the ground-referenced supply, which was handy because all three power supplies share a similar PCB layout.

The floating supplies need not include the 5-V regulated MCU power supply that was part of the ground-referenced supply. The value of the output voltage-scaling network is different from the ground-referenced supply. In this case, potentiometer R10 is set to produce 12.8 V at power-up. This gives a resolution of 100 mV per digital potentiometer step.

The only remaining difference has to do with the value of the current monitor-scaling resistor R6. I increased the value of this resistor from 100 to 220 W to scale the lower current capacity of this supply into a voltage that’s compatible with both the 5-V referenced ADC and digital potentiometer.