Four-quadrant power supply capable of providing voltage and current of any polarity

　　Conventional power supplies can only operate in the first quadrant, providing positive output voltage and current to the load, or, by deliberately misconnecting the output, can operate in the third quadrant statically as a "negative" power supply. However, conventional power supplies cannot operate in the second quadrant (for example, as an adjustable load as a negative power supply) or in the fourth quadrant (for example, battery discharge testing with a specific constant current). In addition, it cannot seamlessly switch between various operating modes as a function of load conditions or control inputs. The circuit shown in Figure 1 uses a "complementary" pass transistor configuration with an output topology similar to that of a common audio power amplifier to achieve full four-quadrant functionality. This complementary section can be a basic op amp output in lower current designs, while an external power MOSFET can be used when higher powers are involved. When the LT1970 power op amp is used to control the circuit's operation, it becomes very simple to control the output in various operating modes due to its internal closed-loop current limiting characteristics.

　　Four-quadrant supplies can provide at least ±16V regulation range with up to ±2A output capability. Figure 1 shows the basic regulator circuit section based on the LT1970. Figure 2 shows the user-controlled analog circuit section, which uses the LT1790-5 reference and the LT1882 quad precision op amp. The entire circuit is powered by a preregulated ±17V main supply (not shown). You can configure the user-controlled potentiometers VSET and ILIMIT to provide buffered command signals VCONTROL and ICONTROL, respectively (Figure 2). You can adjust VCONTROL from -5V to 5V, and the LT1970 regulator circuit then amplifies VCONTROL to form a nominal ±16.5V output. You can adjust ICONTROL from 0V to 5V; 5V represents the maximum user current limit command. The VCSNK and VCSRC trimpots attenuate the ICONTROL signal to set the precise full-scale current for sink and source modes, respectively (Figure 1).
　　A 0.1Ω resistor in the load loop senses the output current and provides feedback to the LT1970 during current limit operation. With this sense resistor, the LT1970 can limit the output current to approximately ±5A by simply setting the current limit trim pot to 100%. However, since the maximum output current required for this circuit is 2A, you can adjust the pot to approximately 40% during calibration. To prevent internal control contention at low output currents, the LT1970 sets a minimum current limit threshold that corresponds to approximately 40mA current across the sense resistor. Another useful feature of the LT1970 is that it can provide status flags, which in this case simply drive a front panel LED indicator when current limit operation is in progress. The LT1970 uses dual power supply connections, allowing the analog control section and the internal output section to be powered independently. The flexibility of this configuration allows you to sense the output current of the op amp directly through the resistor in the V (pin 19) and V- (pin 2) connections. With this capability, current feedback techniques can be used to establish Class B operation of MOSFET output devices, where the op amp output current is converted to a gate drive potential that turns the MOSFET on only to the degree necessary to help the op amp provide the output command.

　　Because the power supply must drive capacitively heavy loads (i.e., circuits with large-value bypass capacitors), and because any overvoltage could damage the circuit, close attention is paid to compensating the op amp for minimum overshoot under various load conditions. As with most op amps, the LT1970's internal-loop feedback and external-loop feedback are both designed to tolerate capacitive loads. In this case, the op amp itself is resistively decoupled from the load. The LT1970's DC feedback uses differential voltage sensing to eliminate regulation errors that would otherwise occur due to current sensing and lead resistance in series with the load. You can connect two inexpensive digital panel meters to the output to monitor the output in real time (Figure 1). (The two digital panel meters do not share "common" wires to complicate their power supply.) Note that the current-sense resistor is selected to optimize the display of a digital panel meter with a common ±200-mV full-scale sensitivity to, for example, ±1.999A. A word of caution: When you use this four-quadrant power supply instead of a normal single-quadrant power supply to power sensitive electronic equipment, it is a good practice to connect a reverse-biased Schottky diode (such as the cathode of a 1N5821) to the positive connection and the output terminal. In addition, you can use disconnect relays and power sequencers in your design to protect the load from strong reverse transients when the main power is turned on and off. An
　　adjustable power supply is an indispensable tool for any electronics laboratory. If the adjustable power supply can be continuously adjusted to 0V in both the sourcing and sinking directions, the current can be adjusted, or both functions can be combined, it will be more useful in many situations. With these additional functions, it is easy to drive or load various circuits being developed or tested, which otherwise require dedicated or custom equipment such as active loading cells or DC compensation generators. You can easily obtain these functions if you use the versatile LT1970 power op amp to design a linear regulator, because the LT1970 has built-in adjustable, closed-loop current limiting and other functions.