Dynamic programming can be connected in parallel

The parallel connection of DC / DC converters can be implemented in different ways

The obvious and easiest way to connect power supplies in parallel would be to simply connect their outputs together. Normally, however, this does not work, as each device has its own regulation for the output voltage and tries to maintain this even with load changes and against the regulation of the other devices.

Key data

There are applications in which the parallel connection of DC / DC converters is advantageous for a developer. However, to implement it, he must be familiar with various topics such as topology and control loop. A common approach for the parallel connection of a DC / DC converter is to group the power supply around a common signal input, which is controlled by a single error amplifier, the signal of which the system distributes to all supplies concerned.

This also applies to devices with traditional internal error amplifiers with reference, where differences in parameters from device to device always result in one device taking on the full load while the others are not supplying power. This ultimately leads to an overload and a potential breakdown of the entire supply.

One solution for this direct interconnection would be for one device to be in constant voltage operation, but the other devices in constant current operation, albeit with a slightly higher value for the nominal output voltage. Developers must keep in mind that not all power supplies allow these operating modes. The supplies, the output voltages of which have been set slightly higher, deliver a constant output current and their output voltage drops until it reaches the value of the device with a constant output voltage. The load must draw enough current so that the power supplies with constant current characteristic also remain in this operating mode.

This solution with directly connected outputs assumes that either the power supply is suitable for this application or that there is a single error amplifier in the control loop. This reports the deviation to all power supplies so that they all share the load. However, this method also requires a so-called share bus for the control signals from the master to the slave devices.

In another approach, small resistors are connected in series to the outputs of each power supply in order to achieve an even distribution of the load current to all devices in a network (Fig. 1). The resistance reduces the load regulation somewhat and also generates power loss, which has a negative effect on the overall efficiency.

Decoupling with OR-ing diodes

Figure 1. One approach for load sharing is the use of low-ohm series resistors at each output of the power supplies. However, this creates losses at the resistors and affects the overall efficiency. Vicor

A seemingly simple solution to the problem of direct interconnection of the outputs is to use a diode between the respective outputs and their common load point. This technique is also commonly referred to as "Diode OR-ing" (Fig. 2). This is an efficient way to prevent a supply from drawing current from the common load point. However, this method is usually not suitable for correcting the different load sharing between power supplies, each with its own internal error amplifier.

Diode decoupling is usually required for power supplies that operate independently and are capable of both delivering and drawing current (two-quadrant mode). If such power supplies are connected directly to one another without decoupling diodes, the effects are far more drastic than with power supplies with one-quadrant mode. It is very likely that one or more power supplies will immediately go into overload.

If these diodes also have a negative temperature coefficient for their forward voltage, this leads to an increase in current in the respective network. One way to prevent this is to use a rectifier with a positive temperature coefficient.

Under certain circumstances, such as a short circuit in an FET or capacitor at the output of a power supply, these decoupling diodes can mean an increase in reliability. You immediately separate this short circuit from the output bus and thus increase the reliability and robustness of the system.

Strategy for regulation

For reliable and predictable operation when interconnected, developers must first develop power supplies for parallel operation. In order to deliver a corresponding current to the load, the control loop must be designed for this operation. A common approach for this is to group the power supply around a common signal input, which is controlled by a single error amplifier, the signal of which the system distributes to all affected supplies.

Figure 2. In principle, the outputs of several DC power supplies can be connected via diodes in order to separate the outputs from one another. However, this configuration can cause problems with balancing as well as with load sharing. Vicor

This strategy provides good regulation of the output voltage and fewer errors in load balancing. However, the use of a single error amplifier and a single control bus also creates the risk of a so-called single point of failure (SPOF). The failure of a single component can therefore be a problem in applications that require the highest level of reliability. In addition, it can be difficult to get parametric deviations in the amplifier stage under control. In a system with a single control loop, differences in load distribution are minimized if the power supplies have the tightest possible tolerances on the inputs for the control. If there are large differences in power distribution, developers must take measures that reduce the performance of the entire group in order to avoid overloading a single power supply due to an imbalance. If this does not lead to a result, you must take further measures.

An uneven load distribution, which comes from small differences in the control loop of the devices, can be remedied by a fine adjustment in production, which compensates for the differences. Another way is an additional current control for each power supply in the group. To measure the current, a shunt is typically required on every power supply.

For isolated DC-DC converters whose regulation is on the primary side, there is the further problem that the system has to transmit the output signal of the error amplifier via the isolation path between the primary and secondary side. This often means additional costs, more space required on the circuit board and can have a negative impact on reliability.

An alternative control technology, which enables different power supplies to be connected in parallel, uses a characteristic that simulates the aforementioned series resistance in the output circuit. With this technique for load sharing, known as inclined characteristic, each power supply has its own reference and integrated control. However, as the output current increases, the reference voltage decreases linearly by a defined value.

The parallel connection of power supplies with this inclined characteristic can have a negative effect on the regulation of load changes or load jumps. External control across this group of power supplies with an inclined characteristic curve can be used to compensate for the effects of the falling output voltage. The static control deviation is then identical to that of a traditional error amplifier, since the external control as an error integrator compensates for the deviations.

Design of a power supply

Fig. 3. The components in the chip housing were developed for parallel operation by simply interconnecting the outputs. No diodes, resistors, or other circuitry for load sharing are required. Vicor

Manufacturers can make it easier to connect power supplies in parallel. The DCM-DC-DC converters from Vicor in chip housings (converter housed in package), for example, already have a built-in negative load characteristic. As the load increases, the regulation built into the DCM reduces the output voltage slightly. This has the same effects as the series resistor mentioned above, but works without a resistor (Figure 3) and the following important differences: no power loss in a resistor and improved dynamic behavior due to the elimination of high-frequency parasitic effects. With a rapid change in the voltage across the resistor, the current also changes accordingly.

The load curve was implemented in the DCM converters by discrete time modulation of the D / A converter, which generates the reference for the error amplifier. As a result, the resistor that the load simulates on the DCM reacts like a resistor with a large capacitance connected in parallel.

This characteristic at the output allows several DCMs to be connected directly in parallel, with each converter having its own internal control active. The distribution of the entire load current between the DCMs is even, so that DCMs connected in parallel behave like a single DCM, but with a higher output current (Figure 4). If one power supply is loaded more than the others, its temperature rises compared to the other supplies, which in turn results in a drop in the output voltage. The output voltages of the other DCMs connected in parallel also drop to this voltage. According to the output characteristic, these DCMs will increase the output current and thus again achieve an even distribution of the load current. The same considerations apply to power ICs for smaller loads. For example, the LT3083, a 3 A low dropout linear regulator (LDO) from Linear Technology / Analog Devices, enables parallel connection through a 10 milliohm resistor between each output and the common output bus.

Figure 4. With DCM converters, several devices connected in parallel behave like a single converter. As can be seen from the characteristic curve, with an N + 1 redundant interconnection for a maximum load, the unit still works perfectly even if a converter fails. Vicor