Synthetic power supply solutions for industrial high voltage applications

Vicor's second-generation DC-DC converters have many features that are integrated together to form a complete power solution. Chief among these features is a unique current sharing method that increases power capability with a 5% current sharing accuracy between modules.

The precise current sharing capability between modules provides designers with the opportunity to increase the system operating voltage by connecting the inputs in series.

This article describes the use of the second generation 48V input series modules to form a 110Vdc input application.

System requirements

A typical multi-converter design connects the inputs of the DC-DC converters in parallel so that the same voltage appears on each converter, and connects the converter inputs in series so that they are configured as a single power supply. This requires that the converters share the same load under all operating conditions, ensuring that the voltage presented to each converter input will be the same.

When the converters are connected in series, the input capacitors used in front of the DC-DC converters must be equally tapped at the input of each converter, and a parallel resistor will help compensate for the leakage current of the capacitors (see Figure 1). The converter uses a control circuit to monitor the voltage balance of the converter and control the operation of the converter.

Figure 1 Basic series input configuration

Input overvoltage protection is necessary to prevent excessive voltage on the capacitor during a fault condition. Transient voltage suppressors are usually used for protection.

Figure 1 is an example of a basic series input connection method. From Figure 1, you can see that the module configuration method is slightly different from the input parallel connection configuration. Note that the module 1 SC pin is connected to the –Sense and –Output pins. This connection forces the module into a “slave” mode. Module 2 is the “master” of this configuration, which means that only module 2 controls the output voltage of this configuration.

To ensure load balance in this configuration, the PR signals must be transformer coupled using a Vicor P/N 22400. Transformer coupling provides the required galvanic isolation and has high noise immunity.

Another bypass capacitor (0.2 F ceramic film capacitor) connected to each module input provides low AC impedance for high frequencies. The power supply wiring should be symmetrical to balance the circuit impedance.

Control Circuit

The control circuit (see Figure 2) monitors the input voltage of each module and ensures that both modules are operating within their rated input range. This circuit also provides startup delay and keeps each module consistent.

Figure 2 Control circuit

The LTC1444 circuit's comparators monitor the input voltage to each converter individually and disconnect the converter if the monitored voltage is outside the operating range.

The comparator reference is derived from the LTC1444 internal reference (pin 8). Reference noise rejection is achieved with R14 and C1. Increasing R12 increases the hysteresis of the voltage comparator (1kΩ/mV hysteresis at the comparator input).

When the bus voltage exceeds the operating range, the comparator is in a low state, the converter is turned off and the timer capacitor is discharged, and when the input voltage is within the operating range, capacitor C2 is charged. When the voltage on C2 exceeds the VR2 reference voltage, the current flowing through the optocoupler is shunted, enabling the converter.

The delay must be set equal to or greater than 500ms. The source voltage for the timer is obtained from Vcc through the voltage divider R7, R8, and a voltage divider is used to reduce the reference for the low Vcc detection of the LTC1444 and to adapt a smaller timer capacitor.

As mentioned earlier, the PR signal must be transformer coupled between modules. Figure 3 shows the required circuit.

110VDC input power application

The series input configuration uses standard off-the-shelf DC-DC converter modules, which feature high power density, high efficiency and increased input voltage.

The DC-DC converter module suitable for this application is the Vicor second generation 48V series, and the modules used must be the same component number.

The standard input of the 48V series is 36Vdc-75Vdc, and in this series input configuration example, the input range is approximately 76Vdc-137Vdc. Based on the series input configuration of the 48V series, it provides a suitable solution for building a power system for industrial or railway 110Vdc applications.