Do´s and Don´ts-Product Application-Minmax Technology - DC/DC Converters and AC/DC Power Modules in Taiwan

Do´s and Don´ts

Absolute Maximum Ratings
Exceeding the specified absolute maximum ratings may severely damage the module. These ratings are intended as guidelines for absolute worst case operating conditions and are not to be interpreted as recommended operating condition.

Fusing Considerations
Encapsulated DC/DC module can be used in a variety of applications, ranging from simple stand-alone operation to an integrated part of a sophisticated power architecture. To preserve maximum flexibility, internal fusing is not included. However, to achieve maximum safety and system protection, DC inputs should always be fused.

In general, a slow-blow fuse with 150% to 200% of the maximum input current is used (see Figure 1).

Whether a fast or slow blow fuse is required depends upon the application. Generally, a slow blow fuse will provide adequate protection and the module's internal circuitry will handle any short period transient faults. A fast blow fuse is recommended for redundant systems to prevent a failed unit from shorting the input bus.

Maximum Operating Temperature
The maximum operating temperature for a power converter is determined by the internal temperature rise of its components. In a DC-DC converter, a small proportion of the input power is not converted to output power but is dissipated as heat inside the module. The amount of power dissipated depends on the efficiency of the converter, defined as the ratio of useful output power to supplied input power.

At an ambient temperature of 70°C, the internal temperature of some components may be over 100°C. The internal temperature of any component must never exceed its maximum operating temperature and for this reason, many DC/DC converters specify derated outputs at higher operating temperatures. In other cases, the power converter is specifically designed with special components and thermal techniques to allow operation at full load to 71°C with no derating.

Whether or not the unit is derated at higher temperatures, it is a good idea to provide additional cooling above 50°C ambient temperature. It is not just to keep a power converter operating within its specified operating area but to increased reliability.

However, for normal operation, the modules should not be run at the maximum allowable temperature since the Mean Time Between Failures (MTBF) will reduce sharply as temperature increases.

Power Line Transients

Power line transients can cause damage to the DC/DC converter. If voltage transients in a given application can exceed the maximum rated input voltage of a DC/DC converter, it may be necessary to provide external protection devices.

Figure 2 shows transient protection methods commonly used. A DC/DC converter input is protected by a fuse and a TVS (or power zener diode). The TVS effectively absorbs and dissipates transient voltages above its breakdown voltage.

Series Connection

One frequent application of the series connection is in using a dual output power converter as a higher voltage single output converter as shown in Figure 3. The outputs are already series connected by means of the common output terminal so it is only necessary to float the common and connect the load directly across positive and negative output terminals. In this manner, 10, 24, or 30V outputs can be realized from ±5, ±12, or ±15 dual output power converters respectively.

In general, DC-DC converters can be operated with outputs connected in series. Note there will be an addition of ripple voltage at the outputs since the power converters in generally will not have synchronous ripple voltages.

The only other limitation on series connection is that the total output voltage should not exceed the working breakdown voltage of any one of the power converters. This may be substantially less than the dielectric test voltage.

A common practice in the series connection of power converters is to connect reverse biases diodes across the output of each series connected power supply as shown in Figure 4.Diodes placed on the outputs of the modules ensure that on start-up, the modules are protected against reverse polarity. This can occur when the modules do not begin delivering power to the load simultaneously.

Parallel Connection
The parallel connection of DC-DC converter outputs is a much more difficult problem than series connection. In fact, as a general rule, it should not be done unless the power converters are specifically designed for parallel operation or the manufacturer says it can be done.

The problem with parallel operation is that it is nearly impossible to get equal load sharing between two power converters. First of all, two output voltages from fixed-output DC/DC converters will not be exactly equal. The converter with the larger output voltage will tend to provide the entire load current.

Even if the outputs can be adjusted so that they are precisely equal, a difference in output impedance and also drift with time and temperature will cause the loads become unbalanced.

Redundancy

A good reason for parallel operation of power converters is in providing power redundancy. In Figure 5, two power converters have their outputs connected in parallel through two diodes. For 100% redundancy each power converter must be capable of supplying the total load. In this case, it does not matter whether the load current is shared equally; however it is desirable for each output to provide at least part of the load current.

A diode should be fitted to the output of each of the paralleled units in order to isolate the modules from the output bus in the event of a failure.

No Load Operation
The problem with no load operation of unregulated DC/DC converter is the output voltage will exceed specified tolerance, maybe +20% or more. In fact, output voltage is undefined when loading below 10% of maximum output current.

Keep 10% loading on the output to ensure the output voltage remains within a specified tolerance. However, to achieve maximum safety, 2% loading is necessary.