Overload Protection
Although the use of filtering will prevent excessive current at power-on, under normal conditions there's no protection against an output circuit taking excessive power or even going short circuit. When this happens, the dc-dc converter will try to supply the output by taking a large input current. If this situation isn't rectified, the device will overheat and, eventually, destroy itself.
The easiest way of preventing overload at the output is to use a fuse with sufficient tolerance to the inrush current, so that it won't blow at power-on. Alternatively, a simple circuit breaker can be used. There exists, though, the potential to add some intelligence to the overload scheme by detecting either the input current or the output voltage. The simplest implementation for overload protection at the input is to have the devices supplied by a linear regulator with an internal thermal shutdown facility. But, this has a significant, negative impact on overall efficiency.
If there's an intelligent power management system at the converter's input, replacing the series inductor with a series resistor allows the voltage drop across the device to be used to signal the management system. A similar scheme can be applied at the output in order to determine the device's output voltage. On the other hand, if the management system is on the input side, then the signal will need to be isolated from the power management controller to preserve the system isolation barrier (Fig. 7).
Additionally, the thermal dissipation in a series resistor on the output can be used to determine overloading, while preserving isolation. A thermistor, or another thermally sensitive device mounted close to the sense resistor, can be used to indicate an overload condition. In such instances, the system temperature must be known in order to provide a suitable offset for different operating environments.
When using packaged dc-dc converters in a DPA application, the designer might find it necessary to calculate the leakage current of the device. In the case of galvanically isolated dc-dc converters, the isolation barrier within the device has a capacitance. This is a measure of the coupling between the input and output circuits. Provided that it's the largest coupling source, a calculation of the leakage current (IL) between input and output circuits can be estimated.
Assuming that there's a known isolation capacitance (CIS) and a known frequency (F) for either the noise or the test signal, then the expected leakage current can be calculated from the isolation impedance. The general equation for isolation impedance at a given frequency is given by:
The leakage current can then be calculated by dividing the test voltage by the value of Z
F.
As the above demonstrates, leakage currents are larger for higher test or noise voltages, and lower for lower isolation capacitances. Therefore, to obtain low leakage current and high noise immunity, designers should select high-isolation dc-dc converters with an appropriately low-isolation capacitance.
Temperature Considerations
Because dc-dc converters are heat-generating devices, temperature specifications are the key to successful DPA design. It's important to distinguish between the terms "operating range" and "specification range," as well. The specification range of a device is the range of temperatures in which it will perform key datasheet specifications, like rated power and noise output. Still, the operating range, which is generally wider than the specification range, is the complete temperature range that the converter will work in, albeit with performance degradation.
This is a critical issue, as some converters may claim operating ranges of −40° to 85°C, and yet only meet specifications between 0° and 70°C. The noise figure, for example, could be more than double outside of the specification range. As a result, performance specifications need to be analyzed carefully.
Keep the following instance in mind. If a datasheet states that a converter with an operating range of −25° to 75°C derates at 3% per degree above 60°C then, at the maximum operating temperature of 75°C, the power will be just above half the datasheet specification. Indeed, there are instances of 15-W converters that will only deliver 7 W at their maximum operating temperature. Because of this, it's vital for the engineer to ensure that the chosen converters offer the requisite performance across the application's full temperature range.
Within the last five years, a large number of surface-mount, low-power dc-dc converters have been launched. But, while most of these devices offer some advantages in power conversion applications, only now are technologies emerging which allow designers to capitalize on the true benefits of surface-mount technology (SMT). Great care should be exercised when selecting SMT converters.
One of the main problems in this area is that many existing surface mount dc-dc converters are, in fact, modified through-hole components. These converters are based upon standard hybrid circuitry, with pins bent to form level contacts. There are a number of disadvantages to using such devices.
First, the physical bending of the pins can introduce an extra level of mechanical stress into the component. And, because they weren't originally designed as true surface-mount components, these devices are rarely able to withstand the vapor phase and infrared reflow soldering processes of the modern automated manufacturing line.
For example, on most modern high-volume assembly lines, an IC must withstand lead temperatures of up to 280°C to achieve compliance with the standard CECC00802 reflow profile. Often, this means a temperature at the component surface in excess of 300°C. Yet, with many hybrid SMT dc-dc converters constructed by using thermoplastic casings and 220°C solders, the maximum reflow temperature is significantly lower than that of an IC.
As a result, designers incorporating these converters into their applications must allow for two separate mounting and soldering processes—standard components and converter hybrids. Naturally, this two-step process adds to assembly time and increases manufacturing costs, thus eroding many of the fundamental benefits associated with the use of surface-mount components.
Strict Coplanarity Requirements
Another problem with hybrid converters relates to what's called pin coplanarity, or alignment. International standard IEC 191-6:1990 defines strict requirements for coplanarity in surface-mount devices, making the positioning and size of the converter leads absolutely vital. Compliance to the standard, however, can be difficult to achieve consistently with the traditional methods used in hybrid manufacture. This can mean time-consuming and costly problems because even a slightly misaligned pin can lead to inaccurate placement on the board, poor soldering performance, and part rejection.
To address this issue, designers should be certain that their dc-dc converter manufacturer has systems in place which will allow them to verify the coplanarity of each and every component that they supply. Newport Components, for instance, has developed its own customized, automated system for testing coplanarity. This system uses a technique that builds a three-dimensional model of the device and calculates x, y, and z coordinates of a lead to within 10 µm. As a result, the company is able to offer surface-mount converters that guarantee 0.1-mm pin coplanarity.