CONVERSI.MAR–Conversion Devices, Inc.–SC–12– –##
Solving typical dc/dc converter application problems
Here are some simple guidelines
to help ensure maximum
performance and reliability
BY ANASTASIOS SIMOPOULOS and STEVE FORRESTER
Conversion Devices, Inc.
Brockton, MA
Dc/dc converters have evolved from simple three-terminal regulator circuits into complex electronic subassemblies that are critical in the power distribution systems of many applications. Good fundamental engineering practices must be followed in applying the converter to achieve optimum system performance and long field life. This is especially true if the application has any special requirements (like pulse loads and load sharing) or is subject to abnormal operating environments (such as elevated temperatures or moisture).
Here are some dc/dc converter applications that sometimes present special problems.
Hot-pluggable applications
Fault-tolerant equipment, where critical system components must always be on-line, is common in the telecommunication industry (and to a lesser degree in many medical, data processing, and data communications systems). Typically, hot-swappable (also called hot-pluggable) supplies are used for fault-tolerant systems. A hot-swappable supply (see Electronic Products, March 1993, p. 35) can be connected to, or disconnected from, a live power bus.
A power converter not designed for hot plug-in may be damaged by high inrush currents if connected to a live power bus. (Inrush current is the peak current required by a dc/dc converter at turn on.)
As shown in Fig. 1, the input stage of a typical dc/dc converter has an input filter that includes capacitors connected across the input terminals. At turn-on, these capacitors present a low impedance to the power line. Under normal operating conditions, the dc power bus ramps up to full power slowly, allowing the input capacitors of the dc/dc converter to fully charge without drawing excessive current.
However, connecting the converter to a high input line causes it to draw high peak currents–typically lasting a few microseconds–that could reach magnitudes of 10 to 100 times the steady-state value. The amplitude and duration of these current peaks may overstress components, adversely affecting the performance and reliability of both the dc/dc converter and its power source.
To avoid damage from peak currents, the user should implement a soft-start circuit. A soft-start circuit limits the rate at which power is transferred to the output stage by controlling the “on-time” of the semiconductor switch. Switching power supplies often use negative temperature coefficient (NTC) resistors to limit inrush currents caused by connection to the ac line. NTC resistors are impractical in a dc/dc converter module because of excessive heat dissipation and the lack of space.
To solve the inrush problem, the user can implement a simple external inrush current limiter circuit, comprised of components as shown in Fig. 2. This circuit can control the rate at which the dc/dc input and output capacitors charge. The capacitor, C1 , is chosen to provide the appropriate delay to eliminate both current spikes.
Hold-up time
In the data-processing industry, the use of high power density dc/dc converters has increased dramatically with the acceptance of distributed power architectures. Distributed power systems carry a host of potential application-related problems, such as thermal management, input/output protection, and parallel connection problems.
One overlooked problem is hold-up time, the period when a dc/dc converter remains operating within specified limits after input power is lost. More commonly specified for ac/dc converters (typically at 16 ms), the hold-up time of a dc/dc converter is normally in picoseconds. This can be a problem in data processing (or other computer-based applications) because adequate hold-up time is needed to provide an orderly system shutdown.
The hold-up time of a dc/dc converter is limited by the physical size and value of the output capacitors that can be used in a small module. To increase hold-up time, the user must install an external capacitor at the converter output. The approximate value for this capacitor is given by the formula:
Cout =I*(
where
t = the desired hold-up time
V = the output voltage drop allowed during the hold-up time
I = the output current
Unfortunately, this simple solution may not work in many applications. If the external capacitor (Cout ) is too large, it may trigger the internal short-circuit protection circuits within the dc/dc converter (the capacitor presents a short to the converter output as it begins to charge). This prevents the converter from starting.
Another problem is that the converter starts, but the initial ramp-up in the converter output voltage is so great that it triggers the system power-fail signal. If one or both of these conditions occurs after the installation of Cout , additional circuitry is required to achieve sharp converter turn-on and extended output hold-up time.
These problems can be eliminated by using the hold-up time circuit shown in Fig. 3. This circuit enables the converter to turn on normally, achieving its full output voltage levels before the external capacitor (Cout ) is connected across the output terminals. The circuit will not increase turn-on time or trigger the converter's short-circuit protection feature.
Remember minimum load
Most dc/dc converter application problems are neither as unique as the first two examples, nor limited to a single market segment. Often, they relate to common practices that are either poorly documented by the manufacturer or easily misunderstood by the user.
One often-ignored specification is minimum load, a specification not mentioned on most dc/dc converter data sheets. Nearly all multiple output dc/dc converters require maintaining a minimum load current on the primary output so that the auxiliary outputs work properly.
In a typical pulse-width-modulated converter, magnetic coupling is used to semi-regulate the auxiliary outputs from the primary output. The primary output is normally the one with the highest output current. This output controls the pulse-width-modulator (PWM) through a direct feedback loop. By controlling the PWM, the primary output, in effect, controls the auxiliary outputs.
An external minimum load is required to keep the PWM feedback loop activated and deliver full rated output power to the auxiliaries. The minimum load is typically set at 10% of the full rated primary output current, but can be as high as 20% to 25% depending on the converter design.
Not maintaining a minimum load causes several application problems, depending on the actual loading of each output. At best, the converter outputs lose regulation and drift out of specification. If one or more of the auxiliary outputs is heavily loaded when the primary current falls below the minimum set point, output voltage levels could rise, triggering the converter overvoltage protection circuits. If overvoltage protection is not a feature of the converter, damage to the converter or load circuitry may occur.
CAPTIONS:
Opening shot:
Dc/dc converters form an integral part of many power distribution systems.
Fig. 1. Most dc/dc converters have an input filter stage with capacitors that charge at a slow rate.
Fig. 2. An external inrush current limiter circuit can be added to a dc/dc converter to mitigate the effects of peak currents.
Fig. 3. A hold-up time circuit enables the converter to turn on normally, achieving its full output voltage levels before the external capacitor (Cout ) is connected across the output terminals.
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