Power Electronics

850W Quarter Brick Bus Converter is 98% Efficient, Works With Online Power Simulation Tool

Vicor’s IB050Q096T80N1-00 Intermediate Bus Converter (IBC) module is an efficient, low profile, isolated, fixed ratio converter for power system applications in enterprise and optical access networks (Fig. 1). It is the newest addition to Vicor’s wide input range VI BRICK™ intermediate bus converters (IBCs). The converter can support up to an 850W load, making it well suited for demanding applications spanning enterprise, optical access and storage networks.

In particular, the converter is suited for powering non-isolated converters in point-of-load applications. For more information, see the sidebar titled “Bus Converters Cut Cost Of Distributed Power Architectures”.

This IBC is also available as a drop-in upgrade for industry-standard 5:1 fixed ratio converters. The module operates from a 36V to 60V input voltage range, with 2,250Vdc isolation from input-to-output while achieving 98% peak efficiency (Fig. 2). Its rating is up to 80 A, 850W from 55 to 60 Vin and 550W at 36Vin. Housed in a standard 2.30 x 1.45 x 0.42-inch quarter-brick module, the converter allows designers to conserve valuable board space and achieve full load operation at 50 °C with 400 LFM airflow.

Designers can optimize the performance of the IB050Q096T80N1-00 IBC module by using Vicor’s IBC Power Simulation tool to interactively model the module’s electrical and thermal performance under application-specific operating conditions and thermal environments. Available to users via Vicor’s PowerBench™ online design center, the IBC Power Simulation tool enables designers to quickly and easily select, simulate and optimize IBC performance under a variety of system thermal and electrical conditions.

You can view a demonstration of Vicor’s IBC Power Simulation tool interactively modeling and testing the electrical and thermal performance of an IBC at http://www.engineeringtv.com/video/Vicor-Design-Tool;search%3Avicor.

Safe Operation

The IBC incorporates a number of safeguards to protect against improper usage. An internal undervoltage/overvoltage lockout function prevents operation outside its normal input operating range. The IBC turns on within an input voltage window bounded by the Input under-voltage turn-on and Input over-voltage turn-off levels. You can protect it against accidental application of a reverse input voltage by adding a diode in series with the positive input.

Connect the IBC to its power source with minimal distribution inductance. If the interconnect inductance exceeds 100 nH, bypass the input with an RC damper to retain low source impedance and stable operation. With an interconnect inductance of 200 nH, the RC damper may be 47 µF in series with 0.3 Ω. A single electrolytic or equivalent low-Q capacitor may be used in place of the series RC bypass.

The IBC’s open-frame construction facilitates airflow above and below the module to minimize temperature rises of downstream components. Using an industry-standard form factor and pin-out, the module provides designers with greater power capability and also frees up valuable board space.

Sine Waves

The IBC050 series modules are pin-compatible with traditional “square wave” bus converters that are limited by their switching losses to low operating frequencies, low power densities and low bandwidth. To improve performance, the IBC050 series modules employ Sine Amplitude Converter™ (SAC) topology. Operating at 1 MHz, the IB050Q096T80N1-00 IBC module cuts transient response time by a factor of 10 and eliminates the need for bulk capacitors across the intermediate bus.

As shown in Fig. 3, the SAC approach is a transformer-based series resonant topology that operates at a fixed frequency equal to the resonant frequency of its tank circuit. Switching MOSFETs in the primary are locked to the natural resonant frequency of the primary and switch at zero crossing points. This eliminates switch power dissipation, boosts efficiency, and cuts generation of high order noise harmonics. Current in the primary resonant tank exhibits a pure sinusoidal waveform, which reduces the harmonic, content and provides a much cleaner output noise spectrum. As a result, the module requires less output voltage filtering than competitive power modules.

Enable Input

The function of the EN depends whether you are using a negative or positive input for the IBC. If the input is negative and you float the EN pin it disables the IBC output. Pulling the EN pin lower than 0.8 Vdc with respect to the negative input enables the output. You can drive the EN pin by a relay, optocoupler, or open collector transistor. This pin should not be toggled at a rate higher than 1 Hz and it should not be driven by or pulled up to an external voltage source.

A positive input and floating EN pin enables the IBC output. Pulling the pin lower than 1.4 Vdc with respect to the negative input disables the output. You can accomplish this using a relay, opto-coupler, or open collector transistor. Do not toggle this pin at a rate higher than 1 Hz. Also,

Do not drive the EN pin or pull it up to an external voltage source. The EN pin can source up to 2 mA at 5 Vdc and should never be used to sink current.

After using the EN pin to disable the IBC, the IBC will attempt to restart approximately every 250ms. Once the module has been disabled for at least 250ms, the IBC delays the turn on after enabling the EN pin.

Total load capacitance at the output of the IBC should not exceed the specified maximum. Because of the wide bandwidth and low output impedance of the IBC, users can add capacitance at the input of the IBC to more efficiently provide low frequency bypass capacitance and significant energy storage.

The IBC will inherently current share when operated in an array. Arrays may be used for higher power or redundancy in an application. Current sharing accuracy is maximized when the source and load impedance presented to each IBC within an array are equal. The recommended method to achieve matched impedances is to dedicate common copper planes within the PCB to deliver and return the current to the array, rather than rely upon traces of varying lengths. In typical applications, the current being delivered to the load is larger than that sourced from the input, allowing narrower traces to be utilized on the input side if necessary. The use of dedicated power planes is, however, preferable.

One or more IBCs in an array may be disabled without adversely affecting operation or reliability as long as the load does not exceed the rated power of the enabled IBCs.

The IBC power train and control architecture allow bi-directional power transfer, including reverse power processing from the IBC output to its input. The IBC’s ability to process power in reverse improves the IBC transient response to an output load dump.

The temperature distribution of the VI Brick can vary significantly with its input /output operating conditions, thermal management and environmental conditions. Although the PCB is UL rated to 130°C, it is recommended that PCB temperatures be maintained at or below 125°C.

For maximum long-term reliability, lower PCB temperatures are recommended for continuous operation; however, short periods of operation at 125°C will not negatively impact performance or reliability.

Thermal and Voltage Considerations

The IBC can operate with surface temperatures and operating voltages that may be hazardous to personnel. Ensure that adequate protection is in place to avoid inadvertent contact.

To take full advantage of the IBC capabilities, the impedance presented to its input terminals must be low from DC to approximately 5 MHz. The source should exhibit low inductance and should have a critically damped response. If the interconnect inductance is excessive, the IBC input pins should be bypassed with an RC damper (e.g., 47 µF in series with 0.3 Ω) to retain low source impedance and proper operation. Given the wide bandwidth of the IBC, the source response is generally the limiting factor in the overall system response.

Anomalies in the response of the source will appear at the output of the IBC multiplied by its K factor. The DC resistance of the source should be kept as low as possible to minimize voltage deviations. This is especially important if the IBC is operated near low or high line as the overvoltage/undervoltage detection circuitry could be activated.

The IBC is not internally fused in order to provide flexibility in configuring power systems. However, input line fusing of VI Bricks must always be incorporated within the power system.


nBus Converters Cut Cost Of Distributed Power Architectures

As shown in Fig. 4, a typical Intermediate Bus Converter delivers an unregulated, stepped down voltage of 9.6 to 14V with at least 2000Vdc input-output isolation. This converter is ideal for a loosely regulated 12Vdc Intermediate Bus Architecture to power a variety of downstream non-isolated, point-of-load regulators. These bus converter modules are suited for computer servers, enterprise networking equipment, and other applications that use a 48Vdc (±10%) input supply.

In many distributed power architecture applications, you can achieve cost savings by replacing multiple 48V in, isolated dc-dc converter bricks with lower cost, non-isolated POL modules or embedded converters fed from a 12V bus converter rail. Implementing one central point of isolation eliminates the need for individual isolation at each point of load, thus reducing costs, reducing board space, and allowing greater design flexibility.

Bus converters achieve high efficiency by limiting the input range and essentially optimizing for a single input voltage. Removing the entire feedback path (reference, error amp, optocoupler, etc.) liberates board area and power. Additional parallel MOSFETs may be added to lower on-resistances. MOSFET duty cycles in the power train are set and maintained at 50%, and all components are optimized for the voltages they will actually experience and not the voltages they may experience. For high efficiency most bus converters employ synchronous rectifier outputs.

Bus converter packages come in many sizes, from SIPs and SMTs to quarter-brick, eighth-brick and sixteenth-brick modules.


Related Articles:

Distribution vs. Efficiency: Power Architect's Dilemma

Distributed Power Architectures Evolve and Reconfigure

Bus Converter Maximizes System Efficiency, Minimizes Heat Losses

Topology Selection by the Numbers Part One

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