Power Electronics

High di/dt Converters Take a Load off External Capacitors

Fast di/dt specifications on new nonisolated point-of-load (POL) converters from Artesyn Technologies and Datel suggest that high-end microprocessors aren't the only applications generating fast load transients these days. Although the new dc-dc converters target somewhat different applications, as evidenced by their different input-voltage ranges and output-current capabilities, each may offer as much as an order of magnitude improvement in transient response over typical POLs. Although both are basically synchronous buck regulators, these POLs employ different design approaches to achieve their dynamic performance.

Before examining the new POLs, consider some of the existing options for servicing fast load transients in board-mounted power applications. Voltage regulator modules (VRMs) have been used for years to provide the high di/dt performance required by advanced microprocessors. But VRMs represent an application-specific form of nonisolated dc-dc POL converters. Often designed to power a specific device, VRMs have their own unique sets of electrical and mechanical requirements. And because output current ratings for VRMs can be fairly high, there's the possibility that these converter will be oversized for other applications.

General-purpose POLs also can achieve fast transient response with the right combination of converter, external capacitors and board layout. Typically, POL performance is specified for load steps in the range of 1 A/µs to 10 A/µs, with settling times at about 100 µs and output voltage deviations of 3% to 5%. When di/dt values jump to 100 A/µs or more, system designers may resort to adding significant amounts of capacitance to their boards to maintain acceptable performance.

Because added capacitance demands more board real estate, this option detracts from the goal of achieving high current and power density with the power converter. It also complicates the system design, which negates some of the POL's ease of use — a primary reason for using POL modules in the first place.

Artesyn Technologies seeks to overcome these limitations with its introduction of the SMT12F, a 12-A POL that steps down a 3-V to 5.5-V input to a lower voltage in the 0.9-V to 3.3-V range. This surface-mount converter measures just 16 mm × 13.46 mm × 7.93 mm and requires just five external 22-µF ceramic capacitors to achieve transient response in the range of 100 A/µs to 300 A/µs. The total board space occupied by the POL and capacitors is 300 mm2.

The SMT12F's datasheet specifies a transient response of 300 A/µs for a full 12-A load step with a voltage deviation of 100 mV (3%) and a 15-µs settling time to ±1%. According to the vendor, the recommended 110 µF of external capacitance is needed to ensure converter stability and to supply energy to the load during transients.

The transient response of the SMT12F has been tested at various rates of di/dt and at various output voltages (see the table). For a given output voltage, the output voltage deviation and recovery time remain fairly constant over widely varying di/dt rates. This performance is illustrated by measurements taken at 3.3-V output. Similar consistency in measurements was obtained at a 1.5-V output, though the table only shows results for a di/dt of 100 A/µs.

Measurements of transient response at the lower voltages (approaching 1 V) are restricted to a di/dt of 100 A/µs. However, according to the company, there is no degradation in dynamic performance at the lower output voltages. If these tests were repeated at the higher transient levels, the same levels of voltage deviation and settling times would be seen as at the higher values of output voltage. The voltage deviations as a percentage of the nominal output remain fairly consistent across different values of output voltage. At the same time, recovery times at the lower voltages are reduced somewhat, perhaps because of the smaller load steps.

A POL's response to load transients can be attributed in varying degrees to the system's decoupling capacitance (high and midfrequency) and bulk capacitance (low frequency), as well as to the response of the dc-dc converter itself. The decoupling capacitance, and to some extent the bulk capacitance, will power the load during the converter's setting time. If the converter's output settles more quickly, less decoupling capacitance may be required in the system to provide energy to the load during the recovery period. The short recovery times achieved by Artesyn's POL suggest a need for less capacitance in the application.

Table. Transient response measurements for Artesyn's SMT12F.
Vout (V) Rise Time (nS) Load Step (A) di/dt (A/µs) Deviation (mV) Recovery Time (µs) Deviation (mV) sink Recovery Time (µs)
3.3 44 0 to 12 300 103 12 75 16
66 0 to 12 200 100 12 75 16
132 0 to 12 100 100 12 75 16
528 0 to 12 25 100 12 75 16
1320 0 to 12 10 90 12 70 16
2.5 50 0 to 10 200 73 12 57 12
1.8 48 0 to 7.2 150 53 10 41 10
1.5 60 0 to 6 100 44 12 32 10
1.2 48 0 to 4.8 100 35 10 33 10
1.0 40 0 to 4 100 30 10 25 10
0.9 36 0 to 3.6 100 27 8 25 8
Note: All plots on file are for VIN = 5 V and load = 250 mΩ. When VIN is changed from 5 V to 3.3 V, the deltas for deviation and recovery time are negligible.

According to Artesyn, the transient response of the SMT12F results from use of a high switching frequency and monolithic integration. The POL switches at a frequency in the range of 600 kHz, which is achieved using a chip that integrates the PWM controller with gate drivers and high- and low-side MOSFETs. Artesyn plans to extend this POL's performance shortly with the introduction of a 15-A version that will specify similar levels of dynamic performance with no external capacitance required. This change reflects a tradeoff in that the 12-A part has been designed to maximize current density. Conversely, the larger industry-standard footprint of the 15-A POL (33 mm × 13 mm) will allow additional room for capacitance on-board the module. Also, unlike the 12-A POL, the 15-A version is designed to operate from a 12-V input.

Meanwhile, Datel has developed its HEN series of nonisolated eighth-brick POLs, which are high di/dt versions of the company's LEN series. Larger than Artesyn's SMT12F, the 2.3-in. × 0.9-in. eighth brick is rated for 25 A to 28 A of output and operates from a 12-V nominal input, while producing outputs at 0.8 V to 5 V. This device specifies a 50-A/µs transient response with the full 25-A load step. For a 50% to 100% load step, settling time to 1.5% VOUT is specified at 30 µs typ. (60 µs max). These values reflect the use of a 3000-µF POS cap and a 300-µF ceramic capacitor in the test setup. The HEN series converters also are distinguished by very low output noise (10 mV p-p typ.) and line/load regulation well below 1% each.

In the case of the Datel POL, the fast transient response also can be attributed to its high switching frequency of 660 kHz. However, this performance is obtained with a 2-phase PWM converter rather than the single-phase operation associated with the Artesyn part. In that regard, the HEN series POL resembles the VRMs, which also exploit multiphase, high-frequency switching to achieve fast transient response.

Another factor contributing to the POL's transient response is the minimization of impedance (particularly resistance) between the converter's input and output. To this end, Datel's fast POLs exploit MOSFETs with low RDS(on) and low-valued inductors that have low series resistance. Naturally, these choices necessitate some design tradeoffs. For example, lower on-resistance in the MOSFETs comes at the cost of higher gate capacitance, which in turn, degrades converter efficiency. That tradeoff can be seen when comparing Datel's HEN series POLs with the existing LEN series. The latter devices quote efficiencies up to 95% at 25 A — about 3% higher than that cited in the HEN datasheet.

Similarly, the choice of lower values for output inductance suggests less filtering of output noise. However, this reduction in filtering can be offset by the choice of low-ESR output capacitors (which also help achieve fast response) in combination with the multiphase, high-frequency switching. Another comparison of the LEN and HEN series reveals that the high di/dt models feature lower output noise than their predecessors (10 mV p-p typ. for the HEN versus 25 mV p-p for the LEN series).

At least one other vendor has indicated its plans to introduce a high di/dt version of its POLs, and others may eventually follow suit. Taken together, these developments may signal an emerging trend toward more stringent power demands in datacom and other applications, and a desire to simplify overall system design wherever these requirements arise.

Datel; www.datel.com CIRCLE 350 on Reader Service Card
Artesyn Technologies; www.artesyn.com CIRCLE 351 on Reader Service Card

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