ANALOG FEEDBACK
High di/dt Converters Take a Load off External Capacitors
B y DDaa vid M
or
Mor
orrr ison, Editor, Power Electronics Technology
ast 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 boardmounted 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.
F
High di/dt POLs are general-purpose power converters
that seek to service fast transients without requiring
large amounts of external capacitance.
Power Electronics Technology April 2004
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
74
www.powerelectronics.com
ANALOG FEEDBACK
Vout
Rise Time
(V)
3.3
2.5
Load Step
di/dt
Deviation
Recovery Time
Deviation
Recovery Time
(nS)
(A)
(A/µs)
(mV)
(µs)
(mV) sink
(µs)
44
0 to 12
300
103
12
75
16
16
66
0 to 12
200
100
12
75
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
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 m⍀. When VIN is changed from 5 V to 3.3 V, the deltas for
deviation and recovery time are negligible.
Table. Transient response measurements for Artesyn’s SMT12F.
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.
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
www.powerelectronics.com
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 lowESR output capacitors (which also help achieve fast
response) in combination with the multiphase, highfrequency 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.
PETech
Datel; www.datel.com
Artesyn Technologies; www.artesyn.com
75
CIRCLE 350 on Reader Service Card
CIRCLE 351 on Reader Service Card
Power Electronics Technology
April 2004