PDF AN:029 Single DCM as an Isolated, Regulated DC-DC Converter

APPLICATION NOTE | AN:029
Single DCM as an Isolated, Regulated DC-DC Converter
Arthur Russell
Applications Engineering
April 2015
ContentsPage
Introduction1
Support Circuitry
2
Input Circuitry
2
Output Circuitry
2
Control Pins
3
Burst Mode
5
Output Capacitor
5
Thermal6
Considerations
Layout and Routing
6
Introduction
The DC-DC Converter Module (DCM) encapsulates isolation, regulation, thermal
management, and fault monitoring in a single module. Its wide input voltage range,
high output power, high density, and high efficiency enable its use in a variety of
industrial and military applications. The DCM is offered in the Converter housed in
Package (ChiP) platform, which delivers high power density through its superior
thermal performance. ChiP platform products use advanced magnetic structures that
are integrated within high density interconnect (HDI) substrates, together with power
semiconductors, control ASICs and a microcontroller. For more information on DCM
capabilities and options, please see the Vicor web page for DCMs:
www.vicorpower.com/dc-dc-converter-board-mount/dcm-dc-dc_converter
The DCM operates on the same high frequency, double-clamped zero voltage switching
(DC-ZVS) topology as the Vicor PFM and the Picor Cool-Power Isolated
DC-DC Converter Modules. The DC-ZVS topology provides a revolutionary increase in
density and efficiency compared to other complete DC-DC converter solutions.
Support Circuitry
Referring to Figure 1, the DCM simplifies the design of the input, output, and control
circuitry.
Conclusion6
DCM
TR
EN
L2
F1
L1
Vin
Load 1
FT
R1
+IN
+OUT
-IN
-OUT
CLOAD
C1
Non-isolated
Point-of-Load
Regulator
Load 2
Figure 1.
Block Diagram of a
Single DCM Circuit
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Input Circuitry
DCMs feature on-board protection for input overvoltage and overcurrent, so the fuse F1
shown in Figure 1 is only needed for applications that must pass safety approvals, such
as CE Mark or UL60950. For more information on fuse selection and recommendations,
refer to the data sheet for the DCM model of interest.
The effective input impedance of the DCM is VIN2/PIN. This form of the equation is the
most relevant because for a given load, the DCM presents a constant power load to its
input power source (up to the control loop bandwidth, which is up to 20 kHz).
To guarantee stability of the DCM control loop, the impedance of the source that
supplies VIN must be less than ½ the effective input impedance of the DCM at the
expected minimum input line condition, over the control loop bandwidth. If the source
cannot satisfy this requirement alone, the input filter circuit must compensate for the
excessive impedance. A large electrolytic capacitor can be used: for example, at least
1000 µF is recommended for a 320 W DC-DC converter operating with line voltages
down to 16 V, such as the MDCM28AP280M320A50 (model specific values are provided
in the corresponding DCM data sheet). However, in cost-sensitive or space-limited
applications, a more efficient solution may be possible. The theory behind the input
impedance requirement is given in the Vicor tech tip:
powerblog.vicorpower.com/wp-content/uploads/2009/10/tech_tip_input_source_impedance.pdf
For an additional optimization to minimize ringing and provide additional margin
between the DCM’s rated low line and its input undervoltage fault protection threshold
(UVLO), the source impedance of VIN should be further limited to be no more than 1/10
the effective input impedance of the DCM.
If the application circuit is designed such that the DCM is not inhibited using the EN
pin, the DCM output voltage will become active during the input voltage ramp, when
it reaches the input undervoltage fault protection (UVLO) voltage VIN-UVLO+. The input
filter needs to be designed according to this extreme low line condition. As the DCM
starts near UVLO+, the source impedance must be low enough that input voltage
transients caused by inrush current do not trigger UVLO.
DCM efficiency is slightly impacted by high frequency ripple voltage developed on the
DCM input due to switching currents. Ceramic input filter capacitors in close proximity
to the +IN and IN leads can form a resonant tank with the package lead inductance and
the internal input capacitors. When the DCM is operating near its maximum switching
frequency (approximately 1 MHz), the resonant tank can amplify ringing voltage,
leading to a slight increase in losses within the DCM. This effect can be minimized by
avoiding the placement of ceramic capacitors directly across the DCM input, or by
reducing the Q of this resultant resonant tank by adding resistors in series with those
capacitors.
Information on the design of the input filter can be found in the following application
note, which emphasizes designing DC-DC converters for low EMI:
http://www.vicorpower.com/documents/application_notes/vichip_appnote23.pdf
Vicor supplies a design tool to assist the user in designing a stable filter , with minimal
peaking:
app2.vicorpower.com/filterDesign/intiFilter.do
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Control Pins
The three DCM control pins are EN, TR and FT. Internally, they are implemented as
GPIOs, each with a pull-up resistor to a constant 3.3 V rail. Each control pin has a simple
internal bias and drive structure, which is maintained throughout the various operating
modes. The logic thresholds, bias levels and bias strengths for the input pins do not
change during startup or fault protection conditions. In general, the control pins can all
be left unconnected to select their default function.
EN is a digital input that enables the powertrain. The powertrain is disabled when EN
is less than the EN disable threshold and enabled when EN is pulled above its enable
threshold; if EN is left open, an internal 10 kΩ pull-up resistor keeps the powertrain
enabled.
TR selects the trim mode and sets the trimmed output voltage of the DCM. The TR pin
is sampled one time at power-up, after VIN exceeds VIN-UVLO+; its voltage at that time
determines the function of TR for as long as the DCM is supplied an input voltage. If TR
is greater than
VTRIM-DIS when it is sampled, the DCM will be in non-trim mode. This is the case when
TR is left open: an internal 10 kΩ pull-up resistor pulls TR up to VCC, selecting the nontrim mode of operation. In this mode, the programmed trim condition will simply be
the rated nominal Vout of the DCM model.
If TR is sampled below VTRIM-DIS at power-up, the internal trim circuitry will be enabled
and TR will control the output voltage setting as long as VIN is applied. If the powertrain
is disabled with EN or stopped by a fault condition, it retains the trim mode which was
previously latched in when it resumes operation. The trim mode is re-evaluated only
after removing and reapplying the input voltage to the DCM.
The nominal relationship between a trim resistor and the trim pin voltage is given by:
VTR = VCC • RTRIM / (RTRIM + 10 kΩ)
A sample circuit to control TR is shown in Figure 2.
Figure 2.
Example of a Circuit to Set TR
Vcc
10k
Output Voltage
Reference,
Current Limit
Reference
and Soft Start Control
TR
RTRIM
Kelvin -IN connection
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When trim mode is active, the programmed VOUT of the DCM maps linearly to the TR
pin voltage. For the example MDCM28AP240M320A50 product, the transfer function is:
VOUT-FL @ 25°C = 9.98 + (18.780 • VTR/VCC)
at full load room and temperature. Each DCM model data sheet provides constants for
this transfer function.
When VTR is set such that the programmed trim value is above the nominal output
voltage rating, the internal current limit threshold will be decreased proportionally thus
maintaining a roughly constant maximum average output power. (See the DCM data
sheet.)
Any time VTR is set such that VOUT is outside the rated output voltage trim range VOUTTRIMMING, the DCM will function, but it may not operate as expected, and none of the
performance specifications in the data sheet are in force. For example, trimming the
VOUT above the high trim threshold decreases the margin before overvoltage protection
(OVP) is activated; depending on the DCM model, output capacitor, and dynamic load
profile, a load dump could trigger OVP. Conversely trimming VOUT below the low trim
threshold may result in excessive ripple on the output, or even shutdown of the DCM in
response to load transients due to output undervoltage fault protection.
FT is the positive-true output generated by the internal Fault Monitoring circuit.
When the DCM activates fault protection, the FT pin is driven high (to 3.3 V) by a low
impedance driver. This configuration helps avoid the need for the system to generate a
separate low voltage supply for powering the fault pin monitoring circuit, since a simple
opto-coupler can be entirely driven from the FT pin.
In Figure 3, the RSERIES resistor acts as a current limiter to keep the maximum current
below the 4 mA limit of the FT pin. With a single DCM, a 560 Ω resistor will limit the
current to about 2 mA, sufficient to light an LED.
Figure 3.
FT pin driving an LED
Vcc
499k
Fault
Monitoring
FT
RSERIES
RSHUNT
Under normal conditions, the FT pin is internally biased to 3.3 V by a 499 kΩ resistor,
but can easily be held low by an external pull-down resistor. In the fault indicating LED
circuit above (Figure 3), RSHUNT is part of a voltage divider that holds the diode voltage
in the OFF state when no fault is detected; a 47 kΩ resistor is sufficient for a single DCM.
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Burst Mode
The DCM enters Burst Mode under light loading conditions, when the internal power
consumption of the converter plus the external output load is less than the minimum
conversion power. This scenario is most pronounced when the DCM input voltage is high,
the trim voltage is low, and there is no output load. During Burst Mode, the DCM alternates
between two operating conditions: initially the error amplifier attempts to regulate
VOUT by enabling the converter powertrain, but the minimum energy per pulse that the
powertrain can supply is greater than the power needed to maintain output regulation, so
the output voltage climbs. As a result of that slight excess output voltage, the error amplifier
momentarily inhibits the powertrain, allowing the output voltage to fall again. Once the
output voltage falls below the error amplifier setpoint, the error amplifier starts the converter
powertrain switching again. The second condition, when the powertrain is not switching,
will typically last for tens or hundreds of times that of a regular switching period.
Due to the primary-referred method of sensing output voltage (see the Functional Block
Diagram in the DCM data sheet), the accuracy with which the error amplifier senses VOUT is
greatly reduced while the powertrain is momentarily inhibited. This results in an effective
boosting of the actual VOUT during Burst Mode operation.
For most DCM models, output voltage boosting can occur when the external output load is
10% or less of the DCM rated load. The output voltage can rise by VOUT-LL above its set point.
The boosting of the output voltage is described by the ΔV term of the Overall Output Voltage
Transfer Function. For the example MDCM28AP280M320A50 DCM, ΔV is 5.01 V in the
transfer function:
VOUT = 9.98 + (18.780 • VTR/VCC) + 1.26 • (1 - IOUT / 13.40) - 3.200 • 0.001 • (TINT -25) + ΔV
To calculate VOUT for other DCMs, see the relevant DCM data sheet.
Another result of the primary-sensed output voltage is that if an output load is suddenly
applied during the momentary inhibit time, there may be an increased delay before VOUT
returns to its regulated level. The choice of output capacitor minimizes this, as covered in the
next section.
Output Circuitry
The output circuit must be designed and laid out to minimize conducted emissions. As
with any high power, high density circuit, thermal management may also be a special
concern. See the Thermal Considerations section below.
Output Capacitor
To ensure proper operation of the DC-DC Converter, the capacitance of the load must be
within the limits that are defined in the data sheet by COUT-EXT for normal operation. The
maximum limit is required to avoid excessive startup time, which could trigger output
undervoltage fault protection. The minimum limit for COUT-EXT, as well as the minimum ESR
of the external capacitor, RCOUT EXT, are needed to ensure control loop stability.
For applications where the DCM sees very light loading, certain DCM models have
increased minimum allowed external output capacitor values. The higher values depend
on the load transients and the trim transients that the application imposes on the DCM.
During Burst Mode, a sudden change in the load – for instance, a step increase from a
light load condition – may not be tracked by the control loop, as explained above. If there
is not enough capacitance on the output, this could cause the output voltage to drop
below the undervoltage fault protection threshold VOUT-UVP, causing shutdown. (See
Output Undervoltage Fault Protection (UVP) in the DCM data sheet.) To prevent this from
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happening, the minimum allowed output capacitance value must be increased so that it
falls in the range given by COUT EXT-TRANS. This effect is amplified for applications which
dynamically trim the DCM output at the same time these load transients are occurring. In
this case, the minimum allowed output capacitance may be further increased, per the limits
of COUT-EXTTRANS-TRIM, to prevent the load transient and dynamic trim event from triggering
fault protection.
Thermal Considerations
The Thermal Specified Operating Area in Figure 1 of the DCM datasheet relates the
maximum load power permissible versus environment temperature for different heat
extraction paths. Figure 1 extends up to the maximum rated output power of the DCM.
It assumes the worst case efficiency h from the data sheets specifications. (Also see the
efficiency curves in the data sheet, under Typical Performance Characteristics.) Data sheet
Figure 1 shows the maximum output power that can be delivered to the load for a given
ambient temperature if the package temperature is controlled at the top of the package;
at the top and leads; and at the top, leads and bottom of the package. Some DCM data
sheets don’t have the “top only” curve because cooling only the top isn’t sufficient for those
modules. (See the analysis in the data sheet, under Thermal Design.)
The DCM is also capable of sustained operation above rated power, up to the current limit
threshold with no derating or overstress to the module, provided it is sufficiently cooled.
Figure 25 “Thermal Specified Operating Area: Max Power Dissipation vs. Case Temp for
arrays or current limited operation” is an extension of Figure 1, which relates maximum
dissipated power to environmental temperature for same the three heat extraction paths.
Any time the DCM is used above rated power, for example at current limit, the thermal
design must be based on the operating efficiency and the appropriate Figure 25 curve.
Vicor offers a selection of DCM-specific heat sinks. Check with your Vicor sales team for the
solution that best fits your needs.
Vicor provides a suite of online tools, including a simulator and thermal estimator, a thermal
calculator, application notes, and white papers. These simplify the task of developing a DCM
thermal configuration that is sufficient for a given set of conditions. These tools can be found
at: www.vicorpower.com/powerbench.
Layout and Routing
Please reference Application Note AN:005 “FPA Printed Circuit Board Layout Guidelines”
for a detailed discussion of PCB layout. Application Note AN:005 details board layout
recommendations using VI Chip components, with details on good power connections,
reducing EMI, and shielding of control signals and techniques. While this application note is
not written specifically for DCMs, many of the concepts and recommendations still apply.
Avoid routing the trim and enable control signals directly underneath the DCM. It is
critical that all control signals are referenced to –IN, both for routing and for pull-down and
bypassing purposes.
Conclusion
A single DCM can provide regulation, isolation and fault protection for applications that
require high power while optimizing space requirements. The ChiP platform allows for
a small circuit footprint while enabling flexibility in the thermal design. Vicor’s power
component design methodology, along with the minimal support circuitry required help
engineers to develop systems quickly and efficiently.
The Power Behind Performance
Rev 1.1
05/15
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