1. Converter will undergo input over-voltage shutdown.
2. Derate output power to 50% of rated power at Tcase = 135ºC. 135ºC is above the specified operating range. See Figure 5.
3. High or low state of input voltage must persist for about 200µs to be acted on by the shutdown circuitry.
4. Current limit inception is defined as the point where the output voltage has dropped to 90% of its nominal value. See Current Limit discussion in
Features Description section.
5. Parameter not tested but guaranteed to the limit specified.
6. Load current transition time ≥ 10µs.
7. Line voltage transition time ≥ 100µs.
8. Input voltage rise time ≤ 250µs.
9. Operating the converter at a synchronization frequency above the free running frequency will cause the converter’s efficiency to be slightly reduced
and it may also cause a slight reduction in the maximum output current/power available. For more information consult the factory.
10. After a disable or fault event, module is inhibited from restarting for 100ms. See Shut Down section of the Control Features description.
11. Only the ES and HB grade products are tested at three temperatures. The C grade products are tested at one temperature. Please refer to the
Construction and Environmental Stress Screening Options table for details.
12. These derating curves apply for the ES and HB grade products. The C grade product has a maximum case temperature of 70ºC.
13. The specified operating case temperature for ES grade products is -45ºC to 100ºC. The specified operating case temperature for C grade products
is 0ºC to 70ºC.
Product# MQHL-28-2R5S
Phone 1-888-567-9596
www.SynQor.com
Doc.# 005-0005898 Rev. 2
08/14/13
Page 4
MQHL-28-2R5S
Output:
2.5V
Current:
20A
Application Section
BASIC OPERATION AND FEATURES
The MQHL DC/DC converter uses a two-stage power
conversion topology. The first, or regulation, stage is a
buck-converter that keeps the output voltage constant over
variations in line, load, and temperature. The second, or
isolation, stage uses transformers to provide the functions of
input/output isolation and voltage transformation to achieve
the output voltage required.
Both the regulation and the isolation stages switch at a fixed
frequency for predictable EMI performance. The isolation
stage switches at one half the frequency of the regulation
stage, but due to the push-pull nature of this stage it
creates a ripple at double its switching frequency. As a
result, both the input and the output of the converter have a
fundamental ripple frequency of about 550 kHz in the free-
running mode.
CONTROL FEATURES
ENABLE:
The MQHL
converter has one enable pin, ENA1
(pin 4), which is referenced with respect to the converter’s
input return (pin 2). It must have a logic high level for the
converter to be enabled; a logic low inhibits the converter.
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The enable pin is internally pulled high so that an open
connection will enable the converter. Figure A shows the
equivalent circuit looking into the enable pin. It is TTL
compatible and has hysteresis.
5V
82.5K
Rectification of the isolation stage’s output is accomplished
with synchronous rectifiers. These devices, which are
MOSFETs with a very low resistance, dissipate far less energy
than would Schottky diodes. This is the primary reason why
the MQHL
converters have such high efficiency, particularly at
low output voltages.
Besides improving efficiency, the synchronous rectifiers
permit operation down to zero load current. There is no
longer a need for a minimum load, as is typical for converters
that use diodes for rectification. The synchronous rectifiers
actually permit a negative load current to flow back into the
converter’s output terminals if the load is a source of short
or long term energy. The
MQHL
converters employ a “back-
drive current limit” to keep this negative output terminal
current small.
PIN4
ENA1
10K
TO ENABLE
CIRCUITRY
PIN2
IN RTN
Figure A:
Circuit diagram shown for reference only, actual circuit
components may differ from values shown for equivalent circuit.
There is a control circuit in the MQHL
converter that determines
the conduction state of the power switches. It communicates
across the isolation barrier through a magnetically coupled
device. No opto-isolators are used.
An input under-voltage shutdown feature with hysteresis is
provided, as well as an input over-voltage shutdown and an
output over-voltage limit. There is also an output current
limit that is nearly constant as the load impedance decreases
(i.e., there is not fold-back or fold-forward characteristic to
the output current under this condition). When a load fault
is removed, the output voltage rises exponentially to its
nominal value without an overshoot. If a load fault pulls the
output voltage below about 60% of nominal, the converter
will shut down to attempt to clear the load fault. After a
short delay it will try to auto-restart.
The MQHL
converter’s control circuit does not implement an
over-temperature shutdown.
The following sections describe the use and operation of
additional control features provided by the
MQHL converter.
SHUT DOWN:
The MQHL
converter will shut down in
response to only five conditions: ENA input low, VIN input
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