MQFL-28-05S Datasheet, PDF (9/16 Pages) SynQor Worldwide Headquarters – HIIGH RELLIIABIILLIITTY DC/DC CONVERTTER

Technical Specification
MQFL-28-05S
Output: 5.0 V
Current: 24 A
BASIC OPERATION AND FEATURES
The MQFL 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 transform-
ers 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 fre-
quency 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 dou-
ble 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.
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 MQFL con-
verters have such high efficiency, particularly at low output volt-
ages.
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 neg-
ative load current to flow back into the converter’s output termi-
nals if the load is a source of short or long term energy. The
MQFL converters employ a “back-drive current limit” to keep this
negative output terminal current small.
There is a control circuit on both the input and output sides of the
MQFL converter that determines the conduction state of the
power switches. These circuits communicate with each other
across the isolation barrier through a magnetically coupled
device. No opto-isolators are used.
A separate bias supply provides power to both the input and out-
put control circuits. Among other things, this bias supply permits
the converter to operate indefinitely into a short circuit and to
avoid a hiccup mode, even under a tough start-up condition.
An input under-voltage lockout feature with hysteresis is provided,
as well as an input over-voltage shutdown. There is also an out-
put current limit that is nearly constant as the load impedance
decreases to a short circuit (i.e., there is not fold-back or fold-for-
ward characteristic to the output current under this condition).
When a load fault is removed, the output voltage rises exponen-
tially to its nominal value without an overshoot.
The MQFL converter’s control circuit does not implement an out-
put over-voltage limit or an over-temperature shutdown.
The following sections describe the use and operation of addi-
tional control features provided by the MQFL converter.
CONTROL FEATURES
ENABLE: The MQFL converter has two enable pins. Both must
have a logic high level for the converter to be enabled. A logic
low on either pin will inhibit the converter.
The ENA1 pin (pin 4) is referenced with respect to the converter’s
input return (pin 2). The ENA2 pin (pin 12) is referenced with
respect to the converter’s output return (pin 8). This permits the
converter to be inhibited from either the input or the output side.
Regardless of which pin is used to inhibit the converter, the reg-
ulation and the isolation stages are turned off. However, when
the converter is inhibited through the ENA1 pin, the bias supply
is also turned off, whereas this supply remains on when the con-
verter is inhibited through the ENA2 pin. A higher input standby
current therefore results in the latter case.
5.6V
PIN 4
(or PIN 12)
1N4148
ENABLE
82K
250K
125K
TO ENABLE
CIRCUITRY
2N3904
PIN 2
(or PIN 8)
RTN
Figure A: Equivalent circuit looking into either the ENA1 or ENA2
pins with respect to its corresponding return pin.
Both enable pins are internally pulled high so that an open con-
nection on both pins will enable the converter. Figure A shows
the equivalent circuit looking into either enable pins. It is TTL
compatible.
SHUT DOWN: The MQFL converter will shut down in response
to only four conditions: ENA1 input low, ENA2 input low, VIN
input below under-voltage lockout threshold, or VIN input above
over-voltage shutdown threshold. Following a shutdown event,
there is a startup inhibit delay which will prevent the converter
Product # MQFL-28-05S
Phone [phone]
www.synqor.com
Doc.# 005-2MQ050S Rev. C 1/21/08
Page 9