Datasheet
Dual Output BWR Models
Mixed Voltage, 5V AND 3.3V, 2” x 2”
33 Watt DC/DC Converters
MDC_BWR 33W Models.A05 Page 5 of 12
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Current Limiting
When power demands from either output fall within 126% to 181% of the rated
output current, the DC/DC converter will go into a current limiting mode. In this
condition both output voltages will decrease proportionately with increases in
output current, thereby maintaining a somewhat constant power dissipation.
This is commonly referred to as power limiting (see Figures 2a and 2b). Current
limit inception is defi ned as the point where the full-power output voltage
falls below the specifi ed tolerance. If the load current being drawn from the
converter is signifi cant enough, the unit will go into a short circuit condition.
See "Short Circuit Condition."
Short Circuit Condition
When a converter is in current limit mode the output voltages will drop as the
output current demand increases (see fi gures 2a and 2b). If the output voltage
drops too low, the magnetically coupled voltage used to develop primary side
voltages will also drop, thereby shutting down the PWM controller.
Following a time-out period of 5 to 15 milliseconds, the PWM will restart,
causing the output voltages to begin ramping to their appropriate values. If the
short-circuit condition persists, another shutdown cycle will be initiated. This
on/off cycling is referred to as “hiccup” mode. The hiccup cycling reduces the
average output current, thereby preventing internal temperatures from rising
to excessive levels. The BWR is capable of enduring an indefi nite short circuit
output condition.
Thermal Shutdown
These BWR converters are equipped with Thermal Shutdown Circuitry. If the
internal temperature of the DC/DC converter rises above the designed operat-
ing temperature, a precision temperature sensor will power down the unit.
When the internal temperature decreases below the threshold of the tempera-
ture sensor the unit will self start.
Output Overvoltage Protection
Both output voltages are monitored for an overvoltage condition via magnetic
coupling to the primary side. If either output voltage should rise to a level which
could be damaging to the load circuitry, the sensing circuitry will power down
the PWM controller causing the output voltages to decrease. Following a time-
out of 5 to 15 milliseconds the PWM will restart, causing the output voltages to
ramp to their appropriate values. If the fault condition persists, and the output
voltages again climb to excessive levels, the overvoltage circuitry will initiate
another shutdown cycle. This on/off cycling is referred to as "hiccup" mode.
Isolation/Case Connection
The BWR 33 Watt Series’ 5V and 3.3V outputs (pins 5 & 7) and return (pin 6)
are isolated from the +V
IN and –VIN inputs (pins 1 & 2) via a transformer and
an opto-coupled transistor. Case connections are made internal to the DC/DC
converter. "D12 & D24" cases are connected to –Input (pin 2), "D48" to +Input
(pin 1).
Figure 2a. Current Limiting Characteristics for 3.3V Output
Figure 2b. Current Limiting Characteristics for 5V Output
4
3
2
1
0
0 2 4 6 8 101214
3.3 V
OUT Average Ouput Current (Amps)
Typical Current Limiting Characteristics for 3.3V Output
Ouput Voltages (Volts)
VIN NOM, VIN LO All Models
V
IN HI D12, D24 Models
V
IN HI
D48 Models
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
0123456789
5 V
OUT Average Ouput Current (Amps)
Ouput Voltages (Volts)
VIN NOM, VIN LO
All Models
VIN HI D12, D24
Models
V
IN HI
D48 Models
Typical Current Limiting Characteristics for 5V Output
(3.3V Output @ 700mA)