SPDPXXD28_09 Datasheet, PDF (8/14 Pages) Sensitron – Programmable DC Solid State Power Controller Module

SENSITRON
SEMICONDUCTOR
TECHNICAL DATA
DATASHEET 5023, Rev D.1
SPDPXXD28 SERIES
The code programmed in the microcontroller acquires the output of the internal A/D converter, squares the
result and applies it to a simulated RC circuit. It checks the output of the simulated circuit to determine whether
or not to trip (turn off the power Mosfets). Because the microcontroller simulates an analog RC circuit, the
SSPC has ‘thermal memory’. That is, it trips faster if there had been current flowing prior to the overload than if
there hadn’t been current flowing. This behavior imitates thermal circuit breakers and better protects the
application’s wiring since the wiring cannot take as much an overload if current had been flowing prior to the
overload.
The watchdog timer operates from its own internal clock so a failure of the main clock will not stop the watchdog
timer. The code programmed in the microcontroller will periodically reset the watchdog timer preventing it from
timing out. If the code malfunctions for any reason, the watchdog timer is not reset and it times out. When the
watchdog timer times out, it resets the microcontroller. Since the code is designed to detect levels and not
edges, the output of the module, and therefore the output of the SPDPXXD28, immediately reflects the
command on its input.
The Power Mosfets used in the SPDPXXD28 Series have been selected for very low R_ds(on) and results in low
voltage drop and low power dissipation. In most applications, the SPDPXXD28 will be operated at 50 – 60% of
rated current to provide a safety margin. As can be seen in Table 1, when the SPDP25D28 is operated at 15
Amps, 60% of rated current, it only dissipates 1.0 Watt at room temperature. No heat sinking is required for this
condition. However, if the SPDP25D28 is to be operated at maximum rating and/or at elevated temperatures,
the dissipation can exceed 4 Watts and heat sinking is required. Some heat sinking can be accomplished by
adding copper area to the LINE and LOAD pins, a heatsink can be epoxy attached to the surface of the module
or a flat copper or aluminum heatsink can be sandwiched between the SPDP25D28 and the printed circuit board
using a thermal pad to maximize heat transfer. Each application should be evaluated at maximum expected
constant current. The lower current models in the SPDPXXD28 Series do not require heat sinking under all
conditions.
For overloads, no heat sinking is required provided the SPDPXXD28 Series is allowed some time to cool down.
The SPDPXXD28 has sufficient thermal mass that the temperature will rise only a few degrees under the worst-
case overload. Repetitive overloads should be avoided. When the SPDPXXD28 reports a trip condition, the
controller driving the SPDPXXD28 should allow no more than four repetitions and then allow thirty seconds to
cool down before trying to turn on again.
The SPDPXXD28 will trip on overloads in the ALWAYS TRIP region shown in Figure 1 and will never trip when
in the NEVER TRIP region. The SPDPXXD28 can be reset by bringing the CONTROL pin to a logic low. When
the CONTROL pin is brought back to logic high, the SPDPXXD28 will turn back on. If the overload is still
present, the SPDPXXD28 will trip again. Cycling the 5 Volt BIAS power will also reset the SPDPXXD28. If the
CONTROL pin is at logic high when the BIAS power is cycled, the SPDPXXD28 will turn back on when the BIAS
power is re-applied.
Status Outputs
The LOAD and GATE status outputs of the SPDPXXD28 show whether or not the load is drawing current and
Power Mosfet switch is on. A logic high on the LOAD status output shows that the load draws < 5% of rated
load and a logic low shows that the load draws > 15% of rated current. A load that draws between 5% and 15%
of rated current could result in either a high or low logic level on the LOAD status output. Logic high on the
GATE output indicates that the Power Mosfet switch is on while a logic low indicates that the switch is off.
As can be seen in Table 6, of the 8 possible states for the combination of CONTROL, LOAD and GATE, only 3
states represent valid SSPC operation. The other 5 states indicate either a failed SSPC or, more likely, a short
to ground or a short to the BIAS supply of one of the logic outputs. By comparing the CONTROL input with the
LOAD and GATE outputs, the user can determine whether or not the load is supposed to be ON (GATE),
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