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参数资料
| 型号： | RP-21225D2-470 |
| 厂商： | DATA DEVICE CORP |
| 元件分类： | 模拟信号调理 |
| 英文描述： | SPECIALTY ANALOG CIRCUIT, XMA20 |
| 文件页数： | 20/20页 |
| 文件大小： | 228K |
| 代理商： | RP-21225D2-470 |
9
Data Device Corporation
www.ddc-web.com
RP-21200
D-02/04-0
wire’s safe operating curve for all values of time. For the RP-
21200 series and continuous currents, this is simple. Just spec-
ify the SSPC’s rated current to be below the wire’s continuous
current rating by the required safety factor times 1.45. The 1.45
comes from the fact that the RP-21200 series is guaranteed to
trip at 145% of rated current. Since the RP-21200 series are
available in three programmable ranges, it’s easy to tailor the
STAR Series to the wire.
It’s even easier to select an RP-21200 series SSPC for transient
ratings. The
STAR Series I2t ratings are tailored to the continu-
ous ratings so that, except for rare cases, the SSPC I2t ratings
will be below the wire’s ratings. This can be verified by compar-
ing the RP-21200 series I2t rating in TABLE 3 against the wire’s
I2t rating. Since the Instant Trip level has an effect on the shape
of the I2t curves as shown in FIGURE 5, TABLE 3 shows the
STAR Series I2t rating for three levels of Instant Trip. Simply
select the RP-21200 series I2t rating for the Instant Trip level
being used and multiply by the programmed Rated Current
squared to obtain the I2t rating for the application.
PRECAUTIONS
While circuit inductance reduces current transients when a short
circuit occurs with the
STAR Series of SSPCs already on, it will
create voltage transients as the SSPC turns off. The voltage
transient is caused by the rapid change in current in the induc-
tance. If the inductance is on the power input side of the SSPC,
the voltage at the Power In pin will rise when the SSPC turns off.
Similarly, when the inductance is on the output side, the Power
Out pin will swing negative when the SSPC turns off.
In the RP-21200 series, internal circuitry slows the turn off time
of the SSPC to minimize this voltage transient until the induc-
tance, in microhenries, exceeds a critical inductance of 10,000
divided by the Maximum Rated Current.
(For an RP-21225
SSPC with a Maximum Rated Current of 25 Amps, this equates
to 400H.) At that level of inductance, the transient voltage will
be about 45 V. For larger circuit inductances, the voltage tran-
sient will grow linearly with inductance.
In most cases, the inductance on the Power In side of the SSPC
will be, at most, a few microhenries. Therefore, the Power In side
will have only small transient voltages. On the other hand, the
Power Out side can have considerable inductance from loads. A
prime example is a motor. To protect the SSPC with large induc-
tive loads, a power diode should be placed between the SSPC
Power Out pin and the load return to prevent the SSPC Power
Out from swinging negatively. This diode should be rated to han-
dle the same maximum transient current as described above for
short circuit conditions. It should be connected with short leads;
otherwise, the inductance of its leads will negate its benefits.
Since short circuits can create large transient currents, it’s pos-
sible to magnetically couple those transients to the control side if
wiring or circuit board traces of the power side run near those of
the control side. It’s not impossible to couple many volts to the
control side with poor layout. This will cause improper operation
and possibly oscillation of the SSPC which could damage the
SSPC. After all, coupling the output current to the control circuit
is a form of feedback. Since there is no control of the phase of
the feedback, oscillation is possible. This may be hard to trace
since, when there is no short circuit, everything appears normal
but as soon as a short circuit occurs, the SSPC fails. Good lay-
out practice will prevent this.
Likewise, large transient currents could magnetically couple voltage
transients elsewhere in the system causing hard-to-trace system
faults. Large voltage transients could couple into high impedance
system circuits, another possible system problem. Also, these tran-
sients could radiate elsewhere; while the system seems to pass its
EMI tests, it may only do so under certain conditions and fail under
other conditions. Routing of high current wiring and control signals
must always be made while considering these effects.
CAPACITIVE LOADS
Applying a voltage to a capacitor causes an inrush current to flow
which depends on the rate-of-rise of the voltage and the value of
the capacitor. Since the
STAR Series has a typical risetime of
500S, the charging current will never intersect the I2t portion of
the trip curve, but can intersect the Instant Trip level. If it does, the
SSPC will trip. TABLE 3 specifies the capacitive load that the RP-
21200 series can handle without tripping with 28 Volt power input.
Higher power input voltage will proportionally reduce that capaci-
tive load capability since the risetime is independent of voltage. By
carefully controlling the risetime of the output voltage, the RP-
21200 series provide a form of current limiting for capacitive loads.
However, if an SSPC is already on and an uncharged capacitive
load is applied, i.e., through a switch, the SSPC will trip. If, due
to system requirements, this possibility exists, the solution is to
use another SSPC to switch the capacitive load. The risetime
control of the second SSPC will limit the current. As described
earlier, the Slew Control must be connected to AC ground for this
current limiting to function.
In some situations, there may be a critical system load that must
have power if the primary power bus fails. In that case, a second
SSPC may be used to switch power from a secondary power bus
when the primary power bus drops. If the load cannot withstand
the time for the second SSPC to rise, the Slew Control on the
second SSPC may be left open which will speed up its risetime
to about 75S. However, this will reduce the second SSPC’s
capacitive drive capability by a factor of approximately 7.
Risetimes between 75S and 500S can be programmed by
using a voltage divider between the Power Out pin and “AC”
ground and connecting the Slew Control to the mid-point of the
voltage divider. The calculated resistance of the two resistors of
the voltage divider in parallel must be less than 1000 ohms. The
capacitive load capability of an SSPC used this way will be
reduced by the same factor as the speed-up of risetime.
RESET
When 5 Volt bias power is applied, internal circuits are automat-
ically reset. As a result, the SSPC will correctly respond to the
Control Command input and Lockout Input.
When a trip occurs, the RP-21200 series are latched off. To reset
the SSPC, simply bring the Control Command input to a logic
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