参数资料
| 型号： | CL-90A |
| 厂商： | GE Sensing |
| 文件页数： | 3/4页 |
| 文件大小： | 0K |
| 描述： | NTC THERM CURR LIMIT INRUSH |
| 特色产品： | CL Series Current Limiters |
| 标准包装： | 2,000 |
| 系列： | CL |
| R @ 25°C： | 120 欧姆 |
| R @电流： | 1.18 欧姆 |
| 电流 - 稳态最大值： | 2A |
| 容差： | ±25% |
| 直径： | 0.93"（23.62mm） |
| 引线间隔： | 0.328"（8.33mm） |
| 包装： | 散装 |
| 其它名称： | AB15190 CL-90-A CL-90-A-ND Q2025121 Q2320551 |
GE
Sensing
Inrush CurrentLimiters In Switching Power Supplies
The problem of current surges in switch-mode power
supplies is caused by the large filter capacitors used to
smooth the ripple in the rectified 60 Hz current prior to
being chopped at a high frequency. The diagram above
~
R I
-t o
illustrates a circuit commonly used in switching power
supplies.
In the circuit above the maximum current at turn-on is
the peak line voltage divided by the value of R; for 120 V,
it is approximately 120 x √ 2/R I . Ideally, during turn-on R I
should be very large, and after the supply is operating,
should be reduced to zero. The NTC thermistor is ideally
suited for this application. It limits surge current by
functioning as a power resistor which drops from a high
cold resistance to a low hot resistance when heated by
the current flowing through it. Some of the factors to
consider when designing NTC thermistor as an inrush
or in differential form:
Pdt = HdT + δ (T – T A )dt
where:
P = Power generated in the NTC
t = Time
H = Heat capacity of the thermistor
T = Temperature of the thermistor body
δ = Dissipation constant
T A = Ambient temperature
Typical power
supply circuit
current limiter are:
During the short time that the capacitors are charging
?
?
?
?
?
Maximum permissible surge current at turn-on
Matching the thermistor to the size of the filter capacitors
Maximum value of steady state current
Maximum ambient temperature
Expected life of the power supply
(usually less than 0.1 second), very little energy is
dissipated. Most of the input energy is stored as heat in
the thermistor body. In the table of standard inrush
limiters there is listed a recommended value of maximum
capacitance at 120 V and 240 V. This rating is not
intended to define the absolute capabilities of the
Maximum Surge Current
The main purpose of limiting inrush current is to prevent
components in series with the input to the DC/DC
convertor from being damaged. Typically, inrush
protection prevents nuisance blowing of fuses or
breakers as well as welding of switch contacts. Since
most thermistor materials are very nearly ohmic at any
given temperature, the minimum no-load resistance of
the thermistor is calculated by dividing the peak input
voltage by the maximum permissible surge current in the
power supply (V peak/Imax surge ).
Energy Surge at Turn-On
At the moment the circuit is energized, the filter caps in a
switcher appear like a short circuit which, in a relatively
short period of time, will store an amount of energy equal
to 1/2CV 2 . All of the charge that the filter capacitors store
must flow through the thermistor. The net effect of this
large current surge is to increase the temperature of the
thermistor very rapidly during the period the capacitors
are charging. The amount of energy generated in the
thermistor during this capacitor-charging period is
dependent on the voltage waveform of the source
charging the capacitors. However, a good approximation
for the energy generated by the thermistor during this
period is 1/2CV 2 (energy stored in the filter capacitor). The
ability of the NTC thermistor to handle this energy surge
is largely a function of the mass of the device. This logic
can be seen in the energy balance equation for a
thermistor being self-heated:
Input Energy = Energy Stored + Energy Dissipated
thermistors; instead, it is an experimentally determined
value beyond which there may be some reduction in the
life of the inrush current limiter.
Maximum Steady-State Current
The maximum steady-state current rating of a thermistor
is mainly determined by the acceptable life of the final
products for which the thermistor becomes a
component. In the steady-state condition, the energy
balance in the differential equation already given reduces
to the following heat balance formula:
Power = I 2 R = δ (T – T A )
As more current flows through the device, its
steady-state operating temperature will increase and its
resistance will decrease. The maximum current rating
correlates to a maximum allowable temperature.
In the table of standard inrush current limiters is a list of
values for resistance under load for each unit, as well as
a recommended maximum steady-state current. These
ratings are based upon standard PC board heat sinking,
with no air flow, at an ambient temperature of 77° (25°C).
However, most power supplies have some air flow, which
further enhances the safety margin that is already built
into the maximum current rating. To derate the
maximum steady state current for operation at elevated
ambient temperatures, use the following equation:
Iderated = I derated = √ (1.1425–0.0057 x T A ) x I max @ 77°F
(25°C)
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