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Off-the-Line Switching Power Supplies
Published in Nihal Kularatna, DC Power Supplies Power Management and Surge Protection for Power Electronic Systems, 2018
One of the most important protection features in the power supply is current limiting. When designing a current limiter, one should think of two main aspects: measure the current and develop a limiting circuit using the current signal. Current limiter design necessitates trade-offs among cost, complexity, reliability, and performance. There are several possible current-limiting schemes, as shown in Figure 7.20. In constant current limiting (Figure 7.20[a]), the output voltage drops sharply beyond the limit of the current. In an LDO or a common linear regulator, if such a scheme is applied at the limit, the voltage across the pass element will exceed the normal operation value (from [Vin – VO] to [Vin]) and the dissipation limit of the transistor and the heat sink can be exceeded, and the designer should allow for such excess dissipation. Figure 7.20(b) shows foldback technique. An advantage in this scheme over the constant current-limiting method is that dissipation within the regulator circuits is minimized. The ratio of ISC/Imax is an important parameter for this scheme, where a smaller value means better performance.
Design of DC Power Supply and Power Management
Published in Nihal Kularatna, Electronic Circuit Design, 2017
One of the most important protection features in the power supply is current limiting. When designing a current limiter, one should think of two main aspects: measure the current and develop a limiting circuit using the current signal. Current limiter design necessitates tradeoffs among cost, complexity, reliability, and performance. There are several possible current-limiting schemes, as shown in Figure 3.57. In constant current limiting (Figure 3.57a), the output voltage drops sharply beyond the limit of the current. In an LDO or a common linear regulator, if such a scheme is applied at the limit, the voltage across the pass element will exceed the normal operation value (from [Vin − VO] to [Vin]) and the dissipation limit of the transistor and the heat sink can be exceeded, and the designer should allow for such excess dissipation. Figure 3.57b shows foldback technique. An advantage in this scheme over the constant current limiting method is that dissipation within the regulator circuits is minimized. The ratio of ISC/Imax is an important parameter for this scheme, where a smaller value means better performance. However, unless the circuit is designed to reset automatically on removal of excess load, this scheme may need manual intervention at overcurrent. Figure 3.57c shows the concept of hiccup current limiting. It incorporates overcurrent shutdown but adds an automatic restart mechanism. The power supply shuts down for a limited period of time and automatically restarts after a timeout.
Electric Power Transmission and Distribution
Published in Muhammad H. Rashid, Ahmad Hemami, Electricity and Electronics for Renewable Energy Technology, 2017
The purpose of a current limiting device is to reduce the current that will flow through a device, and a system or its parts in case of a fault, which can cause damage. A common current limiting device for this purpose is a neutral grounding resistance (NGR). Such a resistor, nonetheless, must be carefully selected based on a number of criteria including the involved voltage and the allowed delay for the protecting device (e.g., a circuit breaker). A low resistance neutral grounding or a high resistance neutral grounding can be employed. In the former, the allowed current is higher. A current of 50 A or more (up to 400 A) is allowed for 10 sec. In the latter the maximum allowed current is 25 A for potentials of 600 V or less. A current of 5 A is more common. For higher voltages (13.8 kV and up), solid grounding is normally employed.
Low-cost digital microfluidic approach on thin and pliable polymer films
Published in Instrumentation Science & Technology, 2022
Dongping Chai, Jiaxi Jiang, Yiqiang Fan
The simplified circuit design is shown in Figure 4a, including a Si NPN transistor (NEC D882P), a current-limiting resistor R (1 kΩ), and a transformer T. The windings L1, L2, and L3 are magnetically coupled. When the transistor is on, its L1 winding is excited and the transistor is forward-biased by the voltage induced in the L1 winding and is turned on. Next, the voltage induced in the L2 winding causes the increase of the emitter voltage; within a short time, the transistor is turned off. As a consequence, the energy stored in the transformer is transferred to the load through the L3 winding. The circuit repeats this switching process with an operating frequency that varies with the input voltage and the state of the output load. The designed circuit converted 3.7 V DC voltage provided by lithium battery to a high-frequency (around 11 kHz) sine wave AC voltage with a peak to peak value of approximately 340 V. The cost of the oscillation boost circuit is less than $10, and the measured waveform is shown in Figure 4b. The proposed boost circuit is simple, low-cost, and able to provide sufficient high alternating voltage for the droplet manipulation. However, one obvious drawback is the unchangeable output frequency when the circuit is in operation. The output frequency can be changed with current-limiting resistor R and the windings on transformer T.
Impact of Transients Caused by PV Transformer Energization on Active Industrial Loads
Published in IETE Technical Review, 2021
Ayush Chandel, David Lee Lubkeman
In this strategy, current limiting resistor is used for inrush current mitigation and control. Figure 12 shows the current limiting resistor before the point of interconnection of PV plant with distribution grid. Additional controls and by-pass switch are also visible in the figure. The upgradation is intended to limit the huge inrush current drawn by the transformer. Subsequently, the distortion in current and voltage waveforms is recorded. The optimal resistor selection depends upon its power handling capacity, ability to withstand peak current and does not cause voltage sag in the system. After the insertion resistance is included in the system, the inrush parameters obtained are enlisted in Table 4. With the introduction to a resistor in series the immediate voltage rise across the transformer is deliberately reduced and the switching angle is increased. As the switching angle is dependent on source resistance, the initial inrush current can be limited [8–10].
Comprehensive investigation on doubly fed induction generator-Wind farms at fault ride through capabilities: technical difficulties and improvisations
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2021
Preeti Verma, Seethalekshmi K, Bharti Dwivedi
A basic SSFCL circuitry consists of a solid-state switch, a current-limiting impedance, an overcurrent detector, and a control device. However, these limiters introduce switching losses as the power flows through the power-electronic switches during steady-state operation. The long-term reliability of these devices is questionable because of the continuous switching (Fereidouni, Vahidi, and Mehr 2012). The impedance-based SSFCLs are classified into three types: Type-1-R-type SSFCL [Figure 8(a)]Type 2-L-type SSFCL [Figure 8(b)]Type 3-LR-type SSFCL [Figure 8(c)]