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MC3423 bảng dữ liệu(PDF) 6 Page - ON Semiconductor |
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MC3423 bảng dữ liệu(HTML) 6 Page - ON Semiconductor |
6 / 10 page MC3423 http://onsemi.com 6 OV Sense OV Sense *Needed if supply not current limited Vin Vin Vout Vout DC Power Supply DC Power Supply Cout Cout + + * (11A) (11B) Figure 12. Typical Crowbar OVP Circuit Configurations lpk Surge Due to Output Capacitor Current Limited Supply Output di dt l t Figure 13. Crowbar SCR Surge Current Waveform R & L EMPIRICALLY DETERMINED! ESR ESL RLead LLead Output Cap To MC3423 R L Figure 14. Circuit Elements Affecting SCR Surge and di/dt The usual design compromise then is to use a garden variety fuse (3AG or 3AB style) which cannot be relied on to blow before the thyristor does, and trust that if the SCR does fail, it will fail short circuit. In the majority of the designs, this will be the case, though this is difficult to guarantee. Of course, a sufficiently high surge will cause an open. These comments also apply to the fuse in Figure 11B. The value of di/dt that an SCR can safely handle is influenced by its construction and the characteristics of the gate drive signal. A center−gate−fire SCR has more di/dt capability than a corner−gate−fire type, and heavily overdriving (3 to 5 times IGT) the SCR gate with a fast <1.0 ms rise time signal will maximize its di/dt capability. A typical maximum number in phase control SCRs of less than 50 A(RMS) rating might be 200 A/ ms, assuming a gate current of five times IGT and < 1.0 ms rise time. If having done this, a di/dt problem is seen to still exist, the designer can also decrease the di/dt of the current waveform by adding inductance in series with the SCR, as shown in Figure 13. Of course, this reduces the circuit’s ability to rapidly reduce the DC bus voltage and a tradeoff must be made between speedy voltage reduction and di/dt. Surge Current If the peak current and/or the duration of the surge is excessive, immediate destruction due to device overheating will result. The surge capability of the SCR is directly proportional to its die area. If the surge current cannot be reduced (by adding series resistance − see Figure 13) to a safe level which is consistent with the systems requirements for speedy bus voltage reduction, the designer must use a higher current SCR. This may result in the average current capability of the SCR exceeding the steady state current requirements imposed by the DC power supply. A WORD ABOUT FUSING Before leaving the subject of the crowbar SCR, a few words about fuse protection are in order. Referring back to Figure 11A, it will be seen that a fuse is necessary if the power supply to be protected is not output current limited. This fuse is not meant to prevent SCR failure but rather to prevent a fire! In order to protect the SCR, the fuse would have to possess an I2t rating less than that of the SCR and yet have a high enough continuous current rating to survive normal supply output currents. In addition, it must be capable of successfully clearing the high short circuit currents from the supply. Such a fuse as this is quite expensive, and may not even be available. CROWBAR SCR SELECTION GUIDE As an aid in selecting an SCR for crowbar use, the following selection guide is presented. Device IRMS IFSM Package 2N6400 Series 16 A 160 A TO−220 Plastic 2N6504 Series 25 A 160 A TO−220 Plastic 2N1842 Series 16 A 125 A Metal Stud 2N2573 Series 25 A 260 A Metal TO−3 Type 2N681 Series 25 A 200 A Metal Stud MCR3935−1 Series 35 A 350 A Metal Stud MCR81−5 Series 80 A 1000 A Metal Stud |
Số phần tương tự - MC3423_05 |
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Mô tả tương tự - MC3423_05 |
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