Optimal design of the protection circuit of the ho

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Optimization design of protection circuit of power supply equipment


safety and reliability should be the first principle to evaluate the quality index of switching power supply. Under the condition that the electrical technical indicators meet the requirements of normal use, in order to make the power supply work safely and reliably in harsh environments and sudden failures, a variety of protection circuits must be designed, such as surge proof soft start, overvoltage, undervoltage, overheating, overcurrent, short circuit, lack of equal protection circuits

1. Principle and application of active electromagnetic interference filter

active electromagnetic interference filter is referred to as active EMI filter. It is a filter that integrates active devices into micro encapsulated chips and is specially used to suppress electromagnetic interference. Compared with traditional passive EMI filters, active EMI filters not only have excellent noise attenuation characteristics, but also have powerful functions. Some of them can also realize hot plugging, which can greatly save the space of printed boards (PCBs). They are suitable for power supply, electronic equipment, medical instruments and other fields

typical products of active EMI filters are QPI (quiet power input) series produced by VICOR company in the United States, mainly including qpi-3l ~ qpi-6l, qpi-8l. See Table 1 for product classification of QPI series EMI filters. Among them, qpi-8l is an active EMI filter with hot swap function. Table 1 product classification of QPI series EMI filter

1.1 performance characteristics of qpi-8l active EMI filter

qpi-8l conform to 4 The vulnerable parts and consumables of the equipment supplied all year round ensure that the instrument can use the DC 48V or 60V bus for a long time, and can attenuate the conducted noise (common mode noise and differential mode noise) from 150 kHz to 30MHz. At 250 kHz, the attenuation capacity of common mode noise is greater than 40dB, and the attenuation capacity of differential mode noise is greater than 70 dB. It can work continuously under 80V DC voltage and can withstand 100V DC surge voltage. Its insulation voltage to ground is 1500V, the maximum working current is 6a, and the maximum load is 200W. Compared with passive EMI filter, common mode noise attenuation ability can be increased by 20dB, and differential mode noise attenuation ability can be increased by 10 ~ 30dB. It has the functions of surge current limitation and open circuit, programmable undervoltage/overvoltage protection, normal indication of power supply voltage, etc. The default undervoltage threshold is 34V (the hysteresis voltage at shutdown is 2V); The overvoltage threshold is 76V (the hysteresis voltage at shutdown is 4V); The threshold values of undervoltage and overvoltage can also be changed by using external voltage dividing resistors. It is easy to use and has the function of hot plug, allowing the active EMI filter to be plugged in or unplugged with power. The efficiency at full load is higher than 99%, which is especially suitable for filtering the electromagnetic interference of dc/dc power converter. Using the qpi-eval1 software provided by the manufacturer, the installed qpi-8l and terminal equipment can be easily tested

1.2 the working principle of qpi-8l active EMI filter

qpi-8l internal block diagram is shown in Figure 1. Bus + and bus - terminals are respectively connected to the positive and negative poles of the bus. SW end receives full amplitude negative pressure controlled by hot plug function. Shield is the shielding end, which is connected with the shielding end of the load and the common end of the Y capacitor. QPI + and QPI - connect the positive and negative input terminals of the load respectively. Pwrgd (power good) is the output end of the normal indication of the power supply voltage (open collector output). When the power supply voltage is abnormal, this end outputs and uses a more effective and robust way to lower the level. Connect a resistance voltage divider at the uven end and ov end respectively, and set the undervoltage and overvoltage thresholds respectively. It mainly includes the following five parts

Figure 1 internal block diagram of QPI - 8L

(1) hot plug function circuit

(2) EMI filter

(3) internal voltage dividing resistors (R1, R2) for undervoltage detection

(4) internal voltage dividing resistors (R3, R4) for overvoltage detection

(5) p-channel MOSFET

when the power supply voltage is abnormal, the MOSFET is turned off, which can disconnect the load and play a protective role

qpi-8l has a maximum attenuation of about 70 dB for common mode noise and 82 dB for differential mode noise in the frequency range of 0.01 ~ 30 MHz. This is a difficult index for passive EMI filters. The insertion loss curve of qpi-8l active EMI filter and three passive EMI filter products to differential mode noise is shown in Figure 2. In Figure 2, curve a represents qpi-8l, and curves B, C and d represent three typical passive EMI filter products respectively. Obviously, qpi-8l can suppress differential mode noise in a wide frequency range

Figure 2 insertion loss curve of several EMI filters to differential mode noise

1.3 typical application of active EMI filter

typical wiring diagram of qpi-8l is shown in Figure 3, which is inserted between bus power supply and dc/dc power converter. The dc/dc power converter is directly installed on the printed board and shielded. C2 ~ C6 are bypass capacitors, with a capacity of 0.047 μ F。

Figure 3 typical wiring of qpi-8l

2. Integrated overvoltage protection device

in recent years, with the progress of submicron manufacturing technology, the working voltage of many new integrated circuits is getting lower and lower, and the ability of chips to withstand overvoltage is also declining, which makes the role of protection circuits more important. Overvoltage protection is referred to as OVP (over voltage protection). Typical products of integrated overvoltage protection devices include the new overvoltage protection integrated circuit ncp345 launched by American ONSEMI semiconductor company, and max4843 series produced by Maxim company (including four models: max4843, max4844,

max4845, max4846). This kind of new device has high integration and small volume, which can greatly reduce the number of external components and reduce the cost of overvoltage protector. It can be widely used in, digital cameras, laptops, personal digital assistants (PDAs), portable CD players, spare chargers for cars and portable medical devices that help you solve your worries. Next, taking ncp345 as an example, the principle and application of integrated overvoltage protection devices are introduced

2.1 principle of ncp345 integrated overvoltage protector

ncp345 adopts advanced Bi CMOS manufacturing process and can withstand 30V transient voltage. It can be less than 1 μ Turn off the p-channel MOSFET quickly within s to ensure that the load is not damaged. Its turn off speed is much faster than that of low-voltage CMOS monitoring circuit, which can only withstand 12V transient voltage under the same load, and the turn off time is up to 200 μ s。 It is suitable to be connected between the ac/dc power adapter (or battery charger) and the load. The battery charger can be Li ion battery charger or NiMH battery charger. It has overvoltage power-off and undervoltage locking functions, which can detect overvoltage conditions and quickly cut off the input power supply, so as to avoid damaging electronic equipment due to overvoltage or power adapter failure. The typical value of its rated overvoltage threshold is 6.85v

the internal block diagram of ncp345 is shown in Figure 4. It mainly includes input stage resistance voltage divider (R1, R2), voltage regulator, bandgap reference voltage source, under power or △ e ≤ 1.7 voltage locking circuit, control logic with hysteresis characteristics, MOSFET Driver. As long as one of the following three conditions occurs, the out end will output a high level to turn off the MOSFET

Figure 4 internal block diagram of ncp345

(1) UCC drops below the undervoltage lockout threshold voltage (2.8V)

(2) the input voltage at the in end is higher than the overvoltage threshold (6.85v)

(3) CNTRL end input is high level

as long as a resistance voltage divider is connected between the in end and UCC end, the overvoltage threshold uov can be adjusted. Since the input resistance Rin at the in end is dozens of K Ω, it is only when R1 and R2

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