Overcurrent Protection Methods Commonly Used in Switching Power Supply

As the power supply equipment of all electronic products, in addition to meeting the requirements of power supply products, its own protection measures are also very important, such as overvoltage, overcurrent, overheating protection, etc. In case of failure of electronic products, such as short circuit at the input side or open circuit at the output side of electronic products, the power supply must turn off its output voltage in order to protect the power MOSFET and output side equipment from being burned, otherwise it may cause further damage to electronic products and even electric shock and fire of operators. Therefore, The overcurrent protection function of switching power supply must be perfect.


Overcurrent protection methods commonly used in switching power supply

There are many forms of overcurrent protection, as shown in Figure 1, which can be divided into rated current droop type, i.e. フ type; Constant current type; Constant power type, most of which are current drooping type. The set value of overcurrent is usually 110% 130% of the rated current. It is generally automatic recovery type.

In Figure 1, â‘  represents current droop type, â‘¡ represents constant current type and â‘¢ represents constant power type

Figure 1 overcurrent protection characteristics

1.1 current limiting circuit used in primary direct drive circuit of transformer

In the design of primary direct drive circuit of transformer (such as single ended forward converter or flyback converter), it is easy to realize current limiting. Fig. 2 shows two methods of realizing current limiting in such a circuit.

The circuit of Fig. 2 can be used for single ended forward converter and flyback converter. In Fig. 2 (a) and Fig. 2 (b), a current limiting resistor RSC is connected in series in the source of MOSFET. In Fig. 2 (a), RSC provides a voltage drop, and the driving transistor S2 is turned on. In Fig. 2 (b), the current limiting voltage comparator connected across RSC can short-circuit the driving current pulse when overcurrent occurs to play a protective role.

Fig. 2 (a) compared with Fig. 2 (b), the response speed of the protection circuit in Fig. 2 (b) is faster and accurate. Firstly, it presets the threshold voltage of the current limiting drive of the comparison amplifier in a more accurate range than the threshold voltage VBE of the transistor; Second, the preset threshold voltage is small enough, and its typical value is only 100mV 200mV. Therefore, the value of current limiting sampling resistance RSC can be small, which reduces power consumption and improves the efficiency of power supply.

(a) Transistor protection

(b) Current limiting comparator protection

Fig. 2 current limiting circuit in single ended forward or flyback converter circuit

When the AC input voltage changes in the range of 90 264v and outputs the same power, the peak current at the primary of the transformer varies greatly, resulting in serious drift of high and low overcurrent protection points, which is not conducive to the consistency of overcurrent points. A pull-up resistor R1 from VH is added to the circuit to make the base of S2 or the in-phase end of the current limiting comparator have a preset value, so as to achieve the consistency of overcurrent protection points at the high and low ends as far as possible.

1.2 current limiting circuit for base drive circuit

In general, the base drive circuit is used to isolate the control circuit of the power supply from the switching transistor. The output part of the converter is grounded with the control circuit. The current limiting circuit can be directly connected with the output circuit, and its circuit is shown in Fig. 3. In Fig. 3, the control circuit is grounded in common with the output circuit. The working principle is as follows:

Fig. 3 current limiting circuit for various power converters

When the circuit works normally, the voltage drop generated by the load current IL flowing through the resistance RSC is not enough to turn on S1. Since IC1 = 0 when S1 is turned off, the capacitor C1 is in an uncharged state, so the transistor S2 is also turned off. If the load side current increases so that IL reaches a set value, so that ilrsc = VBE1 ib1r1, S1 is turned on to charge capacitor C1, and its charging time constant Ï„= R2c1, the voltage after full charge on C1 is Vc1 = ib2r4 VBE2. When the circuit detects the occurrence of overcurrent, R4 should be selected in order to enable the capacitor C1 to discharge quickly

1.3 current limiting circuit without power loss

The above two kinds of overcurrent protection are more effective, but the existence of RSC reduces the efficiency of the power supply, especially in the case of high current output, the power consumption on RSC will increase significantly. Fig. 4 circuit uses current transformer as detection element, which creates certain conditions for improving power efficiency.

The working principle of the circuit in Figure 4 is as follows: the current transformer T2 is used to monitor the load current IL. When IL passes through the primary of the transformer, the change of the current is coupled to the secondary, resulting in a voltage drop on the resistance R1. Diode D3 rectifies the pulse current, and after rectification, it is smoothly filtered by resistor R2 and capacitor C1. When overload occurs, the voltage at both ends of capacitor C1 increases rapidly, making Zener D4 turn on and driving transistor S1 turn on. The signal of S1 collector can be used as the driving signal of power converter regulation circuit.

Fig. 4 current limiting circuit without power consumption

The current transformer can be wound with ferrite core or MPP ring core, but repeated experiments shall be carried out to ensure that the magnetic core is unsaturated. The ideal current transformer should reach the turn ratio, which is the current ratio. Generally, NP = 1, ns = npipr1 / (vs VD3) of transformer. Finally, the specific winding data should be adjusted by experiments to make its performance reach the best state.

1.4 555 is used as current limiting circuit

Fig. 5 is a basic block diagram of 555 integrated time base circuit.

Fig. 5555 basic block diagram of integrated time base circuit

555 integrated time base circuit is a novel and multi-purpose analog integrated circuit, including LM555, rca555, 5G1555, etc. its basic performance is the same. The delay circuit, monostable oscillator, multivibrator and various pulse modulation circuits composed of it are widely used, and can also be used in the control circuit of direct converter.

The 555 time base circuit is composed of voltage dividers R1, R2 and R3, two comparators, R-S trigger and two transistors. The circuit can work in the range of 5 18V. The voltage divider provides a bias voltage to the inverting input of the comparator 1 with a voltage of 2vcc / 3, and the in-phase input voltage provided to the comparator 2 is VCC / 3. The other two input pins 2 and 6 of the comparator are trigger and threshold respectively. The comparator output controls the R-S trigger, and the trigger output is supplied to the output stage and the base of the transistor v1. When the trigger output is set high, V1 is turned on and the discharge circuit of pin 7 is connected; When the trigger output is low, V1 is cut off, the output stage provides a low output impedance, and inverts the trigger output pulse. When the trigger output is set high, the voltage output by pin 3 is low. When the trigger output is low, the voltage output by pin 3 is high. The maximum current that the output stage can provide is 200mA. The transistor V2 is a PNP tube, and its emitter is connected to the internal reference voltage VR. The value of VR is always less than the power supply voltage Vcc. Therefore, if the base (pin 4 reset) of V2 is connected to VCC, the base emitter of V2 is reverse biased and the transistor V2 is cut off.

Figure 6 shows the circuit with 555 as current limiting protection. Its working principle is as follows: uc384x forms a basic PWM converter circuit with S1 and T1. Uc384x series control IC has two closed-loop control loops. One is that the output voltage Vo is fed back to the error amplifier to generate the error voltage after comparison with the reference voltage Vref (in order to prevent the self excitation of the error amplifier, pin 2 is directly shorted to the ground); The other is the voltage at R8 and C7 detected by the current in the primary inductance of the transformer at the secondary T2, which is compared with the error voltage to generate the pulse signal of the modulation pulse. Of course, these work at the fixed frequency set by the clock. Uc384x has good linear adjustment rate, which can reach 0.01% / V; It can obviously improve the load adjustment rate; The external circuit compensation network of the error amplifier is simplified, the stability is improved, the frequency response is improved, and the gain bandwidth product is larger. Uc384x has two shutdown technologies; First, raise the voltage of pin 3 by more than 1V, causing the overcurrent protection switch to close the circuit output; Second, reduce the voltage of pin 1 below 1V to make the PWM comparator output high level, reset the PWM latch and close the output. The circuit can not be restarted until the next clock pulse arrives and the PWM latch is set. The current transformer T2 monitors the peak current value of T1. When overload occurs, the peak current of T1 rises rapidly, causing the secondary current of T2 to rise. After D1 rectification, R9 and C7 smooth filtering, it is sent to pin 3 of IC1 to reduce the level of pin 1 of IC1 (Note: R3 and C4 connected to pin 1 of IC1 must be connected to open-loop mode. If connected to closed-loop mode, the discharge end of pin 7 of 555 cannot discharge during overcurrent). Pin 1 of IC1 is connected with pin 6 of IC2 to reduce the voltage at the in-phase input end of comparator 1 of IC2, trigger Q outputs high level, V1 is turned on, and pin 7 of IC2 discharges, so that pin 1 level of IC1 is pulled lower than 1V, IC1 output is closed, S1 is closed due to no grid drive signal, so that the circuit is protected. If the overcurrent is not eliminated, repeat the above process, and IC1 will re-enter the cycle state of startup, shutdown, restart and re shutdown, i.e. "burping". Moreover, during the overload period, the vibration start and stop are repeated, but the vibration stop time is long and the vibration start time is short, so the power supply will not overheat. This overload protection is called periodic protection mode (when the input voltage at the input end changes widely, the overcurrent protection points at the high and low ends can still be basically the same). The oscillation period is determined by the RC time constant of 555 monostable multivibrator Ï„ Decision, in this case Ï„= R1c1, the circuit can not resume normal operation until the overload phenomenon disappears. The selection of current transformer T2 is the same as the transformer calculation method in 1.3.

Figure 6 using 555 as current limiting protection circuit

The circuit in Fig. 6 can be used in single ended flyback or single ended forward converter, or in half bridge, full bridge or push-pull circuit. As long as IC1 has feedback control terminal and reference voltage terminal, when overcurrent occurs, the monostable characteristic of 555 circuit is used to make the circuit work in the "burping" state.

1.5 comparison of several overcurrent protection modes

The comparison of several overcurrent protection modes is listed in Table 1.

Table 1 Comparison of several overcurrent protection modes

2 Conclusion

After long-term R & D and production, the author compares various overcurrent protection methods used in switching power supply. It can be said that almost no overcurrent protection method is omnipotent, and only 555 protection method has better performance price ratio. Generally speaking, the choice of overcurrent protection mode should be made in combination with the specific circuit transformation mode. Only after careful analysis and a large number of experiments can we find the most suitable overcurrent protection mode. The reasonable and effective design of protection means that the reliability of products can be higher.

Overcurrent Protection Methods Commonly Used in Switching Power Supply 1

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