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   Abstract: When the output short-circuits either the overload carries on the protection to the power source or the load, namely as current protection, abbreviation overflow protection. Introduced overflow protection’s several patterns, like フ the font, the constant flow, the permanent power and so on, and has carried on the comparison.

    Key word: Overflow protection; Examination; Compared with

Introduction

The power source takes all electronic products the generating plant, must satisfy the power supply product besides the performance the request, its own protective measures are also important, like pressure, overflow, overheated protection and so on. Once the electronic products present time the breakdown, when like the electronic products input side short-circuits or the output side opening, then the power source must close its output voltage, can protect power MOSFET and the output side equipment and so on is not burnt down, otherwise possibly causes the electronic products further damage, even causes phenomena and so on operators’ receiving an electric shock and fire, therefore, switching power supply’s overflow protection function needs certainly to consummate.

in 1 switching power supply commonly used overflow guard mode

Crosses the current protection to have many kinds of forms, as shown in Figure 1, may divide into the nominal current sagging, namely フ font; Constant flow; Permanent power, most for electric current sagging. Crosses the electric current the setting value usually to amount decides the electric current 110%~130%. Generally for self-recovery.

In Figure 1①Expresses the electric current sagging,②Expresses the constant flow,③Expresses the permanent power.

1.1 use in the transformer primary direct drive electric circuit’s current limiting electric circuit

In the transformer primary direct drive’s electric circuit (for example single end is stirring up-like converter or instead stirs up-like converter) in the design, realizes the current limiting is quite easy. Figure 2 is realizes the current limiting two methods in such electric circuit.

Figure 2 the electric circuit may use in the single end stirring up-like the converter and instead stirring up-like the converter. Figure 2 (a) (b) enters limiting resistor Rsc with Figure 2 in the MOSFET source even string, in Figure 2 (a), Rsc provides a loss of voltage actuation transistor S2 breakover, (b) bridges in Figure 2 on Rsc the current limiting voltage comparator, when produces the overflow, may short-circuit the drive current pulse, gets up the protective function.

Figure 2

    Figure 2 (a) (b) compares with Figure 2, Figure 2 (b) the protection circuit reaction rate is quicker and is accurate. First, it compares amplifier’s current limiting actuation threshold voltage initialization in one compared to transistor’s threshold voltage Vbe more precise scope; Second, it the threshold voltage which initializes obtains is young enough, its typical value only then 100mV~200mV, therefore, may obtain the current limiting sample resistance Rsc value is small, like this reduced the power loss, raised power source’s efficiency.

When the AC input voltage changes in the 90~264V scope, and outputs time the same level power, then the transformer primary peak electric current difference is very big, causes high, a low end overflow protection serious drifting, does not favor a overflow uniformity. Increases one in the electric circuit to be from VH on to pull resistance R1, its goal is causes S2 the base electrode or the current limiting comparator’s synchronism end has a pre-value, achieves the height end as far as possible consistently overflow protection.

1.2 use in the base electrode driving circuit’s current limiting electric circuit

Generally, is isolates using the base electrode driving circuit power source’s control circuit and the switch transistor. Converter’s output unit and control circuit altogether. The current limiting electric circuit may direct and the output circuit docking, its electric circuit as shown in Figure 3. In Figure 3, control circuit and output circuit altogether. The principle of work is as follows:

When electric circuit normal work, load current IL winds through the pressure drop which resistance Rsc produces to be insufficient to cause the S1 breakover, because S1 in closure time IC1=0, capacitor C1 is at has not charged the condition, therefore transistor S2 also cuts off. If the load side electric current increases, enables IL to achieve an hypothesis the value, causes ILRsc=Vbe1 Ib1R1, then the S1 breakover, causes the capacitor C1 charge, its charging time constant τ=R2C1, on after C1 fills the electric charge the voltage is VC1=Ib2R4 Vbe2. When the electric circuit examines has the overflow occurs, is enables capacitor C1 the rapid discharge, must choose R4 jie 3. R2 selects the principle is Ib1max= (Vin-Vbe1) /R1, IC1=β×Ib1max, then R2≥ (Vin-Vcesat1) R1/(V1-Vbe1). If the parameter design is correct, has the bias by VC1 to cause S2 to enter the conducting state fast sufficiently, may further close PWM through the S2 collecting electrode output the driving signal. After the overload phenomenon relieves, the electric circuit may the self-recovery to the normal work condition.

    1.3 non-power loss current limiting electric circuit

The above two kind of overflow protection is quite effective, but the Rsc existence cut power source’s efficiency, particularly in the big electric current output’s situation, on the Rsc power loss will obviously increase. Figure 4 the electric circuit takes the detecting element using the current transformer, has created certain condition for the power source efficiency’s enhancement.

Figure 4 the electric circuit principle of work is as follows: Using current transformer T2 surveillance load current IL, IL when through mutual inductor primary, arrives at secondarily the electric current change coupling, has the pressure drop on resistance R1. Diode D3 carries on the rectification to the pulse current, after the rectification carries on the smooth filter by resistance R2 and electric capacity C1. When has the overload phenomenon, the capacitor C1 both sides voltage increases rapidly, causes the zener to manage the D4 breakover, actuates the transistor S1 breakover, the S1 collecting electrode signal to be possible to use for to take the power source converter regulation circuit’s driving signal.

The current transformer may use the ferrites magnetic core or the MPP annular magnetic core circles the system, but must pass through tests repeatedly, guaranteed that the magnetic core is not saturated. The ideal current transformer should achieve the number of windings ratio is a current ratio. Usually mutual inductor Np=1, Ns=NpIpR1/(Vs VD3). Circles the system data also to specifically adjust finally after the experiment, enables its performance to achieve the optimum condition.

1.4 use 555 to make the current limiting electric circuit

Figure 5 is 555 integration time base circuit’s basic diagram.

555 integration time base circuit is one kind novel, the multipurpose simulation integrated circuit, has LM555, RCA555,5G1555 and so on, its key property is the same, composes the delay circuit, the monostable oscillator, the multivibrator and each kind of impulse modulation electric circuit with it, the use is very widespread, may also use in the direct converter’s control circuit.

Figure 4

    555 time base circuits by voltage divider R1, R2, R3, two comparators, the R 睸 trigger as well as two transistors and so on are composed, the electric circuit can work in the 5~18V scope. The voltage divider provides the bias to the comparator 1 opposition input end, the voltage is 2Vcc/3, provides for the comparator 2 synchronism input terminal voltage is Vcc/3, comparator’s another two input end foot 2, feet 6 respectively be triggering and the threshold, the comparator output control R 睸 trigger, the trigger output supplies output stage as well as the transistor V1 base electrode. When the trigger outputs sets is high, the V1 breakover, puts through the foot 7 discharging circuits; When the trigger outputs for low, V1 cuts off, the output stage provides a low output impedance, and trigger output pulse opposition. When the trigger outputs sets is high, the foot 3 output’s voltages are the low level, when the trigger outputs for low, the foot 3 output’s voltages are the high levels. The output stage can provide the maximum current is 200mA, transistor V2 is the PNP tube, its emitter electrode meets internal voltage reference Vr, Vr the value is always smaller than supply voltage Vcc, therefore, if (foot 4 replacements) receives the V2 base electrode on Vcc, V2 base - emitter for instead, transistor V2 closure.

Figure 5

    Figure 6 is uses 555 to make the current limiting protection the electric circuit, its principle of work is as follows: UC384X and S1 and T1 compose a basic PWM converter electric circuit. The UC384X series controls IC to have two closed-loop control return route, one is the output voltage Vo feedback to the erroneous amplifier, after using in the same voltage reference Vref comparison, produces the error voltage (to prevent erroneous amplifier’s autoexcitation phenomenon production, directly foot 2 pair of place pipe nipples); Another is in the transformer primary inductance electric current, in T2 examines secondarily the magnitude of current on R8 and the C7 voltage, carries on the comparison after the error voltage produces the modulating pulse signal impulse. Certainly, these in the clock establishes under the fixed frequency works. UC384X has the good linear relative regulation, can achieve 0.01%/V; May improve the load relative regulation obviously; Enabled the erroneous amplifier’s external circuit compensating network to obtain the simplification, the stability enhances and improved the frequency sound, had the bigger gain-bandwidth product. UC384X has two kind of closure technologies; First, surpasses 1V foot 3 voltage magnifications, causes the overflow protection switch closure electric circuit to output; Second, the foot 1 loss of voltage to 1V below, causes the PWM comparator to output the high level, the PWM latch repositions, the closure output, until the next clock pulse’s arrival, the PWM latch setting, the electric circuit can start. Current transformer T2 is monitoring the T1 peak magnitude of current, when has the overload, the T1 peak electric current rises rapidly, causes T2 the secondary current rise, after the D1 rectification, R9 and the C7 smooth filter, delivers IC1 the foot 3, causes IC1 the foot 1 level to drop (the attention: Meets IC1 foot 1 R3, C4 to meet the split-ring pattern, if meets time the closed loop pattern overflow 555 foot 7 electric discharge ends are unable the electric discharge). The IC1 foot 1 6 connects with the IC2 foot, causes IC2 comparator 1 synchronism input end’s stepping down of voltage, trigger Q to output the high level, the V1 breakover, the IC2 foot 7 electric discharges, causes IC1 the foot 1 level to pull is lower than 1V, then the IC1 output closure, S1 closes because of the non-electronics grid driving signal, enables the electric circuit to obtain the protection. If the overflow does not eliminate, is redundant the above process, IC1 enters the recurrent state which the start, the closure, start again, close again, namely “makeing a belch” phenomenon. Moreover, overloading period, repeatedly carries on is opening inspires with the failure of oscillations, but the failure of oscillations time is long, opens inspires the time to be short, therefore the power source cannot overheated, this kind of overloading protection is called the cycle guard mode (, when input end input voltage range of variation is big, still might cause high, low end overflow protection basic same). Its duration of oscillation Tau decided by 555 list steady multivibrator’s RC time-constant, in this example τ=R1C1, until overload phenomenon vanishing, the electric circuit only then may the rehabilitation. Current transformer T2 choice with 1.3 mutual inductor computational method.

Figure 6

    Figure 6 the electric circuit, may use, in the single end instead stirs up-like or the single end is stirring up-like in the converter, also available in semi-bridge system, entire bridge-type or in push-pull circuit, so long as IC1 has the reaction control end and the datum voltage terminal then, when has the overflow phenomenon, causes the electric circuit work with 555 electric circuit’s monostable characteristics at “makeing a belch” under the condition.

1.5 several kind of overflow guard mode comparison

Several kind of overflow guard mode’s comparison like table 1 arranges in order.

Table 1 several kind of overflow guard mode comparison

2 conclusions

The author undergoes the long-term research and development and the production, compared with each overflow protection method which the switching power supply in used, might say, did not have one overflow guard mode is nearly multi-purpose, only then used 555 guard mode performance price compared to is good. Generally speaking, chooses what overflow guard mode, must unify the concrete electric circuit to transform the pattern to make the corresponding choice. Only then undergoes the earnest analysis, the massive experiments could find the most suitable overflow guard mode. The guard mode design is reasonable, is effective, means that the product the reliability possibly is only then higher.