Given the inherent instability in power supply circuits, circuits designed to prevent such instability from affecting circuit performance are called protection circuits. Protection circuits are ubiquitous in various electronic products, including overcurrent protection, overvoltage protection, overheat protection, no-load protection, and short-circuit protection. This article compiles some common protection circuits.
Motor overheat protection circuit
Accidents caused by motor overheating or temperature controller malfunction frequently occur in continuously operating electromechanical equipment such as automatic lathes , electric ovens, and ball mills used in production, as well as other unattended equipment, necessitating appropriate safety measures. PTC thermistor overheat protection circuits can conveniently and effectively prevent such accidents.
The diagram below shows a control circuit consisting of a PTC thermistor and a Schmitt trigger circuit, using motor overheat protection as an example. In the diagram, RT1, RT2, and RT3 are three step-type PTC thermistors with identical characteristics, each embedded in the windings of the motor stator. Under normal conditions, the PTC thermistors are at room temperature, and their total resistance is less than 1 kΩ. At this time, V1 is cut off, V2 is turned on, relay K is energized and its normally open contact is closed, allowing the motor to operate under AC mains power.
When the motor overheats locally due to a fault, if any PTC thermistor is heated above the preset temperature, its resistance will exceed 10KΩ. This causes V1 to conduct, V2 to cut off, VD2 to display a red alarm, K to de-energize and release, and the motor to stop, thus achieving the protection purpose.
The selection of a PTC thermistor depends on the insulation class of the motor. Generally, the Curie temperature of the PTC thermistor is selected within a range approximately 40°C lower than the extreme temperature limit corresponding to the motor's insulation class. For example, for a motor with B1 insulation class, whose extreme temperature limit is 130°C, a PTC thermistor with a Curie temperature of 90°C should be selected.
Protection circuit in inverter power supply
Inverters frequently need to perform current conversion. If the current in the circuit exceeds the limit, it will cause great damage to the circuit and key components. Therefore, protection circuits are particularly important in inverter power supplies.
Reverse connection protection circuit
If an inverter lacks a reverse connection protection circuit, reversing the input battery connection can often lead to disastrous consequences, ranging from blowing a fuse to destroying most of the circuitry. There are three main types of reverse connection protection circuits in inverters: one consisting of a reverse-parallel Schottky diode, as shown in the diagram below.
As shown in the diagram, when the battery is connected in reverse, the Schottky diode D conducts, and F is burned out. If the subsequent main converter circuit is a push-pull structure, the parasitic diodes of the two push-pull switching MOSFETs are also equivalent to being connected in parallel with D, but their voltage drop is much larger than that of the Schottky diode, and their ability to withstand instantaneous current surges is also lower than that of the Schottky diode D. This avoids large currents flowing through the parasitic diodes of the MOSFETs, thus protecting the two push-pull switching MOSFETs.
This reverse polarity protection circuit has a simple structure and does not affect efficiency, but it will burn out fuse F after protection and needs to be replaced to restore normal operation.
The reverse connection protection circuit using relays has the following basic circuit:
As shown in the diagram, if the battery is connected in reverse, D will be reverse-biased, no current will flow through the coil of relay K, the contacts will not close, and the inverter power supply will be cut off. This reverse connection protection circuit is quite effective and will not burn out the fuse F, but it is relatively large, and the lifespan of the relay contacts is limited.
The reverse polarity protection circuit using MOSFETs is shown in the following basic circuit diagram:
In the diagram, D is the parasitic diode of the reverse-biased MOSFET, which is shown for easy analysis of the principle. When the battery polarity is not reversed, D is forward biased and conducts. The gate-source terminals of Q return to the negative terminal of the battery from the positive terminal through F, R1, and D, thus becoming forward biased and conducting. The voltage drop of Q after it conducts is much smaller than the voltage drop of D, so Q conducts and D does not receive enough forward voltage and is thus cut off.
When the battery polarity is reversed, diode D will be cut off due to reverse bias, and diode Q will also be cut off due to reverse bias of gate and source (GS), preventing the inverter from starting. This reverse polarity protection circuit has a longer lifespan because it does not use mechanical contact switches, and it will not burn out the fuse F like a reverse polarity protection circuit composed of anti-parallel Schottky diodes. Therefore, it is widely used. The disadvantage is that there is some loss when the MOSFET is conducting. It can smoothly carry a relatively large current while maintaining relatively low loss.
Battery undervoltage protection
To prevent battery damage from over-discharge, the inverter needs to stop operating when the battery voltage drops to a certain level. It's important to note that if the battery undervoltage protection is too sensitive, it will activate when starting a sudden load. This makes it difficult for the inverter to start such loads, especially when the battery charge is not sufficient. Please see the battery undervoltage protection circuit below.
It can be seen that the addition of D1 and C1 enables the battery sampling voltage to be established quickly, providing delay protection.
Lithium battery charging protection circuit
Overcharging and over-discharging lithium batteries can both affect their lifespan. During the design phase, attention must be paid to the lithium battery's charging voltage and current. Then, a suitable charging chip should be selected. Care must be taken to prevent overcharging, over-discharging, and short-circuit protection issues. Furthermore, the design must undergo extensive testing after completion.
Design of lithium battery charging circuit
This example uses the TP4056 chip. The maximum charging current can be controlled by varying the connected resistors. A charging indicator light and a specific charging temperature range (a certain range of degrees Celsius) can be designed.
Charging protection circuit
By choosing the combination of chips DW01 and GTT8205, short circuit protection and overcharge/over-discharge protection can be achieved.
Overcurrent protection circuit in switching power supply
Common overcurrent protection methods in switching power supplies
Overcurrent protection comes in various forms, as shown in Figure 1. It can be categorized into rated current droop type (F-type), constant current type, and constant power type, with the droop type being the most common. The overcurrent setting is typically 110% to 130% of the rated current. It is generally an automatic reset type.
In Figure 1, ① represents the current droop type, ② represents the constant current type, and ③ represents the constant power type.
Figure 1 Overcurrent protection characteristics
Current limiting circuit for direct drive circuit of transformer primary.
In the design of circuits directly driven by the transformer primary (such as single-ended forward converters or flyback converters), current limiting is relatively easy to implement. Figure 2 shows two methods for implementing current limiting in such circuits.
The circuit in Figure 2 can be used in both single-ended forward and flyback converters. In both Figure 2(a) and Figure 2(b), a current-limiting resistor Rsc is connected in series with the source of the MOSFET. In Figure 2(a), Rsc provides a voltage drop to drive the transistor S2 to turn on. In Figure 2(b), a current-limiting voltage comparator connected across Rsc can short-circuit the drive current pulse when an overcurrent occurs, thus providing protection.
Compared to Figure 2(b), the protection circuit in Figure 2(b) reacts faster and more accurately. First, it presets the threshold voltage of the comparator amplifier's current-limiting drive within a more precise range than the transistor's threshold voltage Vbe. Second, it sets the preset threshold voltage sufficiently small, typically only 100mV to 200mV. Therefore, the value of the current-limiting sampling resistor Rsc can be smaller, thus reducing power consumption and improving power supply efficiency.
Figure 2 shows the current limiting circuit in a single-ended forward or flyback converter circuit.
When the AC input voltage varies within the range of 90 to 264V and the output power is the same, the peak current of the transformer primary side differs greatly, causing severe drift of the overcurrent protection points on the high and low sides, which is detrimental to the consistency of the overcurrent protection points. Adding a pull-up resistor R1 taken from +VH to the circuit aims to provide a pre-set value to the base of S2 or the non-inverting input of the current-limiting comparator, thereby achieving as much consistency as possible between the high and low side overcurrent protection points.
Current limiting circuit for base drive circuit
In general, the base drive circuit is used to isolate the power supply control circuit and the switching transistor. The converter output and control circuit share a common ground. The current limiting circuit can be directly connected to the output circuit, as shown in Figure 3. In Figure 3, the control circuit and output circuit share a common ground. The working principle is as follows:
Figure 3 shows a current limiting circuit used in various power converters.
When the circuit is operating normally, the voltage drop generated by the load current IL flowing through resistor Rsc is insufficient to turn on S1. Since IC1=0 when S1 is off, capacitor C1 is in an uncharged state, therefore transistor S2 is also off. If the load-side current increases, causing IL to reach a set value such that ILRsc=Vbe1+Ib1R1, then S1 turns on, charging capacitor C1. The charging time constant τ=R2C1, and the voltage across C1 after it is fully charged is VC1=Ib2R4+Vbe2. When the circuit detects an overcurrent, R4 should be selected to ensure that capacitor C1 discharges quickly.
Current limiting circuit with no power loss
The two overcurrent protection methods mentioned above are quite effective, but the presence of Rsc reduces the efficiency of the power supply, especially under high current output conditions, where the power consumption on Rsc will increase significantly. The circuit in Figure 4 uses a current transformer as a sensing element, which creates certain conditions for improving the efficiency of the power supply.
The circuit in Figure 4 operates as follows: Current transformer T2 monitors the load current IL. When IL passes through the primary winding of the transformer, the current change is coupled to the secondary winding, generating a voltage drop across resistor R1. Diode D3 rectifies the pulse current, and the rectified current is then smoothed and filtered by resistor R2 and capacitor C1. When an overload occurs, the voltage across capacitor C1 increases rapidly, causing Zener diode D4 to conduct, which in turn drives transistor S1. The signal at the collector of S1 can be used as the drive signal for the power converter's regulation circuit.
Figure 4. No-power-consumption current-limiting circuit
Current transformers can be wound with ferrite cores or MPP toroidal cores, but repeated experiments are necessary to ensure the core remains unsaturated. Ideally, the turns ratio of a current transformer should equal the current ratio. Typically, the transformer's Np = 1, and Ns = NpIpR1/(Vs + VD3). Specific winding data must be adjusted experimentally to achieve optimal performance.
Use a 555 timer as a current limiting circuit.
Figure 5 shows the basic block diagram of the 555 integrated timer circuit.
Figure 5. Basic block diagram of the 5555 integrated timer circuit.
The 555 integrated timer circuit is a novel and versatile analog integrated circuit, including LM555, RCA555, and 5G1555, etc. Their basic performance is the same. It is widely used to construct delay circuits, monostable oscillators, astable multivibrators, and various pulse modulation circuits. It can also be used in the control circuits of direct converters.
The 555 timer circuit consists of voltage dividers R1, R2, and R3, two comparators, an RS flip-flop, and two transistors. The circuit operates within a 5-18V range. The voltage dividers provide a bias of 2Vcc/3 to the inverting input of comparator 1 and Vcc/3 to the non-inverting input of comparator 2. Pins 2 and 6 of the comparators are the trigger and threshold inputs, respectively. The comparator outputs control the RS flip-flop, which in turn supplies power to the output stage and the base of transistor V1. When the flip-flop output is high, V1 conducts, activating the discharge circuit at pin 7. When the flip-flop output is low, V1 is off, providing a low output impedance and inverting the flip-flop output pulse. When the flip-flop output is high, the voltage at pin 3 is low; when the flip-flop output is low, the voltage at pin 3 is high. The output stage can provide a maximum current of 200mA. Transistor V2 is a PNP transistor. 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 of V2 (pin 4 reset) is connected to Vcc, the base-emitter junction of V2 is reverse biased, and transistor V2 is cut off.
Figure 6 shows a circuit using a 555 timer for current limiting protection. Its working principle is as follows: The UC384X, along with S1 and T1, forms a basic PWM converter circuit. The UC384X series control IC has two closed-loop control loops. One is where the output voltage Vo is fed back to the error amplifier, used to compare with the reference voltage Vref to generate an error voltage (to prevent self-oscillation of the error amplifier, pin 2 is directly shorted to ground); the other is where the current in the primary inductor of the transformer is detected at the secondary side of T2, and the voltage across R8 and C7 is compared with the error voltage to generate a modulation pulse signal.
Of course, all of these operate at a fixed frequency set by the clock. The UC384X has excellent line regulation, reaching 0.01%/V; it significantly improves load regulation; it simplifies the external circuit compensation network of the error amplifier, improves stability and frequency response, and has a larger gain-bandwidth product. The UC384X has two shutdown techniques: one is to raise the voltage at pin 3 above 1V, triggering the overcurrent protection switch to shut down the circuit output; the other is to drop the voltage at pin 1 below 1V, causing the PWM comparator output to go high, resetting the PWM latch, and shutting down the output until the next clock pulse arrives, setting the PWM latch and restarting the circuit. Current transformer T2 monitors the peak current value of T1. When an overload occurs, the peak current of T1 rises rapidly, causing the secondary current of T2 to rise. After rectification by D1 and smoothing and filtering by R9 and C7, the current is sent to pin 3 of IC1, causing the voltage level at pin 1 of IC1 to drop. (Note: R3 and C4 connected to pin 1 of IC1 must be connected in open-loop mode. If connected in closed-loop mode, the discharge terminal at pin 7 of the 555 timer will not be able to discharge during overcurrent.)
Pin 1 of IC1 is connected to pin 6 of IC2, reducing the voltage at the non-inverting input of comparator 1 in IC2. This causes the flip-flop Q to output a high level, V1 to conduct, and pin 7 of IC2 to discharge, pulling the voltage at pin 1 of IC1 below 1V. IC1 then shuts off, and S1 shuts off due to the lack of a gate drive signal, thus protecting the circuit. If the overcurrent is not eliminated, the above process repeats, and IC1 re-enters a cycle of starting, shutting down, restarting, and shutting down again—a phenomenon known as "hiccups." Furthermore, during overload periods, oscillations repeatedly begin and stop, but the stopping time is long and the starting time is short, preventing the power supply from overheating. This overload protection is called periodic protection (when the input voltage varies significantly, the overcurrent protection points at the high and low ends remain essentially the same). Its oscillation period is determined by the RC time constant τ of the 555 monostable multivibrator; in this example, τ = R1C1. The circuit only returns to normal operation after the overload disappears. The selection of current transformer T2 follows the same calculation method as described in section 1.3.
Figure 6 shows a current-limiting protection circuit using a 555 timer.
The circuit in Figure 6 can be used in single-ended flyback or single-ended forward converters, as well as in half-bridge, full-bridge, or push-pull circuits, as long as IC1 has a feedback control terminal and a reference voltage terminal. When an overcurrent occurs, the monostable characteristic of the 555 circuit is used to make the circuit work in a "hiccup" state.
Comparison of several overcurrent protection methods
The following table compares several overcurrent protection methods.
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