UC3832/3 improves the performance characteristics of linear regulated power supply

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Basic types and problems of overcurrent protection

The commonly used overcurrent protection methods are mainly limited flow type, current reduction type and cut-off type.

The current limiting type is characterized in that when the load current reaches the current limit value, the regulated power supply enters a constant current state to limit the load current to the current limit value. The advantage of this circuit is that it is relatively easy to implement and can achieve full load start-up. The biggest disadvantage is that under the overcurrent protection state, the power supply regulating tube will withstand the excessive loss caused by the current limit value and the input power supply voltage. In order to ensure that the power supply regulating tube is not damaged during overcurrent, the power consumption in the overcurrent protection state should be selected as the basis for selecting the power supply regulating tube and the thermal design, which will increase the cost, volume and weight of the regulated power supply, and be reliable. Reduced sex.

The characteristic of the cut-off type is: When the load current reaches the current limit value, the over-current protection circuit causes the regulated power supply to enter the cut-off state and is no longer restored, so that the regulated power supply and the load are effectively protected. The advantage is that the power consumption of the power supply regulating tube is zero at this time. The biggest disadvantage is that the impact load easily causes the regulated power supply to malfunction and enter the overcurrent protection state. Once the overcurrent protection state is entered, even if the overcurrent state is released, it cannot be automatically reset.

The characteristic of the current-reducing type is that after the regulated power supply enters the over-current protection state, the output current decreases with the output voltage, so that the loss of the power supply regulating tube is not too large even in the overcurrent protection, and its typical application is as follows. The terminal is integrated with a voltage regulator. This overcurrent protection method, although suitable for many applications, may have a latch up effect under full load constant current characteristics and power supply characteristics of most analog and digital circuits, ie, the output voltage is locked below normal regulation. The low voltage of the value does not start properly. In order to ensure the impact load or full load constant current start, a large margin must be set between the overcurrent protection point and the normal maximum operating current, and the overcurrent load is easily burned out without being protected by the power supply.

Combined overcurrent protection

The best protection method for the linear regulated power supply should have both the startup and auto-reset characteristics like the current-limiting protection, and the low power loss of the power supply regulator after overcurrent protection like the cut-off overcurrent protection. A combined overcurrent protection method is proposed.

The basic requirement of the combined overcurrent protection for the linear regulated power supply is: when the power is turned on, it is started by the current limiting protection mode to ensure smooth start during the impact load; if the regulated power supply works normally, if an overcurrent occurs The protection circuit first enters the current limiting protection state to eliminate the influence of the instantaneous inrush current; if it is still in the overcurrent state within the limited time, the protection circuit recognizes the fault overcurrent and enters the cut-off protection state; after a certain time ( If the current limit protection time is 20~30 times, the current limit protection state is restored. When the load returns to normal, the regulated power supply should start normally. If the overcurrent fault remains, the protection circuit enters the cutoff state again. As shown in Figure 1. In this overcurrent protection state, the loss of the power supply regulating tube is 1/20 to 1/30 of the current limiting type. The overcurrent protection circuit will periodically detect whether the load overcurrent is released. Once the load current is detected to be less than the current limit protection value, the normal output voltage is restored.

Implementation of combined overcurrent protection

The easiest way to achieve a linear regulated power supply combined overcurrent protection is to use the ASIC UC3832/3.

UC3832/3 circuit principle

In addition to the functional blocks necessary for the internal regulator of the linear regulator circuit control IC, the error amplifier and the driver stage, the most important thing is that the UC3832/3 has a switching overcurrent protection function. The block diagram and auxiliary circuit are shown in Figure 2. As shown in Figure 3, only the principle of overcurrent protection is introduced here.

The current-limiting amplifier (CSAMP) is set to a bias voltage of 130mV. When the input voltage is below 130mV, the current-limiting amplifier does not work, and the regulated power supply operates in a regulated state. When the load is over-current or short-circuited, the voltage between the input terminal +Vin and C/S(-) of the current-limiting amplifier is limited by the closed-loop control of the current limiting amplifier driver, the external power supply regulating tube and the current detecting resistor Rs. 130mV, that is, the current limiting regulator input current (approximate output current) is:

The current limit comparator (CSCOMP) is set to a 100mV bias voltage that shares the input with the current-limit amplifier. Since the bias voltage of the current-limit comparator is 30mV low, the current-limit comparator is guaranteed to operate before the current-limiting amplifier to ensure the implementation of the switching over-current protection function. Once the voltage difference at the input of the current-limit comparator is greater than 100mV, when the overcurrent occurs, the current-limit comparator output changes from high to low. The timer starts to work and the regulated power supply enters the overcurrent protection state. When the overcurrent is released, the voltage difference at the input of the current-limit comparator is less than 100mV, and the current-limit comparator outputs a high potential to block the timer. At this time, the regulated power supply operates in a normal regulated state.

When the timer controlled by the limited current comparator operates, its internal constant current source charges the TRC terminal capacitor. When the charging voltage of the timing capacitor CT is lower than 1.8V, the timer output is low, and the regulated power supply operates in the current limiting state; when the charging voltage of the CT reaches 1.8V, the constant current source inside the timer is turned off, and the CT passes. The resistor RT is discharged in parallel with it, the timer output high potential drive T1 is turned on, the output stage of the circuit is cut off, the power supply regulating tube is turned off, the regulated power supply is in the off state; when the discharge voltage reaches 0.9V, the timer is internally The constant current source recharges the CT and outputs a low potential to turn off T1, and the regulated power supply re-operates in the current limiting operation state. If the load continues to overcurrent or short circuit, the current limiting state of the regulated power supply will cycle cyclically. When the load overcurrent is released, the regulated power supply will return from the current limiting state to the normal state. The UC3832 differs from the UC3833 in that the bias voltage of the current limiting amplifier of the UC3832 can be externally externally from 130 to 300 mV, and the starting capability is strong. The UC3833 is suitable for driving PNP transistors and P-channel FETs, and the UC3832 is suitable for driving NPN transistors and N-channel FETs.

High current low input/output differential voltage implementation

Minimizing the minimum input/output differential voltage of the power supply regulator is important for improving power supply efficiency and minimizing the minimum input voltage. Low dropout voltage (usually defined as: Vinmin-Vout ≤ 1V) means improved performance of the regulated power supply. . In UC3832/3, the minimum input/output voltage difference is the saturation voltage drop Vsat of the power supply regulator, or the product of the on-resistance Rds(on) and the current Io flowing. Bipolar transistors can be less than 1V (but higher than 0.5V), while power FETs can easily achieve very low input/output voltage differences below 0.3V, such as 30~40V input voltage, 10A output current can be used with IRF3205 ( 55V, 0.08W).

Practical application of UC3832/3

The parameter selection of the main components (power supply regulating tube, output capacitor, timing capacitor, timing resistor) in Figs. 2 and 3 will be given below.

Power adjustment tube rating

The rated voltage of the power regulating tube is the highest input voltage, the rated current should not be higher than the rated current or current of the collector/drain, and the collector/drain dissipation power should be greater than the product of the highest input voltage and the output current limit Iolim. .

Output capacitor selection

If the capacitance of the output capacitor is too small, the output ripple may be too large, and if the capacitance is too large, the maximum load of the constant current will not start. Therefore, the maximum capacity of the output capacitor Cmax is:

Timing capacitor selection

The timing capacitor Ct determines the time of the start-up and current-limit operation states, and the relationship between Ct and the on-time Ton is:

The Ton and Ct units are s and F, respectively.

The on-time Ton is determined by the output voltage and the duration of the impact load. The on-time determined by the output voltage is:

Timing resistor selection

The timing resistor determines the duration of the cut-off protection in overcurrent/short circuit protection.

The loss of the power adjustment tube in the short circuit condition is:

Conclusion

The UC3832/3 is easy to implement combined overcurrent protection for linear regulated power supplies. It is superior to any existing overcurrent circuit in performance and can achieve very low input/output differential voltage regulation for large currents.