ECRI Microelectronics

Design of Output Short Protection Circuit of High Power and Wide Input military DC DC converter

Design of Output Short Protection Circuit of High Power and Wide Input military DC DC converter

Abstract: This article firstly analyzes the operating principle of the traditional output short protection circuit and give its deficiency occurred in the high power and wide input  military DC DC converter application. Then a new output short protection circuit is proposed. It is more suitable for high power and wide range input military DC DC converter. And the operating principle and key part of thecircuit are analyzed in detail. Finally, the simulation analysis of comparing between the traditional circuit and proposed circuit arecarried out by utilizing of saber simulation software
Key Words: short protection, current transformer, threhold compensation, wide range voltage input

1 Introduction
The input current of the high-power and wide-range input military DC DC converter is usually very large when it is working, if the short-circuit fault occurs suddenly in the working process, and the power supply does not have the short-circuit protection function, the power device in the power supply may be burned out due to instantaneous over-current, thus leading to the damage of the whole power supply.However, the traditional output short-circuit protection circuit is not suitable for high-power and wide-range input military DC DC converter because of some shortcomings. This paper presents an improved output protection circuit for high-power and wide-range input military DC DC converter.
Traditional output short circuit protection circuit
The operation principle of the circuit is shown in Figure 1. The input current of the circuit is sampled with Rs, and the sampled AC voltage is rectified and detected by D1, R2, C2, and then connected to the positive input terminal of the comparator. The comparator is supplied with a reference voltage (VREF terminal) in a PWM control chip, and the reference voltage is connected to the negative input terminal of the comparator after being divided by resistors R3 and R4, as a threshold voltage for short-circuit protection.The forward input voltage of the comparator is V + and the reverse input voltage is V-. When a short circuit fault occurs, V + > V-, the comparator output is high, the transistor VI is on, the COMP end of the PWM chip is pulled down, the output pulse of the PWM chip is turned off, and the power supply enters the protection.When the short circuit fault is eliminated, V + < V-, the comparator output is low, triode VI 1 is turned off, and the circuit returns to normal operation. Through the analysis of the circuit, if it is applied to high-power and wide-input military DC DC converter, there are mainly the following shortcomings:
1) When the output power of the power supply is large, the input current will be relatively large, and the loss on Rs will be large, which will reduce the efficiency of the power supply.
2) when the input voltage range of the power supply is relatively wide, the rising slope of the step wave of the primary peak current detected by Rs varies greatly with the input voltage, and when a short circuit fault occurs, since the rising slope of the primary peak current at the input high end is much larger than the rising slope of the input low end primary peak current, a short circuit protection threshold voltage (point B voltage)Under the same circumstances, the value of short-circuit protection current will vary greatly, if the threshold voltage of short-circuit protection is not set properly, it will even appear that the input voltage can not be protected at the low end, and the input high end can not be restored after entering the protection, which will greatly increase the difficulty of t he debugging of short-circuit protection circuit.


Figure 1. Conventional Output Short Circuit Protection Circuit


Improved output short circuit protection circuit
In view of the traditional output short circuit protection circuit, there are two shortcomings in the application of high-power wide-range input military DC DC converter, this paper puts forward an improved output protection circuit can solve these two problems, the circuit schematic diagram is shown in Figure 2, its greatest characteristics are:
1) The current transformer is used to sample the input peak current, which can not only reduce the power loss caused by Rs, but also realize electrical isolation and reduce interference.
2) Compensation measures are adopted for the threshold voltage of the short-circuit protection, that is, a compensation resistor R1 is connected between the input voltage and the reverse input end (point B) of the comparator, so that when the input voltage changes, the threshold voltage of the short-circuit protection will be adjusted correspondingly, and when the input voltage of the product is in the high and low end, the value of the short-circuit protection current does not differ greatly, thus making the debugging of the short-circuit protection function easier.



Figure 2 Improved Output Short Circuit Protection Circuit

The circuit uses a current transformer to sample the input current of the circuit, wherein R2 resets the current transformer, the current passes through D1 and R3 to obtain a pulsating DC voltage, the voltage passes through a peak detection circuit composed of D2, R5, R6 and C2, and is connected to an input positive terminal of a comparator, and the reference voltage of the PWM chip passes through the voltage divisions of resistors R1, R7 and R8 to obtain a voltage VT1.After the voltage dividing of R1, R7 and R8, VIN obtains a compensation voltage VT2, T1 and VT2 are superposed and connected to the reverse input end of the comparator as the threshold voltage of short-circuit protection, the positive terminal voltage of the comparator is V +, the negative terminal voltage of the comparator is V-, when the power supply has a short-circuit fault, V + > V-, the output level of the comparator is high, the transistor VI 1 is on, the COMP pin of the PWM chip is pulled down, and then the output pulse is turned off.When the short circuit fault disappears, V + < V-, the comparator output is high, and the circuit works normally.The design of current transformer and short circuit protection threshold compensation circuit are the two key parts of the circuit, the following will focus on the analysis of these two parts of the circuit.
3.1 Analysis and Design of Current Transformer
The current transformer is divided into the alternating current transformer and the pulse direct current transformer, both of them use the principle of electromagnetic isolation of transformer to realize the detection of the measured current. The difference between them is that in the alternating current transformer, the alternating signal will make the magnetic core of the transformer magnetized symmetrically, while in the direct current pulse transformer, the magnetic core is unidirectional magnetized, which belongs to the working mode of forward converter. Usually, in the application of switching power supply, the pulse direct current transformer is used. When designing the pulse direct current mutual induc, it needs to meet the following requirements:
(1) The materials with small residual magnetic and high permeability should be selected to make the transformer, and the annular magnetic core is usually used to make the magnetic core of the transformer work on the local magnetization curve.
2) The core reset circuit must be designed for the mutual inductor, because in the DC pulse mutual inductor, the magnetic core is unidirectional magnetized, which can easily cause the saturation of the magnetic core. Usually, the simplest core reset method is to connect a large resistance to the ground in front of the secondary diode of the mutual inductor, such as the resistance R2 in Figure 2.
(3) In order to reduce the loss, the current transformer with high turn ratio should be chosen, but the turn ratio should not be too large, because if the number of turns on the secondary side is too large, its distributed capacitance will also become large, and when the core is reset, the resonance period between the secondary side inductance and the distributed capacitance will become longer, so when the duty is relatively large, the secondary side coil will not have enough time to release energy to reset the core, therefore, the turn ratio of the primary and secondary sides of the current transformer usually takes 1:30 to 1:50.
Analysis and Design of 3.2 Short Circuit Protection Threshold Compensation Circuit
The short-circuit protection threshold circuit of this circuit is realized by connecting a compensation resistor R1 between the input voltage and the reverse input of the comparator, and the value of RI is determined according to the actual circuit requirements.
As shown in FIG. 2, if the voltage of point B, I. E., the threshold voltage of short-circuit protection, is VB, then according to the superposition theorem in the basis of circuit analysis, it can be obtained.


The VB value which is not difficult to find from the formula (1) is composed of two parts. If the value of the first part is VB1 and the value of the second part is VB2, the following equation can be obtained.

From the formula (3) (4), it can be seen that when R1, R7, R8 is given, the value of VB1 changes in direct proportion to the change of input voltage VIN, and the value of VB2 is a fixed value.So in general VB and VIN values are positively correlated changes, low input voltage, short-circuit protection threshold voltage will be reduced, high input voltage, short-circuit protection threshold will be increased.According to the actual engineering experience and the previous analysis, when the power supply uses the traditional short-circuit protection circuit, the magnitude of short-circuit protection current and the value of input voltage VIN are negatively correlated, the lower the input voltage, the greater the short-circuit protection current, the higher the input voltage, the smaller the short-circuit protection current, and the wider the input voltage range, the greater the variation of short-circuit protection current between high and low input voltage.In this circuit, because of adopting the compensation measure to the threshold value of the short circuit protection, the threshold voltage value of the short circuit protection at the input low end of the power supply is lower than that at the input high end. Compared with the traditional protection circuit, it can make the short circuit protection current at the input low end of the power supply increase appropriately, and finally reach the goal that the value of the short circuit protection current does not differ too much when the power supply is input in a wide voltage range.
Results of simulation experiment
The traditional short-circuit protection circuit and the short-circuit protection circuit proposed in this paper are applied to a 65W single-ended flyback converter respectively, and the experimental results show that the design specifications of the flyback converter are as follows:

(1) Comparison of power efficiency results
Table 2 Comparison table of power supply efficiency between traditional protection circuit and improved protection circuit

As can be seen from Table 2, the efficiency of the power supply with the improved protection circuit is obviously higher than that with the traditional protection circuit.
(2) Comparative analysis of short circuit protection current of power supply
1) The situation when the traditional protection circuit is used


Figure 3 Threshold Voltage Using a Conventional Short-Circuit Protection Circuit

Figure 4 Waveform of Supply Input Current for Conventional Output Short Circuit Protection
Figure 3 is the threshold voltage of the traditional short-circuit protection circuit, from the figure can be seen, when the input voltage changes, its value is fixed, about 0.42V, Figure 4 is the use of traditional output short-circuit protection circuit, VIN at 15V, 28V, 50V, power short-circuit, input current waveform, from the figure can be seen, when VIN = 15V, the average input current of the power supply is about 1.69A, when VIN = 28V, the average input current of the power supply is about 0.81 A; when VIN = 50V, the average input current of the power supply is about 5; Conclusion: 0.41A. It can be seen that in the power supply using traditional short-circuit protection circuit, when the input range of the power supply is wide, the variation of the input short-circuit protection current of the power supply with the input voltage is very large.
2) The short circuit protection circuit improved in this paper is adopted.

Figure 5 Change in threshold voltage with improved output short circuit protection

Fig. 5 is a case where the threshold voltage of the improved short-circuit protection circuit changes with the input voltage, and it can be seen from the figure that the threshold voltage of the short-circuit protection circuit changes with the change of the input voltage and increases with the increase of the input voltage, and FIG. 6 is a waveform of the input current when the input voltages are 15V, 28V, 50V and the power supply is short-circuit, and it can be seen from the figure that when the power supply is short-circuit protected, when V = 15V,The average input current during a short circuit to the power supply is about 1.26A, when V = 28V, the average input current during the short circuit of the power supply is about 0.80A, when VN = 50V, the average input current during the short circuit of the power supply is about 0.75A, so that when the input range of the power supply is wide, the variation of the short circuit protection current of the power supply with the input voltage is obviously improved compared with that of the power supply using the traditional short circuit protection circuit.



Figure 6 Waveform of Supply Input Current Using an Improved Output Short Circuit Protection Circuit
5 Conclusion
In view of the limitation of the traditional output protection circuit when it is used in the high-power and wide-range input military DC DC converter, this paper presents an improved short-circuit protection circuit, which not only reduces the loss, but also reduces the variation amplitude of the short-circuit protection current with the input voltage, and greatly reduces the difficulty of debugging the short-circuit protection.















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