Research on 28V DC input overvoltage surge circuit suppression method

1. Introduction

The 28V DC power supply is the earliest power supply used in aircraft. Its rated voltage is 28V, and the steady-state variation range is 18 to 36V. In the aviation 28V DC power supply, the electrical load is required to withstand an overvoltage surge of 80V/50ms and an undervoltage surge of 8V/50ms. Voltage surges often occur in large generator switches, engine startups, transient loads, etc. For example, sudden unloading or sudden load addition will cause the generator bus voltage to rise or fall for a short time, thereby generating an overvoltage surge or undervoltage surge. These surge voltages usually appear at the distribution bus, and the surges referred to in this article are all overvoltage surges. The surge voltage greatly exceeds the steady-state power supply voltage. When it strikes the electrical equipment, it often causes misoperation and equipment damage, and may cause the entire system to pause and communication to terminate.

In view of the harmfulness of the surge mentioned above, in order to protect these electrical equipment from damage caused by surge voltage impact, it is necessary to consider the impact of surge in the design of DC power electronic equipment, increase protection measures, design effective surge protection circuits, and perform surge protection on the power circuits of electronic equipment. Since the volt-second product of the 80V/50ms overvoltage surge is very large, it cannot be simply suppressed by traditional energy storage methods, otherwise the inductor and capacitor components will be too large.

2. Principle and design

This article summarizes some methods of peak surge suppression, which are described as follows:

2.1 Passive Surge Suppressor

The most basic method of using a surge suppressor is to directly connect the voltage clamping device in parallel with the protected electrical equipment in order to transfer energy when the voltage exceeds the predetermined voltage value of the protected equipment. Among them, the voltage clamping devices mainly include varistors and transient voltage suppressors. Under normal circumstances, the fluctuation range of the power supply voltage is lower than the action voltage of the clamping device. In this case, there is also a voltage drop on Zs, which increases the loss.

The clamping device has no response, which is equivalent to an open circuit; when a surge occurs in the power supply, once the surge voltage is higher than the operating voltage of the clamping device, the clamping device quickly turns on, limiting the power supply voltage to a safe range, thereby protecting the electrical equipment.

2.1.1 Zinc Oxide (ZnO) Varistor

ZnO varistor is a polycrystalline semiconductor ceramic component made of ZnO as the main body, with various metal oxides added, and made by typical electronic ceramic process. The volt-ampere characteristic of the varistor is shown in Figure 1, which is basically the same as the performance of two back-to-back Zener diodes with the same characteristics. In the circuit, the varistor is usually connected in parallel to the input end of the protected electrical appliance, as shown in Figure 2. In the figure, the role of Zs is to limit the current during overvoltage, and the Zv of the varistor and the total impedance of the circuit form a voltage divider, so the limiting voltage of the varistor is

When the voltage across the varistor is within the nominal voltage, its resistance is almost infinite, in a high-resistance state, and the leakage current is much less than 50μA; when the voltage across it exceeds the rated voltage, its resistance drops sharply, the varistor is turned on, the working current increases by several orders of magnitude, and the reaction time is in milliseconds. It can be seen that when a large current flows through Zv in an instant, Zs will bear most of the surge voltage, making the voltage across the electrical appliance relatively stable, so it can play a protective role [2], but under normal input conditions, there is also a voltage drop on Zs, which increases the loss.

2.1.2 Transient Voltage Suppressor

Another type of surge suppression component is the transient voltage suppressor (TVS). When the two poles of the TVS are subjected to a reverse transient high-energy impact, it can change the high impedance between the two poles to a low impedance at a speed of 10-12s, quickly absorbing surge power of up to several kilowatts, clamping the voltage between the two poles to a predetermined value, and effectively protecting electronic circuit components from damage by various forms of surge pulses [2]. Transient voltage suppressors have the characteristics of small size, easy installation, fast response time, large transient power, low leakage current, small breakdown voltage bias, and easy control of clamping voltage. TVS diodes are available in the following different powers: (ProTek)

The forward surge current allowed by TVS diodes can reach 50-200A at 250°C, but the longest suppression time can only reach 10ms, so TVS cannot effectively suppress the continuous surge voltage of 80V/50ms. In addition, since the voltage clamping device absorbs the energy of the surge voltage, it often withstands high-power surge shocks, which will accelerate the aging of the device. After working for a certain period of time, the performance and reliability will decrease, and the protection ability will be weakened, which may cause itself and the electrical equipment to be damaged by the surge shock.

2.2 Active surge suppression circuit

In order to prevent the surge suppression device from being damaged by a high-power surge for a long time, the parallel energy absorption method is not considered, but a power switch device is used to control the power input, as shown in Figure 3. Based on this principle, three active surge suppression circuits are proposed.

2.2.1 Buck type surge suppression circuit

Buck type surge suppression circuit consists of Buck main circuit and control circuit. As shown in Figure 3, R1 and R2 sample the input voltage to obtain voltage Vf, and R3 and R4 sample the output voltage to obtain voltage Vr. The specific control principle is as follows: the sampled voltage Vf is compared with the reference voltage V10 through comparator cmp1, and the output signal INH is output; the sampled voltage Vr and the triangular wave are passed through comparator cmp2, ​​and the output signal Vv is output; the signal INH and Vv are operated through the OR gate, and the output signal Dr is used to control the power tube in the main circuit to achieve the purpose of suppressing the surge voltage. Figures 4 and 5 are simulation waveforms. It can be seen from the simulation analysis that when the input voltage is 28V, the output voltage is 27.9V; when the input voltage exceeds 36V, as the input voltage increases, the duty cycle decreases, suppressing the increase of the output voltage. When the input voltage is 80V surge voltage, the output voltage can be suppressed to 40V.

The advantages of this circuit are: Q1 works in a switching state, with low loss; the disadvantage is that two main circuit components are added, which increases the size. Under normal voltage, the main load current flows through the two newly added components (Q1, L1), affecting the system efficiency under normal conditions. However, the inductor size can be reduced by increasing the switching frequency.

2.2.2 Dual transistor controlled surge suppression circuit

The dual-transistor controlled surge suppression circuit is shown in Figure 6. The power device Q1 uses a P-channel MOS tube. The function of the voltage regulator diode D1 is to protect the gate-source voltage of Q1 within a safe range to prevent breakdown.

The dual-transistor controlled surge suppression circuit is similar to a step-down switching voltage regulator circuit. The change of the output voltage is fed back to the front end in real time, and the power tube Q1 is controlled to be in a linear state or an open state to ensure that the output voltage is stable within a certain range, eliminating the impact of the surge voltage. When the input voltage is normal, the voltage divider value of R5 and R6 is less than the base conduction voltage of Q3, and Q3 is cut off; the base voltage of Q2 is equal to the emitter voltage, and Q2 is cut off; the voltage divider of R1 and R2 makes the gate-source voltage of Q1 greater than the conduction voltage. At this time, Q1 is turned on, and the power supply to the electrical equipment is normal through Q1. When the power supply surges, the surge voltage charges C1. When the voltage of C1 is higher than the surge protection voltage value, the voltage divider value of R5 and R6 is greater than the base conduction voltage of Q3, Q3 is in the linear amplification area, Q2 is turned on, Q1 is cut off, and the power supply is disconnected. At this time, C1 is relied on to maintain the power supply of the electrical equipment. When the voltage at the C1 terminal drops to a normal range, the voltage divider value of R5 and R6 is less than the base conduction voltage of Q3, Q3 is turned off, Q2 is also turned off, Q1 is turned on, and power supply is restored.

Figures 7 and 8 are simulation waveforms. From the simulation analysis, it can be seen that when the input voltage is 28V, the output voltage is 27.9V; when the input voltage exceeds 36V, as the input voltage increases, the voltage drop on the power tube increases, suppressing the increase in output voltage. When the input voltage is an 80V surge voltage, the output voltage can be suppressed at 40V.

Advantages and disadvantages analysis: The circuit structure is simple, but the on-resistance of the P-channel MOS tube is large, which affects the efficiency under normal conditions.

2.2.3 Charge Pump Driven Surge Suppression Circuit

The charge pump driven surge suppression circuit shown in Figure 9 is improved on the basis of the filter used in Vicor's product V24A28C400AL. When the input voltage is normal, the MOS tube is turned on and outputs normal voltage; when there is a surge in the input voltage, the feedback voltage control circuit controls the MOS tube drive to make it in a linear working state to suppress the surge voltage. The high-voltage, low-energy spikes are absorbed by the capacitor across the input end and the transient suppressor. The remaining circuits are used to handle high-energy surges.

The specific working principle is as follows:

The input voltage charges capacitor C3 through resistor R5, and the charging voltage is clamped by D2. If D2 is a 12V voltage regulator, the voltage of C3 is clamped at 12V. Since there is no discharge loop on the voltage of capacitor C3, the voltage on C3 can be stabilized at 12V. The 12V voltage is connected to the Vcc (pin 8) of the timer 555 and the V+ (pin 7) of the op amp LM10C as the power supply voltage for the two chips. At normal input voltage, the timer generates a high-frequency square wave. According to the charging pump principle [4], R3, C4, D1 and D3 perform peak detection and level shifting, which can change the voltage at the node connected by C4 and D1 into a square wave with a low level of 28V and a high level of 38V. The square wave charges capacitor C7 through D1, and D4 limits the maximum charging voltage. Since there is no discharge circuit after capacitor C7 is charged, the charging voltage does not exceed the maximum voltage of D4, so capacitor C7 can maintain the maximum charged voltage. The voltage of C7 supplies power to the gate of the MOS tube, and there is a voltage difference between the gate and the source (this voltage difference can be controlled by changing the driving voltage of the timer), which is higher than the turn-on voltage of the MOS tube. At this time, the MOS tube works in the saturation region, and the output voltage is the input voltage. Since the voltage divided by R13 and R14 is fed back to the 3rd pin of LM10C, and the voltage value of the 1st pin of LM10C is designed to be 2V, the voltage of the inverting input terminal of LM10C is 2V. If the feedback voltage is also 2V, the output voltage of the op amp LM10C is 0V, and the transistor Q2 is in the off state. After the voltage is built up, the voltage on C7 is stable, D1 and D3 are both in the off state, Ugs is greater than the turn-on voltage, and the MOS tube is always turned on. When a surge voltage appears in the input voltage, the voltage divider value of R13 and R14 is greater than 2V. After passing through the op amp LM10C, the output voltage is a certain value, which drives the transistor Q2 to turn on, making it in the amplification area. Since the MOS drive voltage Ugs is less than the turn-on voltage, the MOS tube is in the linear area, and the output voltage is the surge voltage minus the Uds at both ends of the MOS tube. The simulation results of the charge pump driven surge suppressor are shown in Figure 10. At 4ms, a surge voltage appears in the input voltage, and the surge suppressor stabilizes the output voltage at 40V. The simulation verifies that its surge suppression performance is good.

The advantages of this circuit are: compared with the Buck type surge suppression circuit, when working normally, the current only flows through one switch tube, and the loss is smaller; the disadvantages are that the circuit complexity increases. Comparing the performance of the three active surge suppression circuits, as shown in Table 2, the dual transistor control type surge suppression circuit has a simple principle and fewer devices, but uses a P-channel MOS tube. In high-power applications, the switch device has a large on-state loss, so it is suitable for low-power applications; Buck type surge suppression circuit and charge pump driven surge suppression circuit can be used for various power levels, but from a cost perspective, the charge pump driven surge suppression circuit has more advantages.

For low-power applications, dual-transistor controlled spike suppressors are better, and for higher-power applications, Vicor type spike suppressors are more suitable.

3. Experiment

Design example: input voltage 18 ~ 36V; peak voltage: 80V/50ms; output power: 0 ~ 40W; output voltage during peak voltage is controlled at 40V; startup impact current is not higher than 5A. There is no specific requirement for the values ​​of R1, R2, and C2, but it is necessary to ensure that the 555 timer can output a high-frequency square wave, so take R1 = 5.1kΩ, R2 = 2.2kΩ, C2 = 1nF, R3 = 68Ω, C4 = 10nF, and substitute the parameters into the formula

Then f=150kHz and the amplitude is 10V.

Due to the working principle of LM10C, the voltage value of pin 1 is 2V, so R11=2.7kΩ, R12=0.3kΩ, and substituting into the formula ( ) 1 12 11 12 2.5V = V × RR + R (3) to get V1=2V. The values ​​of R13 and R14 determine the amplitude of the output voltage when there is a spike voltage. If the voltage needs to be suppressed to 40V, R13=3.4kΩ, R14=64.6kΩ, so that the voltage divided by R13 is 2V when the output is 40V.

Figure 12 Input voltage and output voltage waveforms of charge pump driven surge suppression circuit when there is a surge input voltage. The experimental results show that under 28V input voltage, the output voltage can track the input voltage normally; when the input voltage surges, the surge suppressor can suppress the voltage within the range of 40V, thus achieving a good surge suppression function.

4. Conclusion

This paper studies the suppression of input voltage surges in 28V DC power supply systems. In view of the problems of passive surge voltage suppressors, such as easy aging, easy damage, and low clamping voltage accuracy, three active surge voltage suppression methods are studied, all of which are simulated and verified, and a charge pump driven surge suppression circuit is selected for experimental verification. The study shows that the charge pump driven surge suppression circuit has a good performance in suppressing surge voltage.