Design secrets for DC/DC switching power supplies that can make the solution one-stop

introduction

With the rapid development of electronic technology, modern electronic measuring devices often require negative power supplies to power their internal integrated circuit chips and sensors, such as integrated operational amplifiers, voltage comparators, Hall sensors, etc.

The quality of the negative power supply greatly affects the performance of the electronic measuring device. In serious cases, the measured data will deviate greatly from expectations. At present, the negative power supply of electronic measuring devices usually adopts a switching power supply with strong anti-interference ability and high efficiency. The previous isolated switching power supply technology achieves negative voltage output through a transformer, but this will increase the volume of the negative power supply and the complexity of the circuit. With the emergence of more and more dedicated integrated DC/DC control chips, non-isolated negative voltage switching power supplies with simple circuits and small volumes have been increasingly widely used in electronic measuring devices. Therefore, the research on non-isolated negative voltage switching power supplies has high practical value.

There are two circuit topologies of traditional non-isolated negative voltage switching power supplies, as shown in Figures 1 and 2. Figure 3 is the charging current waveform of its filter output capacitor. As can be seen from Figure 3, the negative voltage power supply with a smaller output ripple can be obtained by using the structure of Figure 2, and its load capacity is stronger under the same inductor peak current. Since the switch device of Figure 2 is connected to the negative pole of the power supply, this will make its control circuit more complicated than Figure 1, so there is no negative voltage switching power supply control chip that implements the circuit structure of Figure 2 (similar to the function of the linear voltage regulator chip 7915) in the market.

In order to make up for the shortcomings of the existing non-isolated negative voltage switching power supply technology and obtain a non-isolated negative voltage switching power supply with strong load capacity and small output ripple, this paper proposes a new non-isolated negative voltage DC/DC switching power supply based on peak current control, which adopts the Boost switching power supply control chip LT1935 and discrete components to realize the principle shown in Figure 2.

Figure 1 Conventional non-isolated negative voltage switching power supply circuit structure 1

Figure 2 Conventional non-isolated negative voltage switching power supply circuit structure 2

Figure 3 Charging current waveforms of filter capacitors of two switching power supplies

Working principle analysis

The non-isolated negative voltage DC/DC switching power supply designed in this paper is shown in Figure 4. The negative power supply works in continuous current mode. When the power transistor inside the power controller LT1935 is turned on, the DC power supply charges the output inductor L1 and the output capacitor C1. When the power transistor inside the power controller LT1935 is turned off, the current in the output inductor L1 continues to charge the output capacitor C1 through the low impedance loop provided by the Schottky diode VD1 until the power transistor inside the power controller LT1935 is turned on again in the next cycle. It can be seen that the capacitor C1 is charged in the process of storing and releasing energy in the output inductor L1, thereby reducing the output ripple voltage. At the same time, under CCM conditions, the output current passes through the output inductor L1 during the on and off periods of the power transistor inside the LT1935, which greatly suppresses the fluctuation of the output current and reduces the impact of the output ripple current, thereby greatly increasing the load capacity and efficiency of the system.

The feedback control loop uses peak current control. Compared with traditional voltage control, peak current control can improve the dynamic response of the power supply on the one hand, and can also achieve fast over-current protection on the other hand, which greatly improves the reliability of the system. Due to the use of the power controller LT1935, which integrates the peak circuit control circuit and the slope compensation circuit, the feedback loop design of the non-isolated negative voltage DC/DC switching power supply is converted into a compensation network design, which greatly simplifies the design of the feedback loop.

In order to prevent the excessively high DC power supply from harming the power controller, voltage regulator tubes VD2 and VD3 are used here to achieve overvoltage protection.

Figure 4 Non-isolated negative voltage DC/DC switching power supply hardware circuit diagram

Compensation Network

1. Small signal modeling of non-isolated negative voltage switching power supply

In essence, the non-isolated negative voltage DC/DC switching power supply introduced in this article is a non-isolated negative voltage Buck switching power supply, and its equivalent power stage circuit schematic is shown in Figure 5, where the influence of the equivalent series resistance Resr of the output filter capacitor on the system is considered.

Figure 5 Non-isolated negative voltage Buck switching power supply equivalent power stage circuit schematic

Figure 6 shows the CCM large signal model of the non-isolated negative voltage Buck switching power supply established using the average circuit method in Figure 5. Let Vi be the steady-state value of the input voltage, Vo be the steady-state value of the output voltage, Vpc be the steady-state value of the voltage across the controlled voltage source, Ii be the steady-state value of the input current, IL be the steady-state value of the output inductor current, and D be the steady-state value of the duty cycle.

Figure 6 Non-isolated negative voltage Buck switching power supply CCM large signal model

Introduce a small signal disturbance corresponding to the above steady-state value.

make:

It can be deduced that:

If the small signal interference satisfies D, ignore the quadratic term and simplify equations (3) and (4), the linear expression of is:

According to equations (5) and (6), the CCM small signal model of the non-isolated negative voltage Buck switching power supply represented by an ideal transformer as shown in FIG7 can be obtained.

Figure 7 Non-isolated negative voltage Buck switching power supply CCM small signal model 2. Compensation network design

Figure 8 is a system block diagram of a non-isolated negative voltage Buck switching power supply with peak current control in continuous current mode (CCMCPM). The control loop consists of two parts: the inner current loop and the outer voltage loop. The compensation network belongs to the outer voltage loop, so the design of the compensation network requires the establishment of a small signal model that includes the inner current control loop.

Figure 8 CCM-CPM type non-isolated negative voltage Buck switching power supply system block diagram

Assuming that the system is stable and ignoring the influence of the output inductor ripple voltage and artificial slope compensation, the output inductor current is equal to the control current, that is:

According to the CCM small signal model of the non-isolated negative voltage Buck switching power supply shown in FIG7, and by simplifying equation (7), the dynamic equation of the CCM-CPM type non-isolated negative voltage Buck switching power supply is:

Using equations (8) and (9), the small signal model of the CCM-CPM type non-isolated negative voltage Buck switching power supply shown in Figure 9 can be easily established.

Figure 9 CCM-PWM type non-isolated negative voltage Buck switching power supply small signal model

Considering that the control current and control voltage satisfy:

Where Rs is the current sampling resistor; k is the sampling current amplification factor. Substituting equation (10) into equation (9), the transfer function Ap (s) between the control voltage and the output voltage is obtained as follows:

From the analysis, it can be seen that the control object Ap (s) is a single-pole control object, and is affected by the equivalent series resistance. Its high-frequency characteristics are poor and its ability to suppress high-frequency noise is weak.

Therefore, according to the CCM-CPM type voltage outer loop system block diagram shown in FIG10 , the designed compensation network should not only improve the steady-state characteristics and response speed of the system, but also enhance the anti-interference ability of the system.

Figure 10 CCM-CPM voltage outer loop system block diagram

FIG. 11 is a hardware circuit diagram of an actual non-isolated negative voltage DC/DC conversion circuit compensation network.

Figure 11 Compensation network hardware circuit diagram

The static gain of the compensation network is proportional to the static operating voltage Vf of the feedback pin of the power controller relative to its reference ground. The static operating voltage Vf satisfies the following relationship:

Note that the value of Vf should be in a moderate range. If the value is too large, the signal-to-noise ratio of the system will be reduced. If the value is too small, the sensitivity and steady-state characteristics of the system will decrease.

The dynamic characteristics of the compensation network are compensated by capacitors C2, C3, and C4. Capacitor C2 introduces advance correction, which effectively improves the dynamic stability of the system. Capacitor C3 increases the bandwidth of the system. Capacitor C4 plays the role of bypassing high-frequency noise. Therefore, by reasonably selecting the capacitance values ​​of C2, C3, and C4, the system can obtain a more satisfactory dynamic compensation effect.

Experimental studies

The circuit shown in Figure 4 is experimentally studied. The main parameters of the experimental circuit are: input voltage Vi = -24V, output voltage Vo = -15V, output inductor L1 = 33μH, output capacitor C1 = 10μF, and diode VD1 is a Schottky diode 1N5819.

From the output voltage waveform shown in FIG12, it can be seen that the stable output of negative voltage can be easily achieved by using the non-isolated negative voltage DC/DC switching power supply shown in FIG4. In addition, the static gain of the feedback loop is very large, so that the output negative voltage has a good steady-state characteristic.

Figure 12 Schematic diagram of the collector voltage and output voltage waveform of the internal power transistor of LT1935 when fully loaded

Figure 12 shows the waveform of the collector output voltage of the power transistor inside the power controller LT1935 under full load. It can be seen that the system will not generate too high peak current under full load, and the fluctuation of the inductor current is small, that is, the output ripple current is well suppressed, which is conducive to the high efficiency and load capacity of the non-isolated negative voltage DC/DC switching power supply. At the same time, the switching frequency of the system is very high, and the bandwidth of the feedback loop is guaranteed.

FIG13 shows the waveform of the output ripple voltage under full load. Obviously, the output ripple voltage has a small fluctuation and no pulsation, and the output ripple voltage is effectively suppressed.

Figure 13 Schematic diagram of output voltage and output ripple voltage waveform at full load

in conclusion

A new non-isolated negative voltage DC/DC switching power supply design based on peak current control is proposed. In the continuous current mode, the output capacitor is guaranteed to be continuously charged through the output inductor, so that the output ripple is effectively suppressed, thereby achieving the purpose of improving the system's load capacity and efficiency. At the same time, the small signal model of the switching power supply under CCM conditions is constructed by combining the average circuit method, and the compensation network of the voltage outer loop is designed to enhance the overall performance of the system. Experimental tests show that this scheme is simple, reasonable, feasible, and has certain engineering practical significance.