A design of low ripple switching power supply

introduction

In order to meet the requirements of ignition of explosive bridge wires with different resistance values ​​in a narrow space, a switching power supply with small ripple, adjustable output, small size and high reliability is required for a test ignition device for a certain project. The basic requirements are a switching power supply with an input of AC220V, a continuously adjustable output of DC0~100V, a maximum output current of 100A, a low-frequency ripple Vrms≤O.1%, a voltage regulation rate≤0.5%, and a stability≤0.1%. The design concept mainly complies with the requirements of reliability and volume. After a comprehensive analysis of the existing soft and hard switching converter power supply technology, a pulse width modulation converter design is adopted, which has the advantages of being simple and reliable. By optimizing the power supply link, feedback control, absorption circuit, component selection and production, and process structure of the power supply, the problems of large output ripple under high current, poor power supply stability under large output range adjustment, pipe explosion and oscillation are solved, and a qualified power supply is developed.

l Main power circuit

The main circuit of the power supply is shown in Figure 1. The power supply consists of an input circuit, a converter, a DC output, and a control drive. The input circuit contains a delay circuit to suppress the closing surge, an EMI filter, a single-phase rectifier, and a filter capacitor bank. The converter adopts a pulse width modulation "H" bridge topology. The high-frequency transformer, bridge rectifier, inductor, and capacitor bank constitute the DC voltage output circuit. The control circuit uses PWM to adjust the output voltage and current scheme.

1) Input circuit

The input circuit is composed of input delayed start, EMI filter, bridge rectifier circuit B1 and filter capacitor group C1. The delayed start uses a relay to reduce the current impact on the filter capacitor group C1 when the power is turned on. The capacitor is the DCMCE inverter dedicated input filter capacitor of CDE. The rectifier bridge is the MDQ75 single-phase rectifier bridge module of TECHSEM. Due to the short working time of the power supply each time and the size limitation, the power factor correction circuit is not added to the input circuit.

2) Converter and drive circuit

The converter adopts the pulse width modulation "H" bridge topology, which consists of four IGBTs (VT1-VT4). When working, its output changes the pulse width in 1-60μs. The IGBT uses Mitsubishi's CM75BU-12H single-phase full-bridge module. The advantage is that the switching characteristics of each IGBT are consistent, and there is no need to consider the balance of the circuit. For the drive circuit, there are many integrated drive circuits at present. Since it directly affects the reliable operation and reliable protection of the IGBT, after comparison, POWERXE's two-channel IGBT drive module BG2A based on VLA502-02 was selected. Figure 2 is the schematic diagram of VLA502-02. Another similarity with the commonly used hybrid drive module is that it integrates a DC/DC power supply, and there is no need to power each drive circuit separately. Actual use shows that the reliability of BG2A is much higher than that of drive circuits such as EX840, and it has never been damaged.

3) Transformer and output rectifier filter circuit

The transformer has a large power, and it is difficult to make a ferrite core, so iron-based nanocrystalline strips are used. Compared with ferrite materials, iron-based nanocrystalline strips have high saturation magnetic induction, high magnetic permeability and low coercivity, which can reduce the size of the transformer, increase the frequency response and improve the efficiency. At the same time, it is easy to process and install, and easy to adjust parameters. Since IGBT is a hard switch, when the operating frequency is higher than 15kHz, the switching loss becomes the main loss of the power supply. Therefore, the operating frequency of the transformer and converter is designed to be 15kHz. The output rectifier diode uses Motorola MUR20020 fast recovery diode.

The output filter circuit uses an LC filter. Due to the large inductance and power of the inductor, there are no suitable product specifications for iron powder cores and sendust cores, so the inductor core uses an iron-based amorphous material. In order to reduce the output ripple, the capacitor uses a 101-type electrolytic capacitor from the American CED company. Its ESR (equivalent series resistance) and ESL (equivalent series inductance) are the lowest among all brands of electrolytic capacitors, and the temperature range is very wide.

4) Control and safety protection

The power supply is a DC regulated power supply with an output voltage between 0 and 100V and can be adjusted continuously. The overload protection is designed as a high-precision current limiting protection form, and its current limiting value can be adjusted continuously between 0 and 100A. For reliability, the feedback control adopts traditional analog control, that is, an error amplifier is used to reduce the error between the output voltage and the reference voltage. The control chip uses TL494, which is designed as a constant voltage and constant current dual closed-loop control system. The two closed loops share a pulse width modulation real-time processing to achieve constant voltage regulation and constant current regulation functions. The two error amplifiers of TL494, one for voltage stability control and the other for current limitation. The current sampling resistor uses a 100A/75mV standard shunt. The amplifier used for voltage feedback and current feedback of TL494 is designed to have an "or" relationship. The output voltage is adjusted by changing the voltage divider ratio of the voltage feedback sampling resistor, and the current limit adjustment method is the same.

2 Output voltage ripple

The key to suppressing low-frequency AC ripple and switching noise ripple in the power supply output voltage is to find the cause and then determine the solution. Usually, the solution is from two aspects: device and circuit.

1) Suppression of spike noise

Spike noise is generated at the moment of transistor ON/OFF. To suppress spike noise, capacitors with low ESR and ESL and diodes with soft recovery characteristics are selected, and the spikes during diode switching are suppressed by C4~C7 and R4~R7. In addition, amorphous beads are inserted into the diode, which has a much better spike suppression effect than the RC circuit, but it generates too much heat and is not adopted. For the spikes caused by the IGBT bus inductance, the usual LCR absorption circuit is relatively complex and the parameters are not easy to coordinate. Since the converter uses an H-bridge module and the bus is very short, only C2 is used to complete the spike absorption. R2 and R3 absorb the spikes caused by the transformer leakage inductance. The peak level generated by the switching waveform is only slightly higher than the DC level, which greatly reduces the spike noise on the IGBT. C2 and C3 use CDE's 930 type polypropylene film non-inductive capacitor.

2) Output ripple suppression

The low-frequency AC ripple in the switching power supply is generally said to be introduced from the AC power grid. The output filter cannot filter it out, and it is mainly suppressed by the system closed-loop negative feedback, but this can easily cause circuit oscillation and damage the IGBT. In fact, when the power supply capacity is sufficient and the input filtering is appropriate, the ripple brought to the DC output by the AC input cannot be measured. Due to the rich harmonics in the switching power supply circuit, there are many reasons for the low-frequency ripple: process structure, such as improper wiring; device reasons, such as the ESR of the capacitor, the inductance parameters (straightening, air gap, and the selection of the magnetic core); the coordination between components, such as the product value of L and C, etc. In particular, the inductor affects the stability and output ripple of the power supply. Among them, the anti-saturation air gap is an important reason for the low-frequency ripple caused by the inductor at the output, which is often difficult to judge. The frequency of this ripple varies with the size of the air gap, ranging from several hundred Hz. The amplitude has a certain relationship with the value of the output voltage and is not proportional, as shown in Figure 3. However, the inductor core has no air gap and is easy to saturate. The non-closed magnetic core makes the volume very large. Relatively speaking, the problem is solved by designing a magnetic core with enlarged power capacity and no air gap.

3 Conclusion

After the formal test of the power supply by the relevant measurement units, whether it is no-load or loaded, when the output voltage changes from 1 to 100V, the corresponding ripple is between 0.6 and 50mV, and the other work and indicators of the power supply also meet the requirements. The power supply has successfully passed the assessment and verification of the engineering control system self-operation, ignition and detonation test, measurement and control system joint test, engine ignition hot test and other work at the project site, and has been officially put into use. This shows that the high-power wide-width converter type voltage-stabilized power supply can well achieve a wide range of continuous adjustment and ultra-low ripple requirements, broadening the use of high-power ordinary hard-switching power supplies.