Design of a new medium frequency power supply scheme based on PWM power amplifier

With the development of power electronics technology and devices, especially the emergence of new devices such as power MOSFET, IGBT, MCT, IPM and monolithic integrated pulse width modulation power amplifier, voltage-type SPWM inverter has received extensive attention, development and application.

Traditional medium-frequency power supply generally includes two links: pre-stage voltage regulation and post-stage medium-frequency inverter, which basically adopts discrete components. Although there are no traditional rotating parts, the volume is still large, the efficiency is low, the structure is complex, the debugging is troublesome, and there are many unreliable factors, which directly affect the reliability of power supply and equipment. Various new shipborne electronic equipment have put forward more stringent requirements on the volume, weight and performance indicators of medium-frequency power supply, which can no longer be met by traditional medium-frequency power supply. The only solution is to seek more advanced frequency conversion technology.

Sine Pulse Width Modulation SPWM (Sine Pulse Width Modulation) technology uses the conduction and shutdown of power devices to convert DC voltage into a voltage pulse sequence with equal amplitude and width changing according to the sine law, and achieves the purpose of voltage and frequency conversion by controlling the width of the pulse and the period of the pulse sequence. The main advantages of power supplies using SPWM technology are: high efficiency, small size, low noise, low distortion, and fast response.


Working Principle

In the early days, SPWM intermediate frequency power supplies were mostly composed of discrete devices; the oscillators, comparators, dead zone generators, drivers, etc. in the system required very careful adjustment; and the reliability was not high; this solution is rarely used now; later, single-chip integrated SPWM controllers appeared; such as HEF4752V, etc.; oscillators, comparators, op amps, etc. were integrated into a single-chip IC; greatly simplified system design; system reliability was greatly improved; with the emergence of high-speed single-chip microcomputers and low-cost DSP controllers; digital intermediate frequency power supplies began to be widely used; at the same time, programmable digital SPWM generators such as SA838 also appeared; further simplified system design; improved system reliability. However, all these control schemes require at least three isolated power supplies when forming a complete power supply system; the size of the power supply is difficult to further reduce.

The American company APEX combines power electronics technology with power amplifier technology to design and produce pulse width modulation power amplifiers. This series of amplifiers integrates oscillators, PWM generators, dead zone generators, bootstrap drivers, IGBTs and freewheeling diodes, overcurrent/overheat protection and other functional components into one, making it a complete PWM power amplifier. Users only need to provide the required analog modulation signal as required. After the amplifier, the high-frequency and high-voltage SPWM signal sequence with the fundamental wave as the modulation signal will be output. After high-frequency filtering, the modulation signal with amplified amplitude and power can be obtained. If the amplifier and filter are regarded as a whole, its function is equivalent to that of a traditional linear power amplifier, but its efficiency is much higher.

Typical parameters of the representative model of PWM power amplifier SA08:
power input voltage 16 ~ 500VDC; single-phase full-bridge output; maximum output current 20A; maximum transmission power 10KW; internal oscillation and control circuit to achieve 22. 5kHz switching frequency; module efficiency up to 98%; with internal overcurrent and overheating protection; while providing users with external current limiting settings and shutdown functions; MO-127 package; operating temperature range - 55 ~ 125 ℃.

SA08 appearance and pin distribution diagram is shown in Figure 1; internal function diagram is shown in Figure 2.

Since the oscillation circuit, control circuit, drive circuit and power devices are all integrated into the module; the reliability of SA08 is greatly improved; and the interface is very simple. The prototype power supply is based on SA08; it consists of rectifier circuit, auxiliary power supply, intermediate frequency signal generator, feedback control circuit and input and output filter circuit. The schematic diagram is shown in Figure 3.

One path of single-phase 220V AC voltage is used as the power supply of SA08 after rectification and filtering by a rectifier bridge; the other path is used as the control circuit and SA08 auxiliary power supply after AC/DC conversion and common mode suppression; the signal generator generates a 400Hz frequency and adjustable amplitude sine signal; the signal generator achieves output frequency stability by optimizing circuit design and selecting low temperature drift components; the control circuit biases, amplifies, PIDs and filters the input command signal, and outputs limiting, etc.; finally, it is used as the input signal of SA08 to control its power output. Overvoltage protection and overcurrent protection are also realized through the control circuit; the high-frequency and high-voltage SPWM wave output by SA08 is supplied to the load after passing through the LC filter; the feedback circuit directly samples the voltage and current from the load to ensure the accuracy of control.

Solution of key technology

Narrow pulses contain rich spectrum, and their spectrum is roughly inversely proportional to their pulse width: the narrower the pulse, the richer its spectrum. For an SPWM pulse sequence with an amplitude of 280V and a frequency of 22.5KHz, it has very serious interference with its peripheral circuits. At the same time, the huge voltage change rate du/dT and current change rate di/dT generated by the high-speed switching of power devices will have an inestimable impact on the power supply and control circuit through stray capacitance and lead inductance. Therefore, how to achieve electromagnetic compatibility between strong and weak electricity in the system becomes a key technology.

In terms of implementation methods, electromagnetic compatibility mainly includes three methods: grounding, shielding and filtering. On the basis of theoretical analysis, through a large number of experiments, sensitive circuits, sensitive components and sensitive points in the system were found, and targeted measures such as filtering, strict grounding, reasonable layout and wiring were taken.

First, in the overall layout, according to the principle of separation of strong and weak electricity, the signal generator, control and protection circuits are separated from the power circuit. Secondly, local layout and wiring are carried out according to the signal flow and power flow to make it smooth and without overlap. Input and output are separated. At the same time, the wiring strictly follows the principle of minimum loop. Third, the heating device is arranged close to the casing, and the components with strict requirements on temperature drift are kept away from the heating device. Fourth, the power supply is fully decoupled. Since the high-frequency SPWM wave contains rich high-frequency harmonic components, the power circuit and the weak current circuit need strict high-frequency decoupling. Fifth, strict grounding. The power ground and the weak current ground are separated and wired; large ground loops are strictly eliminated; at the same time, the continuity of the power ground impedance is ensured.

Test results

A medium-frequency power supply prototype was systematically tested; the prototype achieved the following main performance indicators:
Considering the following factors: pay attention to the physical location of the AP, the control and management of the AP, user identity authentication and billing management, simplify network security management, and do not allow non-professionals to build a wireless network.

(1) Grid regulation rate: input 187 ~ 242VAC, output 115V ±3%
(2) Load regulation rate: input 220VAC, load 0A ~ 1A, output 115V ±3%
(3) Output frequency 400Hz ±0.5%, waveform distortion ≤ 2%
(4) With overcurrent protection, overvoltage protection and overheating protection functions
(5) Volume: 160 × 97 × 45 (mm3)

Conclusion

The use of new power devices and control technology has achieved the technical requirements of miniaturization and high performance of medium frequency power supply. The volume of the prototype is reduced to one-fifth of the traditional medium frequency power supply with the same electrical indicators, and the number of components is reduced to one-half of the original. The new medium frequency power supply is small in size, light in weight, low in noise, fast in response, low in distortion, high in reliability, and has strong practical value.