Design and implementation of a 1KW 27VDC/190VDC current-mode controlled flyback DC/DC converter-EEWORLD

Collect

1. Introduction

Static converters generally adopt a 27VDC/190VDC/115VAC 400Hz conversion structure. The front stage converts the input 27V DC into 190V DC, and the back stage inverter converts the 190V DC into the 115V/400Hz AC required by the electrical equipment. Since the flyback converter has the advantages of simple circuit topology, wide input voltage range, input and output electrical isolation, small volume and weight, it will reduce the volume and weight of the entire static converter as the front stage circuit topology of the static converter to achieve higher power density.

2. Current-controlled flyback DC/DC converter

(1) Power circuit

Because the input is low voltage and high current, a single-tube flyback topology is selected, as shown in Figure 1. For this structure, a buffer circuit must be added to the main power switch, otherwise the leakage inductance energy will have nowhere to be released when the switch tube is turned off, which will cause a voltage spike to break through the power tube. Commonly used buffer circuits include LCD, RCD and active clamp. Considering the high switching frequency of the circuit (80kHz), LCD buffer is not advisable; if active clamp is used, the clamp switch tube must have a large current rating, and the primary of the transformer has a large circulating current, which is not conducive to improving efficiency, so active clamp is also not available; RCD buffer is passive and has a simple circuit. If the parameters are adjusted reasonably, it will not have much impact on the efficiency, so it is adopted in the experimental prototype.

Figure 1 RCD buffer single-tube flyback converter circuit topology

(2) Control scheme

The flyback converter has the characteristics of a current source, and the open loop cannot be open, otherwise the output voltage will be extremely high. When used as the DC link of a static converter, when the four power tubes of the rear inverter bridge are all turned off, the output end of the flyback converter of the front stage is equivalent to an open circuit, so voltage closed-loop control must be adopted; at the same time, in order to improve the performance of the power supply, current-type control technology is adopted. This current and voltage dual closed-loop control enables the system to have the advantages of fast transient response, high stability, and inherent current limiting capability.

(3) Circuit composition

The developed 1KW prototype circuit is shown in Figure 2. The core of the entire circuit is the current-type PWM chip UC3843N from UNITRODE, which was originally used to design low-power switching power supplies below 200W, and is used here in a flexible way. Q1 and Q2 form a totem pole to increase the driving capability, Q3 acts as an emitter follower to extract the sawtooth wave of UC3843, CT1 samples the current signal of the main switch tube, R7 and R8 superimpose the current signal with the sawtooth wave for slope compensation to eliminate the inherent subharmonic oscillation of the current control regulator when the duty cycle is greater than 50%, and the ratio of R7 and R8 can determine the depth of slope compensation. R13 and D6 form a reference voltage source, Q5 is an error amplifier tube, and the photocoupler U3 feeds the error voltage back to UC3843N to form a voltage closed loop while ensuring electrical isolation.

Figure 2 Principle prototype circuit composition

[page]3. Design of key circuit parameters

(1) Energy storage transformer

Assuming that the power at the critical continuous current is 1/5 of the total output power, then

(1)
In formula (1), Bm is the maximum magnetic flux density, B is the magnetic flux density at the bias point, and the magnetic flux density change is:

(2)
The number of turns on the original side is

(3)
In formula (3), Ui min is the minimum input voltage, Ton max is the maximum on-time of the switch tube, and S is the cross-sectional area of the magnetic core.

(4)
In formula (4), Pomin is the critical continuous power, Ts is the switching period, and η is the conversion efficiency.
The air gap of the energy storage transformer core is

(5)
In formula (5), I1p is the maximum peak current of the primary side, and μ0 is the magnetic permeability of vacuum.

(2) Power switch tube

The voltage stress and current stress of the switch tube are respectively

In formula (6.b), I1avg is the average value of the primary inductor current, and ∆I is the primary current ripple value.

(3) Rectifier diode

The voltage stress and current stress of diode D5 are respectively

(4) RCD clamping circuit

Clamping capacitor C6 is

(8)
In formula (8), Ureset is the initial voltage of the clamping capacitor C6.
The clamping resistor R3 must satisfy

(9)
In formula (9), Toff is the cut-off time of the switch tube.

(5) Selection of dead load R10

Since the duty cycle is very small when no-load and will cause gap oscillation, a dead load needs to be added. Its value is determined during debugging. Under the premise of system stability, the larger the resistance, the better.

4. Principle prototype test

Design example: rated output power 1000W, input voltage 27V, output voltage 190V, switching frequency 80kHz, energy storage transformer core R2KBD PM74, winding turns N1/N2=4/28, core air gap 3.2mm, maximum duty cycle 0.6, clamping resistor R3 is 51Ω, clamping capacitor C6 is 5.6μF, clamping diode D2 is DSEI60-06, rectifier D5 is DSEI30-10, dead load R10 is 10kΩ; the turns ratio of current transformer CT1 is 1/250, and the core adopts Φ27 ultra-fine gold magnetic ring.

The prototype test waveform is shown in Figure 3. Figure 3 (a) and (b) are the current waveform and drain-source voltage waveform of the switch tube when no-load, respectively. The current waveform is measured from the sampling resistor R6; Figure 3 (c) and (d) are the current waveform and drain-source voltage waveform of the switch tube when fully loaded, respectively. When the power supply changes from no-load to full-load, the output voltage fluctuation is less than 1%Uo, indicating that the load regulation rate of the power supply is quite high.

5. Conclusion

The flyback converter can be used in high-power applications and has the advantages of small size and light weight. The successfully developed prototype has comprehensive performances such as high power density, high stability, high conversion efficiency, and inherent overload and short-circuit current limitation, and has important application value in various power application fields.

References:

[1] Chen Daolian. Research on high-frequency link aviation static converter [Postdoctoral research report]. Nanjing University of Aeronautics and Astronautics. 2001

[2] Unitrode's Product Application Handbook. 1995~1996

Reference address：Design and implementation of a 1KW 27VDC/190VDC current-mode controlled flyback DC/DC converter

Previous article：Study on nonlinear chaotic phenomena of DC-DC converter
Next article：A New Soft-Switching DC-DC Converter

Popular Resources
Popular amplifiers