Design of main circuit parameters of electric operation power supply

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Design of main circuit parameters of electric operation power supply [Copy link]

Introduction

With the development of power electronics technology, phase-shifted full-bridge soft-switching control technology has been gradually applied to power supply, because it can not only reduce the switching loss and electromagnetic interference of the power supply, but also improve the output characteristics of the circuit, and improve the efficiency, stability and reliability of the circuit. In the research of phase-shifted full-bridge soft-switching power supply, many aspects are involved, such as the parameter selection and design of the main circuit, the design of the control circuit, the design of anti-electromagnetic interference and the influence of parameters. This paper only analyzes the main circuit parameter design of the power supply.

Figure 1 Operational power module block diagram

Working principle of electric operation power supply

The principle block diagram of this power supply module is shown in Figure 1. The three-phase AC input power is converted into DC through input three-phase rectification and filtering. The full-bridge conversion circuit converts the DC into high-frequency AC, and then it is converted into a stable DC output through high-frequency transformer voltage isolation, rectified by a rectifier, and filtered by a filter. The main circuit of the power supply adopts a phase-shifted ZVS full-bridge soft switching conversion circuit, and each bridge arm uses two power tubes in parallel.

The converter has a total of 12 working states in one conversion cycle, and the four switch tubes are turned on in turn to achieve zero voltage turn-on and zero voltage turn-off, thereby reducing power consumption. At the same time, in order to suppress the DC component, an isolation capacitor is used; in order to reduce the duty cycle, a saturated inductor is used to make the power supply work more reliable.

Design of the main circuit parameters of the power supply

Main circuit design main indicators

1. Input three-phase AC voltage:
,
2. Output DC rated voltage: 220V, continuously adjustable within the range of 180V~320V
3. Output current: 10A
4. Output maximum power: 3200W
5. Output ripple factor: ≤0.2%
6. Operating frequency: 34KHz
7. Comprehensive efficiency: ≥90%

Selection of input filter capacitor

The high-power switching power supply uses three-phase 380V AC input, and obtains a pulsating DC voltage after full-bridge rectification. The input filter capacitor C _in is used to smooth this DC voltage to reduce its pulsation.

Phase voltage effective value:
=304V-437V
In order to ensure that the minimum value of the DC voltage after rectification and filtering Vin _(min) meets the requirements, the energy provided by C in each cycle is approximately: _After sorting

, the input filter capacitor can be obtained as:
.

Design of resonant inductor

When designing resonant inductor, in order to realize zero voltage switching of lagging bridge arm, the following formula must be satisfied:

Wherein, Lr is resonant inductor, I is the magnitude of primary current when lagging switch tube is turned off, CMOS is drain-source capacitance of switch tube, and Vin is DC voltage after rectification and filtering.

In actual design, considering that lagging tube realizes zero voltage switching when it is more than 1/3 full load, Vin should take the maximum value. At the same time, when the load current is 1A, the current ILf of filter inductor is critically continuous, that is, its pulsation ΔiLf is 2A. At 1/3 load,

(A), drain-source capacitance C _MOS of switch tube IXFX2780Q =750pF, V _in(max) =618V, L _r =39 μ H.

The resonant inductor core uses Siemens' G42 pot-type core, and the air gap δ=2mm. Then according to the formula:

Where: μ ₀ is the magnetic permeability, which is 4π×10-7H/cm; A _e is the magnetic cross-sectional area of the core, which is 388mm ^2. Substituting μ ₀ , A _e and δ into the above formula, we can get: The number of winding turns N=4. In practice, 6 wires with a wire diameter of 0.62mm are twisted and wound.

High-frequency transformer design

1. The primary and secondary side ratio of the main transformer
When designing a high-frequency transformer, the primary and secondary side ratio should be as large as possible. At the same time, the transformer ratio should be selected according to the lowest input voltage Vin. Assuming that the maximum duty cycle of the secondary side is 0.85, the secondary side voltage V _S(min) can be calculated :

Where, V _o(max) is the maximum output voltage; V _D is the on-state voltage drop of the output rectifier diode; V _Lf is the DC voltage drop on the output filter inductor.
Therefore, the transformer primary-to-secondary ratio is:

2. Material selection of magnetic core
In the converter, the high-frequency transformer transmits a high-frequency square wave voltage with a steep front edge of more than 34KHz. Therefore, the transformer core uses the N27 series ferrite material produced by Siemens. Since the output power of this power supply is 2.2kW, according to the core specification and power relationship, the PM74 type magnetic core can be used. According to the B~H temperature characteristic curve, the maximum working flux density BM selected in this design is:

3. Primary and secondary turns
After selecting the PM74 type magnetic core of the N27 series, the secondary side can be determined by the following formula:

Where _Ae is the effective magnetic cross-sectional area of the magnetic core.

According to the manual, Ae ₌ 790mm2, Vs _(min)= Vin _(min) /K=396V, so it can be obtained that Ns _{= 20, and Ns} =21 is used . According to the transformation ratio K=1, and according to the principle that the transformer transformation ratio should be maximized, the primary turns Nf ₌ 23.

4. Determine the wire diameter and number of strands of transformer windings
. When determining the wire diameter of the winding, the effective value of the current should be used:

At the same time, the skin effect of the wire should also be considered. At a frequency of 34KHz, the penetration depth Δ=0.35mm, so the winding should use a copper wire with a wire diameter less than 0.70mm. If the current density is 4A/mm2 ^, the cross-sectional area of the wire is:

According to the table, 6 wires with a wire diameter of 0.62mm and a cross-sectional area of 0.3019mm2 can be used for ^twisting and winding. At the same time, because the primary and secondary sides of the transformer are both a set of windings, the transformation ratio is 1, that is, the primary side is also made of 6 wires with a wire diameter of 0.62mm twisted and wound.

5. Check the window area
Because Nf ₌ 23, Ns ₌ 21, A1 ₌ 6 × 0.62 = 3.7mm2 ⁼ A2 _, the window area S0 ≥ 790mm2 (core area) can be estimated ^{, and the duty cycle} _Km can be obtained as:

This shows that the window area of the designed transformer can accommodate all windings.

Design of output inductance

From the output filter side, the PWM type DC/DC full-bridge converter is actually similar to a buck converter, except that its operating frequency is twice the switching frequency. Therefore, when designing the output filter inductance and output filter capacitor of the PWM type DC/DC full-bridge converter, the calculation formula of the buck converter can be used, but its switching frequency fs must _be changed to 2fs _. The output inductance can be calculated as follows:

The maximum input voltage of this power supply is 618V, and the minimum output voltage is 180V. Assuming Vlf ₊ VD≈5V _, it is calculated that: _Lf = 920μF .


(1-Ip:2A/div 2-Up:200V/div t:10us/div) (1-Is:2A/div 2-Us:200V/div t:10us/div)
Figure 2 Transformer primary current/voltage waveform Figure 3 Transformer secondary current/voltage waveform

The selection of output filter capacitors

is based on actual requirements. The ripple factor should be ≤0.2%. Since the rated output voltage of this power supply is 220V, the output voltage ripple value ΔV _out <0.44V. Considering the voltage spikes caused by the on/off of the power switch tube and the on/off of the output rectifier diode and the residual ripple of the DC bus voltage, the AC ripple of the output voltage can be: =100mV, and I _o =10A. The minimum output capacitor can be calculated using the following formula:

I _o : output current; △ _Vout : allowable output voltage ripple peak-to-peak value; f: operating frequency.

The capacitor value calculated in this way is the minimum value. Considering the actual needs, a 560mF/400V output filter capacitor is selected. Selection of

main power devices and
output rectifier diodes

Usually, the selection of switch tubes and rectifier diodes depends on their voltage stress and current stress. The voltage and current levels are determined according to the requirements of the maximum output current when the input voltage is the highest, and 1.5~2 times the voltage and 2~3 times the current margin are reserved. Since the maximum DC voltage after rectification and filtering is 618V, and the maximum current of the primary side of the transformer is I _p =12A/K=12A, the power tube uses the IXFX27N80Q power tube produced by IXYS, with a rated voltage of 800V and a rated current of 27A.

Because the secondary side of the transformer adopts a full-bridge rectifier circuit, the maximum reverse voltage on the rectifier is V _VD =618V, and the maximum current flowing through the rectifier is: I _0VDmax ≥2I _0max =24A, so the DSEI30 diode produced by IXYS is selected, which can withstand a maximum current of 26A (higher than 2 times the output rated current) and a maximum reverse voltage of 1200V.

Experimental results and analysis

In order to examine whether the selected parameters meet the design requirements, this paper uses the TDS3012 memory oscilloscope to collect the transformer primary and secondary voltage/current waveforms and the secondary rectifier output waveforms for analysis.

Figure 2 shows the transformer primary voltage/current waveforms when the input AC voltage is 380V and the output is 220V/6.5A. It can be seen from the figure that the voltage waveform of the primary transformer is very pure, and the primary current has no turn-on current spikes due to the presence of the resonant inductor. At the same time, the primary current has a large slope when it is up or down, and quickly rises to the load current, indicating that the resonant inductor is almost in saturation at this time, resulting in a significant reduction in duty cycle loss.

Figure 3 shows the transformer secondary voltage/current waveforms when the input AC voltage is 380V and the output is 220V/6.5A. It can be seen from the figure that the secondary current waveform is very pure, and when commutating, the spike is not as large as the primary current. At the same time, there is a little oscillation in the secondary voltage, which is caused by the reverse recovery of the output rectifier tube and the secondary leakage inductance of the transformer.

Figure 4 is the output voltage waveform of the secondary rectifier. It can be seen from the figure that the output voltage of the secondary circuit has almost no trailing edge spike, which also indirectly illustrates that the leakage inductance of the main transformer is small and the use of PM type magnetic core can reduce leakage magnetic flux.

Conclusion

In the study of electric operation power supply, the design of the main circuit plays a very important role in the performance of the whole power supply. According to actual measurement, the performance of the whole power supply is stable, the output voltage peak is greatly reduced, and the efficiency is high when the main circuit is designed by the above design method.

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