Design of main circuit parameters of electric operating power supply

Introduction

With the development of power electronics technology, phase-shifted full-bridge soft switching control technology is gradually applied to electric power operating power supplies 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 increase the efficiency of the circuit. , stability and reliability. In the research of phase-shifted full-bridge soft-switching power operation, many aspects are involved, such as parameter selection and design of the main circuit, design of the control circuit, design of anti-electromagnetic interference, and the influence of parameters. This article only analyzes the power operation here. Power supply main circuit parameter design.

Figure 1 Principle block diagram of operating power module

Working Principle of Electric Power Supply

The principle block diagram of this power module is shown in Figure 1. The three-phase AC input power is converted into DC through three-phase rectification and filtering. The full-bridge conversion circuit then converts the DC power into high-frequency AC power, and then passes through the high-frequency Transformer isolation, rectification by rectifier, and filtering convert it into a stable DC output. 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 connected in parallel.

The converter has a total of 12 working states in one conversion cycle, and the four switching 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 more reliable.

Design of main circuit parameters of power supply

Main indicators of main circuit design

1. Input three-phase AC voltage:
, 
2. Output DC rated voltage: 220V, continuously adjustable in the range of 180V~320V
3. Output current: 10A
4. Output maximum power: 3200W
5. Output ripple coefficient: ≤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 pulsating DC after full-bridge rectification voltage, the input filter capacitor C _in is used to smooth this DC voltage and reduce its pulsation.

Effective value of phase voltage:
 =304V-437V _{In order to ensure that the minimum value of the DC voltage V in (min)}
after rectification and filteringthe energy provided by _{C in} in each cycleAfter sorting, the input filter capacitor can be :. Design of resonant inductor When designing a resonant inductor, in order to achieve zero-voltage switching of the lagging bridge arm, the following formula must be satisfied:where, Lr is the resonant inductance, I is the primary current when the lagging switch is turned off, and CMOS is the leakage of the switching tube. -Source capacitance, Vin is the rectified and filtered DC voltage. In actual design, considering that the hysteresis tube achieves 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 the filter inductor is critically continuous, that is to say, its pulsation amount ΔiLf is 2A. At 1/3 load, _{(A), the drain-source capacitance C MOS} of the switch tube IXFX2780Q=750pF, V _in(max) =618V, L _r =39 μ H. The magnetic core of this resonant inductor uses Siemens' G42 model pot core, and the air gap δ=2mm, then according to the formula: where: μ ₀ is the magnetic permeability, the size is 4π×10-7H/cm; A _e is the magnetic The magnetic cross-sectional area of ​​the core 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, it is twisted and wound with 6 wires with a diameter of 0.62mm. High-frequency transformer design 1. Primary and secondary side transformation ratio of main transformerWhen designing a high-frequency transformer, the primary and secondary side transformation ratio should be as large as possible. At the same time, the transformation ratio of the transformer 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 primary and secondary transformation ratio of the transformer is:

2. Magnetic core material selection
In the converter, the high-frequency transformer transmits a high-frequency square wave voltage with a steep front edge above 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, the PM74 model core can be used based on the core specification and power relationship. According to the B~H temperature characteristic curve, the maximum operating magnetic flux density BM selected in this design is:

3. After selecting the PM74 core of the N27 series for the number of primary and secondary turns
, the secondary side can be expressed by the following formula Determine:

where A _e is the effective magnetic cross-sectional area of ​​the magnetic core.

According to the manual, A _e =790mm2, V _{s (min) =} V _{in (min)} /K = 396V, so it can be concluded that N _s =20, adopt N _s =21, according to the transformation ratio K = 1, and at the same time according to the application In order to maximize the transformation ratio of the transformer, take the number of primary turns N _f =23.

4. Determine the wire diameter and number of
strands of the transformer winding wire. 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 must also be considered. At the frequency of 34KHz, the penetration depth Δ=0.35mm, so , the winding should use copper wires with a wire diameter less than 0.70mm. If the current density is 4A/mm ² , the cross-sectional area of ​​the wire is:

Looking up the table, 6 twisted wires with a wire diameter of 0.62mm and a cross-sectional area of ​​0.3019mm 2 ^{can be used.} At the same time, since the primary side and secondary side of the transformer are both a set of windings, the transformation ratio is 1, that is, the primary side is also twisted and wound with 6 wires with a wire diameter of 0.62mm.

5. Check the window area
because N _f =23, N _s =21, A ₁ =6×0.62=3.7mm ² =A ₂ , the window area S0≥790mm ² (core area) can be estimated, and the duty cycle can be obtained K _m is:

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

Design of Output Inductor

From the output filter side, the PWM 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 inductor and output filter capacitor of the PWM DC/DC full-bridge converter, the calculation formula of the buck converter can be used, but the switching frequency f _s must be changed to 2f _s . The output inductance can be calculated according to the following formula:

The maximum input voltage of this power supply is 618V, and the minimum output voltage is 180V. When V _lf +V _D ≈5V, the calculation is: L _f =920 μ F.

                
     (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 The selection of the transformer secondary current/voltage waveform

output filter capacitor

is based on actual requirements. The ripple coefficient must be ≤0.2%. Since the rated output voltage of this electric power supply is 220V, the ripple secondary value of the output voltage ΔV _out <0.44V , taking into account the voltage spike caused when the power switch tube is turned on/off and the output rectifier diode is turned on/off, as well as the residual ripple of the DC bus voltage, the AC ripple of the output voltage can be: =100mV, and I _o =10A, minimum The output capacitance can be calculated by the following formula:

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

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

Selection of main power devices and
output rectifier diodes

Usually, the selection of switching tubes and rectifier diodes depends on their voltage stress and current stress. Determine its voltage and current levels based on the requirements for outputting the maximum current when the highest voltage is input, and reserve 1.5 to 2 times the voltage and 2 to 3 times the current margin. Since the maximum DC voltage after rectification and filtering is 618V, and the maximum primary current of the transformer is I _p =12A/K=12A, the power tube uses the IXFX27N80Q power tube produced by IXYS Company, with a rated voltage of 800V and a rated current of 27A.

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

Experimental results and analysis

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

Figure 2 shows the primary voltage/current waveform of the transformer when the input AC voltage is 380V and the output is 220V/6.5A. As can be seen from the figure, the voltage waveform of the primary transformer is very pure, and the primary current does not have the turn-on current spike that occurs in traditional hard-switching converters due to the presence of resonant inductance. At the same time, the primary side current has a very large slope when it overshoots or undershoots, and quickly rises to the load current, indicating that the resonant inductor is almost in a saturated state at this time, resulting in a greatly reduced duty cycle loss.

Figure 3 shows the secondary voltage/current waveform of the transformer 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 during commutation, the peak is not as large as the primary current. At the same time, the secondary voltage has a little oscillation, which is caused by the reverse recovery of the output rectifier 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 peaks. This also illustrates from the side that the leakage inductance of the main transformer is small, and the PM type core can reduce leakage. magnetic.

Conclusion

In the research of electrically operated power supplies, the design of the main circuit plays a very important role in the performance of the entire power supply. It is known from actual measurements that by using the above design method to design the main circuit, the performance of the entire power supply is stable, the output voltage spike is greatly reduced, and the efficiency is high.