Design and Analysis of Control System Power Supply Based on PEMFC

The PEMFC hydrogen generator emits direct current with a wide range of variation. It must be converted by voltage stabilization, inversion, etc. to obtain a stable output voltage before it can be applied to the load. When the control system power supply of the PEMFC generator is self-generated, the power supply system needs to adapt to the output characteristics of the generator. The normal operation of the control system is an important condition for the safe and reliable operation of the generator, and a reliable power supply is the basis for the stable operation of the control system. Therefore, it is very necessary to study the control system power supply that adapts to the output electrical characteristics of the PEMFC power generation system.

1 Overall structure design of the PEMFC control system power supply

This paper analyzes an input/output isolated DC/DC conversion circuit structure, as shown in Figure 1. The circuit adopts a single-ended flyback structure. The 36-72 V DC voltage output by the PEMFC is first inverted into a high-frequency square wave in a PWM manner, and then stepped down by a high-frequency transformer, and then rectified and filtered to obtain a stable 24 V and 5 V DC voltage. It is mainly composed of Mitsubishi intelligent power module (IPM), high-frequency transformer, rectifier filter capacitor, Hall voltage sensor and PWM control board. The PWM control board is implemented by DSP.



2 Design of the main circuit

2.1 IPM power module

IPM, or intelligent power module, is a module that encapsulates IGBTs, their drive circuits and multiple protection circuits in the same module, freeing system designers from the cumbersome design of IGBT drive and protection circuits.

IPM selects Mitsubishi intelligent power module PM300HHA120, which contains a 300 A/1 200 V IGBT, which contains gate drive control, fault detection and multiple protection circuits, and has a built-in current sensor. The

protection functions that IPM can achieve include: control power supply undervoltage protection (UV); overheat protection (OT); overcurrent protection (OC); short circuit protection (SC). It should be emphasized that the protection function of IPM itself cannot eliminate faults. When designing the circuit, the fault output signal FO should be used so that the system can block the input signal of IPM and shut down when a fault occurs. The control input and output of PM300HHA120 are isolated by optical coupling, as shown in Figure 2, and an isolated power supply is used to supply power separately to ensure safety and reliability.



2.2 High-frequency transformer

The design of high-frequency transformer is a key technology for the development of switching power supplies. The transformer of a single-ended flyback switching power supply is actually a coupled inductor, which realizes the functions of DC isolation, energy storage and voltage conversion. Its performance not only has a great influence on the power supply efficiency, but also is related to the technical indicators such as the electromagnetic compatibility of the switching power supply.

Known parameters: maximum DC input voltage VIN=72 V; minimum DC input voltage VINmin=36 V; switching frequency fs=20 kHz; output voltage Vo1=5 V, Vo2=24 V; output current Io1=1 A, Io2=0.5 A; output power Po=5×1+24×0.5=17 W; power supply efficiency η=80%; loss distribution coefficient Z=0.5, Z is the ratio of secondary loss to total power; the ratio of primary ripple current Ir to primary peak current Ip Krp=0.4.

(1) Calculation of primary inductance

The expression of primary peak current Ip is:


Substituting the numerical value into it, we can get Ip=1.17 A.

In each switching cycle, the magnetic field energy transmitted from the primary to the secondary varies from LpI2p/2 to Lp(Ip-Ir)2/2. The primary inductance is determined by the following formula, and substituting the number to get:



( 2) Selection of magnetic core. Ferrite soft magnetic material is a composite oxide sintered body with high resistivity, especially suitable for use at high frequencies, and it is cheap. Therefore, the high-frequency transformer in this switching power supply uses a magnetic core made of R2KB manganese-zinc ferrite material. Its saturation magnetic induction intensity Bs=350 mT at 25°C. The working magnetic induction intensity of the magnetic core can be selected as 0.7 times the saturation magnetic induction intensity, Bw=0.7Bs=245 mT.

According to the power and operating frequency, select the E135 type magnetic core, whose Ap=1.52 cm4, Ae=1.04 cm2, Aw=1.46 cm2.

(3) Determine the number of turns of each transformer winding. After determining the magnetic core of the transformer, the number of turns of the primary side of the transformer can be calculated according to the following formula:



Calculated: Np=100.2 turns, actually 101 turns.

For a 5 V output transformer, the secondary voltage Vs1=Vo1+Vl1+Vf1=5+0.3+0.4=5.7 V, where the transformer secondary winding voltage drop Vl1 is 0.3 V, and the output Schottky rectifier conduction voltage drop VF1 is 0.4 V.

For 24 V output, the transformer secondary voltage Vs2 = Vo2 + VL2 + Vf2 = 24 + 0.6 + 0.7 = 25.3 V,

where the transformer secondary winding voltage drop VL2 is 0.6 V, and the fast recovery rectifier voltage drop Vf2 is 0.7 V.

Calculate the number of turns of the secondary winding:

For 5 V output:


actually 10 turns.

For 24 V output:

actually 42 turns.

2.3 Rectification and filtering

(1) Design of output filter inductor. In addition to the DC component, the current in the output filter inductor also has a small AC component. The design of the output filter inductor generally requires that the maximum pulsation of the inductor current is 10% to 20% of the maximum output current.

For the output voltage Vo = 5 V, the output current Iomax = 1 A, and the maximum duty cycle Dmax = 0.63.

Substituting these values ​​into: L = 462.5μH.

For the output voltage Vo = 24 V, repeat the above calculation to get: L = 0.004 4 H.

(2) Selection of output filter capacitor. Ripple current on the output filter capacitor:



According to the data obtained in the previous section, substitute ISRMS1 = 1.712 A and ISRMS2 = 0.856 A into the above formula respectively, and we can get Iri = 11.39 A and Iri2 = 0.695 A. The ripple current of the filter capacitor at 20 kHz should be greater than or equal to Iri. The

output ripple voltage is determined by the formula Vri = IsprO. The filter capacitors C2, C3, and C4 are selected as 330 μF/50 V, and C5 is selected as 100 μF/25 V.

3 Design of control circuit

3.1 PWM control circuit

Here, with the digital signal processor (DSP) TMS320LF2407 as the core, a fully digital PWM control system is designed, as shown in Figure 3, which has better real-time performance and can well adapt to the output characteristics of the PEMFC generator.



The output voltage is isolated and sampled by the Hall voltage sensor and sent to the ADC module of the DSP for analog-to-digital conversion. After these values ​​are controlled by a series of digital PI within a certain period of time, a new comparison value is generated for the full comparison unit. The comparison value will change the duty cycle of the PWM waveform in the next switching cycle, thus achieving the purpose of controlling the output voltage to the required value.

There is no function of automatically generating PWM signals in the DSP. It must be realized by programming. The required PWM signal is realized through a single comparison 1 output pin PWM1. The following is a detailed introduction to this method. The single comparison unit has a comparison register to store the comparison value. When the counter is equal to the comparison value, the corresponding PWM output pin level jumps. How it jumps depends on the working mode of the PWM pin.

The working mode of the PWM output pin: effective high mode, effective low mode, etc. When timer 1 works in continuous increase and decrease counting, the level is: if the output pin is set to effective high, when the counter is zero, the output pin level is zero, and the counter starts to count up. When it is equal to the comparison value, the output pin is in an effective state and the level becomes high. After the counter reaches the period value, it starts to count down. When the count is down to the comparison value, the output pin is in an invalid state and the level becomes low. If the output pin is set to effective low, the level change at this time is exactly opposite to the effective high state. This paper adopts the effective high working mode.

T1CNT is the count value of counter 1, and T1PER is the period value of counter 1. When the value of T1CNT increases to be equal to T1PER, counter 1 starts to count down, and when the value of T1CNT decreases to 0, the counter counts up. The change of the counter value over time is shown in Figure 4. When the count value of the counter is equal to the comparison register value (SCMPR1) of each comparison unit, the output pin level changes. The waveform is shown in Figure 4. It can be seen from the figure that the counter value can adjust the PWM pulse width by comparing with the real-time changing comparison register value (SCMPR1), thereby changing the duty cycle of the power tube and achieving real-time control of the output voltage of the DC/DC converter.


3.2 Design of isolated sampling circuit

In order to ensure the reliable operation of the circuit, the voltage sampling is preferably isolated from the control circuit, so as to avoid the interference caused by the voltage drop when the large current flows through the ground wire in the main circuit. In this machine, the isolation of the control circuit and the main circuit is achieved through the voltage Hall element. The principle of the Hall voltage element is: a large resistor is connected in series with the voltage and the primary side of the Hall element to obtain the primary current, which can generate a certain proportion of the secondary current on the secondary side. The voltage drop generated by the secondary current flowing through the resistor can reflect the voltage value of the main circuit. The sampling circuit of the output DC voltage of the designed DC/DC converter is shown in Figure 5.



From the parameters in Figure 5, it can be seen that:

U _ADC1 =Uo/10 After the isolation of the Hall element and the processing of the operational amplifier, the A/D conversion voltage sent to the DSP is isolated from the main circuit, which improves the anti-interference ability of the entire circuit. 3.3 Parameter selection of PI regulator The control circuit of the DC/DC converter adopts a voltage single closed-loop control, and Gv(s) is designed as a PI controller. Its parameter selection determines the performance of the DC/DC converter to a large extent, so their selection is crucial in the research and development process of the machine. In the process of developing the machine, this paper selects parameters as follows: first select the parameters of the main circuit and the sampling circuit, and establish the model of the DC/DC converter in Matlab, and then make a rough estimate of the parameters of the PI according to the general principle, and continuously adjust the parameters of the PI. After obtaining satisfactory results, the parameters are programmed into the DSP, and after actual operation, they are slightly adjusted according to the experimental results. The final results are as follows: Gv(s)=5+20/s Under this parameter, the full load result obtained by Matlab simulation is shown in Figure 6. It can be seen from the simulation waveform that under this PI parameter setting, the output voltage of the designed switching power supply basically reaches the required 5 V. 4 Experimental results and analysis The developed switching power supply has an input voltage of 48 V, an output of 24 V at no load, and a voltage waveform measured at 5 V and an output current of 0.5 A as shown in Figure 7. The measured output voltage is 4.96 V, and the peak-to-peak value of the ripple voltage is Vp-p=35 mV. When the PEMFC generator is running. Its output terminal is connected to a switching power supply, and the voltage waveform measured when the 5 V output is connected to a 0.5 W load is shown in Figure 8. After measurement, the voltage output of the switching power supply is 5.01 V, and its peak-to-peak voltage ripple is measured to be Vp-p=80 mV. It can be seen from the test waveform that the developed DC/DC switching power supply has a stable output voltage, which can adapt to the output characteristics of the PEMFC generator and basically meet the control system's demand for power supply. There are many burrs in the DC voltage waveform output by PEMFC, and the manufacturing process of the switching power supply needs to be further improved. Therefore, the output ripple of the switching power supply is large. 5 Conclusion The main circuit structure of the switching power supply, as well as the filtering, rectification and other circuits of the switching power supply are designed, the design method of the high-frequency transformer of the switching power supply is given, the parameters of the components are calculated and the models are selected, and a power supply prototype is developed. The developed switching power supply is tested for performance, which can adapt to the output characteristics of the PEMFC generator and meet the needs of the control system.