Journal of Beijing Institute of Technology , 2000 , V ol.9, No .4
Article ID :1004-0579(2000)04-0465-07
Principle , Modeling and Control of DC-DC
Convertors for EV
ZHANG Cheng-ning , SUN Feng-chun , ZHANG Wang
(School of V ehicle and T ransportation Engineering , Beijing Institute of T echnology , Beijing 100081)
Abstract :DC-DC convertors can convert the EV' s high-voltage DC power supply into the low-voltage DC power supply .In o rder to design an ex cellent conv erto r one must be guided by theo-ry of automatic control .The principle and the method of desig n, modeling and control for DC-DC converto rs of EV are introduced.T he me thod of the system-respo nse to a unit step-function input and the frequency-response method are applied to researching the convertor' s mathematics model and control characteristic .Experiments show that the designed DC-DC convertor' s out-put voltage precision is hig h, the antijamming ability is strong and the adjustable performance is fast and smooth .
Key words:EV ;DC-DC conv erto rs;automatic co ntrol ;mathematics model;Bode drawing
CLC number :U 469.72 Document code:A
Generally there are tw o power supplies in EV .One is the DC high-voltage pow er supply that is used by high pow er devices such as traction motors and air conditioners etc .The o ther is the DC low-voltage pow er supply that is usually used in some control circuit and low-voltage electrical devices such as the instrument and lighting .I ts rating voltage is 24 V or 12 V .The low-voltage pow er supply can be gained from the hig h-voltage power supply by a DC-DC convertor .
In this paper , the main perform ance of the designed convertor is that the input voltage range is from DC 250 V to DC 450 V , the output voltage is DC 24 V , the m aximum output current is DC 20 A , and the output precision is 1 %.
1 Principle of the Convertor
1.1 The Block Diagram of the DC-DC Convertor
The block diag ram of the DC-DC convertor is show ed in Fig .1.The battery series provide the DC high-voltage input U s .The low-voltage output of the convertor is U o .The setting value Ui of the converto r is equal to o r is in proportion to the demanded output voltage U o .The conver-tor is a closed-loop negative feedback-sy stem w ith voltage feedback .
Received date:2000-06-12
Biography:ZHANG Cheng-ning(1963 -),male, associate professor, doctor .
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Journal of Beijing Institute of Technology , 2000 , V ol.9, No .4
1.2 Power Switch Circuit
The power sw itch circuit with semi-bridge mode is show ed in Fig .2.L1 and C1 constitute an input filter to avoid high-frequency impulses flowing backw ards .Capacito rs C2 and C3 consti-tute the partial-voltage circuit w hile resistances R1 and R2 do so .IGBT1 and IGBT2 are semi-conductor sw itch devices.C6 is a separation-DC capacitor .T1 is a transformer that reduces the voltage .L2 and C7 constitute an output filter .RL is the load resistance .When the PWM signals in the reverse semi-w aves are inputted onto IGBT1 and IGBT2' s co ntrol poles , the corresponding DC voltage can be yielded from the convertor .
Fig .2 Principle circuit of pow er switch w ith semi-bridge mode
1.3 Control Circuit
The chip SG3525 is used in the PWM control circuit showed in Fig .3.V cc is the power volt-age applied to the chip , it is 12.0 V .A base-voltage of 5.1 V is yielded on pin16 of the chip that is partially used as parameter voltage input U i .The chip includes a saw tooth-w ave generator .R t and Ct are the external resistance and capacity that determine the saw tooth-w ave' s frequency . Pin2 of the chip is a positive-phase input po rt .Voltage input U i is putted to the port , here U i = 2.5 V .Pin1 of the chip is the neg ative-phase input port w here the feedback voltage is inputted . Pin9 of the chip is the output end of the inside amplifier of the chip .The proper resistance and ca-pacito r are connected betw een the pin1 and pin9 to realize compensation of the DC-DC convertor . C8 is the integ ral capacito r .The integ ral compensator is adopted as the system-compensation of the sy stem .The PWM impulses are yielded from pin11 and pin14 of the chip .When the PWM control circuit operates normally , Ui on the pin2 and U b on the pin1 should be balanced .When Ub is not equal to U i , the PWM w idth can be automatically adjusted by the PWM control circuit
to m ake U b equal to U i .By this w ay we can control the output voltage of the convertor .
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Z HAN G Cheng-ning et al ./ Principle , Modeling and Control of DC-DC Convertors for EV
Fig .3 The connection circuit for the PWM control chip SG3525
1.4 Drive Circuit
The drive circuit of IGBT usually adopts a pulse-transformer o r an opto-coupler to isolate the pow er circuit from the control circuit .An individual pow er supply is needed if an opto-coupler is used , w hich increases the complexity of the system .So the isolation-circuit adopts a pulse-trans-former show ed in Fig .4.Transistors BG1 and BG2 in Fig .4 compose a complementation power amplification circuit .T2 is the pulse-transformer that isolates the pow er circuit from the co ntrol circuit .R5 and C8 compose the acceleration circuit .The diode D6 eliminates negative impulses . The diode D7 and transistor BG3 compose the rapid discharge circuit of the distributing capacitor at the control pole of IGBT .
Fig .4 Principle circuit for IGBT drive
2 Modeling and Control
2.1 Modeling
The DC-DC convertor is a voltage negative feedback-system .Aiming to obtain the better dy-namic and static characteristic we must model and analyse it in theory .According to Ref .[1] , DC-DC convertors are the appro ximate second-order systems .In order to obtain accurate parame-ters , the method of the sy stem-response to a unit step-function input is adopted in this paper .
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Journal of Beijing Institute of Technology , 2000 , V ol.9, No .4
2.1.1 Measuring the Open-Loop Sy stem' s Response to a Unit Step-Function Input
The block diag ram for measuring is shown in Fig .5.The concrete method is described as fol-lows :①The voltage feedback signal is cut off ;② The setting value of the chip SG3525 adopts the middling value U i0 to make the width of an impulse be about 0.5 T ;③ U i0 is superimposed w ith dU i that is composed by positive and negative rectang le w ave impulses .The amplitude of dU i is taken to be equal to 0.2U i0 .It should make dU o be easy to be observed to select the rect-angle w ave frequency , adopting f 1 =400 Hz ;④ The output wavefo rm of U o(=U o 0 +dU o)is show n in Fig .6.As show n in Fig .6w hen f 1 =400 Hz , period T =2.5 ms (5 g rills), the time for the max im um voltage value is about 0.2 g rills .dU o ' s stable voltage amplitude is 2 g rills . Peak overshoo t is 1 g rill .Every grill in the vertical direction represents 5 V .By this w ay the data of system-response to a unit step-function input can be obtained as follow s :
peak time t p =0.1 ms ;peak overshoot σp =1/2 =50 %;
output and input' s incremental ratio K0 =dU o/dU i =10/1 =10 .
Fig .6 The system-response to a unit step-function input
2.1.2 Determining the Open-Loop Transfer Function
According to Refs .[2 ,3] , we have the damping ratio ξ, undamped natural frequency transfer function of controlled object Gp(s)as follow s :
ωn and
(1 )
(2 )
(3 )
In order to ensure that w hen the output voltage U o =24 V the feedback voltage to pin1 of
the SG3525 is 2.5 V to balance the input voltage U i =2.5 V , we take the feedback and measur-
ing factor as
Kb =U b/ U o =2.5/24 =0.104 . (4 )
2.2 Design of the PID Regulator
2.2.1 The Principle Scheme and Transfer Function of the PID Reg ulator
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Z HAN G Cheng-ning et al ./ Principle , Modeling and Control of DC-DC Convertors for EV
To resist the disturbance of the pow er supply voltage and load current to the DC-DC conver-tor so as to im prove control precision , an integ ral com pensato r is adopted .The principle scheme of the integral com pensato r is show n in Fig .7.
characteristic , logarithmic phase-frequency characteristic of the feedback and measuring circuit
Fig .8 The Bode draw ing of the system open-loop transfer function
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