A_FULLY_PROTECTED_PUSH-PULL_CURRENT-FED

A FULLY PROTECTED PUSH-PULL CURRENT-FED DC-DC CONVERTER Alexander L. Rabello, Mircio A. C6, Gilbert0 C. D. Sousa and JosC L. F. Vieira Universidade Federal do Espirito Santo Departamento de Engenharia Eletrica - LEPAC - CP: 01-901 1 Vitdria - ES - Brazil - 29060-970 Phone: 55.27.335.2699 - Fax: 55.27.3 3 5.2650 - E-Mail: j oseluiz @ ele.ufes .br Abstract - This paper presents an isolated dc-dc converter designed to operate as a primary stage of an on-board dc-ac power supply for diesel-electric engines. The dc-ac power supply must provide a sinusoidal waveform of 127V rnls from 72V dc bus of the engine. The dc-dc push-pull current-fed converter has been used to obtain an isolated 200V dc output voltage from the available 72V dc bus engine. This converter operates as a boost converter with constant frequency and duty cycle greater than 50%. Two control loops operating in cascade mode were used. A fast inner current loop regulates the boost inductor current. Whereas an external control loop keeps a constant output voltage. The experimental results have been obtained for a 300W laboratory prototype operating at SOkHz. The converter efficiency was 93% at full load, and the output voltage regulation was less than 1 % for 10% to 100% load changes. I. INTRODUCTION Due to diesel-electric engine automation, different on-board control equipments are required, which often need a regulated ac power supply. The diesel-electric engine provides 72V dc bus, which varies between 58 to 85V. To function as an useful power source this dc bus voltage must be converted to an ac voltage. An ac power supply is essential to feed the measurement instruments, which are employed to verify the behavior of all on-board engine control equipments, under different operating conditions, as well as the switching noise interferences caused by the motor-generator set. The block diagram of the complete dc-ac power supply is presented in Fig.1. This paper discusses with the dc-dc converter block only. V i i i y f l DC-DC 1 dc 1-c 58-85 V ISoLA'IED 200V INWRTER 127V EILm BOOST dc CONVERTER -I I Fig. 1 - Block diagram of the dc-ac power supply. A dc-dc push-pull current-fed topology has been chosen, since it provides isolation and low cost with high reliability. Also, it can ensure high efficiency at low power levels[l,2,3,4,5]. The dc-dc push-pull current-fed topology operates as a boost converter with duty cycle greater than 50%. As can be seen in Fig. 2, the electrical isolation between the engine's dc bus and the output voltage achieved through by a high frequency transformer. The control strategy is based on two cascade loops dynamically separated, in which the current loop is faster than the voltage loop [6]. This control strategy results in faster responses to load changes, as well as to input voltage variations compared to the conventional single voltage control loop. The use of a conventional single voltage control loop in a boost converter results in a second-order transfer function with a right half-plane zero. To prevent instabilities, a slowly control-loop must be implemented. As the two cascade loops present dynamic response faster than the conventional one, low output-capacitance values can be used, reducing the converter weight and size. The converter reliability is improved by employing protection circuits to prevent over-voltage, short-circuit and over-current. 11. CIRCUIT DESCRIPTION A. Power Stage: The power stage diagram of the dc-dc push-pull current-fed converter is shown in Fig.2. Its operation with duty cycle greater than 50% is like the conventional boost converter. The converter operation can be described by two stages, as follows: both switches are ON (b < t < tl and t2 < t < t3): the voltage across the transformer primary is zero, and the boost inductor current increases linearly, following the equation: (1) one switch is ON (tl < t < t2 and t3 < t < t4): the voltage across the transformer primary is q,, and the energy stored by the boost inductor is delivered to the load. During this stage the boost inductor current decreases linearly according to the equation: v, -V; iL (t)= iL (ti)+ - (t - t l) (2) b b r, 0-7803-3932-0 587 1 Yo Fig.2 - Power stage diagram of the dc-dc push-pull current-fed converter. where: SI, S2 - switches; D1, Dz - output diodes; Lb - boost inductance; Vi" - input voltage; vo - output voltage. vb NI, Nt toll - switch conduction time; tc - charging time; td - discharging time; T - switching period; D - output voltage reflected to the transformer primary - transformei windings (n = NI&); (V$VirJ; - duty cycle ( D = t, / T). The main waveforms of this converter are shown in Fig.3. I I * t e t '"p 1 w t . i t t z t r L Fig. 3 - Main waveforms: a, b) gate drive signals; c) boost inductor current; d, e) switches currents; f) transformer secondary current; g, h, i) voltage across: Lb, SI and Dz respectively. B. Control Stage: The control scheme is based on two cascade loops dynamically separated. A faster inner current loop regulates the average current the boost inductor. For this control loop, the output voltage reflected to the transformer primary can be considered constant and equal to Vb. The external loop maintains a constant output voltage, independently of the load changes and the input voltage variations. In this case, the output capacitance can be considered fed by a constant current source. By using this control strategy the two loops are reduced to first order systems. These two cascade loops can be described by two independent control loops, as shown in Fig. 4. I Fig. 4 - Control loop scheme: (a) - current loop; (b) - voltage loop C. Protection Stage: Protection circuits, shown in Fig. 5, were implemented to operate in case of anomalous conditions, such as: over-voltage circuit: if the output voltage increases beyond 15% of the rated value, the gate drive signals of the MOSFETs are disabled by the over-voltage circuit. over-current and short circuit: by using a voltage comparator with hysteresis, the boost inductor current can be maintained between a lower and an upper bounds. If the output voltage drops bellow a specified value, characterizing over-current or short-circuit, the converter is shut-down after a given time period. 58 8 under-voltage lockout circuit: the MOSFElTs gate circuits are kept out, at the converter start-up, until the IC supply voltage has risen to 1OV. - vcc $12v vcc t to MOSFET gate circuit I ....... 14 Fig. 5 - Protections circuits: (a) over-voltage circuit, (b) over-current and short-circuit, (c) under-voltage lockout circuit (d) shutdown circuit D. Complete Diagram of the dc-dc Converter: The complete diagram of the dc-dc converter is shown in Fig. 6. As can be seen, the control scheme is implemented using just two ICs. The PWM regulator UC3527 IC provides the gate drive signals to the MOSFETs. The control loop regulators are accomplished by employing a Quad operational amplifier TL074. 111. RELEVANT ANAJ..,ISYS The average current through of the boost inductor can be obtained from Fig. 3.c as: - Z(t1) + Z(t2) 'Lb,avg - 2 (3) The output current reflected to the transformer primary, obtained from Fig. 3.f, is given by: O T The discharging time is given as follows: (4) From (3), (4) and (5) , it is obtained: z; = 2 . z . ( l -D) U, "8 Considering Pi, = Pout, results in: (7) Substituting (6) in (7) the converter dc gain is obtained: As can be seen from (8), in order to operate in the boost mode (V: > Via), the following condition must be satisfied: IV. EXPERIMENTAL, RESULTS A laboratory prototype was built using the following data specifications: Input Voltage: V, = 58 to 85 Volts Output Voltage: Vo = 200 +/- 5% V Output power: Po = 300 W Switching frequency: & = 50 kHz 58 9 71.8- I Fig. 6 I 1 i startingcimrit VCC 4 - Complete diagram of the dc-dc The experimental results were obtained for: v,=5ov vo=200v Po=300W f,=SOkHz The steady state waveforms are shown in Fig. 7 and 8. The switch S1 voltage and current are shown in Fig. 7. The switch currents can be seen in Fig 8. The transient waveforms are shown in Fig. 9 to Fig 13. Fig. 9 shows the comp1t:te converter start up. A detailed view of' the output voltage and the boost inductor current at start up is shown in Fig. 10. The output voltage and boost inductor current behavior under load changes are shown in Fig. 11, Fig. 12 shows the over current and short circuit protection actuation. A complete start-up at short-circuit is shown in Fig. 13. Fig. 14 shows the converter efficiency as a function of the output power. converter. Fig. 7 - S1 voltage and current; scales: voltage: lOOV/div; current: SA/div time: 2,Spddiv. 590 Fig. 8 - Switch currents; scales: current: Wdiv., time: 2,5ps/div. (a) TeK Stop: sinele Sea 1oo)cys 17JW 1996 Tek Stou: Sinaie Sea 5.00kS/s 17Jun 1006 Fig. 9 - Complete output voltage and boost inductor current start up; scales: voltage: 1 OOV/div.; current: 2A/div.; time: 50mddiv. TeK Stou: sinde sea 1 o o ~ / s 17iun IQ96 Fig. 10 - Output voltage and boost inductor current start up detail; scales: voltage: lOOV/div., current: 2A/div., time: 2.5ms/div. (b) Fig. 11 - The output voltage and the boost inductor current for load steps: (a) from 10% to 100% of the rated load; (b) from 100% to 10% of the rated load; scales: voltage: 2 V/div., current: 2A/div., time: 2,5ms/div. TeK stop: smia sa I.OO~Q/S 24 JIM 10D6 Fig. 12 - The output voltage and boost inductor current for the over current and the short protection circuits actuation; scales: voltage: 5OV/div., current: SNdiv., time: 25Oms/div. TBK Stop: Single Seq 2.5OkSls XJun 1006 b- 10:53:57 Fig. 13 - The output voltage and the boost inductor for a complete start up at short circuit; scalcs: voltage: lOV/div. current: SA/div., time: 100ms/div. Fig. 14 - Efficiency as a function of the output power. V. CONCLUSIONS This paper presented. a dc-dc isolated converter, which can be employed as a primary stage for an on-board diesel-electric engine dc-ac power supply. Two control loops operating in cascade mode were used. This control strategy results in faster responses to load changes, as well as a better output voltage regulation. These characteristics are achieved in spite of the fact that a low value of output capacitance in use if compared to that of conventional single voltage control loop. The converter reliability is increased by incorporating over- voltage, over-current and short-circuit protections. The converter performance has been verified for a 300W laboratory prototype operating at 5OkHz. The measured efficiency was 93% and the output voltage regulation was less than 1% for 10% to 1013% load changes. Consequently, it is an attractive choice for on-board ac power supply. ACKNOWLEDGMENT The authors would like to express their gratitude to “Thornton Inpec Eletrhica” Ltda. for contributing the magnetic core for this project, and to “Autom6tica Tecnologia” for the studentship support. REFERENCE [l] T.S.Latos and D.J.Bosac, “A High Efficiency 3kW Switch Mode Battery Charger”, IEEE - PESC’82, [2] R.Redl and N.O.Soka1, “Push-pull Current-Fed Multiple-Output (regulated) DC-DC Power Converter with only One Inductor and 0-100% Switching Duty Ratio”, IEEE- [3] C.P.Henze, J.A.Smith and D.S. Lo, “A transformer Isolated AC to DC Switch-Mode Power Converter with Resistive Input Current”, IEEE Conference Publication, [4] V.J. Thottuvelil, T.G. Wilson and H.A. Owen Jr, “Analysis and Design of a Push-pull Current-Fed Converter”, IEEE-PESC Record 1981, pp. 192-203. [5] V.J. Thottuvelil, T.G. Wilson and H.A. Owen Jr, “Small-Signal Modeling of a Push-Pull Fed-Current”, IEEE- PESC Record 1982, pp. 225-239. [6] Unitrode Integrate Circuits, “Products Applications Handbook”, 1993-1994, Application Note, pp.9.457-9.468. pp.341-349. PESC’80, pp. 341-345. N-.291, July 1988, pp. 428-439. 592