1. Introduction

Boost converter are widely used in industry for the following applications such as discharge lamp driver, UPS, Motor drivers, and PV system [1, 2]. The boost converter or step up converters is simple in structure, but the voltage conversion ratio is less, whereas the fly-back converter possesses a high voltage conversion ratio but the corresponding leakage reactance is large. The voltage conversion ratio can be improved when the number of inductor in the circuit is increased, and these inductors are connected in series during their demagnetizing period [3, 4].

Normally isolated boost converters boosts the voltage to much higher voltages with wide input ranges typically in the range of 30-60V. Since the DC-AC converter operates at high voltage and is widely used in high power applications such as in UPSs, motor drives, solar converters etc., it is required to have low voltage to high voltage DC-DC converter with high efficiency.

High boosted output voltage is obtained by discharging the energy stored in the inductor and capacitor along with input voltage into the output terminals. In this paper, a brief illustration of the operation of high voltage boosting converter is given along with some simulation results provided to demonstrate the effectiveness of such converters. In [5-8], high voltage conversion ratios are achieved by coupling inductors, but the voltage spikes due to the accompanying leakage inductances and the complexity in the corresponding circuit analysis are unavoidable.

2. Proposed Converter Topology

In this paper a new high voltage boosting converter is proposed. The circuit is shown in figure 1.

Figure 1. Proposed High voltage boosting converter

The converter contains three MOSFET switches S1, S2, and S3, two bootstrap capacitors C_b and C_e, three bootstrap diodes D_b, D₁, and D₂, one output diode D_o, two inductors L₁ and L₂, one output capacitor C_o, and one load resistor R_L. In addition, the input voltage is signified by V_i, the output voltage is represented by Vo.

3. Basic Operating Principles

3.1. Continuous Current Mode Operation

Figure 2 shows the illustrated necessary waveforms of vgs1, vgs2, vgs3, v_L1, v_L2, i_L1, and i_L2 for the proposed circuit operated in CCM with L1 equal to L2.

Figure 2. Waveforms of CCM operation

3.1.1 Mode 1 [t₀ - t₁]: The current flow in the proposed circuit during mode 1 in CCM is shown in figure 3. As in the figure in mode 1, S2 and S3 are turned on, but S1 is turned off. Diode Do is reverse biased, but D_B, D₁ and D₂ are forward biased. The capacitor C_B and the inductor L₁ charges through switch S2. And the capacitor C_e and the inductor L₂ charges through switch S3. The capacitor voltages abruptly reach to its peak value Vi. At the same time, the voltages across L₁ and L₂ both are Vi, thereby causing L₁ and L₂ to be magnetized. Also, Co releases energy to the output.

Figure 3. Operation of the proposed circuit in mode 1

In this mode, the voltages across L₁ and L₂, v_L1−ON and v_L2−ON, can be written as

(1)

(2)

3.1.2 Mode 2 [t₁- t₂]: Mode 2 operation is shown in figure 4. The switches S2 and S3 are turned off, but S1 is turned on. At the same time, the input voltage plus the energy stored in C_b and C_e plus the energy stored in L₁ and L₂ supplies the load, thereby causing C_o to be charged, C_b and C_e to be discharged, and L₁ and L₂ to be demagnetized. By doing so, the output voltage is boosted up, and is higher than the input voltage. According to the voltage-second balance, the voltages v_L1−OFF, v_L2−OFF, and V_o can be expressed as

Figure 4. Operation of the proposed circuit in mode 2

(3)

(4)

(5)

Since , and from equation (1)-(4), Equation (5) can be written as

(6)

CCM voltage conversion ratio is then given by:

(7)

3.2. Discontinuous Conduction Mode Operation

Figure 5 shows the illustrated key waveforms v_gs1, v_gs2, v_gs3, v_L1, v_L2, i_L1, and i_L2 for the circuit operated in DCM with L₁ equal to L₂.

Figure 5. Waveforms during DCM mode operation

3.2.1 Mode 1 [t₀- t₁]: In this mode, the operating principle is the same as that of circuit operated in CCM in mode 1. Hence, the associated equations can be written as

(8)

(9)

Since L₁ is equal to L₂ and set to L,

(10)

3.2.2 Mode 2 [t₁- t₂]: In this mode, the circuit operates same as CCM in mode 2. Therefore, the related equations can be written as

(11)

(12)

3.2.3 Mode 3 [t₂ - t₃]: In this mode, all the switches and diodes are turned off, and the currents in two inductors are zero. Hence, the energy needed by the load is supplied from Co. The output voltage is expressed as

(13)

Since , and from equation (11) (12)

(14)

Simplifying the above equation results

(15)

4. Simulation Results

The circuit simulated in SIMULINK/MATLAB in open loop. The various parameters given according to design as explained earlier are shown in Table 1.

Table 1. Simulation Parameters

The simulated circuit in MATLAB/SIMULINK is shown in Fig.6. The input DC voltage is given using DC voltage supply block. There are three switching MOSFETs in the circuit. Gating pulses are given to the switches using pulse generator. The signals so formed are given to a scope to be verified. The input voltage, output currents, current through the inductors, voltage across the output load is analyzed.

Figure 6. MATLAB simulation of proposed topology

4.1. Input Voltage

DC voltage of 24V is given to the converter input terminals.

4.2. Output Voltage

The output voltage obtained is as shown in Fig.7. The output voltage of 155 V is obtained and the ripple content in the output voltage is very low. The model is simulated by setting duty ratio as 0.65 to achieve this boosting.

Figure 7. Output voltage waveform

4.3. Output Current

The output current obtained is as shown in Fig 8. The output current of 0.38A is obtained.

Figure 8. Output current waveform

4.4. Inductor Current and Gate Pulse

The inductor current and gate pulse waveform when L1=L2 is shown in Fig.9. Here the inductors are of same value. The inductor and current waveform when L1>L2 is shown in Fig.10.

Figure 9. Inductor current and gate pulse (L1=L2)

Figure 10. Inductor current and gate pulse (L1>L2)

5. Conclusions

A high voltage boosting converter based on inductors connected in series with bootstrap capacitors is proposed in this paper. From the experimental results, such converters exhibit good performances even with different inductances, and hence are suitable for industrial applications, such as the energy-recycling burn-in test of the buck-type converter, isolated or non-isolated. From the detailed simulations an experimental analysis, it is clear that the presented converters have the following advantages.

1. The proposed system is simple and easily constructed to achieve expected voltage conversion ratios with less loss.

2. For each converter, the power switches are easy to drive, as this converter only needs one PWM control signal to drive all three switches making its controlling simple.

3. From the simulation results, such converters exhibit good performances even with different inductances, and hence are suitable for industrial applications.

4. Proposed converters need lesser components making it less weight, low cost and compact.