DESIGN AND PERFORMANCE OF ZCS-ZVS COMBINED SNUBBER NETWORK IN A DC-DC APPLICATION

Mohd Hasanul Arifin Mashod , Nor Zaihar Yahaya

School of Electrical and Electronic Engineering Universiti Teknologi Petronas, Bandar Seri Iskandar,

31750 Tronoh, Perak Darul Ridzuan, Malaysia. E-mail: mashod_jr@yahoo.co.uk, norzaihar_yahaya@petronas.com.my

ABSTRACT In this paper, a combined zero-current switching and zero-voltage switching (ZCS-ZVS) snubber network is proposed for a DC-DC boost converter. This technique can reduce the ringing and spiking on drain-source voltage (VDS) during switching. Thus, it will reduce the EMI radiation and the voltage and current stress encountered by switches. A boost converter with 60 V input voltage, 80 V output voltage and 100 kHz switching frequency adopting this technique is presented as an example, to illustrate the circuit operation principles and derive the design procedures. Simulation and hardware implementation of a combined snubber boost converter has been made to validate the operation and the efficiency. Experimental result shows an improvement in output power efficiency of 94%. This is an increment of 20% compared to the conventional boost converter. KEY WORDS Pulse-Width Modulation, Soft Switching, Snubber Network and DC-DC Converter. 1 Introduction

Many DC-DC applications demand high power density power converters. This leads to the research of converters that can provide high efficiency and high power operation, which is a great concern due to the switching stresses and losses in semiconductor devices during the power conversion stages. Reducing these losses without increasing device voltage and current stress will be the main factor in producing efficient soft-switching scheme [1]. Several circuit topologies have been proposed in recent years to achieve this target and most of them commutate in the range of line voltage or load current during a small resonant transient and operate in constant-frequency pulse width modulation (PWM) with minimum voltage and current stresses [2,3].

In order to propose better topology, zero-voltage-switching (ZVS) converter can be applied. It contains an active snubber circuit to achieve soft switching [4, 5]. The snubber circuit normally consists of several auxiliary components that can be connected across the switch or at any parts in the circuit. This component layout configuration will generate soft switching and hence reduce switching stress and also losses [6].

On the other hand, hard switching will lead to an increase of switching loss during the turn-off of the switch and during its turn-on, the switch requires an

elaborate cross-zero detection circuit otherwise the switch may suffer from high current stresses [7]. A zero-current-switching (ZCS) converter can be used to solve this problem however it will then again suffer from increased voltage stress [8, 9].

Using ZVS circuit system alone in the design is not desirable to achieve high power efficiency especially at light load and large duty cycles [10]. Moreover when the current stresses during turn-on are increased, the designs become relatively complicated. The soft switching cannot be achieved easily once the output voltage exceeds twice the input voltage [11] which makes boost converter topology as the preferred test circuit. Duty cycle selection range for the boost converter design is also limited by the soft-switching resonant commutation [12].

A new soft-switching method is proposed in this paper using a passive dissipative snubber to achieve zero-current turn-on and zero-voltage turn-off for the active switches, as well as soft switching for the passive switches, without increasing their voltage and current stresses. Its operation principle is analysed and the design issues are discussed. The performance of this soft-switching method is demonstrated and verified with the experimental results. 2 Operation of the boost converter

To demonstrate the viability of the ZCS-ZVS combined snubber network in a DC-DC application, boost converter topology is chosen to be the test circuit due to its requirement in achieving soft switching whenever its output voltage is at least twice or more than the input. The conventional boost converter circuit is shown in Figure 1.

During the time the switch is closed, energy is transferred to the inductor while the diode prevents the capacitor to discharge through the switch. When the switch opens, current, which initially has been charged, will discharge through the inductor. This current continues to flow in the same direction as during the previous cycle. This will forward bias the diode and both input voltage source and the inductor transfer energy to the load. Hence, a voltage boost occurs across the load, which causes the output voltage to be higher than the input voltage. The capacitor value is large enough to keep the output voltage approximately constant. Also, the system is designed to function in the continuous conduction mode (CCM) and at any time, the inductor current will never reach a zero value.

Figure 1 : Convention

3 Snubber network

A snubber network reduceone or more power switches ieither turned-on or turned-offnetwork in the converter desigcurrent stress (a switch encocurrent simultaneously durinencountered by switches and dPWM design [14]. A passivebeen considered with resistor (C) and inductor (L) to controwhile the energy stored is dissorder to achieve ZCS conditisnubber inductor in the circuit the snubber diode is used to disof the snubber inductor. Thprocess is normally arrangsemiconductor switch to ach[15]. It is usually used in a num

a. Control the rate of risevoltage level acrossduring turn-off, whisnubber.

b. Control the rate of switch and diode ducalled a current snubb

c. Damp ringing of voldiodes, which is called

Figure 2 shows a combined reduce ringing and oscillation.

D1

al Boost topology

s switching losses when n DC-DC converters are [13]. Adding a snubber n reduces the voltage and unters high voltage and g switching transition) iodes experienced in the dissipative snubber has (R), diode (D), capacitor l the voltage and current ipated in the resistor. In

on, it usually places the to control the di/dt while charge the stored-energy e capacitor discharging ed to occur over the ieve the ZVS condition ber of ways such as to: of voltage, or maximum

the switch and diode ch is called a voltage

rise current through the ring turn-on, which is

er. tage across switches and a RC snubber.

snubber configuration to

I

Figure 2 : Combined

The circuit configuratioreduce the ringing and oswitching, and hence could The circuit configuration shohand is used to reduce thswitching. Hence, it will redstress encountered by swDC-DC converter.

IDS

Figure 3 : Combined snubb

spik

DS VDS

snubber configuration

n of Figure 2 is used to scillation on VDS during reduce the EMI radiation. wn in Figure 3 on the other e spiking on VDS during uce the voltage and current itches as experienced in

VDS

er configuration to reduce ing

3.1 Circuit operation A circuit of the boost converter is designed as shown in Figure 4 using the combined snubber network.

Figure 4 : Resonant boost converter with combined

snubber

The resultant waveforms of the converter from simulation are presented in Figure 5 and the modes of operation of the circuit are shown in Figure 6.

Figure 5 : Relevant waveforms for boost converter

with combined snubber network

Each mode is explained with the corresponding circuit of Figure 6.

Mode 1 (T1 – T2)

In this mode, switch (M1) in the condition from ON to OFF state. In this stage, drain-source current (IDS) still is not equal to zero while VDS is already in zero position. When M1 is totally in OFF state, both IDS and VDS across the switch are zero. For this reason, the circuit is called zero-current switching and zero-voltage switching (ZCS-ZVS).

Output voltage

Input current

Mode 2 (T2 – T3)

Entering this mode, switch M1 is already in OFF condition. Current in D1 (ID1) starts to increase gradually. On the other side, current in D (ID) decreases gradually. In this stage, VDS, voltage across D1 (VD1), voltage across D (VD) and the voltage across Llk (VLlk) are not in zero condition. This is the period where C clamps the peak voltage due to the Llk during switch OFF to the safe level. This condition illustrates the voltage clamping snubber in operation. Mode 3 (T3 – T4)

Oscillations occur between Llk and CP during this mode. RS is used to damp the oscillation until T4. This condition illustrates the RC damping snubber in operation. Mode 4 (T4 – T5)

In this mode, switch M1 in condition from OFF to ON state. In this stage, at T4, VDS is still not equal to zero while IDS is already in zero position. All other waveforms are in transition period during this mode.

VGS M1

VLlkMode 5 (T5 – T6)

In this mode, the switch M1 is going to the ON state condition whereas VDS, ID1, VD1, VD, ID and VLlk equal to zero, except IDS.

IDS

VD1

ID1

VD

ID

VDS

Figure 6 : Stages of operation of boost converter with combined snubber

4 Results and discussion

Figure 7 shows the Pspice simulation of VDS during turn on and turn off of the switch. As can be seen, reduction in spiking and ringing occur in VDS during the turn off. There are some power losses during the transition of switch, however they are minimal.

Figure 7 : Waveform of VDS of resonant boost converter with combined snubber based on Vpulse of the

switch, M1

An experiment is conducted to test the functionality of the switch. Figure 8 shows voltage across VDS of the switch (MOSFET IRF 640).

Figure 8 : Waveform of VDS from oscilloscope

(Graph scale on oscilloscope = Y:50V/div, X:5µs/div)

The VDS waveform of Figure 8 is not really same compared to the simulation waveform as in Figure 7. This waveform shows more spiking during the switching and reduced ringing (oscillations). This reduction in the oscillations also reduce the electromagnetic interference (EMI) effect. The spiking on VDS is caused by the output capacitor of the switch during turn-on. This does not affect much the output

power efficiency since the duration of this spike is very minimal.

4.1 Evaluations

The input ripple current shown in Figure 9 and the output ripple voltage as in Figure 10 of snubber network are evaluated to see the variation compared to their base values. Comparing from simulation, experimentally, both input ripple current and output ripple voltage are only in difference of 2.5% and 0.7% respectively.

VDS (Simulation) a) Input ripple current

(G

b)

(Gr

Ripple = 3.3%

A

Figure 9 : Waveform ofraph scale on oscilloscope =

Output ripple voltage

Figure 10 : Waveform ofaph scale on oscilloscope = Y

Ripple = 5.8%

input current Y:5V/div, X:5µs/div)

Ripple = 1.2%

o:1

Ripple = 1.9%

82.5V

80.9V

5.3

5.0A

utput voltage 0V/div, X:2µs/div)

4.2 Discussion

A comparison between conventional boost converter and combined snubber network is presented in Table 1.

Table 1 : Summary of performance analysis

Criterions Experiment on

conventional boost

converter

Experiment on boost converter with combined

snubber

% of input ripple

current 6.5% 5.8%

% of output ripple voltage 2.4% 1.9%

Spiking on VDS High Reduced

Ringing on VDS High Low

Efficiency, ηPO 0.75 0.94

From Table 1, it is observed that with a combined snubber network, the percentage of input current is reduced compared to the conventional boost converter. Also a reduction in output ripple voltage. Even though the spiking on VDS does not fall within the targeted range, the value is reduced. In addition, the ringing of VDS has been successfully minimized and consequently, indicates the reduction in the generated EMI in the circuit. The output power efficiency shows an improvement from 75% to 94%.

5 Conclusions

In this paper, a combined zero-current switching and zero-voltage switching (ZCS-ZVS) snubber network has been presented for DC-DC application. The passive dissipative snubber is found to be applicable in reducing the ringing and spiking on VDS of the switch during switching. Thus, this reduces EMI radiation, the voltage and current stress encountered by switches. A boost converter with 60 V input voltage, 80 V output voltage and 100 kHz switching frequency adopting this technique has been developed to illustrate the operation principles and the design procedure is derived. Hardware implementations of the converter have been constructed to validate the operation. The experimental work shows output power efficiency of 94%, an increment of 20% from the conventional boost converter.

Acknowledgement The authors wish to thank the Universiti Teknologi PETRONAS (UTP) for providing financial support for presenting the paper. Also to Mr. Musa Md. Yusuf and Miss Siti Hawa Tahir for the technical assistance in accomplishing the project. References [1] M. M. Jovanovic, A technique for reducing rectifier

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