Zero Voltage Switching Converters

In this paper zero voltage switching converters are investigated. The investigation starts by discussing the RCD charge-discharge snubber. The concept of resonant and quasi-resonant DC link converters is discussed. One of the most promising quasi-resonant DC links reported in the literature is implemented and tested in a battery charger application. Simulated resonant link voltage and current waveforms are analyzed. IGBT switching waveforms under zero voltage conditions are investigated. Measured waveforms are shown and the converter and the overall battery charger efficiency are measured.


INTRODUCTION
Several problems associated with hard switching are reported in the literature. The main problems are the semiconductor losses due to the finite duration of the switching transients and the electromagnetic compatibility (EMC) problems associated with the high voltage derivative with respect to time, occurring especially at the turn-off transient. Power electronic converter manufacturers strive towards increased switching frequencies in order to omit the audible noise and reduce the output current harmonic content. For such high switching frequencies, the switching losses dominate, at least if insulated gate bipolar transistor (IGBT) technology is employed. IGBT technology is the most common choice for mid-power converters due to its ease of drive, high ruggedness and favorable combination of VOLUME 1, ISSUE 1 NOV VOLUME 1, ISSUE 1 NOV VOLUME 1, ISSUE 1 NOV VOLUME  In Fig. 2.2, the IGBT is exposed to a current spike at turn-on due to reverse recovery of the freewheeling diode. It is seen that the IGBT is exposed to simultaneously high current and voltage during the switching transients. This causes high switching losses, especially at turnoff since the IGBT exhibits a collector current tail there.

SOFT SWITCHING BY MEANS OF SNUBBERS
To partly overcome the previously mentioned problems and to use the semiconductor devices in a more efficient way, snubber circuits are introduced. Various snubber circuits are used for different purposes, for example to reduce the semiconductor switching losses. One such snubber is the RCD (resistor, capacitor, and diode) charge-discharge snubber. From now on, this snubber is referred to as the RCD snubber.
Step down Converter

Half Bridge Converter
To investigate the switching waveforms for a three-phase converter with RCD snubbers, one across each IGBT, an entire half bridge has to be considered, see Fig. 3.3. The upper and lower RCD snubber provide soft turn-off for the upper and lower IGBT, respectively. The simulated collector current and collector-emitter voltage are shown in Fig. 3 Fig. 3.3, denoted C s1 , should be discharged and the lower, denoted C s2 , should be charged. The snubber resistor R s1 limits the discharge current of C s1 as in the previous case but the charging current of C s2 is not limited by any other component.   turn-off of the upper IGBT in the converter consisting of a bridge leg. One RCD snubber is used across each IGBT. Note the high collector current peak at turn-on. Also note the poor behavior at turn-off.
One of the main problems related with soft switching appears due to poor understanding of power semiconductor physics, since it is assumed that data sheet information is still valid for soft switching. However, data sheet information for IGBTs is in most cases given for inductively 6 | P a g e clamped load, i.e. constant load current during the switching transients. Also, the information is only valid for certain constant DC link voltage.In the literature, several problems associated with soft switching is discussed.
One of the most frequently discussed phenomena is the current tail bump occurring at IGBT zero voltage turn-offs. In Fig. 6 the current tail bump is clearly seen. According to, the reason for this bump is that during ZVS turn-off the excess carriers stored in the drift region are not forced out by the expanding depletion region, as is the case for hard-switched turn-off. Consequently, after the channel is removed, the collector current continues its decrease and no current tail is observed until the IGBT collector-emitter voltage begins to increase. The current tail bump results in higher losses for ZVS turn-off than expected from data sheet information. Nevertheless, the turnoff losses are lower for ZVS than for hard switching.

THE RESONANT DC LINK CONVERTER
An important step in resonant converter technology was taken in 1986 when the resonant DC link converter was invented. For the resonant DC link converter, one resonant circuit is used to provide soft switching for the entire converter. As the name resonant DC link indicates the DC link is forced to oscillate. The basic three phase resonant DC link converter is shown in Fig. 4.1.   In the first case, there is excess energy stored in the resonant inductor due to the previously high current through L r , corresponding to the DC link current. This energy must decrease to meet the new DC link current, which implies that the energy must be transferred to the resonant capacitor C r .
In the second case, a too small amount of energy is stored in the inductor, which results in a prolonged zero voltage intervals. The length of the zero voltage intervals is determined by the time needed for the inductor current to reach the same level as the converter current. When the inductor current reaches the level of the converter current, the resonant capacitor is charged, since the inductor current continues to increase. Another problem of this circuit is that carrier wave pulse width modulation (PWM) cannot be used since the possible switching instants are determined by the resonant circuit. Instead, other modulation strategies must be applied. However, these modulation strategies require that the resonance frequency is much higher than the switching

CONCLUSIONS
Zero voltage switching converters are investigated. The quasi-resonant converter was introduced to cope with problems of ZVS converters based on snubbers or resonant links. The quasi-resonant DC link converter investigated at first seems promising since it can be triggered on demand, and operates without excessive over voltage due to the clamping network. However when implemented, measurements show that the converter losses increase compared to the hard-switched counterpart at a switching frequency of 5 kHz. Despite the low switching frequency, the measurements show that the commanded switching's are delayed, causing low order harmonics