# Floating Step-Down Converter with a Novel Lossless Snubber
## 1 Introduction
### Switching Loss Reduction Based on the Snubber  
Usually, a snubber is mainly constructed by a resonant inductor and a resonant capacitor. The structure of a turn-on snubber connects the resonant inductor in series with the main power switch, while the structure of a turn-off snubber connects the resonant capacitor in parallel with the main power switch. The structure can be categorized into active and passive types according to whether the structure contains an active power switch or not.  
As for the active snubber, it will have an additional auxiliary switch and its auxiliary circuit, and before the main power switch is turned on or/and off, the auxiliary switch will act first, together with the auxiliary circuit, to transfer the energy from the main power switch to the auxiliary circuit so as to achieve soft switching. The concept is called zero voltage transition (ZVT) and zero current transition (ZCT) and can also reduce the stress on the main power switch. But the disadvantage is that it requires additional switching costs and complex control strategies. Reference [9] presents the ZVT technology; the auxiliary switch is not yet able to achieve soft switching, resulting in additional switching losses and limited efficiency improvement. Reference [10] proposes a dual-switch structure that can realize high boost voltage; however, its analysis is too complicated and requires an additional equivalent circuit.  
As for the passive snubber, it is composed of passive components and will be the trend today because it simplifies the complexity of the circuit and the control strategy. Depending on whether this snubber uses a resistor or not, it can be categorized as lossy or lossless. The most common of the former are the RC snubber [11] and the RCD snubber [12]. Reference [11] proposes an exhaustive design method for the RC snubber by deriving the frequency-domain equations of conduction and cutoff and checking the position of the poles to reduce the oscillations. Although parasitic components are taken into account, this algorithm requires a large amount of memory and does not analyze the efficiency when the value of the parasitic components is too small. Reference [12] presents a new type of RCD snubber for the full-bridge DC-DC converter. Although the lossy snubber has a certain degree of effect on suppressing spikes and oscillations, the efficiency improvement of the circuit is limited to a certain extent due to the existence of its resistance, which causes a certain degree of loss. References [13; 14; 15] propose a boost circuit with a passive lossless snubber. Although the equations of the three modes in [13] are analyzed, only mode 2 can achieve ZVS cutoff, and the other two modes are only near-ZVS cutoff. The circuit mentioned in [14] has both ZVS turn-off and ZCS turn-on for the main power switch. But such a circuit requires additional snubber components in the power transfer path, so there is a higher conduction loss, and furthermore, due to the need to withstand high stresses, the component cost is high. Reference [15] proposes two snubbers to minimize the stress without giving extra stress to the primary power switch, but they use too many componentsand cause extra losses, so the efficiency improvement is limited, and the main improvement in the stress problem is only for light-load conditions.  
In this paper, a novel lossless snubber is proposed and applied to the floating step-down converter with an LED string used as a load. This circuit is mainly derived from [13], which presents a boost converter topology. The proposed circuit structure is very suitable for being applied to low-noise optoelectronic systems such as SPADs [16; 17; 18], time-resolved circuits [19; 20; 21], or wearable EMG platforms [22; 23; 24]. Incidentally, the proposed circuit can be controlled by the fixed frequency with a smaller number of components used to achieve high efficiency by ZVS cutoff.