WIRELESS power transfer has been widely used for body implantable medical device to address power supply problem. However, there are concerns regarding the implant location. The size of an implantable system with a wireless power receiving coil should be sufficiently small to fit in the location. The coupling coefficient is proportional to the ratio of the size of the power receiving coil and power transmitting coil . Therefore, coupling coefficient in the body implantable device is usually extremely low . In extremely low coupling condition, voltage gain is susceptible to the implant depth. Since the thickness of the skin varies for each person, the received voltage of the receiver varies a lot for each person. Therefore, a DC converter is needed to provide stable power even when the implant depth varies in extremely low coupling condition.
A switching converter is more efficient than a linear converter in a wide voltage conversion ratio condition.Therefore, a switching converter is preferred as DC converter in a wireless power transfer system for body implantable applications . This converter type uses many external components such as inductor, storage capacitor, bootstrap cap, and components for the compensation circuit. The use of manyexternal components should be avoided in body implantable medical devices as these increase the size of the device. The inductor and storage capacitor are essential components of a switching converter and cannot be removed. The switching converter for the body implantable device operates in discontinuous mode (DCM) since the device is operated at low power. In DCM, the number of components for the compensation circuit can be further reduced and integrated on the chip so that such components are not used as external components. To remove bootstrap capacitor, the level shifter should be the only component of the driver for a high-side power switch. However, the level shifter is always-on circuit and its high power consumption when it is used in a driver degrades power conversion efficiency.
1.1.0 Conventional Switching Converter Drivers
A driver circuit is used to operate the power MOS switch of a DC converter and class D amplifier. A taper buffer with scale factor that is proportional to the input capacitance to output capacitance ratio is generally used as driver . The taper buffer is made up of an inverter chain. However, if the input voltage of the converter is greater than the driving voltage of the buffer, the buffer alone will not be able to drive the power switch. Several driver techniques can be applied for high input voltage such as using a bootstrap capacitor, using a high-side ground, and using a standalone level sifter.
The most popular method to drive a high-side MOS switch is using a bootstrap capacitor. Bootstrapping is used when the high-side MOS switch is an NMOS. The principle of bootstrapping is charging the voltage to drive the MOS switch to the capacitor using a diode and operating the low-voltage taper buffer using the charged voltage. The cathode of a bootstrap capacitor is connected to the source of a high-side MOS switch to realize an identical reference. Since the reference voltage is the same, the voltages of the bootstrap capacitor and taper buffer swing together with the high-side MOS switch. The driving signal is provided by the level shifter. The advantages of using bootstrapping is that it uses an NMOS as a power switch. An NMOS has a lower on-resistance compared to a PMOS, which has a low conduction loss. Also, driving loss is low because input capacitor is smaller within identical on-resistance. Bootstrapping can be applied whenever the supply voltage varies. However, a bootstrap capacitor cannot be integrated on a chip because a high capacity capacitor is required to support bootstrapping. Therefore, the use of an external capacitor is necessary. Applications with size constraints, such as body implantable devices, are not appropriate for bootstrapping.
1.1.2 High-side ground method
The high-side ground method is applied when PMOS is used in a high-side MOS switch. The high-side ground method is similar to bootstrapping; however, the process of selecting the reference voltage is different. In bootstrapping, the reference voltage is the source of the MOS switch, while an extra voltage regulator is needed to generate the reference voltage in the highside ground method. The power consumption of the driver is low since a low voltage taper buffer is used as the driver. However, flexibility with respect to the supply voltage variations is poor owing to its fixed reference voltage. The driver can be designed with adaptive reference voltage, but its efficiency is lower than that of bootstrapping.
- Standalone level shifter driver
A level shifter is a circuit that increases a low reference level swing to a high reference level with the same swing voltage. Using a level shifter as a driver eliminates the need for an external capacitor because a low voltage gate driver such as an inverter chain is not required. In addition, since it only increases the reference level of the voltage swing, it can operate properly even with variable supply voltage.
The power consumption of a level shifter differs from that of a digital inverter. Fig. 1 shows the schematic and waveform of a conventional level shifter. The level shifter always consumes current alternating between IF and IR. This current consuming property is a serious drawback of the converter driver because it lowers the power conversion efficiency (PCE). The larger the driving capacitor, the faster the switching frequency; thus, the level shifter becomes the dominant component that reduces thePCE.
- Size of the converter is increase.
- More power consumption.
In a recent technology of Wireless power transfer has been widely used for body implantable medical device to address power supply problems. To avoid this power supply problems here this paper will introduced an instantaneous power consuming level shifter to increase the DC converter efficiency. In this level shifter which have a capability to remove high power driver instead of capacitor bootstrap technique and delay cell, its consume less power consumption during the transition period. Here, proposed work of this paper using this modified level shifter to verified asynchronous discontinuous conduction mode buck converter at 45nm and 130nm CMOS Technology in TANNER EDA Tool. Finally this work will compared all the parameters in terms of area, delay and power.
DC-DC converters are widely used to efficiently produce a regulated voltage from a source that may or may not be well controlled to a load that may or may not be constant. This paper briefly introduces DC-DC converters, notes common examples, and discusses important datasheet parameters and applications of DC-DCconverters. DC-DC converters are high-frequency power conversion circuits that use high-frequency switching and inductors, transformers, and capacitors to smooth out switching noise into regulated DC voltages. Closed feedback loops maintain constant voltage output even when changing input voltages and output currents. At 90% efficiency, they are generally much more efficient and smaller than linear regulators. Their disadvantages are noise and complexity DC-DC converters come in non-isolated and isolated varieties. Isolation is determined by whether or not the input ground is connected to the output ground.
A rising edge is the transition of a signal from a low state to a high state. The falling edge detection is done in a similar manner as the rising edge. The only difference is that we check if the current value of the signal is lower than the value it had one time step before.
- Reduced the speed of transistors with technology basic and also time and voltage resolutions
- Less Power consumption.