The metal to semiconductor diode as laid out by the theories of Walter Schottky in the 1930s is used for its attractive electrical parameters such as a low forward voltage (VF) and fast switching behavior (tRR).
But these advantages also come with a cost. The main complaint about Schottky Diodes is their relatively high leakage current. This leakage current, denoted as ‘IR’ (current in reverse direction), is usually in the μA (10-6 A) range for small Schottky diodes and can reach mA (10-3 A) for larger power diodes. In comparison, a low leakage PN junction Diode (semiconductor to semiconductor junction) operates in the nA (10-9 A) range, such that even large diodes have only μA of leakage.
With battery driven applications such as cell phones, tablets and smart watches, this downside of the Schottky diode can easily become a major concern for battery life. To respond to these concerns, the industry tried to approach this by introducing transistor based ‘Schottky-like’ devices which promise similarly low VF but with significantly lower leakage current. While this approach worked well in some cases, it required a sacrifice of another hallmark parameter of Schottky diodes - namely the fast switching time. It also added more complexity in the fabrication process of such a device, as more complicated CMOS based processes had to be used.
Is it now time to say goodbye to the Schottky Diode?
Most definitely not! ON Semiconductor has continued to invest in research into the Schottky diode and now has the industry’s only small signal trench Schottky solution, thus providing a real Schottky for power sensitive applications. While trench based Schottky Diodes are well established in the industry for high power Applications, ON Semiconductor did expand this technology into the small signal world, in order to also have an advanced Schottky product for applications such as LED booster, battery management and wireless charging.
This new line of small signal trench diodes offers the renowned low VF and fast tRR of the Schottky, whilst also maintaining low leakage which is comparable, and in most cases better than, available ‘Schottky-like’ products. The combination of low VF and low IR to optimize the power dissipation for energy sensitive applications is one of the hallmarks of these small signal trench Schottky diodes.
This technology in turn enables the engineer to use the improved benefits in applications which place a high demand on efficiency and power dissipation. One example is the use in wireless charger applications.
Schottky bridge in Wireless Charging application
As the energy coupled through wireless transmission into the power receiving Unit (PRU) is relatively small, all further losses in converting the energy should be minimized in order to get the best charging rate. A critical element in this chain is the bridge rectifier which converts the alternating current waveform into a DC power signal. This is then further processed by a DC/DC converter to bring the voltage to compatible levels for charging the battery of the wireless device. The bridge rectifier needs to therefore have the lowest impact on power dissipation, which means that forward voltage and current losses should be minimized as they reduce the precious limited power transmitted by the Power Transmitting Unit (PTU).
How the Vf and the Ir impacts the overall effectiveness of the Full bridge.
As an example we look at a positive half wave of the receiving antenna coil. The voltage amplitude of the wave (Vwave) will be reduced by the forward voltage drop of the Diode D1, resulting in an effective voltage of (Vres= Vwave-Vf) which is then fed into the DC/DC converter. On the other hand, the received current wave, Iwave will be reduced mainly by the leakage (Ir2) of diode D4 and partially also by the leakage of diode D2.
The resulting current useful for the receiving circuitry is therefore Ires=Iwave –(Ir2+Ir4). The new Trench Schottky products are optimized for this case in a way that the product of forward voltage drop (VF) and reverse current losses (IR) are optimized to have the lowest power dissipation (PD).
Why is this so important?
Think about a Schottky diode that has an excellent Vf of 0.2V but an Ir of 3mA . The excellent minimal forward voltage drop would mean nothing in a bridge rectifier if the rectified pulse were literally eaten by the leakage Ir of the other diodes in reverse direction. Conversely, if you would have an extremely small leakage of 1nA (Remember in the first section I mentioned PN Diodes?) but your forward voltage drop is in the 0.8V range. You lose too much on the initial Voltage and will suffer from this when trying to boost the DC/DC voltage conversion. So the goal is to have a balance between Ir and Vf so that the power losses are minimized and the signal voltage is as close as possible to the value on the receiving coil. Optimizing the power losses in the new small signal Trench Schottky portfolio is where ON has invested R&D money.
The advantages described did not come at the cost of more complex processing which in turn might lead to the risk of reliability problems. ON Semiconductor’s research team instead focused on the ease of manufacturability with highest demand for quality and reliability, so that markets such as automotive could be addressed. The first series of the new small signal trench Schottky diodes are already available (NSR05T).
A follow-up for an improved generation is in development and will push the combined boundaries of low Vf and Ir with extremely low power dissipation.