Recently, we have seen an increasing demand for 900 V capability for their switching power converter ICs. We were initially surprised at this trend as, on the face of it, 900 V seemed to be greatly in excess of the application requirement. So we investigated further and came up with two challenges:
- How can the appliance industry cope with unstable mains electricity supply?
- High line and multi-phase industrial applications need to be very rugged.
Unstable mains may seem to be an odd problem to be discussing within the pages of a magazine such as ECN. But in fact, many products are now designed to be sold in the global market, so a washing machine that is sold in a New York may be the same model that can be bought in London or Tokyo…or Delhi. And right there is the challenge: India—as the world’s sixth largest economy—is a huge and potentially highly lucrative market for goods, yet its utility health is ranked only 102nd in a 2018 IMF study. This is not because India does not have some very clever design engineers, it is simply that the Indian economy is expanding so quickly that infrastructure cannot keep up.
Line faults, phase imbalance, and system short-circuits lead to frequent line sags/surges, and it is common to see ≥350 VAC on a 240 VAC line. Typical line-voltage distortions are shown in Figure 1. Put simply, power conversion solutions for the India market—and others such as Brazil, all across Africa, and in parts of SE Asia—must be very robust. In such circumstances, 900 V MOSFET-based converters deliver field-reliability benefits to all types of mains powered products and typically the higher the value of the equipment, the more likely it is to incorporate a robust power solution. However, even in relatively inexpensive products, the loss of brand-trust due to a line-induced failure may be unacceptable, and so we anticipate increased use of the high-voltage devices as the market matures and regional brand preferences develop.
The second challenge is how to best support high-line and multi-phase industrial applications. Globally, the voltages required in factories and other high line applications commonly range from 380 to over 400 VAC. In this application, the delicate electronic controllers and processors are attempting to sip a few watts from a fire hose capable of milliwatts, so MOSFET robustness and protection features such as overvoltage shut-down are appreciated.
But why specifically 900 V? Most flyback controllers are based around a 650 V or 750 V MOSFET. This is problematic because it does not provide enough headroom for 400 VAC applications. During operation, the MOSFET will be stressed—as a rule of thumb—to a little over twice the nominal operating voltage, which in a 400 VAC system will exceed the breakdown voltage of 650 V or 750 V parts. Figure 2 shows this and illustrates how a 900 V device where the MOSFET stress is around 800 V, supports 400 VAC operation with a 10 percent margin. Such devices will enable continued operation during line overvoltage and ensure safety in the case of faults, such as an incorrect installation or the disconnection of a neutral line.
Industrial products are sometimes specified for operation across a very wide range of voltages, for example 30 VDC to 500 VDC—particularly those operating from battery banks or from intermittent supplies such as from solar panels. It’s a great benefit if the switching power converter IC family can start up and provide reliable power to the load over such a wide dynamic range. Electric vehicles are an example of an application with wide voltage requirements.
