Test Saturation Voltage to Achieve High Efficiency

Mar 1, 2008 12:00 PM
By Richard Dunipace, Fairchild Semiconductor

Efficiency is the name of the game in the design of power supplies. This is a major reason why switch-mode power supplies (SMPSs) are chosen for power-supply designs. However, even SMPSs have their own losses, which are associated with their switching transistors, be they MOSFETs, bipolar transistors or insulated-gate bipolar transistors (IGBTs). Understanding these losses via accurate measurements can minimize the losses and increase efficiencies even further.

This is the first part of a two-part article series on understanding saturation losses in SMPSs. Part one discusses the contribution of saturation losses to power-supply inefficiency, how those losses are a function of a transistor's saturation voltage, and techniques for measuring saturation voltage. The second part, to appear in the next issue, describes in detail how to build a novel low-cost tester for accurately measuring saturation-voltage losses in the presence of high switching voltages or noise.

Forms of Inefficiencies

These inefficiencies generally take three forms: switching losses, saturation losses and the power required to drive the transistors. Let's examine each in more detail.

Switching losses result from the time it takes the transistor to switch from a high to a low level and back again, and are affected by the parasitic capacitances of the circuit and transistor. When switching, the transistor is in a linear mode and hence highly inefficient. These inefficiencies are typically controlled by making sure the switching device transitions quickly and cleanly from one level to the other, and by keeping the duty cycle of the transitions low versus the switching period. Parasitic capacitances are those present in the circuit, but do not contribute to the power output from the power supply. These are controlled by careful layout and selection of the switching transistor and power-supply topology.

Saturation losses are the main sources of inefficiency of the three types of losses mentioned. The saturation voltage is one of the key items that determine the efficiency of SMPSs. Saturation losses are a function of the voltage dropped across the transistor due to the on-resistance of the switching transistor when conducting and the current flowing through the switching transistor. Saturation losses are controlled by selecting a switching transistor with the lowest saturation or on-resistance possible given other constraints such as cost, and by properly driving the switching device.

The power required to drive the switching transistor does not contribute to the overall power output from the power supply and thus lowers the overall efficiency of the power supply. These drive losses vary depending on the specific transistor used, the operating frequency of the supply and the type of transistor. Driving losses associated with MOSFETs and IGBTs are largely due to the input and Miller capacitances of these transistors. Thus, these losses are a function of the required switching and drive voltage, and power-supply switching frequency. There is no constant bias required to keep MOSFETs or IGBTs conducting.

Bipolar junction transistors (BJTs) are different. They require a constant bias and specialized drive waveforms to turn them on and off efficiently. This is the reason MOSFETs and IGBTs have largely displaced BJTs in recent years. This trend has slowed recently and, in some cases, reversed itself as competition has forced many manufacturers to reduce the cost of their power supplies.

BJTs are the lowest-cost switching transistors on the market. They also offer better saturation performance than MOSFETs, especially at high voltages, assuming that comparable transistors are evaluated. BJTs thus offer good value but with increased drive losses and complexity.

Interestingly, drive losses often can be fully offset by the power savings realized from the improvement in saturation performance and by using drive circuits that maximize drive efficiency (e.g., proportional drive). Note that the increased complexity is generally due to the lack of power-supply chips specifically designed to drive BJTs. Some newer power-supply chips offer high drive efficiency with minimum complexity.

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