Power efficiency for high-voltage power conversion

This post sponsored by Texas Instruments.

Power supplies do a fairly good job of providing clean and regulated power at any number of standard and adjustable voltage levels. When the input power characteristics are known with high reliability, it is easier to design an efficient way of converting that voltage level into a higher or lower level at a specified output current rating.

In many cases, voltages are only a few volts different. For example, you may have a clean 5 V supply and need a 3.3 V tap to drive a logic device with different I/O levels. The same holds true in reverse. You may have a steady 1.8 V battery supply and need to generate a clean and stable 2.2 V. In this case, a step-up solution can be designed with very high efficiencies since the input is clean, ripple-free, and known to be stable within a certain range of its discharge curve.

These lower voltage logic and micro levels have steadily and consistently driven the design and refinement to provide well-engineered linear LDO styles as well as switching step-up and step-down (and charge-pump) solutions. Any stray leakage and waste are pretty well confined to fairly low levels of current. But, this situation changes as voltage levels rise. A 5µAmp leakage at 2.2 V dissipates 11µW, while the same leakage current at 30 V dissipates 150uWatts.

Other factors control efficiency and waste. Take FET driving for instance. In order to take full advantage of the low RDS(on) characteristics of a FET, the gate levels must be driven at higher voltage levels than the drain or source terminals. This makes it difficult to switch rail-level signals since a higher-than-rail voltage is necessary. FET drivers overcome these issues by integrating all the necessary biasing internally with the surge current capacities to overcome the initial junction capacitance at the gate. The same benefits are derived when using IGBTs, which basically use an insulated gate drive stage to drive rugged bipolar-style transistors.

As an example, we can look at the Texas Instruments UCC27531 single gate driver. It is a member of the UCC2753x family that provides high-speed switching (typically up to 120 kHz) as driven by TTL- or CMOS-compatible logic signals. With a wide 10-to-35 V VDD range, these drivers can typically source and sink high-peak currents (up to 5 A) with fairly low (17 ns) propagation delays and fast rise and fall times (15 ns and 7 ns, respectively, to an 1800 pF capacitance load).

Both inverting and non-inverting versions are available and these parts can be used in high-side as well as low-side driver designs. The wide – 40 to + 140-degree temperature range is also a plus for harsh environments and fault tolerance. Under-voltage lockout features also help to assure safe operations. Note, no AC line sensing is needed, and these parts also implement soft over-current and cycle-by-cycle peak current limiting.

Test and evaluation systems make it faster and easier to implement this technology. Texas Instruments’ UCC27532EVM-538 Evaluation Board highlights the benefits of using this technology and creates a – 5 V as well as a + 20 V output for designs fed by 10- or 12-V inputs (Figure 1).

Gate driver application information is available online, illustrating several automotive and industrial examples. An Industrial Motor Drive Solutions Guide also serves as a great resource for industrial automation designs.

Full isolation and flexibility allow this gate driver daughter card to be used to fine-tune specific FETs. This includes both positive and negative turn on voltages. (Source: Texas Instruments)

UCC27531
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UCC27524A
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LM5113
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UCC27201
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UCC27532
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UCC28880
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UCC28630
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Author: Jon Gabay, Hearst Business Media