Microtransformer Isolation Benefits Digital Control

Oct 1, 2008 12:00 PM
By Dr. Baoxing Chen, Senior Staff Engineer, Analog Devices, Wilmington, Mass.

Designing a closed-loop, controlled power supply or motor-control system presents two main challenges. First, the design requires sending feedback voltage or current information across an isolation barrier. Second, it requires providing isolated gate-drive signals for high-side switches.

In the past, the possible isolation solutions included optocouplers or discrete pulse transformers. However, they posed significant constraints on system performance, cost and reliability. Linear optocouplers have been used to send analog error signals from the secondary to the primary in isolated power supplies, but their gain varies significantly from part to part and by temperature. Because of this, feedback-loop design becomes difficult, causing phase margins to vary under worst-case conditions.

Today, digitally controlled systems can use microtransformer isolators that provide a better solution, because they are easily integrated with many circuit functions. Such an isolator eliminates the dependence of loop performance on the optocoupler and error amplifier, simplifying the feedback-loop design. Moreover, a shorter propagation delay for microtransformer isolators allows the closed-loop converter to achieve optimum dynamic performance.

Typically, a digital controller is located on the secondary side, so the microtransformer isolator also can provide initial bias for controller startup, which eliminates the need for an auxiliary power supply. Either pulse transformers or optocouplers can be used to provide isolated gate-drive signals for the high-side switches. Unfortunately, optocoupler approaches need a floating supply, and pulse transformers do not provide dc correctness.

Moreover, pulse transformers need additional discrete components for ac-coupled drives, posing significant constraints on duty-cycle variations. However, gate drivers based on microtransformer isolators, with an integrated high-side supply, impose no constraints on duty-cycle variation.

Microtransformer Isolators

An example of a microtransformer isolator is Analog Devices' coreless transformer iCoupler, which provides fully integrated signal and power isolation. Fig. 1 shows a quad-channel isolator with a fully integrated isolated dc-dc converter in a 16-lead SOIC package. Stacked windings are built on top of these CMOS substrates. The chip on the left has high-voltage CMOS switches, while the chip on the right has the rectifier diodes and a converter controller. Two cross-coupled switches and the transformers form the oscillator, with Schottky diodes used for fast, efficient rectification. The transformers sit in the middle. This implementation has the transformers on separate chips, but the transformer process is compatible with standard CMOS processes, so in principle, they can be put on the same chips as the switches or Schottky diodes. On the top transformer chip, the two larger transformers are for power, while the small transformer transmits the feedback PWM signal. The bottom transformer chip holds four additional micro-transformers for the four-channel isolator. The left and right chips also hold the encoding and decoding circuit for the four-channel isolators.

The transfer of logic signals across the isolation barrier occurs by the appropriate encoding on the primary side and decoding at the secondary side that recovers the input logic signals. Specifically, short pulses (about 1 ns) are transmitted across the transformers with two consecutive short pulses indicating a leading edge and a single short pulse indicating a falling edge. A nonretriggerable mono-stable at the secondary generates detection pulses. If it detects two pulses, the output goes high, whereas a single pulse causes the output to go low.

For transmitting power across the isolation barrier, the microtransformers are switched resonantly at high frequency to achieve efficient energy transfer. Energy regulation is obtained by a low-frequency pulse-wide-modulation (PWM) feedback signal, which controls the duty cycle for this high-frequency resonant action. The transformer switches and Schottky diodes used for rectification are all implemented on chip. Isolation up to 5 kV rms is provided by 20-µm-thick polyimide layers sandwiched between the primary and secondary coils.

Similarly, fully integrated half-bridge gate drivers, isolated analog-to-digital converters (ADCs) and isolated transceivers can be integrated. Integrating the isolated dc-dc converter with a signal isolator solves the common problem with optocouplers, which is the need to design a discrete isolated power supply. The discrete dc-dc converter is relatively large, expensive, difficult to design and, in many cases, provides insufficient isolation. Combined signal and power isolation provides possibilities for functional integration that reduce the complexity, size and total cost of isolated systems.

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