Microtransformer Isolation Benefits Digital Control

Oct. 1, 2008
Integrated microtransformer isolators eliminate digitally controlled design constraints caused by the use of optocouplers and discrete pulse transformers.

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.

Isolation for AC-DC Supplies

Digital power supplies provide programmability, ease of use and higher performance. And they can leverage powerful modern processors to implement complex control algorithms that lead to higher efficiency and faster transient response. One of the major barriers in adopting digital control for isolated power supplies is the need to send digital rather than analog information across the isolation barrier. Traditional optocouplers, which are typically used in analog interfaces, are slow and difficult to integrate. On the other hand, microtransformer-based isolators are fast and easily integrated with many circuit functions, including digital interfaces.

Fig. 2a shows the isolation needs for a typical ac-dc power supply implemented with a secondary digital controller. The PWM signals generated from the secondary controller are sent directly across the isolation barrier to drive the full bridge through two half-bridge gate drivers. This example uses two ADuM5230 isolated half-bridge drivers, whose two isolated outputs are also isolated from each other. The red-dashed lines in the figure indicate the isolation barrier. The half-bridge gate drivers provide isolation between the primary and secondary and between the high side and low side. The integrated 200-mW, 15-V high-side supply eliminates the need for a floating power supply to bias the high-side gate driver. This implementation eliminates the need for four discrete isolated pulse transformers, eight discrete catch diodes and eight discrete gate-clamping zener diodes in a traditional gate-drive approach based on pulse transformers.

Fig. 2b shows the complete gate-drive circuit for pulse transformer 2. The gate-drive circuits for the other three pulse transformers are the same. In some cases, only two transformers with two secondary windings may be needed if the same drive signal can be used for two diagonal switches. Two windings can be used for two complementary gate-drive signals also, but only if the duty cycle is fixed at 50%, as in some phase-shift topologies. Four pulse transformers would allow four independent gate-drive signals that are required for many phase-shift and resonant topologies.

The gate-drive amplitude is dependent on the duty cycle of the gate-drive signals with capacitive transformer drivers (Fig. 2b). This amplitude change makes the gate drive inefficient, as the peak-to-peak swing can be twice that of the gate-drive amplitude at 50% duty cycle, so it wastes essentially half of the power. Adding the driver and winding loss, the pulse transformer approach can be inefficient. The microtransformer half-bridge gate drivers encode and decode the gate-drive signals with dc correctness, eliminating the limitation on the PWM duty cycle seen in the discrete pulse transformer implementation.

Optocoupler-based gate drivers do not pose limitations on the PWM duty cycle, but the signal distortion they introduce may be too large to be useful for sending accurate PWM pulses. Also, additional discrete isolated power supplies are needed for powering the optocouplers. The use of a bootstrap circuit again poses limitations on the PWM duty cycle, because it needs bootstrap capacitors to hold enough charge during the high side on cycles. High-speed microtransformer half-bridge gate drivers are very important for transmitting the highly accurate digital PWM with very little distortion. The low-propagation delay will ensure maximum achievable loop response and matching between different gate-drive channels for maximum converter efficiency.

A secondary controller is typically used because of its close proximity to the load. Also, the drive signals for the synchronous rectification switches need not go through the isolation barrier. Implementing secondary control is difficult, in part because of the need to provide initial bias for the controller before the converter starts to deliver regulated power. The ADuM5000 500-mW isolated dc-dc converter uses microtransformers that provide startup power for the controller, eliminating the need for an auxiliary supply at startup.

For power supplies with secondary controllers, there is also a need to communicate primary voltage information to the secondary. For power supplies using the SMBus or PMBus, fully integrated I2C iCoupler devices can be used such as the ADuM1250/1251 with 2.5-kV isolation or the ADuM2250/2251 with 5-kV isolation.

Microtransformer half-bridge gate drivers are also useful for power supplies with primary controllers, because they provide the isolation for the high side even though the low side and the primary controller can share the same ground. Additional two-channel isolators would be needed to send the synchronous rectification switching signals from the primary to the secondary.

Isolation for Motor-Drive Applications

In motor-drive applications, two main parts of the circuit require isolation: the gate drive for IGBTs of bridge inverters and motor-phase-current sensing. Phase-current sensing provides IGBT protection and linear current feedback information for the controller to maintain closed-loop current control. Series shunt resistors, together with high-precision ADCs at the inverter output, are typically used to sense the phase current. Isolated power supplies are needed to provide the bias for the current-sensing ADC and gate-drive circuit, and separate supplies are needed for each phase. The iCoupler devices can simplify this complicated signal and the power isolation needs of ac motor drives.

Fig. 3 is an example of an implementation for a low-power motor drive using microtransformers. The half-bridge gate driver with an integrated 300-mW high-side, 15-V supply provides isolated 15-V gate-drive output for the high-side IGBT and non-isolated 15-V gate-drive output for the low-side IGBT. The 15-V high-side supply, generated through an integrated dc-dc converter, provides power for the buffer circuit to drive the IGBT, and also can be used with a zener diode to generate a 3-V to 5-V supply to power the current-sensing ADC. An isolated second-order sigma-delta modulator converts the analog input to a high-speed single-bit data stream that can interface directly with the controller. It receives a clock signal from the controller and sends the clocked data stream back to the controller.

Without an integrated ADC, multiple optocouplers would be needed; slow optocouplers are usually not suitable for transmitting this high-speed data stream. High-voltage level shifters can be faster than optocouplers, but they do not provide galvanic isolation, which is important to prevent latchup in the presence of negative transients. Both the high-side gate drivers and the current-sensing ADC have their grounds referred to inverter outputs that can be switching very fast. iCoupler isolation with high common-mode transient immunity is important to maintain data integrity for high-side switching and current sensing.

The red-dashed lines in Fig. 3 show the extent of the isolation barrier. The circuit components shown in the blue box can be replicated in bridge inverters for additional phases. The inverter outputs need to be isolated from each other and multiple half-bridge gate drivers will achieve that. Each of the half-bridge gate drivers will generate its own gate-drive signal and high-side supply.

For medium- to high-power motor-drive applications, isolation is also typically required for low-side gate drive, as shown in Fig. 4. Low-side isolation protects the controller from being damaged by IGBT inductive switching transients. The six gate-drive signals are usually isolated through logic isolators. They provide inputs to a gate-drive module, which provides further level shifting or isolation for the high-side IGBTs.

Logic isolation facilitates communication between the controller and dc-link ground, such as passing the dc-link voltage or current-sensing information to the controller. The four-channel isolator provides isolation for four of the six gate-drive signals from the controller. The other four-channel isolator provides reinforced isolation for the other two gate-drive signals. Two unused isolation channels can be used for serial communication between the controller, and a nonisolated ADC can be used for high-voltage direct-current voltage sensing. The 500-mW isolated power from the ADuM5400 can be used to power any logic circuits referenced to the low-side ground, such as the output side of the ADuM2401 and the ADC used for voltage sensing.

Hall-effect sensors are typically used for high-power motor drives because of the power dissipation limitation of shunt resistors. If a series shunt resistor is used for phase-current sensing in medium-power motor drives, an isolated second-order sigma-delta modulator can be used to send the phase-current information to the controller.

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