High isolation DC/DC converter enhances the stability and safety of motor operation

Isolation ensures stable operation of high-power converters

At high power, inverters or converters typically use a "bridge" configuration to generate AC power or provide bidirectional PWM drive for motors, transformers, or other loads, which can be half bridge, full bridge, three-phase, or other configurations. Bridge circuits typically include IGBT or MOSFET (including SiC and GaN) as "high side" switches, with the emitter/source being the switching nodes at high voltage and high frequency. Therefore, the gate driven PWM signal and related driving power rail using the emitter/source as a reference must be isolated from the ground.

The other requirements for high-power converters are that the drive circuit and related power rails should not be affected by high "dV/dt" of the switching nodes, and have very low coupling capacitance. In many cases, bridge circuits require safety mechanism rated isolation from control circuits, so the drive circuit isolation barrier must be sturdy and durable, and not exhibit significant degradation due to partial discharge effects during the design life.

The positive power rail voltage of the gate drive circuit should be high enough to ensure that the power switch is fully saturated/enhanced without exceeding the absolute maximum voltage of its gate. For example, IGBT and standard MOSFET will fully conduct under 15V drive, but typical SiC MOSFET may require a voltage close to 20V to fully enhance.

For the off state, 0 V on the gate is sufficient for all devices. However, negative voltages typically between -5V and -10V can achieve fast switching controlled by gate resistors. The gate threshold of IGBT's conduction state is several volts, usually 5V, but SiC and GaN can be as low as slightly higher than 1 volt.

Negative gate drive also helps overcome the influence of collector/drain on the gate "Miller" capacitor, which can inject current into the gate drive circuit when the device is turned off. During the turn off period, the collector voltage rapidly rises, causing the current peak to flow through the Miller capacitor to the gate, which will result in an opposite positive voltage on the gate resistance. Driving the gate to negative voltage can alleviate this effect. IGBT and all types of MOSFETs have the same effect.

The power demand of the DC-DC converter driving power supply is that the DC-DC conversion only provides the average DC current to the driver circuit, and the peak current is provided by the capacitor near the driver circuit for charging and discharging the gate capacitor in each cycle. It is necessary to consider derating and other losses in the driving process. The Qg of SiC and GaN is lower than that of IGBT, but the frequency may be very high.

Specially designed for gate drive applications

Highly isolated DC/DC converter

Murata has launched a series of high isolation DC/DC converters developed by Murata Power Solutions, including the MGJ series DC-DC converters designed specifically for gate drive applications, which can meet the common high isolation requirements in bridge circuits used in motor drives and inverters, aiming to provide the best driving voltage and isolation for these "high edge" gate drive circuits. The gate is fully charged and discharged during each PWM switching cycle, corresponding to equal positive and negative average currents and peak currents regardless of the positive and negative driving voltages. If the output load has unequal currents (such as through additional protection circuits), the voltage may not remain within the expected tolerance range.

The absolute value of the gate driving voltage is not very critical, as long as it is higher than the minimum value required for switch enhancement, appropriately lower than the breakdown level, and the dissipation is acceptable. Therefore, if the input of the DC-DC is nominally constant, the DC-DC converter providing the driving power may be of a non regulated type, such as the MGJ1 or MGJ2 series. However, unlike most DC-DC applications, the load remains relatively constant when IGBT/MOSFET switches at any duty cycle. Alternatively, when the device is not switched, the load approaches zero. Simple DC-DC typically require minimal load, otherwise their output voltage will sharply increase and may even reach gate breakdown level.

This high voltage is stored on a large capacity capacitor, so when the device starts switching, it may experience gate overvoltage until the level drops under normal load. Therefore, DC-DC converters with clamp output voltage or very low minimum load requirements should be selected.

IGBT/MOSFET should not be actively driven by PWM signals until the voltage rail of the driving circuit reaches the correct value. However, when the gate driven DC-DC is powered on or off, even if the PWM signal is in an inactive state, transient situations may occur, causing the device to be driven and resulting in breakdown and damage. Therefore, DC-DC output should perform well during power on and off, and rise and fall monotonically.

Insulation performance testing is crucial for high-voltage systems

The high side switch emitter is a high-voltage, high-frequency switching node. From the DC-DC input to the output, the full HVDC link voltage can be continuously switched at PWM frequency, which may be very high and have a high rate of change. IGBT can reach about 30 kV/µ s, MOSFET about 50 kV/µ s, SiC/GaN about 50++kV/µ s. The DC-DC input and output are isolated by a coupling capacitor (Cc), which has a high switching voltage at both ends and may cause interference to sensitive input pins. The common mode transient immunity (CMTI) test can be used to indicate this fault level.

In some cases, isolated DC-DC is powered by another linear or switch mode converter, and high transient currents may cause overshoot in the isolated DC-DC input. If the maximum input voltage of the isolated DC-DC is exceeded, it may cause damage. In this case, it may be necessary to use a Zener diode as protection at the input end.

In order to ensure the safety of the power conversion process, DC-DC can be part of a safety isolation system. For example, according to UL60950690 VAC system, a 14mm creepage distance and electrical clearance are required to strengthen insulation. The isolation voltage needs to be verified with a single instantaneous voltage much higher than the operating voltage, such as for one minute. In addition, according to functional requirements, in "high edge" applications, the DC-DC input to output can see the full HVDC link voltage continuously switching at PWM frequency. In this case, a single momentary voltage test of only one minute is not a good isolation indicator, and partial discharge testing in accordance with IEC 60270 is the only way to ensure it.

Discharge occurs because the breakdown voltage of the small gap (~3kV/mm) is much lower than the breakdown voltage of the surrounding solid insulator (~300kV/mm). This "starting voltage" can be measured and used to define the maximum operating voltage to ensure the long-term reliability of the insulator. Partial discharge does not cause significant damage in the short term, but over prolonged use, the phenomenon of partial discharge can reduce insulation performance.

Complete and diverse MGJ series DC-DC converters

The MGJ series DC-DC converter launched by Murata is very suitable for powering the "high side" and "low side" gate drive circuits of IGBT and MOSFET in bridge circuits. Choosing asymmetric output voltage can achieve the optimal driving level, thereby achieving the best system efficiency and EMI. The MGJ series is characterized by meeting the high isolation and dv/dt requirements commonly found in bridge circuits used in motor drives and inverters. The recommended applications of MGJ series include inverters and backup batteries for new energy sources such as wind and solar energy, as well as high-speed and variable speed motor drives, and can meet specific technical requirements through key parameters.

MGJ2 SIP in the MGJ series has a total output power of 2W and uses traditional dual winding methods to provide+ve and - ve gate drive voltage outputs, including+15V/-15V,+15V/-5V,+15V/-8.7V,+20V/-5V, and+18V/-2.5V. Other special outputs can be provided by changing the number of turns, while the MGJ2 industrial grade temperature rating and structure can provide longer service life and reliability.

The total output power of the MGJ3 and MGJ6 series is 3W and 6W, respectively. They use patented technology and can flexibly configure three output voltages, such as 20V/-5V (15V+5V, -5V) and 15V/-10V (15V, -5V). The disable/frequency synchronization pins of MGJ3 and MGJ6 simplify the design of EMC filters, and their protection functions include short circuit protection and overload protection.

The total output power of MGJ1 and MGJ2 SMD is 1W and 2W, respectively. Internal Zener diodes are used to divide and provide specific+ve and - ve gate driving voltages, including+15V/-5V (from a single 20V output),+15V/-9V (from a single 24V output), and+19V/-5V (from a single 24V output). Other special outputs can be provided by changing the Zener diodes. MGJ1 and MGJ2 industrial grade temperature ratings and structures can provide longer service life and reliability.

Conclusion

The DC-DC converter of gate driven power supply is crucial for the safety and stability of motor operation, especially for high-voltage and high-frequency systems, which are critical components. Murata has launched a series of DC-DC converters tailored to different power, coupling capacitance, and packaging specifications, including the MGJ series. These converters are highly suitable for powering the "high side" and "low side" gate drive circuits of IGBT and MOSFET in bridge circuits, and provide strong isolation and insulation performance to ensure the stability and safety of system operation. They will be the ideal solution for developing motor drive applications.