In motor applications, the use of inverters or converters for power conversion is essential. Employing a high-isolation DC/DC converter can contribute to enhancing the stability and safety of motor operation, particularly crucial for high-power, high-speed motor systems. This article will introduce you to the relevant technologies of IGBT/MOSFET/SiC/GaN gate driver DC-DC converters and the features of a series of high-isolation DC/DC converters introduced by Murata.
Isolation ensures stable operation of high-power converters
At high power levels, inverters or converters typically use "bridge" configuration to generate line-frequency AC power or provide bidirectional PWM drive for motors, transformers, or other loads. This configuration can be a half-bridge, full-bridge, three-phase, and so on. Bridge circuits usually include IGBTs or MOSFETs (including SiC and GaN) as the "high-side" switch, whose emitters/sources are the switching nodes at high voltage and frequency. Therefore, the gate drive PWM signals and associated drive power rails referenced to the emitter/source must be isolated from the ground.
Other requirements for high-power converters include that the drive circuit and associated power rails should be immune to the high "dV/dt" of the switch node and should have very low coupling capacitance. In many cases, the bridge circuit needs to be isolated from the control circuitry with a safety agency-rated. Therefore, the drive circuit isolation barrier must be robust and durable, showing no significant degradation due to partial discharge effects during the design lifetime.
The positive power rail voltage for the gate drive circuit should be sufficiently high to ensure complete saturation/enhancement the power switch without exceeding its absolute maximum gate voltage. For instance, IGBTs and standard MOSFETs typically fully ON with 15V drive, but typical SiC MOSFETs may require closer to 20V to fully enhancement.
For the off-state, 0V on the gate is sufficient for all devices. However, a negative voltage between -5V and -10V is often used for rapid switching controlled by a gate resistor. The on-state gate threshold for IGBTs is a few volts, usually around 5V, but can be as low as slightly over 1V for SiC and GaN.
A negative gate drive also helps overcome the influence of the collector/drain to gate "Miller" capacitance. This capacitance injects current into the gate drive circuit when the device is turned off. During turn-off, the collector voltage rises rapidly, causing a current spike through the Miller capacitance to flow into the gate. Applying a negative voltage to the gate helps alleviate this effect. This is effective for both IGBTs and all types of MOSFETs.
The power requirements for the DC-DC converter driving the power supply of the gate driver circuit involve providing average DC current to the driver circuit. Peak current, used to charge and discharge the gate capacitance for each cycle, is supplied by the capacitance near the driver circuit. Consideration must be given to derating and other losses in the drive. While SiC and GaN have lower gate charge (Qg) than IGBTs, the frequency can be significantly higher.
High isolation DC/DC converters designed for gate drive applications
Murata has introduced a series of high-isolation DC/DC converters developed by Murata Power Solutions, where the MGJ series DC-DC converters are specifically designed for gate drive applications. These converters are suitable for the common high-isolation requirements in bridge circuits used in motor drives and inverters. They aim to provide optimal drive voltage and isolation for these "high-side" gate drive circuits. The gate is fully charged and discharged in each PWM switch cycle, corresponding to equal positive and negative average and peak currents, regardless of the positive and negative drive voltages. If the output load has unequal currents (such as through extra protection circuits), the voltage may not be maintained within the expected tolerance.
The absolute value of the gate drive voltage is not critical as long as they are above the minimum required for switch enhancement, appropriately below breakdown levels, and dissipation is acceptable. Therefore, if the input of the DC-DC is nominally constant, the DC-DC converter providing the drive power may be of the unregulated type, such as the MGJ1 or MGJ2 series. However, unlike most DC-DC applications, the load is quite constant when the IGBT/MOSFET switches at any duty cycle. Alternatively, when the device is not switching, the load is close to zero. Simple DC-DC converters usually require a minimal load; otherwise, their output voltage will increase sharply, potentially reaching the gate breakdown level.
The high voltage is stored in bulk capacitors, so when the device starts switching, there may be gate overvoltage until this level decreases under normal load. Therefore, a DC-DC converter with a clamped output voltage or very low minimum load requirements should be chosen.
The IGBT/MOSFET should not be actively driven by the PWM signal before the drive circuit voltage rail reaches the correct value. However, transient conditions may occur when the gate drive DC-DC is powered up or down, even if the PWM signal is inactive, leading to the driving of the device and causing shoot-through and damage. Therefore, the DC-DC output should behave well during power-up and power-down, with monotonic rise and fall, even when the PWM signal is inactive.
Insulation performance testing is crucial for high-voltage systems
Isolated DC-DC converters used for "high-side" IGBT/MOSFET drivers can experience a "DC link" voltage across their barrier. This voltage can reach kilovolts with very fast switching edges of over 10 kV/µs. The latest GaN devices may have switching speeds of up to 100 kV/µs or higher, producing a current of 200 mA with only 20 pF and 10 kV/µs. This current finds an indeterminate return route, returning through the controller circuit to the DC bridge, causing voltage spikes across connection resistances and inductances, potentially disrupting the operation of the controller and the DC-DC converter itself. Therefore, low coupling capacitance is required.
The high-side switch emitter is a high-voltage, high-frequency switch node. The entire HVDC link voltage can be seen from DC-DC input to output, continuously switching at the PWM frequency, which can be high, with a high rate of change. IGBTs can reach about 30 kV/µs, MOSFETs about 50 kV/µs, and SiC/GaN about 50+++ kV/µs. There is a coupling capacitance (Cc) in the DC-DC input-output isolation, with high switch voltage at both ends. This will cause pulse current flow, which may interfere with sensitive input pins. Conducting Common mode transient immunity (CMTI) testing can provide an indication of this fault level.
In certain situations, isolated DC-DC converters powered by another linear or switch-mode converter may experience high transient currents that can lead to overshoot on the input of the isolated DC-DC. If this exceeds the maximum input voltage of the isolated DC-DC, it may cause damage. In such cases, a Zener diode may be needed at the input as protection.
To ensure the safety of the power conversion process, the DC-DC can be part of a safety-isolated system, such as meeting reinforced insulation for a 690 VAC system according to UL60950, which requires a 14mm creepage distance and clearance. Isolation voltage needs to be validated with a single instantaneous voltage much greater than the operating voltage, such as applying for one minute. Additionally, based on functional needs, in "high-side" applications, the DC-DC input to output sees the entire HVDC link voltage continuously switching at the PWM frequency. In such cases, a one-minute single instantaneous voltage test may not be a good isolation indicator, and compliance with local discharge tests according to IEC 60270 is the only reliable way to ensure isolation.
The occurrence of discharge is due to the breakdown voltage of small gaps (~3kV/mm), which is much lower than the breakdown voltage of surrounding solid insulators (~300kV/mm). This "initiation voltage" can be measured and used to define the maximum operating voltage to ensure the long-term reliability of the insulator. Local discharge does not cause significant damage in the short term, but prolonged exposure can degrade insulation performance over time.
The complete and versatile MGJ series of DC-DC converters
The MGJ series DC-DC converters, introduced by Murata, are well-suited for powering the "high-side" and "low-side" gate drive circuits of IGBTs and MOSFETs in bridge circuits. Choosing asymmetric output voltages enables optimal drive levels, leading to improved system efficiency and EMI. The MGJ series is designed to meet the common requirements of high isolation and dv/dt in bridge circuits used in motor drives and inverters. Recommended applications for the MGJ series include inverters in renewable energy sources such as wind and solar power, backup batteries, high-speed and variable-speed motor drives, and it can be tailored to meet specific technical requirements for various applications.
Within the MGJ series, the MGJ2 SIP offers a total output power of 2W. It employs a traditional dual-winding approach to provide +ve and -ve gate drive voltage outputs. The available configurations include +15V/-15V, +15V/-5V, +15V/-8.7V, +20V/-5V, +18V/-2.5V. Additional custom outputs can be achieved by adjusting the winding turns. The MGJ2 industrial-grade temperature ratings and construction ensure long service life and reliability.
The MGJ3 and MGJ6 series have total output powers of 3W and 6W, respectively, and utilize patented technology. They offer flexible configuration of three output voltages, such as 20V/-5V (15V+5V, -5V), 15V/-10V (15V, -5V-5V). The MGJ3 and MGJ6 simplify EMC filter design with their disable/frequency synchronization pins. Protection features include short-circuit protection and overload protection.
The MGJ1 and MGJ2 SMD series have total output powers of 1W and 2W, respectively. They use an internal Zener diode divider to provide specific +ve and -ve gate drive voltages, including +15V/-5V (from a single 20V output), +15V/-9V (from a single 24V output), +19V/-5V (from a single 24V output). Other custom outputs can be achieved by changing the Zener diode. The MGJ1 and MGJ2 industrial-grade temperature ratings and construction ensure long service life and reliability.
Conclusion
The DC-DC converters for gate drive power supply are crucial for the safety and stability of motor operation, especially in high-voltage, high-frequency systems. Murata has introduced the MGJ series of DC-DC converters, tailored to different power levels, coupling capacitor requirements, and packaging specifications. These converters are well-suited for powering the "high-side" and "low-side" gate drive circuits of IGBTs and MOSFETs in bridge circuits. They provide robust isolation and insulation performance, ensuring the stability and safety of system operation. The MGJ series offers an ideal solution for developing motor drive applications.