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

Posted Date: 2024-01-17

High isolation DC/DC converter improvementStability and safety of motor operation

In motor applications, an inverter or converter must be used for power conversion. Using a high-isolation DC/DC converter will help improve the stability and safety of motor operation, which is especially important for high-power and high-speed motor systems. . This article will introduce you to the related technologies of IGBT/MOSFET/SiC/GaN gate drive DC-DC converters, as well as the functional characteristics of a series of high-isolation DC/DC converters launched by Murata (Murata Manufacturing Co., Ltd.).

Isolation ensures stable operation of high-power converters

At high power, inverters or converters often use a "bridge" configuration to generate line-frequency AC or provide bidirectional PWM drive for motors, transformers, or other loads. This can be a half-bridge, full-bridge, three-phase, etc. configuration. Bridge circuits typically include IGBTs or MOSFETs (including SiC and GaN) as "high-side" switches, with their emitter/source being the switching node at high voltages and frequencies. Therefore, gate drive PWM signals and associated drive supply rails that use the emitter/source as a reference must be electrically isolated from ground.

Other requirements for high power converters are that the driver circuitry and associated power rails should be immune to the high "dV/dt" of the switching node and have very low coupling capacitance. In many cases, the bridge circuit requires safety agency rated isolation from the control circuit, so the drive circuit isolation barrier must be rugged and not show significant degradation from partial discharge effects during its design life.

The positive supply rail voltage of the gate drive circuit should be high enough to ensure that the power switch fully saturates/boosts without exceeding the absolute maximum voltage of its gate. For example, IGBTs and standard MOSFETs will fully conduct when driven at 15 V, but a typical SiC MOSFET may require voltages closer 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, enable fast switching controlled by the gate resistor. The on-state gate threshold of an IGBT is a few volts, it is typically 5V but can be as low as just over 1 volt for SiC and GaN.

Negative gate drive also helps overcome the collector/drain effect on the gate "Miller" capacitance, which injects current into the gate drive circuit when the device is turned off. During turn-off, the collector voltage rapidly increases. rises, causing a current spike to flow through the Miller capacitance towards the gate. This causes an opposite positive voltage across the gate resistor. This effect is mitigated by driving the gate to a negative voltage. IGBTs and all types of MOSFETs have the same effect. .

The power requirement of the DC-DC converter driving power supply, whose 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, which is used to charge and discharge the gate capacitance in each cycle, is required Taking into account derating and other losses in driving, SiC and GaN have lower Qg than IGBTs, but the frequency can be very high.

Designed specifically for gate drive applications

High isolation DC/DC converter

Murata has launched a series of high-isolation DC/DC converters developed by Murata Power Solutions. The MGJ series DC-DC converters are designed for gate drive applications and can meet the requirements of bridge type used in motor drives and inverters. The high isolation requirements common in circuits are designed to provide optimal drive voltage and isolation for these "high-side" gate drive circuits. Where the gate is fully charged and discharged every PWM switching cycle, this corresponds to equal positive and negative average and peak currents regardless of positive and negative drive voltages. If the output loads have unequal currents (e.g. through additional protection circuitry), the voltage may not remain within the expected tolerance.

The absolute values ​​of the gate drive voltages are not very critical as long as they are above the minimum required for switching enhancement, appropriately below 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 an unregulated type, such as the MGJ1 or MGJ2 series. However, unlike most DC-DC applications, the load is fairly constant when the IGBT/MOSFET switches at any duty cycle. Or, when the device is not switching, the load is close to zero. Simple DC-DCs usually require a minimum load, otherwise their output voltage will increase dramatically and may even reach gate breakdown levels.

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

The IGBT/MOSFET should not be actively driven by the PWM signal until the driver circuit voltage rail reaches the correct value. However, when the gate drive DC-DC is powered on or off, transient conditions may occur that cause the device to be driven, even when the PWM signal is inactive, causing breakdown and damage. Therefore, the DC-DC output should behave well when powered up and down, rising and falling monotonically.

Insulation performance testing is critical for high voltage systems

An isolated DC-DC used for "high side" IGBT/MOSFET drivers sees the switching "DC link" voltage across its barrier. This voltage can reach kilovolts, with very fast switching edges above 10 kV/μs. The latest GaN devices, which may have switching speeds of 100 kV/μs or higher, can produce 200mA of current at just 20pF and 10 kV/μs. This current finds an uncertain return path through the controller circuit and back to the bridge, causing voltage spikes across the connecting resistors and inductors that can disrupt the operation of the controller and the DC-DC converter itself, thus requiring low coupling capacitance.

The high-side switch emitter is a high-voltage, high-frequency switching node. From the DC-DC input to the output, you can see that the full HVDC link voltage is continuously switched at the PWM frequency. The frequency may be very high and the change rate is also very high. IGBT can reach About 30 kV/µs, MOSFET is about 50 kV/µs, SiC/GaN is about 50+++ kV/µs, DC-DC input and output isolation has a coupling capacitor (Cc), and there is a high switching voltage on both ends of the capacitor , so there will be pulsed current flowing through it, which may cause interference to sensitive input pins. Common-mode transient immunity (CMTI) testing can give an indication of this fault level.

In some cases where the isolated DC-DC is powered by another linear or switch-mode converter, high transient currents may cause overshoot on the isolated DC-DC input. Damage may occur if the maximum input voltage of the isolated DC-DC is exceeded. In this case, it may be necessary to use a Zener diode on the input for protection.

In order to ensure the safety of the power conversion process, DC-DC can be part of a safe isolation system. For example, according to UL60950, a 690 VAC system that meets reinforced insulation requires 14mm creepage distance and electrical clearance. The isolation voltage needs to be much larger than the working voltage. A single instantaneous voltage to verify isolation, such as a one-minute hold. Additionally, in "high-side" applications, the DC-DC input to output can see the full HVDC link voltage continuously switched at PWM frequency, as required by functionality. In this case, a single instantaneous voltage test of just 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 "initial voltage" can be measured and used to define the maximum operating voltage to Ensure long-term reliability of the insulator. Partial discharge will not cause significant damage in the short term, but when used for a long time, the partial discharge phenomenon will reduce the insulation performance.

Complete and diverse MGJ series DC-DC converters

The MGJ series of DC-DC converters introduced by Murata are ideal for powering "high-side" and "low-side" gate drive circuits for IGBTs and MOSFETs in bridge circuits. Selecting asymmetric output voltages enables optimal drive levels for optimal 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. Recommended applications of the MGJ series include inverters and backup batteries on new energy sources (such as wind energy, solar energy), and can also be used for high-speed and variable-speed motor drives, and can meet application-specific technical requirements through key parameters.

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

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

The total output power of MGJ1 and MGJ2 SMD is 1W and 2W, using internal Zener diode voltage 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 special outputs can be provided by changing the Zener diode. The MGJ1 and MGJ2 industrial temperature ratings and construction provide long service life and reliability.

Conclusion

The DC-DC converter of the gate drive power supply is crucial to the safety and stability of motor operation, especially for high-voltage and high-frequency systems. Murata has launched a series of DC-DC converters targeting different power, coupling capacitance and packaging specifications, launching the MGJ series of DC-DC converters, which are very suitable for "high-side" and "low-side" gate driving of IGBTs and MOSFETs in bridge circuits. The circuit is powered and provides strong isolation and insulation performance to ensure the stability and safety of system operation. It will be an ideal solution for you to develop motor drive applications.






Review Editor: Liu Qing


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