Power supply is the foundation of all electronic devices, and the gate driver is the key to stably provide power supply for devices. A gate driver is a power amplifier that accepts a low-power input from a controller IC and produces a high-current drive input for the gate of a high-power transistor such as an IGBT or power MOSFET. This article will show you the technical concepts of gate drive power supplies and DC-DC converters, and the functional characteristics of the isolated gate drive power supply product series introduced by Murata.
A gate driver is the basis of a power conversion circuit
A gate driver is a circuit used to drive power semiconductor switches (such as MOSFET, IGBT, etc). These semiconductor switches are used to control the switching state of power converters, such as DC-AC inverters, AC-DC converters, and DC-DC converters.
The main function of the gate driver is to provide enough current and voltage to control the switching speed and switching time of the power semiconductor switch, thus ensuring the stable operation of the converter. A gate driver generally includes an input drive circuit, output power stage, isolation circuit, and other parts, which can isolate the control signal from the power circuit to improve the security and reliability of the system.
In the design of gate driver, factors such as output power, the characteristics of the power semiconductor, and power supply noise need to be considered. The gate driver can also use different control methods, such as voltage control and current control, to achieve different application requirements.
There are quite a variety of gate drivers, which can operate on-chip or as discrete modules. Essentially, the gate driver consists of a level shifter and an amplifier, and the gate driver chip is used as an interface between control signals (digital or analog controllers) and power switches (IGBT, MOSFET, SiC MOSFET, and GaN HEMT). The integrated gate driver solution can reduce the design complexity, development time, bill of materials (BOM), and circuit board space, while improving the reliability of the gate driver solution implemented separately.
Isolated DC-DC converters improve system security
Take the DC-DC application scenario for a gate driving power supply as an example. The typical application is to provide driving power for the "High side" and "Low side" of a full-bridge motor, which can be in half-bridge, full-bridge, and three-phase modes. The switching emitter at the high side is a high-voltage and high-frequency switching node, which may be IGBT, MOSFET, SiC, or GaN, and it requires a positive and negative dual output voltage of +Ve and -Ve, and the drive and related circuits on the high side must be isolated.
DC-DC 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 capacitor every cycle. When using it, derating and other losses in driving need to be considered. The Qg of SiC and GaN is lower than IGBT, but the frequency may be very high.
According to the data sheet, most devices can be turned off with 0V, but sometimes negative gate voltage is still needed to overcome a parasitic inductance effect and Miller capacitance effect. Due to the existence of source parasitic inductance, when an IGBT is turned off, it will lead to an induced voltage caused by the sudden termination of current, which will cause the peak to be opposite to the gate voltage. On the other hand, during the turn-off period, the collector voltage will rise rapidly, which will cause the current peak to flow to the gate through a Miller capacitor, which will lead to the opposite positive voltage on the gate resistance.
Then why do DC-DC converters need to be isolated? The first consideration is the safety factor. DC-DC can be a part of the safety isolation system. For example, according to the UL 60950, 690VAC system, it needs 14 mm of creepage distance and air clearance, and the isolation is verified by a single instantaneous voltage much larger than the working voltage, such as holding for one minute. In addition, isolation also has functional requirements. For example, in "high side" applications, a DC-DC in full HDVC link voltage will be continuously switched at the PWM frequency from input to output. In this case, a single instantaneous voltage test of only one minute is not a good isolation index, and a partial discharge test conforming to IEC 60270 is the only way to ensure it.
Discharge occurs because the breakdown voltage (~3kV/mm) of the small gap is much lower than that of the surrounding solid insulator (~300kV/mm). This "initial voltage" can be measured and used to define the maximum working voltage to ensure the long-term reliability of the insulator. Partial discharge will not cause great damage in the short term, but it will reduce the insulation performance after long-term use.
The emitter of the high-side switch is a high-voltage and high-frequency switch node. The full HVDC link voltage can be seen from DC-DC input to output, and it can be continuously switched at the PWM frequency. Its frequency may be very high, and the change rate is also very high. For example, the IGBT can reach about 30 kV/µs, the MOSFET is about 50 kV/µs, and the SiC/GaN is about 50+++Kv/µs. DC-DC input and output isolation will have coupled capacitive reactance (Cc). There is a high switching voltage across the capacitor, so a pulse current will flow, which may cause interference to sensitive input pins, so the common-mode transient immunity (CMTI) test will give an indication of this fault level.
DC-DC converters capable of realizing bipolar voltage
Murata has introduced a series of DC-DC converters for gate drive power supplies, which are specially designed for gate drive circuits and are usually used to alternative energy, motion and control, mobility, and healthcare solutions. These products have an ultra-low isolation capacitance of 3pF, which can be used to optimize the bipolar output voltage of IGBT/SiC and MOS gate drivers. The DC Link voltage has a tolerance of up to 3kV, and has a unique partial discharge performance. The dv/dt immunity at 1.6 kV is as high as 80kV/µS. The whole series includes several product lines supporting IGBT, SiC, MOS, and GaN.
All switching devices need different gate voltages, and the levels specified by different manufacturers are also different. Murata adopts different methods to realize bipolar voltage, such as the MGJ2 SIP DC-DC converter, which has a total output power of 2W, and uses the traditional double winding method to provide +ve and -ve gate drive voltage outputs, including supporting +15V/-15V, +15V/-5V, +15V/-8.7V, +20V/-5V, and +18V/-2.5V, and other special outputs provided by changing the number of turns.
In addition, DC-DC converters such as the MGJ3 and MGJ6 DC-DC converters provide total output power of 3W and 6W. With patented technology, they can output three voltages for flexible configuration, such as 20V/-5V (15V+5V, -5V) and 15V/-10V (15V, -5V-5V). The total output power of the MGJ1 and MGJ2 SMD is 1W and 2W, which use an internal Zener diode to 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 Zener diodes.
DC-DC converters supporting GaN devices
GaN devices have become the first choice for high-power applications at present, and Murata has also introduced a new optimized DC-DC converter for GaN gate drive applications. Murata has introduced a series of new compact DC-DC converters by using its proprietary PCB electrical and mechanical design topologies, which are consistent with the increasingly popular wide bandgap technology. The new MGN1 series 1W output DC-DC converter is designed to provide the voltage required by the gate driver of GaN devices.
These devices provide a low-profile, small footprint, surface mount solution that can be easily integrated into space-limited systems. They also have the advantage of being lightweight, which opens up greater deployment opportunities and provides output voltages of +8V, +12V, and +6/-3V.
One of the key attributes of the MGN1 series of DC-DC converters is its ultra-low isolation capacitance of 2.5 pF (typical). In this way, transient coupling on the isolation barrier is minimized, thus preventing signal distortion. In addition, this means that the EMI problem of the system can be alleviated. The >200kV/μs common-mode transient immunity (CMTI) of these units makes them very suitable for higher switching speeds of GaN-based systems, further ensuring the signal integrity of gate drivers. Thanks to partial discharge performance, they can maintain reliable operation under high voltage conditions.
The DC-DC converters in Murata's MGN1 family support a continuous isolation barrier withstand voltage of 1.1kV, with 650VDC basic insulation and 240VAC enhanced insulation to meet the UL62368 standard. These converters feature 6.5 mm of creepage distance and clearance data, and can operate in the working temperature range of -40°C to +105°C, which enables them to be installed in extremely challenging environments. In addition, reverse polarity and short circuit protection mechanisms are combined.
This new DC-DC converter can be used in a variety of GaN-based applications, including electric vehicle fast charging infrastructure, battery storage converters, smart grid implementation, solar inverters, solid-state switch circuit breakers, ICT and data centers, wind turbines, and motor drivers.
Conclusion
In various power conversion processes, the gate driver plays an important role, and the isolated DC-DC converter that drives the power supply through the gate and supports bipolar voltage output is the best choice to provide stable power supply for various electrical devices. Murata has launched a series of isolated DC-DC converters, providing products supporting different technologies such as IGBT, MOSFET, SiC MOSFET, and GaN, which can meet the different power requirements of various applications. The product lines are fairly complete and will be your best partner in product development.