Using Isolated DC DC Converters to Optimize Efficiency and Safety in Gate Drive Power

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DC-DC converters are power supplies that can change a direct current (DC) voltage into another DC voltage. Although DC-DC converters exist without input-to-output isolation, most DC-DC converters use an internal transformer, and the output is electrically (galvanically) isolated from the input. The separated output can provide either a floating power source or generate different voltage rails or dual polarity rails. Isolated DC-DC converters are also useful for breaking up ground loops, thus separating parts of a circuit that are sensitive to noise. Safety requirements are a common reason to use an isolated DC-DC power converter. Isolation separates the output from dangerous voltages on the input and protects against electric shock or short circuit.

Isolated DC-DC converter for Gate Drivers

Isolated DC/DC converters find their way to high speed and high power applications. A preferred use case is to utilize a DC-DC converter for isolation and powering the gate drive circuits of switching devices. Semiconductor switches such as IGBTs and MOSFETs are used extensively to switch power to a load. The switch is controlled by pulse-width modulated (PWM) on-off signals to its gate driver. The on and off drive voltage is critical, so the gate drive circuit has to be carefully designed and powered correctly. This is where the DC-DC converter comes into action—considering the high switching rates to maintain high efficiency and high floating voltages, highly isolated DC/DC converters are needed to power gate drivers.

DC-DC converters providing gate drive power for IGBTs and MOSFETs provide significant advantages over alternative techniques in obtaining optimum efficiency and security under all start-up, transient, and continuous operating conditions. Figure 1 shows the simplified driver circuit for an IGBT with its gate driven on to positive voltage and off to negative volts, controlled by a PWM signal. The DC-DC converter provides the driver's power and isolation from the supply powering the DC-DC labeled Vcc to the switched voltage. An appropriate negative drive ensures that the gate-emitter off-voltage is always actually zero or less. The negative gate drive also helps to overcome the effect of collector/drain to gate 'Miller' capacitance which works to inject current into the gate drive circuit on device turn-off.

Figure 1: DC-DC Converter for gate drive power

At high power, converters typically use 'bridge' configurations (Figure 2) to generate line-frequency AC or provide bi-directional PWM drive to motors, transformers, or other loads. Bridge circuits generally include IGBT or a silicon carbide (SiC) and gallium nitride (GaN) MOSFET as the "high side" switch, in which the emitter/source electrode is the switching nodes under high voltage and high frequency. Therefore, gate drives PWM signals using emitter/source electrodes as the reference, and relevant driving power rails must conduct electric isolation from the ground. The additional requirement is that drive circuits, and relevant power rails should be immune to high "dV/dt" of the switching node with very low coupling capacitance.

Figure 2: Typical End application “high side” and “low side” for full bridge motor drive

To solve these problems, Murata Power Solutions has launched the MGJ series DC-DC converters to provide the optimal driving voltage and isolation for gate drive circuits. The MGJ series DC-DC converter is very suitable for the power supply of "high side" and "low side" gate drive circuits for IGBTs and SiC and GaN MOSFETs in bridge circuits. It chooses asymmetric output voltage to achieve the optimal drive level and the optimal system efficiency and EMI. The MGJ series converter provides various versions such as 1W, 2W, 3W, and 6W to satisfy different customer requirements and has an industrial-grade temperature rating and construction for providing a longer service life and higher reliability. The DC-DC converters from Murata Power Solutions also feature low coupling capacitance, high dV/dt ratings, and safety agency accreditations.

The MGJ1D051505MPC Murata Power Solution

MGJ1D 1W series of the DC-DC converter is an isolated, dual output DC/DC suitable for powering 'high side' and 'low side' gate drive circuits for IGBTs/SiC MOSFETs in bridge circuits. It is rated for 5.7KV isolation in an SMD footprint and has an embedded transformer technology where the ferrite core and windings are integrated into the PCB.

The MGJ1D051505MPC is available with asymmetric output voltages of +15 / -5 VDC, allowing optimum drive levels for best system efficiency. Nominal inputs are available in a choice of 5 VDC with maximum output current rating of 0.05A. The MGJ1 series is characterized by high dV/dt immunity, which aids reliable and continued operation in fast switching circuits and partial discharge performance that contributes to long service life. Also, the converter's low input-to-output coupling capacitance, typically 5 pF, helps reduce the effects of EMI. MGJ1D051505MPC features an industrial-grade temperature rating (-40ºC to +105ºC) and construction, giving long service life and reliability. Murata Power Solutions MGJ1 series of DC-DC converters are all 100% production tested at 5.7kVDC for 3 seconds and have been qualification tested at 5.7kVDC for one minute.

Figure 3: MGJ1D051505MPC Isolated Surface Mount DC/DC Converter  Buy NowBuy Now

The MGJ2D121503SC Murata Power Solution

MGJ2D 2W series of DC-DC converter from Murata Power Solutions is an isolated, dual output DC-DC suitable for IGBT or SiC gate drive. It is rated for 5.2KV isolation in a SIP7 package to occupy a 1.96 cm² footprint and achieve a power density of 0.81 W per cm³. The series comprises 12 models offering nominal input voltages of 5, 12, 15, or 24 VDC. There are three output voltage combinations for each input voltage: +15/-5 VDC, +15/-8.7 VDC, or +20/-5 VDC.

MGJ2D121503SC Murata Power Solution offers a nominal voltage of 12 VDC and an output voltage combination of +15/-5 VDC. The converters have a characterized dV/dt immunity, suiting reliable operation in fast switching applications. The MGJ2 series has an extended operating temperature range of up to 100°C that suits industrial-grade temperature ratings. Typical applications include high-power AC/DC conversion, motor drives, and solar power inverters. MGJ2 series of DC-DC converters are all 100% production tested at 5.2kVDC for one second and have been qualification tested at 5.2kVDC for one minute.

Figure 4: MGJ2D121503SC Isolated Through Hole DC/DC Converter Buy NowBuy Now

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