How to choose the right DC-DC converter for HV gate driver applications
Bridge circuits using ‘high-side’ switches are common in applications from inverters to motor control (Figure 1, right). However, the high-side gate drive return is actually a switching node so the gate driver and its associated power rails are offset from ground by a high voltage, high frequency waveform with fast edges. While you can achieve this offset for the driver power rails with charge pumps, isolated DC-DC converters are often preferred and are actually necessary when safety isolation is required between control and power circuitry. Gate drive power can also be surprisingly high – 2W is typical for small systems but can be 10W+, especially when using large IGBTs. Power requirement scales directly with frequency so even newer technologies such as SiC and GaN with low gate charge need significant power when run at high speed.
Gate drive voltages are often bipolar and asymmetrical
The DC-DC has to be right for this application though – gates of IGBTs and MOSFETs of all technologies are often driven with asymmetric bipolar voltages. Even though data sheets might indicate that 0V is adequate for the OFF-drive, ‘Miller’ capacitance between gate and collector (drain) tends to inject charge opposing the gate drive so a negative drive ensures reliable switching. The same effect occurs with any emitter (source) inductance that is common to the gate drive loop: with high di/dt levels, a voltage transient is generated which again opposes gate drive. With packaged devices, this common inductance is inevitable and even a few nH causes volts of spikes with realistic rates of change of current. The bipolar gate drive voltage can be relatively uncritical for some devices such as Si-MOSFETs with typically +/-12V but SiC and GaN need to be much more exact with +6/-3V often recommended for GaN, for example.
DC-DCs see continuous barrier stress
General-purpose DC-DC converters are often specified with high insulation breakdown voltage and safety agency certifications but are only rated for occasional voltage stress and are expected to see little voltage across their barrier in operation. In high-side gate drive applications, however, the voltage across the barrier may be kV, switched at 100kHz or more with fast edges, over 100V/ns. This stress can lead to unreliable operation and short life of the DC-DC if not specifically designed for the job.
An operational consideration is barrier capacitance: often, general-purpose DC-DCs measure around 100pF but with this value, according to I = C.dV/dt, our 100V/ns edges would pump a massive 10A through the barrier into the control circuitry and back to the power switches through an indeterminate route. The effect would be increased EMI at best and at worst chaotic operation and damage. Barrier capacitances should therefore be a few pF maximum.
Figure 2: Current and voltage transients in high-side gate drives
Even with low capacitance, the barrier of DC-DCs in the application is subject to a high electric field with implications for long term reliability. A phenomenon known as ‘Partial Discharge’ (PD) is a measure of the degradation of insulation when subject to high electric field strength. PD is caused by the gradual and sequential breakdown of micro voids in insulation leaving them carbonised and conductive. As more voids break down, the remaining insulation is subject to yet higher field strength leading to eventual run-away and total failure. PD has a definite ‘inception’ voltage below which no breakdown occurs and a lower ‘extinction’ voltage. In DC-DCs designed for high-side gate drives, the inception voltage should exceed the normal operating voltage by a good margin to indicate that long-term degradation will not be a problem. Testing for PD is complex and dedicated equipment is necessary that can detect inception, typically at several kV, with discharges measured in pico-coulombs.
PD testing proves long-term reliability
Manufacturers of DC-DCs such as Murata have done extensive testing for PD on their products, both on general-purpose types and those intended for high-side gate drive applications. Some test results are shown in Figure 3 for Murata parts showing worst-case inception voltages well over 3kV for their MGJ series, designed for gate drive applications.
The NCM6 series shows lower inception values even though it has high levels of safety agency rating, demonstrating that PD and safety rating are not necessarily correlated. The difference in this case being the construction of the barrier – the MGJ series is solid material whereas the NCM6 is multilayer.
Figure 3: PD test results on Murata DC-DCs
In many instances, the DC-DC converter needs low capacitance, good PD performance and a safety agency rating as well. The control circuitry is often accessible to touch through interfaces, for example, and the DC supply to the power switches could be several kV. Safety rules therefore apply and the DC-DC needs to have reinforced isolation, mandating creepage and clearance distances and an adequate insulation system. For industrial systems utilising three-phase high voltage supplies, the requirements can be extreme – for example 690VAC systems need 14mm creepage for compliance with the common standard EN 61800-5 for safety in motor drives.
Murata features high-side gate driver DC-DCs in its portfolio at power levels from 1W to 6W, including parts with multiple isolated outputs for all high side drives in half, full and three phase bridges. All DC-DCs hold safety agency certifications including a 6W part with 14mm creepage for 690VAC reinforced isolation. Download the brochure to find out more.
Aimtec also has solutions at the 2W power level with high operating temperature to 105°C.
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