Maximize power-device control efficiency with the right gate-driver power converter
typically use a bridge configuration to generate line-frequency AC or to provide bi-directional PWM drive to motors, transformers, or other loads. For user safety and to meet regulatory mandates, the gate-drive PWM signal and associated drive power rails of the high side switches need galvanic isolation from ground with no ohmic path between them. Furthermore, the isolation barrier must be robust and show no significant degradation due to repeated partial discharge effects over the design lifetime.
In addition, there are issues due to capacitive coupling across the isolation barrier; this is analogous
GaN devices. This fast-slewing dV/ dt causes transient current flow through the capacitance of the DC/ DC converter’s isolation barrier. Since current I = C x (dV/dt), even a small barrier capacitance of just 20 picofarads (pF) with 10 kV/μs switching results in a current flow of 200 mA. This current finds an indeterminate return route through the controller circuitry back to the bridge, causing voltage spikes across connection resistances and inductances, which can have the potential to disrupt operation of the controller and the even DC/DC converter. Low coupling capacitance is therefore very desirable. There’s another aspect to basic isolation and associated insulation of the DC/DC converter. The isolation barrier is designed to withstand the rated voltage continuously, but because the voltage is switched, the barrier can potentially degrade more quickly over time. This is due to electrochemical and partial discharge effects in the barrier material that would occur solely as a result of a fixed DC voltage. The DC/DC converter must therefore have robust insulation and generous creepage and clearance minimum distances. If the converter barrier also forms part of a safety isolation system, the relevant agency regulatory
to leakage current between the primary and secondary
windings of a fully insulated AC line transformer. This leads to requirements that the drive circuit and associated power rails should be immune to the high dV/dt of the switch node and have a very low coupling capacitance. The mechanism of this problem is due to the very fast switching edges, typically 10 kilovolts per microsecond (kV/μs), and even as high as 100 kV/μs for the latest
Figure 8: It is critical that the DC/DC converter outputs are well behaved during power- up and down sequences and not have voltage transients. (Image source: Murata Power Solutions)
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