Gate drivers — the critical link in power-device performance: part 3

This section will discuss some of the many subtleties of the gate driver and its power source.

Other gate-driver converter considerations

Gate-driver dc-dc converters have other unique issues. Among them are:

1) Regulation: the load on the dc-dc converter is close to zero when the device is not switching. However, most conventional converters require a minimum load at all times; otherwise, their output voltage can increase dramatically, potentially reaching the gate breakdown level.

What happens is that this high voltage is stored on the bulk capacitors, such that when the device starts to switch, it could see a gate overvoltage until the converter level drops under normal load. A dc-dc converter that has clamped output voltages or very low minimum load requirements should therefore be used.

2) Start-up and shutdown: it is important that IGBTs and MOSFETs not be actively driven by the PWM control signals until the drive-circuit voltage rails are at their designated values. However, as the gate-drive converters are powered up or down, a transient condition may exist where devices could be driven on, even with the PWM signal inactive, leading to shoot-through and damage. Therefore, the dc-dc converter outputs should be well-behaved on power-up and down with monotonic rise and fall as shown in Figure 1.

Figure 1. It is critical that the dc-dc converter outputs are well-behaved during power-up and down sequences and do not have voltage transients. (Image: Murata Power Solutions)

3) Isolation and coupling capacitance: at high power, power inverters or converters 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 requirements, the gate-drive PWM signal and associated drive power rails of the high-side switches require galvanic isolation from ground, with no ohmic path between them. Furthermore, the isolation barrier must be robust and exhibit no significant degradation due to repeated partial discharge effects over the design lifetime.

Additionally, there are issues due to capacitive coupling across the isolation barrier, which is analogous 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/μsec), and even as high as 100 kV/μsec for the latest 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 × (dV/dt), even a small barrier capacitance of just 20 picofarads (pF) with 10 kV/μsec 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 the operation of the controller and even the 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, which 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 regulatory agency mandates apply for the level of isolation required (basic, supplementary, or reinforced), operating voltage, pollution degree, overvoltage category, and altitude.

For these reasons, only gate-drive dc-dc converters with suitable design and materials are recognized or are pending recognition to UL60950-1 for various basic and reinforced levels of protection (and which are generally equivalent to those in EN 62477-1:2012). More stringent recognition is also in place or pending for the medical standard ANSI/AAMI ES60601-1, which includes 1 × Means of Patient Protection (MOPP) and 2 × Means of Operator Protection (MOOP) requirements.

4) Common-mode transient immunity: CMTI is an important gate-driver parameter at higher switching frequencies where the gate driver has a differential voltage between two separate ground references, as is the case for isolated gate drivers. CMTI is defined as the maximum tolerable rate of rise or fall of the common-mode voltage applied between two isolated circuits and is specified in kV/µsec or volts per nanosecond (V/nsec).

Having a high CMTI means that the two sides of an isolated arrangement—the transmit side and receive side—exceed the datasheet specifications when “striking” the insulation barrier with a signal having a very high rise (positive) or fall (negative) slew rate. The dc-dc converter datasheet should have a specification value for this parameter, and designers need to match it to the specifics of the operating frequency and voltage of their circuit.

Summary

Selecting the appropriate MOSFET or IGBT device for a switching power design is a critical step in the design process, but it is only one part of the overall signal chain. There’s also the associated gate driver, which controls the switching device, flipping it between on and off states quickly and crisply. In turn, the driver itself needs a suitable dc-dc converter to provide its operating power. Various vendors offer dc-dc converters with the requisite electrical performance that also meet the many complicated safety and regulatory mandates stipulated for this function.