For this reason, an effective driver design requires both positive and negative voltage rails for the gate- drive function. However, unlike most bipolar DC/DC converters which have symmetrical outputs (such as +5 V and -5 V), the supply rails for the gate driver are usually asymmetrical with a positive voltage that is larger than the negative voltage. Sizing the converter’s power rating A critical factor is how much current the gate-driver converter must provide, and thus its power rating. The basic calculation is fairly straightforward. In each switching cycle, the gate must be charged and discharged through the gate resistor R g . The device’s datasheet provides a curve for the gate charge Q g value, where Q g is the amount of charge that needs to be injected into the gate electrode to turn ON (drive) the MOSFET at specific gate voltages. The power which must be provided by the DC/DC converter is derived using the formula: Where Q g is the gate charge for a chosen gate voltage swing (positive to negative), of value V s and at frequency F. This power is dissipated in the internal gate resistance (R int ) of the device and
switching. However, most conventional converters need a minimum load at all times; otherwise, their output voltage can dramatically increase, possibly up to 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. ■ 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 (Figure 8). ■ Isolation and coupling capacitance: At high power, power inverters or converters
external series resistance, R g . Most gate drivers need a power supply below one to two watts. Another consideration is the peak current (I pk ) required to charge and discharge the gate. This is a function of V s , R int , and R g . It is calculated using the formula: In many cases, this peak current is more than the DC/DC converter can provide. Rather than go to a larger, more costly supply (that is operating at a low duty cycle), most designs instead supply the current using “bulk” capacitors on the driver supply rails, which are charged by the converter during low-current portions of the cycle. Basic calculations determine how large these bulk capacitors should be. However, it is also important that they have low equivalent series resistance (ESR) and inductance (ESL) so as to not impede the transient current they are delivering. Other gate-driver converter considerations Gate-driver DC/DC converters have other unique issues. Among them are: ■ Regulation: The load on the DC/DC converter is close to zero when the device is not
we get technical
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