For high-side gate drivers in power modules that use half-bridge, full-bridge and totem-pole topologies, a common wa y of providing the necessary high-side bias rail is to use a seemingly simple bootstrap circuit. This approach almost appears to offer something for nothing – a bias supply in return for just a handful of simple components. But, as with most cases that seem to offer something for nothing, there’s a catch. And with the bootstrap circuit the catch is that selecting the right combination of components is fraught with difficulties.
Common performance trade-offs
The three essential components of the bootstrap circuit are a diode, a capacitor and, in some cases, a resistor. The circuit is arranged so that when the low-side switch is turned on, the capacitor is charged via the diode almost to the supply voltage Vdd. The capacitor subsequently provides the drive power for the high-side switch. So far, so easy, but looking a little closer reveals that things are not as straightforward as they seem. The diode must have a higher reverse voltage rating than the HV bus and, especially in designs that use a high switching frequency, it must be a fast recovery type. This means it’s likely to be a large and relatively expensive component.
Very often, a resistor is needed in series with the diode to limit the amplitude of the charging current spikes into the capacitor. Without this limiting resistor, the spikes can appear in the low-side current sense signal to the controller and cause false over-current tripping. However, if the value of the resistor is too high or the capacitor is too large, it can take a long time to charge the capacitor at start up, which delays the availability of high-side drives. Conversely, if the capacitor is too small, it will not be able to supply the required gate charge for the high-side switch without allowing the gate voltage to droop too much.
Eliminate issues with a self-powered gate driver
When these points are considered together it becomes clear that optimising that ‘simple’ bootstrap circuit is in reality far from simple: there are always trade-offs relating to component values and costs. Heyday self-powered gate drivers with Power-Thru™ technology eliminate all of these problems. They incorporate internal circuitry to provide the bias supply for the high-side switches, so bootstrap circuitry is no longer needed. The only external component required is a single capacitor and selecting the optimum value for this in a particular application is straightforward.
And there are further benefits. Eliminating the bootstrap not only saves money, it also reduces component count and allows smaller, more compact control boards to be developed.
Bootstrapping is a potentially attractive option for providing bias voltage in gate drivers but achieving an optimised, cost-effective design can be challenging. Heyday self-powered Power-Thru™ gate drivers offer a complete and easy-to-implement solution to this problem. To find out more, watch our webinar on gate drivers. For further information, simply click here.