The full-bridge topology is a popular choice for power modules, but implementing it with conventional gate drivers requires the provision of two auxiliary high-side power rails. These are sometimes supplied by dedicated DC-DC converters, which may be an off-the-shelf component or a discrete design. Alternatively, a bootstrap circuit configuration is often used. As we shall see, however, none of these solutions is straightforward, and all of them inevitably increase the overall complexity and cost of the power module.
Challenges of DC-DC converter
At first sight, it may seem that providing an extra power rail presents few technical challenges, but the reality is rather different. When a DC-DC converter is used, it will add stray capacitance between the control ground and the source terminal of the semiconductor power switch that the gate driver is controlling. For a switch in the high-side leg of a full bridge, this additional capacitance is likely to cause common-mode and EMI issues. In addition, the HF current spikes that flow in the stray capacitance can interfere with correct functional operation.
These issues are likely to occur even if the power device is being driven across an isolation barrier or if the extra power rail is derived from an additional winding on the main system transformer. Further, when an additional winding is used, this complicates the design of the main system transformer and can have implications for safety compliance.
Challenges of bootstrap
When bootstrap circuitry is used to provide the additional power rail, this brings a fresh set of challenges. A potentially expensive and large-size high-voltage fast-recovery diode is needed. The size of the bootstrap capacitor is always a compromise – it must be small enough to recharge quickly, but large enough to supply the required gate charge for the switch without becoming discharged too quickly. And a resistor may be needed to limit the spikes of charging current flowing into the bootstrap capacitor.
There is however a better and much more straightforward solution, and that is to use Heyday Power-Thru™ gate drivers. These need no external auxiliary supply as the bias supply system is embedded within the driver - all of the components and design headaches associated with auxiliary supplies are eliminated.
Over 50% reduction in board volume
The results are impressive. Two full-bridge boards examined by Heyday which used conventional gate drivers needed 53 and 47 components respectively while the equivalent Heyday solution needed only 36 components – a reduction of 32% compared with the most complex board. The board area for the Heyday solution was 26% less than the most complex board, and the overall volume was 56% less. These are big savings that not only make the board less expensive to produce and easier to accommodate, but also increase its reliability thanks to its use of fewer components.
In short, adopting Heyday Power-Thru™ gate drivers not only eliminates major design headaches, it also makes possible the implementation of power modules that are simpler, smaller and less expensive. To find out more, join our free webinar on gate driver bias supplies on 24th June. Click here for full details and to register.