There is often a requirement in an electronic system to provide an isolated voltage rail or multiple rails for powering wired interfaces such as RS485, wired mBus, 4-20mA current loops and various other types. An example application would be a smart electricity meter (eMeter). Instead of producing these rails by using additional windings on the main flyback magnetics, it is sometimes more desirable to produce these rails by taking a low voltage input and performing a low voltage isolated conversion. This avoids the main flyback converter’s magnetics from becoming too large, given the stringent safety isolation voltages and creepage and clearance rules that must be applied.
Various converter topologies are frequently used, like the flyback, fly-buck (and fly-buckboost) and the push-pull converter. This article looks at another topology - the full-bridge (or H-bridge) converter. There are a number of advantages to this topology:
- the four FETs ensure the current always has a path to flow and so there is little or no overshoot voltage spike
- it is not necessary to use a centre-tapped transformer, meaning that low-cost toroid transformers can be used having one primary and one secondary winding
- there is no energy storage and the transformer is fluxed in both positive and negative directions, which means the magnetics’ size is minimized
- integrated motor-control ICs exist that integrate the FETs and current/thermal protection. Additionally, these devices’ dv/dt edges tend to be quite slow and this benefits systems trying to avoid RF emissions which can disrupt sensitive radio receivers that are also in the system.
- Some motor drive ICs have more than one H-bridge integrated and these can be controlled separately, enabling multiple full-bridge converters to be controlled using one IC. This is useful in applications where the outputs must be independent from each other and cannot be made simply by using one transformer having multiple isolated outputs.
Design of the Full Bridge DC-DC Converter
The DRV8848 is a dual H-bridge motor control IC rated up to 18Vin which makes it ideal for 12Vin designs. Each H-bridge is comprised of high-side P-FETs and low side N-FETs. An alternative architecture is a charge pump with high-side N-FETs. The charge pump can be a source of EMI and so for this EMI sensitive application the DRV8848 is a good choice. Each H-bridge is controlled separately by AINx, BINx logic inputs and each H-bridge has a programmable current limit (CL) via using an external sense resistor (Risense) per H-bridge. When triggered, the CL reacts by turning off the FETs that were ramping up the current and then by turning on the opposite pair of FETs, making the current ramp down for 20μs (the PWM Cycle) or until the next PWM switching cycle comes along. There is also a fast internal overcurrent limit, OCP, (set to 2A min) which if triggered is followed by a hiccup retry time of 1.6ms. These protection circuits are useful for isolated converters that have to withstand a fault condition overload on their output(s). Often overlooked when making discrete FET H-bridges is the undervoltage lockout function (UVLO, ~3V in DRV8848). This prevents