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Bridges have come a long way from the old tree fallen across the creek, with man-made bridges getting people safely over all kinds of obstacles.
Bridges connect cities and states separated by rivers (like Portland-Vancouver), enable simple and safe commerce and family travel, provide habitat for trolls, and are just plain fun to ride across on your bike. Bridges can be expensive or cost effective, beautiful and elegant, or clunky/bulky and ugly, safe or wobbly. Everything depends on the designer and the builder.
Bridges are also an important circuit topology in electrical/electronic circuits. The wide variety of bridge circuits include:
- the Wheatstone bridge used in sensors and measurement tools
- the diode bridge rectifier widely used in power supplies converting AC current to DC current
- the H-bridge used for bi-directional DC motor control
- and sometimes you get a solder bridge, but let’s try to avoid those
The H-bridge does not mean half a bridge (though you can have a “half-H” bridge), but is a reflection of the topology with 4 switches arranged with the load bridging between the two legs. Turning on a high-side switch in one leg and the low-side switch in the opposite leg drives current in one direction. Current is driven the opposite direction by changing the pair of switches that are activated.
H-bridges for motor control used to be big and expensive, using multiple discrete components and bulky heatsinks, particularly in high-current applications. But H-bridges today are typically designed using efficient FETs and are often integrated, with the controller logic and the power switches in one relatively small chip.
A perfect example of an integrated H-bridge chip, which Stilwell Baker is using to great advantage, is the Freescale MC33926. This monolithic H-Bridge Power IC provides flexible control of an inductive load (like a DC motor) with up to 5 Amps of load current and a supply voltage of 8-28V. The IC is in a 32-pin QFN package and is packed with features, including:
- low Rds(on) of 0.12 ohms
- internal charge pump
- short circuit protection
- thermal overload protection
- wide industrial temperature range
In fact, a cap for the charge pump is really the only external component you need to add. We, of course, also added bypass caps on the supply voltage and ESD protection on the outputs. Heat sinking was accomplished simply with large copper planes in the printed circuit board (application dependent). This design provided a low cost for 5 Amp support. The chip is used in automotive applications and was readily available in volume.
Stilwell Baker engineers used a number of these MC33926 in a recent project primarily due to the level of integration for fully-protected 5 Amp capability, the flexible MCU interface, the temperature range, and the low cost. This chip enabled us to design an elegant solution and meet all the customer’s requirements. Add a PIC18 micro-controller and we have a safe, cost-effective control system with a lot of power!
Contact us if you have questions about motor control or H-bridge circuits, or need a custom controller solution.