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The Half Bridge

This section briefly covers the half bridge switching topology and why we use gate drive transformers to provide isolated drive signals to the floating FET.

 

Layout

This is quite a common circuit in SSTC use, which generates a voltage swing across the load of +0.5Vcc to -0.5Vcc. One end of the load (an inductor in this case representing the primary winding) is tied to a point between two capacitors to fix it at 0.5 Vcc. The other end is switched between the +ve Vcc rail and the ground.

Below is a diagram of a simplified half bridge showing the power supply, capacitors, load, switches and drive signals.

A full bridge, which is the most common switching circuit in SSTC duty, consists of two half bridges driving each end of the load out of phase with each other.

 

Voltages

The voltage waveform at the centre point (marked Vc on the above diagram) switches alternately between 100V and 0V (as illustrated in the below plot) as the switches alternately connect this point to the positive and then negative supply rails.

This voltage in the above trace represents the voltage on the source of M1, the top FET. To switch this voltage up to 100V, this M1 transistor has to be turned on. Remembering the page on MOSFETs, we know that we need around 12V on the gate, relative to the source, to turn the FET on.

This means we have to generate a voltage greater than this 100V - 112V in this case. See the below waveform, with the required gate drive voltage in red.

This requires the generation of a voltage higher than any we already have in this circuit (12V higher than the maximum VCC rail). This can be done using several methods including...

  • High Side gate drive ICs using bootstrapping like the output stage of the IR2085S from IRF. Also see this TI paper on MOSFET driver design
  • Isolated power supplies and local floating drivers (like those used in this paper)
  • Gate Drive Transformers (GDT) to provide electrical isolation and floating drive signals

 

Implementation

For this page, we are going to focus on GDT design. This is how we could implement a GDT in our half bridge.

One end would be driven with the switching signal and the other end connected between gate and source of the top FET. This effectively shifts the level of the drive signal so that instead of being referenced to ground (like the bottom switch), it is referenced to the source of the top switch - exactly what we want!

It is also worth noting at this point that the voltage across the windings of the transformer is equal to the supply voltage so the wire insulation must be able to stand this voltage without breaking down, otherwise damage to the driver could occur. Include a safety margin, using wire that is rated for twice the highest supply voltage that is used.

However, we would use the GDT to drive both FETs - the low side and the high side - for reasons covered in the leakage inductance and shoot through section.