Figure 1. The basic DAB configuration
Some key characteristics:
✅ It’s a high-power isolated DC/DC converter
✅ It’s bidirectional
✅ Low EMI – Soft switching
✅ Symmetric, simple construction, few parts
✅ Low cost
✅ High efficiency: typically 93-98% depending on operating conditions
As the name suggests, we are talking about x2 full bridges on the left and on the right of the transformer. Looking at Figure 1 the first thing you can notice is symmetry.
- Either V1 supplies power to V2, or the other way around.
- It all has to do with the signals driving the mosfets.
So, the converter is indeed bidirectional. Another very important characteristic is that under conditions all x8 mosfets can experience ZVS at turn-on. Turn-off losses exist, but they are usually minimal, thus allowing the topology to achieve high efficiency numbers with low EMI.
Which applications are a good fit for DAB?
👉 EV Chargers.
The trend is to allow electric cars not only to be charged from the grid, but to be able to supply power back to it, acting like capacitors stabilizing the grid during peak power periods. The nice thing about this topology is that many submodules can be paralleled to achieve high power charger solutions for cars.
👉 UPS systems in general.
Again, taking advantage of the bidirectional power feature: the same thing as EV chargers, but on a smaller power level. A charger usually supplies power to the battery from a high voltage DC bus, thus using the converter in “buck” mode. When the power is down or the batteries need to supply power to the system, the DAB works in “boost” mode maintaining a fixed DC bus voltage. From that point on, the DC bus can supply a DC/AC converter for example.
Basic control waveforms
In Figure 2 the simplest modulation technique is shown. It is called SPS (Single Phase Shift) because there is one control variable in the system, and that is the phase shift between the bridges.
⚠️ Let’s assume that V1 is a high voltage DC line, whilst V2 is a low voltage output. I’ll call Q1-Q4 (from Figure 1) bridge HV bridge and Q5-Q8 LV bridge.
In SPS control method:
⭐️ Each bridge has a 180° phase shift fixed between its legs. For example in the HV bridge Q1,Q4 has a phase difference of 180° with respect to Q2,Q3.
⭐️ Only the phase shift between the two bridges is changing. Maximum power transfer occurs when the Δφ=90° between the bridges.
Power transfer direction?
👉 Power is transferred from HV bridge to LV bridge when there is a positive Δφ, taking the HV bridge as reference for our signals.
👉 If there is a negative Δφ, then the power is transferred from LV bridge to HV bridge.
In Figure 2, taking as reference Q1,Q4 we can check that the corresponding Q5,Q8 are delayed, or in other words, they a positively phase shifted by some degrees. Therefore power is transferred from HV bridge to the LV bridge.
Switching stages
Before getting into each stage, notice that IL is the current of the leakage inductance of the transformer (not shown in Figure 1). Usually that inductance isn’t enough, and we end up using a series inductor at the primary of the transformer.
