Figure 1. The basic DAB configuration
👉 Some basic naming info and assumptions:
In a DAB there are two H-bridges. I named them HV bridge (Q1-Q4) and LV bridge (Q5-Q8), where the primary and secondary of the transformer is connected accordingly. I used this naming, making the assumption that the HV bridge sees a high voltage input (i.e. V1=400V), whilst the LV bridge sees a low voltage V2 (let’s assume that’s the system’s output).
The SPS (single phase shift) control method
☝️ First let’s remember again how the SPS control strategy works.
Looking at each H-bridge independently, they are switching with a fixed 180° phase shift between their legs. In simple words, taking the HV bridge as an example, Q1-Q4 switch ON for half of the switching period, then they turn OFF and then Q2-Q3 switch ON for the second half period. So, the input of the transformer is 50% of the time at +V1 and 50% of the time at -V1. There is a dead time between these polarity changes, but since dead time is very small compared to the switching period, we can ignore it for the moment. Right now, we don’t care about ZVS, we just want to find out how SPS works.
But there is a second phase shift! It appears as a delay in an oscilloscope, between the two bridges.
If you probed Q1-Q4 gate driving signals, using as trigger signal one of these signals, all Q5-Q7 gate driving signal would appear delayed in time.
In the paragraphs above I’ve mentioned 2 phase shifts:
✅ D1=180° fixed (between legs of a bridge)
✅ D2=x° (between the two bridges), where 90°<x<-90° *
*D2=-90° degrees means that power gets transferred in the reverse direction. If the phase shift is negative taking the HV bridge’s control signals as a reference (using one of them as trigger in a scope) the power is transferred from the LV bridge back to HV bridge. Be careful which signals are your reference in tests!
Theory says that if D2=°0 no power gets transferred, whilst |D2|=90° is the point of maximum power transfer in SPS.
💥 All we have to do is to control that phase shift, which practically means controlling the delay time between the two bridges, that’s it!
The ZVS game
For a product to be efficient and produce low EMI, the goal of achieving soft switching in all conditions is about what the engineer strives for. Without debate, we can say that soft switching pretty much ensures high efficiency and low EMI. Of course, pcb layout plays a crucial role on minimizing emissions and passing EMC standards as well.
Now I carefully said “soft switching” and not just ZVS (zero voltage switching). ZVS eliminates turn-on losses, but what about turn-off losses?
⚠️ The truth is that in SPS control method, turn-off losses exist in all switches, but the good thing is that are usually low enough, which helps achieving high efficiency numbers.
Another point is that EMI strongly depends on high dV/dt transitions, as well as parasitic oscillations that extend into the MHz territory, mainly caused by the mosfets Coss capacitances and the parasitic inductors formed by traces etc. ZVS makes sure that all capacitances are charged/discharged before the voltage transitions, ensuring that oscillations can’t occur. That alone goes a long way EMI-wise.
Practically speaking the engineer only needs to take into account the turn off losses and conduction losses to choose the thermal management scheme in his application.
ZVS for SPS control method:
➡️ A definition first:
